Panel 3: Botanics (Contd.)

THE USE OF ENVIRONMENTAL ISOTOPES OXIGEN-18 AND DEUTERIUM IN THE STUDY OF WATER RELATIONS OF PROSOPIS TAMARUGO PHIL.

Ramón Aravena
Environmental Isotopes Laboratory
Chilean Nuclear Energy Commission
Edmundo Acevedo
Soil-Water-Plant Relationship Laboratory
University of Chile

INTRODUCTION

A number of studies conducted at the Prosopis tamarugo Phil. forests of the Tamarugal Pampa, Atacama Desert, have shown that this species has significant potential for forest and livestock development in the area (Habit 1980, Corfo 1983). The present range and future expansion of this species are strongly linked to the water resources existing in the area. This region is characterized by a normal desert climate, with absolute lack of rainfall. Rain falls solely at the Andean High Plateau, during the Altiplanic winter, constituting the source of recharge for the Tamarugal Pampa aquifer, which, in turn, is the only water source for agricultural and mining activities —and human settlements— in the Region. This situation, added to the possible impact that the tamarugo plantations have had or could eventually have on the water balance at the Tamarugal Pampa, have prompted researchers to study the probable water sources for tamarugo. These could be the groundwater table and the environmental moisture. Research studies have shown the existence of a large soil moisture content at the thicker root proliferation layer (20–70 cm below the surface, approximately), decreasing markedly above, below and outside that zone. Trials using tritium as tracer suggested that this water could come from the atmosphere, which was interpreted as tamarugo having the capacity for absorbing water through the leaves and trasferring it to the higher root density zone (Sudzuki 1969). Later, basing on certain physical parameters, such as dew temperature and soil and leaf water potentials, it was postulated that the source for the root zone water is the groundwater table, since the necessary physical conditions for the foliar water absorption process were not present (Acevedo and Pastenes, 1980). Furthermore, a model was proposed which states that the origin of the water stored at the root layer could come both from the aquifer and the atmosphere, and that this stored water could be used by the tree during the period when the static level drops (Mooney et al. 1980).

The need to provide additional data and direct evidence on the subject prompted us to undertake some research using the behaviour of the stable isotopes oxigen-18 and deuterium — constituents of the water molecule— as tracers. The concentration of these isotopes in the water depends on the path the molecule has followed through the water cycle. In general, rainfall and, consequently, groundwater, of a given area are characterized by an isotopic composition which reflects the history of the original water vapor masses, and of geographic parameters such as altitude and latitude. Consequently, the existence of the various processes affecting the isotopic composition makes the isotopic content of the groundwater be different from that from atmospheric humidity, because of their different origin. This feature makes these isotopes excellent tracers for the study of water behaviour in the soil-plant-atmosphere system. In the case of the soil-plant system, the evaporation and transpiration processes give rise to a marked isotopic enrichment at soil and leaf levels, making it possible to follow the dynamics of these processes by the variation of the oxigen-18 and deuterium concentration in the residual water.

The causes of this isotopic enrichment are linked to the different behaviour of the various isotopic species in the liquid-vapor system. There are two effects which produce fractioning when the water evaporates from a surface or is transpired by the leaves. One of them is associated with the different vapor pressures of the species H₂¹⁸O and H² with respect to the species H₂¹⁶O, which brings about an enrichment in ¹⁸O and ²H at the liquid phase. During the change from liquid to vapor phase, at equilibrium, the lighter molecules tend to evaporate more readily. This discrimination is a function of temperature, being for instance the liquid phase 9‰ heavier in ¹⁸O than the vapor phase at 25°C.

The second effect contributing to the isotopic enrichment is known as kinetic effect, associated with the different diffusion rates of the various isotopic species resulting from their different masses.

During the molecular diffusion process of evaporation, the heavier molecules are diffused at lower rates, which tends to accumulate these species in the liquid phase. The magnitude of this effect depends on the aerodynamic conditions of the boundary layer at the leaf-atmosphere interphase, which reaches about 16% under turbulent conditions.

The changes in the isotopic composition of the water —subject to the above described processes— present a characteristic evolution, which depends on the conditions under which the processes took place. These tendencies make it possible to infer the water isotopic content at the beginning of these processes. This, in turn, makes it possible to determine the provenance of the water feeding the tamarugo and stored at the thicker root zone.

This report deals with a trial conducted in March 1984 at the Tamarugal Pampa, Atacama Desert (69° 35' W and 20° 15'–20° 30'S, at 1,200 m altitude).

MATERIALS AND METHODS

Samples were taken of tamarugo leaves and twigs, soil, groundwater and atmospheric humidity. As regards the leaves, they were sampled at two-hour intervals in 12- and 55-year-old trees, growing at different sectors of the Refresco Farm forest. Water was extracted from the leaves by vacuum distillation at 100°C, condensing the water with liquid nitrogen. Atmospheric temperature and humidity were measured concurrently with leaf sampling. The soil, at both trial sites, was sampled by strata, extracting the water by distillation and determining its moisture content by gravimetry. Atmospheric humidity was captured by pumping air through a cylinder filled with “molecular sieve” zeolites type 4A, previously dried and in a vacuum. Water was subsequently recovered from the zeolite by distillation at 450°C. This method for collecting water vapor only permits to carry out analyses for deuterium, as ¹⁸O concentration is affected by the zeolites. It was used on account of the impossibility of having cold mix (-80°C) at the trial site.

The isotopic analyses were made with a Micromass 602C mass spectrometer, with analysis reproducibility of ± 0.5‰ and ± 2.0‰, for oxigen-18 and deuterium, respectively. This includes the errors associated with the treatment of the samples and the isotopic analysis. The techniques involved are described in detail in the literature (Polastri et al. 1983). The concentration unit for oxigen-18 and deuterium is δ‰, defined as:

where:

R = ¹⁸O/¹⁶O, ²H/¹H and V-SMOW (Vienna, Sea Mean Ocean Water) the international standard used as reference.

RESULTS AND DISCUSSION

The research was conducted during the period of greatest physiological activity of tamarugo, which makes it possible to observe the soil-plant-atmosphere system at a moment of high interaction among its components. The findings are described in Tables 1 and 2.

TABLE 1
Daily variation of the ¹⁸O and ²H content in leaf water from Prosopis tamarugo Phil, Refresco, Tamarugal Pampa

TABLE 2
Soil moisture (W) and isotopic content of water sampled from the soil, atmospheric humidity, twigs (tamarugo) and phreatic aquifer. Refresco, Pampa del Tamarugal

* Under crown of a 12-year-old tamarugo.
** Under crown of a ~ 55-year-old tamarugo.

