Chapter 3
Botany (continued)

Prosopis Vegetative Propagation through Cuttings

Paulo César F. Lima
Forester, M. Sc., Researcher with embrapa/cpatsa
Petrolina, Pernambuco

Introduction

P. juliflora (Sw) dc is the only species used in the Brazilian semi-arid zone for afforestation programs with financial incentives from the Federal Government. This species, as a result of being cross-pollinated and auto-incompatible, shows extreme variation in shape, presence of thorns and fruit output.

The propagation of plants through cuttings contributes to the establishment of populations that are homogeneous in terms of resistance against pests and disease, fruit output, timber production, or any other characteristic desired. To obtain greater pod output in afforestation efforts with P. juliflora, planting of seedlings raised from cuttings is a good alternative.

Vegetative propagation methods have shown that material collection time, auxine type and dosage, temperature, environmental moisture, substratum, phytosanitary treatments, sprouting, size of cuttings and fertilizing all have influence on callosity formation, rooting rate and rooting intensity in cuttings.

Hartney (1980) found variations in the rooting capacity of Eucalyptus spp. cuttings linked to collection time. Campinhos and Ikemort (1983) used the intermediate part of Eucalyptus shoots as cuttings for rooting, leaving two pairs of leaves in each cutting. Ikemori (1975, 1976), with the purpose of finding a method for rooting of Eucalyptus spp. found an effect produced by type of cutting, hormone and combined action of both, as well as by the substratum used and greenhouse conditions.

Gislerød (1983) observed influence of temperature and relative humidity in development of Euphorbia pulcherrina roots, in propagation through cuttings. Borba and Correa (1983) described the importance of environmental control in rooting of Eucalyptus spp. cuttings.

For the genus Prosopis, Felker and Clark (1981) obtained a success rate of over 70% in rooting of P. alba, P. articulata, P. chilensis, P. glandulosa var. torreyana, P. pallida and P. velutina, using a mixture of hormones and covering the pots containing the cuttings with polyethylene bags. Their attempts to propagate cuttings of P. velutina in and outside the greenhouse did not succeed outside, even with the added hormone mixture.

Souza and Nascimento (1984), using material from basal sprouts of P. juliflora, obtained 70% rooting rate in cuttings 10 to 15 cm in length and 2.37 to 4.39 mm in diameter. Nascimento et al. (1985) obtained up to 95% rooting rate with three sprouts, leaving 2 sprouts in the aerial portion. Cuttings with 100% foliar area were used, treated with indolbutiric acid (iba) at a concentration of 2,000 ppm.

This paper presents the rooting rates obtained with P. alba, P. chilensis and P. pallida in free outdoor growth, and with P. juliflora cuttings taken from plants undergoing vegetative propagation, raised in pots in a greenhouse.

Material and Methods

The trial was carried out at the Agriculture and Livestock Research Center for the Semi-Arid Tropic (cpatsa) in Petrolina, Pernambuco, in a greenhouse with 30° C–35° C temperature and 70%–80% relative humidity.

The layout used was randomized blocks with ten replications, five cuttings per plot, using shoots of 12-month-old P. alba, P. chilensis and P. pallida growing freely outdoors, and from 18-month-old P. juliflora plants undergoing vegetative propagation, cultivated in pots in greenhouse. All cuttings of each species were taken from the same plant.

The cuttings, with 10 cm in length, 3–5 mm in diameter and 4 sprouts, after application of 2,000-ppm indolbutiric acid (iba), were planted in black polyethylene bags, 6 cm in diameter and 20 cm long, containing a 4:1 sand-vermiculite mix. Two sprouts were left in the cutting's aerial portion, with 100% of the leaves. As fungal treatment, 2.0 g of 4% Captan per liter of water were applied. Foliar fertilization was performed at planting, with 2.0 ml/liter of water for 420 cuttings, and weekly during the first three weeks. Thereafter, fertilization with npk (5-17-3), at a dosage of 0.3 g per plant, was applied weekly with irrigation water until 60 days after planting. After 150 days, rooting percentage, callosity, sprouting, root dry weight and presence of nodulation were evaluated.