One of the important parameters influencing the daily variation in leaf water oxigen-18 and deuterium content is the daily relative humidity cycle, which suggests that there exists a relationship between the isotopic concentration in the leaves and the transpiration cycle. This can be drawn from the experimental data represented in Figure 1, showing the daily variation in leaf water oxigen-18 content. The behaviour observed is cyclic, with a decrease in δ¹⁸O values in the afternoon and at night; a minimum occurs around 7 AM (solar time) and then increases through the morning, reaching a maximum between 14 and 15 h. This evolution coincides with the relative humidity and saturation deficit curves, evidencing a minor lag at the maximum and minimum points. This lag, not observed in temperate zones, has been detected in research conducted at a semi-arid region in Brazil, and has been attributed to a slow response of the system to the changes in relative humidity (Dongmann et al. 1974, Zundel et al. 1978).

As regards the transpiration process, trials made by Förstel (1978) showed a parallel evolution of transpiration rates and the leaf water oxigen-18 content. However, studies made by other researchers have shown no clearcut correlation between these two processes (Farris & Strain 1978). The oxigen-18 data obtained in this study indicate an evolution seemingly linked to the tamarugo transpiration cycle. It may be observed in Figure la that the decrease in oxigen-18 coincides approximately with the stomatal closing process, i.e., a decrease in the transpiration rate. The values for Cwv and Δ Cwv were used to make an estimate of the transpiration rate, the higher values of which were observed between 11–13 h solar time, decreasing subsequently. As regards the ¹⁸O level at the leaves, it begins to increase around 7, coinciding approximately with the beginning of the transpiration process, and reaches a maximum enrichment around 15 h. This lag between the maximum ¹⁸O and the transpiration rate has been explained as a slow system response to environmental alterations, considering that after 13 h transpiration is still significant.

Fig. 1: Daily variation of the ¹⁸O content in tamarugo leaf water, water vapor conductivity, relative humidity and atmospheric saturation deficit. Refresco, Tamarugal Pampa.

It is worth mentioning that the isotopic enrichment observed in the leaf water with respect to the water feeding the tree is only associated with the transpiration process, as experimental evidence in literature shows that water uptake by the roots and subsequent transference to the various parts of the tree is not a fractioning process, i.e., it does not affect the isotopic composition of the water (Zimmermann et al. 1967, Förstel 1978).

As regards the main purpose of this trial —to identify the origin of the water for tamarugo and that present at the high root density zone—, was approached by paying attention to two processes which take place in the tamarugo ecosystem: evaporation from the soil and transpiration from the leaves. These processes can be observed in a δ^2H vs. δ¹⁸O plot, as they generate an evolution of the isotopic composition of the water along a straight evaporation line, which slope depends on the conditions under which the process takes place. In temperate regions, this slope has values ranging from 4 to 6 (in soils), but in arid zones it may have values below 3 (Dincer et al. 1974, Allison et al. 1983).

The data for oxigen-18 and deuterium obtained from the soil profiles (Table 2) are plotted in Figure 2. In this Figure, the straight line δ²H = 8 δ¹⁸O + 10 represents the rainwater isotopic content at local and/or regional level, valid for the great majority of the regions in the world (Dansgaard 1964). Rain falls at the Altiplanic zone in Northern Chile over a slightly different straight line (Fritz et al. 1981). As, generally speaking, the isotopic composition of ground-water represens the rainfall average weighted in time, processes such as evaporation alter the groundwater composition towards areas outside of the meteoric line. The Refresco groundwater has an isotopic composition (δ¹⁸O = -7.7 ‰ and δ²H = -60‰) which falls below the meteoric line, a fact that may be attributed to evaporation at the recharge area or which represents water that precipitated under very different climatic conditions from those present today.

Fig. 2: Water ¹⁸O and ²H content. a) Leaves (Eucalyptus sp), rainfall and “camanchaca” (heavy fog), El Tofo, Region IV. b) Leaves and branches (Prosopis tamarugo Phil.), soil, phreatic aquifer and atmospheric humidity, Refresco, Tamarugal Pampa, Region I.

The interesting feature observed in Figure 2b is the trend shown by the isotopic content of the soil water at the root zone (higher moisture content area, Table 2) and at the other strata. An enrichment in deuterium and oxigen-18 may be observed, greater for the latter isotope, which is typical of the evaporation process. These samples fall into a straight line with a slope nearing 2, consequent with values reported for arid zones (Dincer et al. 1974), and which origin is in the groundwater isotopic composition. This evolution indicates that the soil water and, consequently, that from the root zone, is groundwater, modified by the evaporation process. These evidences rule out the possibility that the water reaching the root zone comes from the atmosphere, and are corroborated by the isotopic analyses of atmospheric moisture, with values of -105‰ for δ²H, against -60‰ for the groundwater table.

Figure 2b shows the isotopic evolution of the water resulting from the tamarugo leaf transpiration process. The methodology for the analysis of the isotopic data is based on trials conducted at Region IV, at the area of Cerro El Tofo. At the top of this hill there is a stand of Eucalyptus sp which thrives solely upon its capacity for collecting camanchaca (coastal fog). This was confirmed by the isotopic evolution of the water in its leaves, which starts from the isotopic composition of the camanchaca and is subsequently modified by the transpiration process (Figure 2a). To offer a point of comparison, the isotopic composition of rainwater in this area is also represented.

A linear relationship can also be observed in tamarugo for the samples of the initial and final stages of the transpiration period. The change in tendency observed at the end of the tamarugo evaporation-from-transpiration straight line in Figure 2b stems from a marked ¹⁸O enrichment in relation with deuterium, which could reflect the extreme climatic conditions to which tamarugo is subjected. The evolution shown by these data suggests that the water reaching the leaves and participating in the transpiration process comes from the groundwater table. This is likewise confirmed by the isotopic analyses of the water at the branches, with similar values as those for the groundwater.

As a conclusion, the evidences found in this study support the statement that the water feeding the tamarugo, and that present at the high root proliferation zone, comes from the groundwater table.

Acknowledgements

The authors wish to express their appreciation for the valuable collaboration offered by Mr. Guillermo Alvarez de Araya, manager of the Refresco Farm, during the research conducted at that location. This acknowledgements are also intended for Mrs. Evelyn Aguirre and Mrs. María Paz Albornoz, for their efforts during the treatment and isotopic analyses of the samples.

Financed by the CONAF/UNDP/FAO/CHI/76/003 Project, Chilean Nuclear Energy Commission and Faculty of Agricultural, Veterinary and Forest Sciences of the University of Chile

REFERENCES

ACEVEDO E., PASTENES J. 1980. Distribución de Prosopis tamarugo Phil. en la Pampa del Tamarugal (Desierto de Atacama). Tercer Congreso Int. Zonas Aridas. La Serena, Chile.

ALLISON G.B., BARNES C.J., HUGHES M.W. 1983. The distribution of deuterium and ¹⁸O in dry soils. 2 Experimental J. Hydrol., 64; 377–397.

CHILE. 1983. Desarrollo de zonas desérticas de Chile. CORFO. Tomo 1, 56 pp.

DANSGAARD W., 1964. Stable isotopes in precipitation. Tellus, 16: 436–468.

DINCER T., AL MUGRIN A., ZIMMERMANN U., 1974. Study of the infiltration and recharge through sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium. J. Hidrol., 23: 71–109.

DONGMANN G., NÜRNBERG H.W., FÖRSTEL H., WAGENER K., 1974. On the enrichment of H₂¹⁸O in the leaves of transpiring plants. Rad and Environm. Biophys. 11: 41–52.

FARRIS F., STRAIN B.R. 1978. The effect of water stress on leaf H₂¹⁸O enrichment. Rad and Environm. Biophys. 15: 167–202.

FÖRSTEL H. The enrichment of H₂¹⁸O under field and under laboratory conditions. Proc. Third Int. Cong. on Stable Isot., Oak Brook, Ill, May 1978.

FRITZ P., SUZUKI O., SILVA C., SALATI E., 1981. Isotope hydrology of groundwaters in the Pampa del Tamarugal, Chile. J. Hydrol., 53: 161–184.

HABIT M. 1980. Contribución al conocimiento del árbol forrajero de desierto Prosopis tamarugo, Phil. FAO. Stgo. de Chile. 124 pp.

MOONEY H.A., GULMON S.L., RUNDEL P.W., EHLERINGER J., 1980. Further observations on the water relations of Prosopis tamarugo of the Northern Atacama Desert. Oecologia (Berl) 44: 177–180.

POLASTRI A., ARAVENA R., SUZUKI O., 1983. Técnicas de Análisis para ¹⁸O y ²H en agua y ¹³C en Carbonatos. Informe Técnico C.CH.E.N. 30 pp.

SUDZUKI F., 1969. Absorción foliar de humedad atmosférica en tamarugo, Prosopis tamarugo Phil. Universidad de Chile, Facultad de Agronomía. Boletín Técnico 30: 1–23.

ZIMMERMANN U., EHHALT D., MUNNICH K.O. 1967. Soil water movement and evapotranspiration: Changes in the isotopic composition of the water. Proc. Symp. on Isotopes in Hydrology, Vienna, 1966, Int., at. Energy Agency, Vienna, pp. 567–584.

ZUNDEL G., MIEKELEY W., GRISI B.M., FÖRSTEL H. 1978. The H₂¹⁸O enrichment in the leaf water of tropic trees: Comparison of species from the tropical rain forest and the semi-arid region in Brazil. Rad. and Environm. Biophys., 15: 203–212.

FOLIAR TISSUE WATER PARAMETERS OF PROSOPIS TAMARUGO PHIL.

Edmundo Acevedo
Drina Sotomayor
Virginia Zenteno
Soil-Water-Plant Relationship Laboratory
Faculty of Agricultural, Veterinary and Forest Sciences
University of Chile

INTRODUCTION

The relationship between total water potential (ψ) of a plant tissue and its relative water content (RWC), known as the presure-volume curve of the tissue, has been widely used to determine physiologically and ecologically important tissue water parameters (1,3,7,19,23). From this curve it is possible to infer the combined solute and matric potentials (ψs + ψm) and the water pressure potential (ψp) of the tissue at various ψ and/or RWC and study physiological adaptations to water stress.

Maintaining pressure or turgor potential in plant tissues as the total water potential decreases is a physiologic adaptation which makes it possible for plants to cope more effectively with water stress (12,13), helping to maintain such processes as photosynthesis, growth, ion absorption and others at low ψ values (10,2,23,8).

The mechanisms by which it is possible for the plant to maintain turgor were analyzed and discussed by Weatherley in 1970 (25), who stressed the possibility of a decrease in cell solute potential and/or an increase of the cell wall elasticity. There is evidence in literature that both processes may take place, the former known as osmotic adjustment (14), embodying a net increase in solutes at cell level. On the other hand, cell wall elasticity makes it possible to keep ψp constant even when RWC decreases (17,9,3), with greater wall elasticity observed in small-sized cells (20).

MATERIALS AND METHODS

Pressure-volume curves were plotted using a pressure bomb (18) as suggested by Tyree and Hammel (24). Six to eight cm long tamarugo terminal shoots were clipped at dawn, between 6 and 7 hours (solar time). Green weight was immediately determined, and total water potential measured, using a pressure bomb. The shoots were subsequently hydrated to maximum turgor by floating them in distilled water for four hours in Petri plates, under light intensity close to compensation point (16). The amount of time necessary to achieve maximum hydration was determined from previous trials using shoots which differed in their water potential. Excess water was removed from the hydrated shoots with blotting paper and subsequently weighed to determine turgid tissue weight. They were then introduced in a pressure bomb whose vapor pressure was kept near saturation by the addition of distilled water at the base of the bomb. Bomb pressure was increased at intervals of 0.3 to 0.5 MPa within a range of 0 to 7 MPa, weighing the tissue once it had reached pressure balance with a precision scale placed in a moist chamber. The balance between the pressure applied in the bomb and the tissue was reached in approximately 10 min. Once the water release process of the tissue was concluded, it was dried in a ventilated oven at 75°C for 48 hours. Using the turgid, green and dry weights, the relative water content (RWC) corresponding to each pressure applied with the bomb was computed. This pressure, in a state of balance with the tissue, and assuming an apoplastic water potential equal to zero, corresponds in absolute value to the symplast water potential.

The above procedure was applied to shoots clipped from a plantation of 12-year-old trees located at Refresco (Atacama Desert) in March and December 1982; from 6-year-old trees propagated naturally in an area near La Tirana (Atacama Desert) that had been flooded in 1976 from excess rainfall coming from the Andes during the altiplanic winter; and from 5- to 12-month-old seedlings grown in pots in a greenhouse at the Antumapu Campus of the University of Chile (Santiago). The seedlings were grown according to techniques indicated by Acevedo et al. (4) for a period of 12 months (January through December 1983). At the age of two months, two treatments were differentiated: one in which the plants were irrigated when tensiometers placed 12.5 cm deep in the pots indicated -0.06 MPa, and another one in which gypsum blocks placed at the same depth and calibrated to -1.5 MPa indicated, by extrapolating the calibration curve, a soil water matric potential of approximately -3.0 MPa.