Results and Discussion

The data presented in Table 1 show low rooting rate for Prosopis, with 20% for P. juliflora. The highest indices were obtained for P. chilensis and P. pallida, with 54%, not diferring statistically from P. alba, with 44% rooting rate.

The number of unrooted cuttings without callosity observed with P. juliflora suggests that the type of material used influences rooting rate, since Souza and Nascimento (1983), using the same method, obtained up to 70% rooting with cuttings from adult P. juliflora.

For P. alba and P. chilensis, Balboa (undated) suggests application of low-strength iba in doses of 6.25 to 100 ppm. Felker and Clark (1981) obtained over 80% rooting with P. chilensis, P. pallida and P. alba using a mixture of several hormones and covering the pots with plastic bags. In uncovered pots, the rates obtained were 30% for P. alba and 100% and 80% for P. pallida and P. chilensis, respectively.

Sprouting was observed only in rooted cuttings. P. pallida and P. chilensis showed the highest indices, with 54% and 48%, respectively. P. alba showed the highest rate of rooted cuttings without sprouts. In unrooted cuttings, the highest callosity indices found were with P. chilensis.

TABLE 1
Rooting Rate, Sprouting and Callosity Observed in Cuttings from Different Species of Prosopis

* Figures followed by the same letter do not differ from each other as per Tukey's test at 5% probability level.

TABLE 2
Values Found for Root and Sprouting in cuttings from Different Species of Prosopis

Shoot length and root dry weight are shown in Table 2 above. P. pallida showed the greatest difference in shoot length, with standard deviation of 20.1 cm from the mean. P. chilensis presented mean of 1.8 sprouts per cutting. The lowest index was observed in P. juliflora, with one sprout per cutting.

Conclusions

In general terms, the rooting rates obtained with the species studied were low. No conclusions may be derived regarding the use of P. juliflora cuttings taken from plants undergoing vegetative regeneration and placed in greenhouse, as no trial was made with cuttings obtained from adult plants under the same conditions.

It is necessary to carry out further studies on hormone mixture and dosage, plant age and cutting quality in order to develop successful techniques for vegetative propagation with Prosopis.

Planting of seedlings raised from cuttings is a good alternative for obtaining higher pod output in afforestation efforts with P. juliflora.

References

balboa, o.: “Prospects and constraints for propagating Prosopis,” Facultad de Ciencias Biológicas, P. Univ. Católica, Santiago, Chile, 3 p., undated, mimeographed.

borba, a.m. de, and correa, r.m., 1983: “Control ambiental para enraizamento de estacas en clima subtropical,” Silvicultura, 8(32): 760–61.

campinhos, Jr., e. and eikemori, y. k., 1983: “Produção massal de Eucalytus spp. através de estaquia,” Silvicultura, 8(32): 770–75.

felker, p. and clark, p.r., 1981: “Rooting of mesquite (Prosopis) cuttings,” Journal of Range Management, 34(6): 466–68.

gislerød, h.r., 1983: “Physical conditions of propagation media and their influence on the rooting of cuttings, iii. The effect of air content and temperature in different propagation media on the rooting of cuttings,” Plant and Soil, 75(1): 1–14.

hartney, v.j., 1980: “Vegetative propagation of the Eucalyptus,” Aust. For. Res., 10(3): 191–211.

ikemori, y.k., 1975: “Resultados preliminares sobre enraizamento de estacas de Eucalyptus spp.,” Aracruz-es. Centro de Pesquisas Florestais da Aracruz, 12 p., illust. (Centro de Pesquisas Florestais da Aracuz, Informativo Técnico, 1).

ikemori, y.k., 1976: “Resultados preliminares sobre enraizamento de estacas de Eucalyptus spp.,” Aracruz-es, Centro de Pesquisas Florestais da Aracruz, 9 p., illust., (Centro de Pesquisas Florestais da Aracruz, Informativo Técnico, 2).

nascimento, c.e. de; lima, p.c.f. and silva, h.d. da, 1985: “Influencia do número de gemas no enraizamento de estacas de algaroba,” Petrolina-pe., embrapa/cpatsa, 3 p., (embrapa/cpatsa, research in progress, 39).

souza, s.m. de, and nascimento, c.e.s., 1984: “Propagação vegetativa de algaroba por estaquia,” Petrolina, embrapa/cpatsa, 3 p., (embrapa/cpatsa, research in progress, 27).

Comparative Performance of Prosopis juliflora (Sw.) dc and of Several Other Tree Species in a High-Alkali Soil

H. S. Gill
I. P. Abrol
Central Soil Salinity Research Institute
Karnal, 132 001 India

Introduction

High alkali soils are a major constraint for the successful establishment and growth of most plants. For this reason, the greater part of highly alkaline soils are devoid of vegetation of economic significance. Afforestation of these desertified lands would help to maintain a healthy ecological balance, which has been severely disturbed by indiscriminate and large-scale felling of the forests in the past, as well as helping to create resources to meet impending needs of fuelwood, forage and other products.

Alkaline soils are of wide occurrence in the arid and semi-arid regions of India and in many other countries of the world. These soils are characterized by the presence of small to measurable amounts of sodium carbonate. Hydrolysis of this salt imparts the soils high pH and high exchangeable sodium percentage (esp) throughout the soil profile. High pH and esp cause these soils to have poor physicochemical and biological conditions. The presence of a calcic horizon in the profile of such soils is another severe impediment that checks downward growth and proliferation of plant roots and moisture transmission within the soil profile.

In view of these constraints, establishment of trees in high-alkali soils requires correct choice of species in addition to any special treatment or site preparation. Considering the vast scope for afforestation on alkaline soils, lack of experimental evidence has a serious bearing on formulation of plans and programs. Therefore, a field trial was conducted to investigate the comparative performance of selected tree species in a highly alkaline soil.

Materials and Methods

To evaluate the comparative performance of Prosopis juliflora (SW) dc, Albizia lebbec (Linn.) Benth. (“Siris”), Azadirachta indica Juss. (“Neem”), Dalbergia sissoo Roxb. (“Shisham”), Morus indica var. alba Linn. (“Shahtoot”), Populus deltoides Bartr. (“Poplar”), Syzygium cuminii Wall. (“Jaman”) and Syzygium fructicosum DC. (“Jamoa”) in a highly alkaline soil (Table 1), robust saplings of these species were planted in shallow and deep postholes. Shallow postholes were 30 cm in diameter and 60 cm deep, and were dug out using a mechanically operated posthole digger. Deep postholes were made by deepening shallow ones to 120 cm with 15 cm in diameter, using manually operated soil augers. Each posthole was refilled with a mixture of alkaline soil from the same site, 8 kg farm yard manure (fym), 2 kg gypsum, 25 g n (urea) and rice husk at a ratio of 3:2 by soil volume. Another n dose of the same amount was applied to each plant with irrigation water 180 days after planting.

The experiment was replicated four times in a factorial randomized block design layout. Each replication included four plants. Row to row and plant to plant spacing was kept at 3 m. Thus, plant stocking rate was 1,111/ha.

Plants were watered according to detected need for about four months after planting to aid their establishment. Periodic observations on survival rates and growth indices (height and stem girth diameter at 5 and 30 cm height above ground level) were recorded. The experiment was ended 720 days after planting to evaluate the surviving species in terms of their primary biomass yields.

Results and Discussion

Survival Rates

The genera selected showed varying mortality to planting. All seedlings of Populus, Morus and of the species Syzygium cuminii Wall. were dead as of 540 days after planting (Table 2). Survival rates for Dalbergia, Prosopis, Albizia, Azadirachta and the species Syzygium fructicosum dc were 100, 87, 75, 75 and 50% in shallow pits and 100, 87, 62, 50 and 50% in deep postholes at 540 and 720 days after planting, respectively. This clearly indicated greater tolerance of Prosopis and Dalbergia to soil alkali.

Height Growth

During the observed growth period, maximum height gain was recorded for Prosopis (Fig. 1) and was followed by Albizia, Dalbergia, Azadirachta and S. fructicosum dc. Height growth of Azadirachta and Albizia was markedly greater in shallow postholes than in deep ones. Similar behaviour was also observed with these species regarding survival rates. This indicates that some species may require amelioration of soil alkali to a greater degree for initial establishment, as application of the same amount of gypsum in shallow and in deep postholes resulted in varied amelioration. Height growth of Prosopis in deep postholes was greater than in shallow ones. However, there was no noticeable difference in the case of Dalbergia and Syzygium fructicosum dc.