RESULTS AND DISCUSSION

Figure 1 shows the average pressure-volume curve for seven 12-year-old tamarugo shoots. At the time of clipping (dawn) these shoots had an average water potential of -1.15 MPa. By plotting the inverse of the tissue water potential (1/ψ) against the relative water content for each value of the water potential, a curve is obtained which becomes straight from the point where water pressure potential becomes zero. This straight line corresponds to the change of the inverse solute potential plus the inverse matric potential of the tissue for its various relative water contents; the solute potential at maximum turgor (ψSMT) is obtained when the straight line intercepts a RWC of 1.0 (24). For tamarugo from Refresco this value was -2.5 MPa, similar or slightly higher than the one obtained for Atriplex repanda by Silva in 1982 (19). The difference between the values of ψ and (ψs + ψm) gives the turgor or pressure potential (ψp) value for each RWC. The pressure potential stayed above zero for RWC values between 0.92 and 0.72. Total water potential corresponding to ψp = 0 was -3.7 MPa for the Refresco trees.

Figure 1: Average pressure-volume curve for 7 terminal sprigs of 12-year-old Prosopis tamarugo trees. Refresco Farm, December 1982.

Literature indicates that many plants subjected to water stress show a change in the relationship of total water potential and RWC of their tissues, so that for a given ψ they have a higher RWC as the water deficit increases. This behaviour has ben observed in apples, maize and sorghum (15), soy bean hypocotyls (6), sorghum (22), foliar tissues of Atriplex repanda (19), wheat and other species (23).

Figure 2 shows that the relationship between these parameters was identical for foliar tissue of 12-year-old tamarugo from Refresco and those of the greenhouse seedlings without water stress (soil ψm of -0.06 MPa), evidencing a clear difference with the water release curves of foliar tissues from those seedlings subjected to water deficit (soil ψm of -3.0 Mpa) for a period of 210 days (Figure 2).

Figure 2: Relative water content (RWC) vs. total water potential (Ψ) in P. tamarugo terminal sprigs. Antumapu, December 1983; Refresco, December 1982.

By altering the water release curve, the tissue can maintain positive ψp values under conditions of water stress, and this constitutes the adaptation mechanism. As in other species, adaptation in tamarugo is gradual, as the curves in Figure 3 show for three dates of the acclimation period of the greenhouse seedlings.

Figure 3: Relative water content (RWC) and water potential (ψ) in sprigs from P. tamarugo seedlings with a) ψm soil = -0.06 MPa and b) ψm soil = -3.0 MPa. Antumapu, 1983.

Figure 3a shows a slight displacement of the curve caused by age for the seedlings not subjected to water deficit. This effect is very minor, however, if compared to the change induced by the deficit (Figure 3b). The result is that the tissue water relations change to maintain a given pressure potential at lower total potential (Figure 4). It is striking to observe that the foliar tissues of 12-year-old trees from the Refresco plantation have a water release curve corresponding to plants not subjected to water stress, which evidences that these trees must be absorbing water from an easily accessible source, such as the groundwater table, which in this area is located about 9 m below the ground surface. This water may be easily absorbed by the tap roots of this tree (21), and not from the upper root mass, where total soil water potential is about -2.28 MPa (5).

Figure 4: Pressure potential (ψp) vs. total water potential (Ψ) in P. tamarugo leaves in three adjustment periods (78, 185 and 210 days). Antumapu, 1983.

Naturally propagated trees, whose success in establishing depends on the liquid continuum between the ground surface and the groundwater table level (3), in their slow growth process at the expense of the water stored at the soil profile, show a water release curve resembling that for seedlings subjected to water stress under greenhouse conditions (Figure 5). The change in the water release curve, both in this case and in seedlings under water stress conditions, stems from an osmotic adjustment or decrease of the solute potential at maximum turgor, more than to a change in cell wall elasticity. The osmotic adjustment detected in 210 days of acclimation to water deficit in the greenhouse seedlings was -0.8 MPa, which was reflected by a change of the total water potential at which ψp became zero, and which fluctuated from -3.0 to -4.0 MPa as the plants grew acclimated to the water deficit. The elastic modulus of the tissue remained without significant change during the adjustment period, unlike what has been reported for other species, such as maize, sorghum (17), and Atriplex repanda (19).

Figure 5: Relative water content (RWC) vs. total water potential (Ψ) in 12-year-old P. tamarugo terminal sprigs, planted at the Refresco farm and 6 year-old trees propagated naturally in the vicinity of La Tirana. 1982.

It is concluded that the water release curves of shoots for adult (12-year-old) planted tamarugo trees indicate that the foliar tissues of the tree are not subjected to water stress; that 12-month-old seedlings of this species can adjust their tissues osmotically to cope with water deficit within a range of 0.8 MPa, managing to maintain a positive pressure potential; and that in this turgor maintenance the changes in cell wall elasticity have no effective participation.

Acknowledgements

The authors wish to acknowledge the cooperation of SACOR (CORFO) personnel during the field experiments. Thanks are alsao due to Mr. Quintiliano Gómez for the art work.

* Financial support was obtained from CORFO-SACOR (Chile)

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10. FERERES E. 1976. Growth, development and yield of sorghum in the field under variable water supply. Ph.D. Thesis, Davis, California.

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17. SANCHEZ-DIAZ M.F., KRAMER P.J. 1973. Turgor differences and water stress in maize and sorghum leaves during drought and recovery. J. Expl. Bot. 24:511–515.

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25. WEATHERLEY P.E. 1970. Some aspects of water relations. Adv. Bot. Res. 3:171–206.

CO₂ FIXATION MECHANISM OF PROSOPIS TAMARUGO PHIL.

Edmundo Acevedo
Drina Sotomayor
Virginia Zenteno
Soil-Water-Plant Relationship Laboratory
Faculty of Agricultural, Veterinary and Forest Sciences
University of Chile
Ramón Aravena
Applications Department
Chilean Nuclear Energy Commission

INTRODUCTION

From a physiological standpoint, it is interesting to know the CO₂ fixation mechanism and carbon metabolism for a given species. Three large plant groups are distinguished with regard to CO₂ fixation: C₃, C₄ and CAM. Each of these groups has physiological characteristics which, within certain ranges, are common (20, 7), such as the daily stomata opening cycle, water use efficiency (WUE), daily titrable acidity fluctuation of the photosynthetic tissue, the first compound formed when CO₂ is fixed and the catalyzing enzyme for the process. The latter can be determined through isotopic discrimination between carbon-12 and carbon-13 (25, 5), and the other characteristics by traditional techniques.

The following table shows both anatomic and physiologic characteristics of the three plant groups mentioned.

TABLE 1

A large proportion of opened stomata has been reported for Prosopis tamarugo during the night and a lesser proportion during the day (8), which would suggest inverted stomatal rhythm, characteristic of plants with crassulaceous metabolism (CAM). Furthermore, it has been suggested in India (13), basing on changes in titrable acidity of Prosopis juliflora leaves, that this species could have CAM behaviour under natural conditions.

The desert environment in which Prosopis tamarugo grows (1), together with the above evidence, suggested that this species could have adopted physiological strategies that would optimize water use efficiency, among them, the CO₂ fixation mechanism.

This work studied —under both natural and greenhouse conditions, and under two extreme water regimes—daily stomata rhythm, leaf tissue δ¹³C, foliar tissue titrable acidity fluctuations and WUE. The last parameter was studied only under greenhouse conditions.

MATERIALS AND METHOD

Field measurements were carried out at an adult (50-year-old) tamarugo stand of the Refresco Farm, Tamarugal Pampa (19°33' S and 69°–70° W). Five to twelve month-old tamarugo seedlings raised in a greenhouse in Santiago were also used. Seeds were planted in a 3:1 (by volume) coarse sand-organic mix, where the plants were kept for approximately 60 days. The seedlings were subsequently transplanted into pots 25 cm in length and 16 cm in diameter. The pots were lined on the inside with black polyethylene bags perforated at the bottom and then filled with 4 kg of sandy clay loam soil sieved to 4 mm; electric conductivity of the saturation extract was 0.5 mmhos/cm at 25°C, pH 7.1 and 2.4% organic matter. One seedling was placed per pot, and the soil surface was covered with a 1.5 cm quartz sand layer and a plastic cover to prevent water loss by direct evaporation.

The pots were placed in the greenhouse on wooden benches 20 cm high in a totally randomized experimental arrangement, with two treatments and 6 replications. Twelve pots were used per treatment in each replication. One of the treatments consisted of irrigating the pots when tensiometers placed at 12.5 cm depth inside the pots read -0.06 MPa matric potential (wet treatment). In the second treatment the pots were irrigated when gypsum blocks placed at the same depth as the tensiometers indicated soil water matric potential of around -3.0 MPa, basing on an extrapolation of the gypsum block calibration curve made previously in a pressure plate up to -1.5 MPa (dry treatment). The amount of water applied in each irrigation was such as to bring the soil to field capacity by weight. Disturbed soil field capacity had been previously determined by the method suggested by Silva (22). Treatments were conducted over a period of seven months, from June through December 1983; greenhouse climatic conditions for this period are presented in Tables 2 and 3.

TABLE 2
Greenhouse relative humidity (%)

TABLE 3
GREENHOUSE TEMPERATURES (°C)

Stomata behaviour. The daily course of stomata opening was determined with a LI-COR diffusion porometer equiped with an LI 20S sensor with a 2-cm² opening. As the tamarugo leaflets did not cover totally the opening, the corresponding area correction was made. Readings were taken over 24 h periods at approximately 1 h intervals. Three to four readings were made each time, calibrating the porometer immediately in situ and correcting the values obtained to a temperature of 25°C. Results were expressed in terms of water vapor conductivity (Cwv). Simultaneously, photosynthetically active radiation (PAR) bearing upon a horizontal plane was measured with an LI 170 radiometer (LI-COR Inc.) equipped with a quantic sensor, and relative humidity with an Assman psychrometer with forced ventilation. This latter reading made it possible to know the atmospheric saturation deficit (SD).

Titrable acidity. At 1 h intervals over a period of 24 h, 100 g of tamarugo leaflets were sampled in triplicate, then macerated in mortars with 2 g of quartz sand (previously washed with hydrochloric acid and distilled water); 10 ml of distilled water were added for maceration and the process was carried out for three minutes. Twenty additional ml of distilled water were then added and the macerated sample was titrated with 0.01 N NaOH at pH 6.4 with a Lilliput Digital portable pH meter model 765, according to the method described by Hartsock and Nobel (14). Once the moisture percentage of the leaves was known, titrable acidity was expressed in μeq of acid per g of dry matter. The PAR bearing upon a horizontal plane was simultaneously determined, together with total water potential in terminal shoots by means of the pressure bomb method (21).

Carbon-13. Tamarugo leaves and shoots were oven-dried at 75°C, ground and burned. The resulting gas, CO₂, was then analyzed with a Micromass 602C mass spectrometer, with analysis reproducibility of ± 0.1 ‰, according to the technique described by Aravena (3). Carbon-13 concentration was expressed in δ‰ defined as

where R = ¹³C/¹²C. The international standard used as reference was PDB (Pee Dee carbonate, South Carolina).

All the above measurements were made both at Refresco trees (50-year-old plantation) and greenhouse seedlings, with the two water treatments.

Water use efficiency (WUE). WUE determination requires knowing the amount of dry matter produced and the water transpired by the plant. It is convenient, for this purpose, to use seedlings growing in pots. The amount of water transpired was measured gravimetrically, taking each pot as a phytometer. Weight was measured using a scale with ± 25 g sensitivity, which, for pots with an approximate weight of 5,400 g, gives an error of around 0.02%. Dry matter was obtained from periodic harvests, every 45 days, sorting out leaves, stems and roots, which were oven-dried at 70°C for 48 hours. The slope of the curve, relating the dry matter produced to the water transpired, corresponds to the WUE.

RESULTS AND DISCUSSION

Figure 1 shows the values for water vapor conductivity, atmospheric saturation deficit, photosynthetically active radiation and total water potential of shoots of an adult tamarugo at Refresco. It is evident from Fig. la that stomata open during the day, with a maximum opening between 8 and 9 h (solar time), to close subsequently in a gradual process during daylight hours, staying closed during the night. Associated with the beginning of the stomatal closing process there is an increase in the atmospheric saturation deficit and a decrease in foliar tissue water potential. Stomata remain open only in conditions which determine a relatively low transpiration, at low SD of the atmosphere (0.6 μmol/cm³) and a high photosynthetic efficiency (9, 17) at low PAR levels (500 μE/m²s). Thereafter they begin to close, probably affecting more transpiration than photosynthesis (12, 15). It is likely that this behaviour tends to optimize water use efficiency in this species, according to Farquhar and Sharkey (11) and Cowan and Farquhar (10). The stomatal behaviour observed differs from that observed by Botti (8) and Sudzuki et al. (23), as Cwv decreases during the night with regard to its maximum value, which takes place during the day (0,02 vs. 0,2 cm/s). The behaviour reported here is typical of a plant with C₃ or C₄ metabolism, and by no means one with CAM metabolism. The discrepancy with the above authors regarding stomatal behaviour can be attributed to different research techniques, as they used impressions on silicone (Sil 21) which could induce a reaction in the stomata as they were taken, or else give a false impression of aperture (4).