Girth Growth

Periodic measurements of girth diameter at 5 and 30 cm height above the ground level (Fig. 2) showed better performance for Prosopis than for the rest of the surviving species. The stump girth diameter (dsh) of Prosopis and Albizia 210 days after planting was equal. But subsequent girth growth in Prosopis occurred at a faster rate than in Albizia. Albizia girth growth, however, was significantly greater than that of Azadirachta, Dalbergia and S. fructicosum dc throughout the observed growth period. The effect of the two types of posthole on the girth growth parameters of these species was similar to that of height growth.

Primary Biomass Production

The data (Table 3) of average biomass yield per plant over the 720-day growth period showed large differences among the five tree species. Prosopis yielded the highest amount of dry matter, while S. fructicosum dc yielded the least. Mean total for Prosopis was 17,718 g, several times larger than the yield per plant for Albizia (5,577 g), Azadirachta (654 g), Dalbergia (349 g) and S. fructicosum dc (293 g). A similar order was observed for the aerial (foliage and woody matter) biomass yield, roots and woody matter components. Results also showed the significant effect the two types of postholes had on total biomass yield. Prosopis produced significantly higher biomass when planted in deep postholes (23,252 g/plant), than in shallow ones (12,183 g/plant). This trend was the opposite in Albizia, but not significant in other species.

The findings prove the superiority of Prosopis in comparison to other species as regards performance in highly alkaline soils. Its survival rates in both types of postholes point to its greater tolerance to alkaline soil conditions. But significantly more biomass production in deep than shallow postholes evidences the role of mechanical impedance of the soil. Albizia and Azadirachta suffered more mortality in terms of soil alkalinity, but showed better growth in shallow postholes than deep ones, confirming their greater tolerance to soil mechanical impedance. Despite the high survival rates of Dalbergia, its growth, like that of S. fructicosum dc, was unsatisfactory.

TABLE 1
Soil Characteristics of the Experimental Site

* Measured after shaking soil water suspension in a ratio of 1:2 for half an hour.

TABLE 2
Periodic Per Cent Survival of Selected Tree Species in a Highly Alkaline Soil

Figure 1. Periodic height growth of selected species with respect to posthole type.

TABLE 3
Biomass Yield (g/plant) after 720 Growth Days of Selected Species Planted in Shallow and Deep Postholes in a Highly Alkaline Soil

Figures in parenthesis denote corresponding values in kg/ha.

Growth period, days past planting

Figure 2. Periodic girth growth of selected species as measured by diameter at 5 cm (a) and 30 cm (b) height above ground level.

Pollination Efficiency of P. juliflora (Sw) dc in Petrolina, Pernambuco

Viseldo Ribeiro de Oliveira
Forester, embrapa/cpatsa, Petrolina

Ismael Eleotério Pires
Forester, M. Sc.
Assistant Professor, Forestry Department
Universidade Federal de Vic,osa, Minas Gerais

Introduction

In recent years, P. juliflora (Sw) dc has been planted in large scale for production of wood for different end uses, as well as for fruit production for fodder, on account of the pods' high protein content, as Alves (1972) points out. Fruit production ranges from 0.00 to 120 kg per tree, observing, however, that at inflorescence level fruit yield is low compared to the high number of flowers contained in each inflorescence.

On the other hand, wide spacings are needed, generally 10 × 10 m, for the trees to show satisfactory fruit output. This reduces considerably the number of trees per hectare and, consequently, production volumes.

In view of the above, it becomes necessary to study factors related to the P. juliflora reproductive system, in search of alternatives capable of increasing fruit output. This research study aims, therefore, at defining the reproductive system and the possible causes for the low fruit output per inflorescence, i.e. the low pollination efficiency.

Review of Literature

The characteristics of drought hardiness and the good performance of P. juliflora in the different ecological regions of the Brazilian Northeast (Pires and Ferreira, 1983) make this species a prime candidate for use in afforestation programs aimed at timber and/or fodder production, as well as for use in programs targeted to small farmers, such as provision of windbreaks, shading, firewood, construction timber, etc. Most of the plantations existing at present have the purpose of producing fodder.