Figure 1: Photosynthetically active radiation (PAR) (a), Atmospheric saturation deficit (SD) (b) and water vapor conductivity (Cwv) (c) of Prosopis tamarugo leaves. Refresco, March 1984.

The stomatal behaviour was similar at greenhouse and Refresco: maximum opening took place at around 7–8 h (solar time) in the greenhouse plants, but closing was much faster. It is probable that the Refresco trees had gone through a stomatal adjustment process with regard to the environment as compared to the greenhouse seedlings. Stomatal adjustment has been observed for several species (16).

Figure 2 : Total water potential (Ψ) in terminal shoots (a), atmospheric water saturation deficit (SD) (b), water vapor conductivity (Cwv) in P. tamarugo leaves with ψm soil = -0.06 MPa (c) and water vapor conductivity (Cwv) in P. tamarugo leaves with ψm soil = -3.0 MPa (d). Antumapu, January, 1984.

Figure 3 shows the daily course of the PAR and titrable acidity to pH 6.4 in leaves from adult Refresco trees and from greenhouse seedlings subjected to wet and dry treatments. In both cases the titrable acidity fluctuates between 30 and 60 μeq/g of dry matter. Plants with crassulacean acid metabolism show a much higher daily organic acid fluctuation than the values found in this study, in the range of 100 to 200 μeq/g of green weight (24), with a clearly defined sequence of a minimum at dusk and a maximum at dawn (refer, for example, to Acevedo, Badilla and Nobel, 1983 [2]). The values in Figure 3 do not show this tendency, and their dispersion may be attributable to experimental errors. Some species show a shift from C₃ metabolism to CAM as they undergo water stress (24); this behaviour, however, was not observed in titrable acidity changes for tamarugo seedlings under -3.0 MPa soil water potential. The plotted points for seedlings with or without water deficit in Figure 3 are hard to tell apart.

Figure 3: Daily course of tritable acidity to pH 6.4 for P. tamarugo. 7–8 (x) and 17–18 (o) December 1982, Refresco (a); December 1983, Antumapu (b).

Table 4 includes the values for δ ¹³C obtained in leaves from seedlings grown in greenhouse with and without water stress, together with an analysis of leaves and twigs of adult tamarugo from Refresco. δ ¹³C values (Table 4) fluctuate between -24.9‰ and -26.3‰.

TABLE 4

Carbon-13 content in leaves and twigs from Prosopis tamarugo seedlings and adult trees with and without water deficit

These findings point to the fact that tamarugo uses the Calvin-Benson cycle for CO₂ uptake, i.e., it is a plant with C₃ metabolism. The data obtained rule out a C₄ metabolism, whose values for δ ¹³C fluctuate between -9‰ and -16‰, as well as CAM metabolism, since if this were the case, seedlings subjected to water stress would have shown more positive δ ¹³C values (6, 19).

Figure 4 shows that the water use efficiency for this species is low —1.2 mg dry matter/g of water transpired—, a value which falls within the range of C₃ plants. Figure 5 shows that in plants subjected to water stress this efficiency tends to increase slightly, a fact probably associated to anatomical and morphological changes in the leaves (18).

Figure 4: Dry matter produced and water transpired in P. tamarugo. The straight line joins points estimated by a regression (Δ—Δ) with r² = 0.892, obtained with data from the moist treatment (.) and the dry treatment (x). Water use efficiency was estimated at 1.2 mg DM/g H₂O transpired. Antumapu, 1983.

Figure 5: Water use efficiency in 5- to 12-month-old P. tamarugo seedlings under two water treatments: a) irrigation at ψm soil = -0.06 MPa (.—.) and b) irrigation at ψm soil = approximately -3.0 MPa (.— — —.).

From this research it is concluded that Prosopis tamarugo is a legume tree with C₃ metabolism. It apparently can survive in extremely arid environments, as the Atacama Desert, due to the availability of groundwater and to its strategy of partially closing the stomata during the day.

Acknowledgements

The authors wish to thank Prof. Herman Silva for his help in the discussion of the water use efficiency study, and Mr. Quintiliano Gómez for the drawings.

* Financial support was obtained from CORFO-SACOR (Chile) and the CONAF/UNDP/FAO/CHI-76-003 Project.

REFERENCES

1. ACEVEDO E., PASTENES J. 1980. Distribución de Prosopis tamarugo Phil. en la Pampa del Tamarugal (Desierto de Atacama). III Congreso Internacional de Zonas Aridas. La Serena, Chile. Enero, 1980.

2. ACEVEDO E., BADILLA I., NOBEL P.S. 1983. Water relations, diurnal acidity changes and productivity of cultivated cactus, Opuntia ficus-indica. Plant. Physiol. 72:775–780.

3. ARAVENA O. 1983. Técnica de análisis de carbono-13, oxígeno 18 e hidrógeno-2 en materia orgánica.

4. BARRS D. 1968. Determination of water deficits in plant tissues. In: T.T. Kozlowski ed. Water deficit and plant growth. pp. 236–368.

5. BENDER M. 1971. Variations in the ¹³C/¹²C ratios of plants in relation to the pathway of photosynthetic carbon dioxide fixation. Phytochemistry 10:1239–1244.

6. BENDER M., ROUHANI I., VINES M., BLOCK C. 1973. ¹³C/¹²C ratio changes in Crassulacean acid metabolism. Plant Phisiol. 52:427–430.

7. BOTINI R. 1981. Fotosíntesis. Sus implicaciones agronómicas. Serie Ciencias. Cuadr. No. 1. Univ. Nacional de Río Cuarto. Argentina, 24 p.

8. BOTTI, C. 1970. Relaciones hídricas del tamarugo (Prosopis tamarugo Phil.) en la localidad de Canchones. Stgo., Univ. de Chile, 65 p. (Degree thesis for Agr. Eng. mimeo).

9. CHANG Y.H. 1968. Climate and Agriculture. (Aldine Publishing Co.), 304 p.

10. COWAN I.R., FARQUHAR G.D. 1977. Stomatal function in relation to leaf metabolism and environment. In: Integration of Activity in the Higher Plant. D.H. Yennings ed. pp. 471–505.

11. FARQUHAR G.D., SHARKEY. 1982. Stomatal Conductance and Photosynthesis. Ann. Rev. Plant Physiol. 33:317–345.

12. GAASTRA P. 1959. Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature and stomatal diffusion resistance. Medad Landbank Wageningen 59:1–68.

13. GAUD Y.D. 1968. Preliminary studies on titrable acidity in xerophytic plants: Salvadora persica. Linn. and Prosopis juliflora.

14. HARTSOCK T.L., NOBEL P.S. 1976. Watering converts a CAM plant to daytime CO₂ uptake. Nature. 262:574–576.

15. HSIAO T.C., ACEVEDO E. 1975. Plant responses to water deficits, water use efficiency and drought resistance. In: F.G. Stone ed. Plant Modification for more Efficient Water Use (Elsevier) pp. 59–84.

16. LUDLOW M.M. 1980. Adaptative significance of stomatal responses to water stress. In: Adaptations of Plant to Water and High Temperature Stress. N.C. Turner and P.J. Kramer eds. pp. 123–138.

17. MILTHORPE F.L., MOORBY J. 1974. An Introduction to Crop Physiology (Cambridge University Press). 202 p.

18. NOBEL P.S. 1980. Leaf anatomy and water use efficiency. In: Adaptations of Plant to Water and High Temperature Stress. N.C. Turner and P.J. Kramer eds. pp. 43–55.

19. OSMOND C.B., ALLAWAY W.G., SUTTON B.G., TROUGHTON J.H., QUEIROZ O., LUTTGE V., WINTER K. 1973. Carbon isotope discrimination in photosyntesis of CAM plants. Nature, 246:41–42.

20. SALISBURY F.B., ROSS C.W. 1978. Plant Physiology. (Second. Ed. Wadsworth Pub. Co. Inc.) Belmont. California. 422 p.

21. SCHOLLANDER P.F., HAMMEL H.T., BADTREET E.D., HEMMINGSEN E.A. 1965. Sap pressure in vascular plants. Sci. 148:339–345.

22. SILVA H. 1982. Acondicionamiento a la sequía, crecimiento y eficiencia en el uso del agua en Atriplex repanda Phil. Thesis for M.S. Fac. de Ciencias, Univ. de Chile. 197 p.

23. SUDZUKI F., BOTTI C., ACEVEDO E. 1973. Relaciones hídricas del tamarugo en la localidad de Canchones. Bol. Téc. Est. Exp. Agr. Univ. Chile 37:2–23.

24. TING I.P., RYDER L. 1982. Regulation of C₃ to CAM shifts. In: I.P. Ting and M. Gibbs eds. Crassulacean Acid Metabolism. pp. 193–207.

25. TROUGHTON I.H. 1972. Carbon isotope fractionation by plants. In: Proc. 8th Conf. Radiocarbon Dfing. Wellington. N.Z. pp. E20–E57.

WATER UPTAKE AS AFFECTED BY THE ENVIRONMENT IN PROSOPIS TAMARUGO PHIL.

Edmundo Acevedo
Drina Sotomayor
Virginia Zenteno
Soil-Water-Plant Relationship Laboratory
Faculty of Agricultural, Veterinary and Forest Sciences
University of Chile

INTRODUCTION

The notion that Prosopis tamarugo Phil. might have the capacity for absorbing atmospheric moisture through the leaves (11) gave rise to considerable interest in the water relations of this species, which grows at an area with virtually total absence of rainfall. Acevedo and Pastenes (1) provided data indicating that tamarugo only grows on areas where the groundwater table is located between 4 and 16 m below the surface, and concluded that it was unlikely that the atmospheric conditions would warrant foliar water uptake of any significance, suggesting that the tap roots described by Toro (12) and Sudzuki (11) must reach the water table (or its capillary fringe) to supply the water required by the tree. Other data reported by these authors (1) indicate that the water rising through the soil as vapor from the water table —at a place where the latter is located 9 m below the ground, representative of the type of site where tamarugo occurs most frequently — only amounts to 9 mm a year, a negligible amount when the growth of this leguminous tree is considered. At the same time, Mooney et al. (8), based on existing data and some field measurements, concluded that this was a phreatophyte species, although they did not provide any direct evidence, nor did they discard the possibility that in Springtime a reverse water flow from the leaves to the dense root proliferation zone could take place; this denser root zone occurs usually under the crown projection area of the tree at a depth ranging from 30 to 100 cm (12, 11, and Acevedo, personal observations). As regards foliar uptake, Went (13) concluded, in a climatic analysis of the Tamarugal Pampa, that there was enough moisture in the air column under the crown of the tree to supply a water loss to support its growth, providing this water condenses as dew on the leaves.

This work analyzes the water relations for tamarugo, considering the possibilities of dew formation on the leaves, furnishing relevant additional information on the environmental parameters of the soil, climate and water where this species grows, in order to characterize its water uptake.

MATERIAL AND METHODS

The research was conducted at Refresco and La Tirana, both located at the Tamarugal Pampa, in March and December 1982, with 15-day field observations in each occasion.

Outer foliole temperature was measured for a 12-year-old tamarugo tree with 0.3-mm-diameter copper-constantar thermocouples inserted in the leaves; it was read in an MJ-55 microvoltmeter (Wescor). Thermocouples were inserted right below the epidermis of the adaxial face of leaves having a maximum emissivity. Simultaneously, relative atmospheric humidity was read with a forced-ventilation Assman psychrometer, and total water potential and its components measured in 6–8 cm terminal shoots with a pressure bomb (10). Readings were taken both on overcast and cloudless days at approximately 1-hour intervals over 24-hour periods. With these background data, the water vapor conductivity (Cwv) at the turbulent air and leaf mesophyll level was determined, together with the dew point of the leaves.

Pits were dug both beneath and outside the crown projection area, taking triplicate soil samples at various depths to be subsequently analyzed at the laboratory for moisture content (w), electric conductivity of the saturation extract and total water potential. The soil samples were placed in aluminum boxes which were then hermetically sealed with adhesive tape. Total water potential (ψ) was determined with a Richards & Ogata type psychrometer, and the electric conductivity of the saturation extract (CE) with a conductivity cell.

RESULTS AND DISCUSSION

Figure 1 shows representative leaf, ambient and dew point temperature curves for cloudy and cloudless days. Except for one point —and for a very short period of time— the leaves did not reach the dew point, confirming findings previously reported by Acevedo and Pastenes (1) for February and August. From the evidence gathered, it may be concluded that the probability of dew formation at the periphery of the tree is low and, therefore, that the liquid water absorption by the aerial organs of the plant is unlikely under natural conditions, both in summer and in winter.

Figure 1

P. tamarugo leaf temperature, ambient temperature and dew point temperature in three days: a) Dec. 18 (cloudless); b) March 3 (cloudy); and c) March 5 (cloudless). Refresco, 1982.

Assuming that the stomata open for some length of time during the night, water could enter the mesophyll as vapor, if the leaf water potential were negative enough and its temperature low enough to induce a water vapor concentration gradient towards the inside. Figure 2 shows that such is not the case, and that the difference in the water vapor concentration always favors flow from the inside to the outside of the leaves. These figures also coincide with those found by Acevedo and Pastenes (1), when other seasons of the year were included (winter). It was later shown that stomata remain closed during the night (2), which makes it virtually impossible for water vapor to pass into the inside of the leaf due to its low conductivity.

Figure 2

Water vapor concentration difference between the intercellular spaces and turbulent air. Positive values indicate leaf-turbulent air flow direction in P. tamarugo. Refresco, 1982.

Figure 3 shows the daily course of total water potential in tamarugo terminal shoots, indicating that at the end of spring it fluctuates between -1.3 and -3.0 MPa. The highest potential reading was at dawn, in agreement with the water recovery of the tree during the night. No decrease in leaf water potential during the night was detected, which should occur if there is reverse sap flow, since there is no water recharge from the atmosphere.

Figure 3

Daily course of the total water potential (Ψ) in P. tamarugo sprigs. Refresco, 1982.

The possibility of reverse sap flow during the spring was not discarded by Mooney et al. (8). Figure 4 shows that the pressure potential is maintained through the night, which stresses what has been stated above. Any water movement from the aerial part to the ground would be immediately reflected by a decrease in pressure potential, inasmuch as tamarugo foliar cell wall tissues have low elasticity (3). Solute potential (Figure 4), in fact, increases through the night, which, together with increasing total water potential, indicates that the foliar tissue is undergoing a hydration process.

Figure 4

Values for total potential (Ψ), pressure potential (ψp), solute potential (ψs) in P. tamarugo sprigs.
Refresco, 1982.

The decrease shown by the pressure potential between 8 and 10 h is remarkable (Figure 4), associated with the decrease in total leaf water potential. This induces partial closing of the stomata at an early hour of the day. The relation of stomatal resistance and Ψ is shown in Figure 5, and explains in part the stomatal behaviour of this species during the day, when the stomata stay fully open for a short period of time early in the morning (2).

Figure 5

Foliar resistance to water vapor diffusion (r) and total potential (Ψ) in P. tamarugo seedlings subjected to ψm = - 0.06 MPa and ψm = - 0.3 MPa. Antumapu, 1983.

Soil water. Figure 6 shows typical water profiles found immediately beneath the crown of artificially planted tamarugo. The root mass is spread solely beneath the crown and is associated with a significant moisture content. The figure shows that right outside the crown projection area (where no roots occur) soil moisture content decreases markedly (by approximately 80% with respect to the root zone). It is worth mentioning, however, that despite the high moisture content in the high root density zone, total soil water potential is low, in the range of -2.0 MPa, as a result of the high salt content existing there. Above and below the root proliferation zone the soil water potential becomes even more negative, with values at 20 cm depth of about -10.0 MPa and at 70 cm depth of about -5.0 MPa.

Figure 6

Moisture content on a weight basis (W%) in two soil profiles of the same trial pit, one beneath crown projection area and another farther away from the crown. Refresco, 1982.

At Tamarugal Pampa areas prone to flooding with water draining off from the Andes during the Altiplanic winter (area around La Tirana), tamarugo propagates naturally; soil salinity is considerably lower than that found at the salt flat area, where the man-made plantations have been established. Figure 7 compares two salinity profiles for plantation areas (Refresco) and natural propagation areas (La Tirana). At this latter location, only tap roots were observed for growing tamarugo, from the soil surface to a depth of over 140 cm. Total soil water potential fluctuated between -12.0 and -8.0 MPa from 10 to 120 cm depth in the soil profile; water potential of the shoots was -4.0 MPa at 11:30 (solar time). In this case water came from a soil depth greater than that of the pit dug (140 cm deep).

Figure 7

Salinity profiles in soils with Prosopis tamarugo.

The observations made at the La Tirana tree indicate that the tamarugo roots explore the moist soil for their water supply through tap roots, and that the proliferation of a relatively superficial root mass is somehow conditioned to the salinity in the soil profile. Preliminary observations in seedlings raised in greenhouse, using saline soils from the Pampa, seem to lend some support to this latter hypothesis on account of the high root proliferation observed under these conditions.

Aravena and Acevedo (4) showed directly that the water found both in tamarugo and in the soil at the root level comes from the groundwater table, i.e., that this species is a phreatophyte. The path of the groundwater to the dense root area must be through the tap roots, as there exists a 0.2 MPa/m water potential gradient between the groundwater table and the first meter below the soil surface, caused by the high salinity of the soil where the root mass is located. The resistances involved in the process are not known and should be the subject for further research.

Transpiration. One of the important problems to solve is the assessment of tamarugo transpiration rates. They are difficult to establish directly, as the trees are tapping their water from an aquifer which is undergoing recharge and discharge processes, difficult to assess accurately. Fisher and Turner (16) observed that the water use efficiency (WUE), defined as mg of dry matter produced/g of water transpired, was relatively constant for a species if considered over a relatively long span of time and was corrected by the atmospheric saturation deficit. Acevedo et al. (2) measured the WUE for 6- to 12-month-old tamarugo seedlings, finding a value of 1.2 mg MS/g of water transpired within a vast range of water states for the plant (soil water potential between -0.06 MPa and -3.0 Mpa). Starting from this parameter, it is possible to estimate transpiration providing that the total dry matter produced by the tree is known. Oberpahuer (1982) measured the total weight of a representative 50-year-old tamarugo, obtaining a value of 1,533.7 kg of DM (the authors do not know of similar measurements). If a constant WUE of 1.2 is assumed, and a stocking rate of 100 trees/ha is considered, mean annual transpiration amounts to 256 mm. Additionally, if the fruit and leaf production curve —obtained by Elgueta and Calderón (5)— is considered to estimate the growth of the tree, it may be speculated that tamarugo transpires between 100 and 400 mm per year from the age of 10 to 40 years. To draw a curve relating tree transpiration and age, data on total annual dry matter yield rates is needed.

Financed partly by CORFO (Chile) and the Faculty of Agricultural, Veterinary and Forest Sciences of the University of Chile.

REFERENCES

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10. SCHOLLANDER P.J., HAMMEL H.T., BRADTREET E.D., HEMMINGSE E.A. 1965. Sap pressure in vascular plants. Sci. 148:339–345.

11. SUDZUKI F. 1969. Absorción foliar de agua atmosférica en tamarugo. Bol. Téc. Ext. Exp. Agron. U. de Chile. 30:3–23.

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