The number of fruits per inflorescence observed normally in P. juliflora populations in the Northeast ranges from 1 to 3, for a high number of flowers. Solbrig and Cantino (1975) found an average of 220 to 240 flowers per inflorescence in Prosopis flexuosa and Prosopis chilensis, respectively, also with a low number of fruits per inflorescence.

These same authors point out that the high number of flowers per inflorescence can be a strategy for attracting pollinating insects. Meanwhile, low pollination efficiency (Solbrig and Cantino, 1975) and other factors such as protogyny (Habit, 1981) cannot be ruled out as the reason for the low fruit yield. A question arises in this regard: Are all flowers in an inflorescence hermaphrodite and viable?

Augspurger (1980), working with Hybanthus prunifolius, concluded that the low percentage of fruit produced by that species is not due solely to the phenologic pattern of floral production, but also to factors such as compatibility of reproductive systems, forms of development and temporal and spatial density of the populations.

Another aspect is the presence of pollinating agents at the moment of greater pollination viability. Within this context, Haber and Frankie (1982) carried out controlled pollinations in daytime and nighttime periods with Luehea candida, and found 92% fruit yield for nighttime pollination and 47% for daytime pollination. This difference is due, probably, to the absence of pollinators during the night, when the flowers are more receptive, or even to the effect of protogyny.

According to Koptur (1984), an inflorescence of the genus Inga contains around 40 flowers and produces 4 to 5 fruits, a fact which may be related to physical, chemical or spatial factors.

Bawa and Webb (1983) found a correlation in Mutingia calabura between the amount of fruits produced and ovary size. This leads to the conclusion that, possibly, in the case of the genus Prosopis, a physical restriction may exist; i.e. the size of the ovary or of the pollinical tube varies from flowers to flower, being fertilized only those flowers with size above a certain minimum.

Material and Methods

The experiment was carried out in Petrolina, Pernambuco, at the Bebedouro experimental station of the Agriculture and Livestock Research Center for the Semi-Arid Tropic (cpatsa), in a 16-year-old P. juliflora (Sw) dc population.

Twenty trees were selected at random and identified, collecting 20 ripe inflorescences from each tree identified in order to determine number of flowers per inflorescence (nfi) and to measure inflorescence length. The purpose was to devise a regression equation to estimate the number of flowers. The inflorescences collected were stored in a freezer, both for conservation purposes and to facilitate flower counting in subsequent days.

Inflorescence length was measured with a cm ruler. Then, 10 of the 20 trees identified initially were selected, identifying 15 inflorescences at random from each of them, leaving ten free and storing in bags the remaining five. The length of each inflorescence (il) was measured with a common ruler.

For each inflorescence stored in bags, another free one was identified from the same bunch with the purpose of assessing the occurrence of effective pollination with external pollen. The bags were made of kraft paper, 35 cm in height and 16 cm in length, with a string to tie them up and spiral wire and cotton padding to protect the bunch. Those inflorescences were controlled until the end of the pollination phase.

In the other 10 trees, ten inflorescences were identified and measured with 2 controls per week, to estimate free pollination efficiency and fruit persistence, until their physiological ripening occurred.

Results and Discussion

Number of flowers per inflorescence and inflorescence length

Table 1 shows the average number of flowers per inflorescence, with the corresponding standard deviation per tree, resulting from direct counting in the first stage.

The number of flowers per inflorescence ranged from 269 to 456, while inflorescence length varied from 7.09 to 14.08 cm.

The average number of flowers per inflorescence was 344, with inflorescence length of 11.45 ± 1.77 cm, by direct counting.

Based on these data, a linear regression equation was arrived at = 117.80919 + 19.79285 il, with a correlation coefficient (r) of 0.77985, which makes it possible to estimate the number of flowers per inflorescence () according to inflorescence length (il). Figure 1 shows shows the data distribution observed and the line resulting from the regression equation.

Table 2 shows mean number of flowers per inflorescence for the 20 trees used in this trial. estimated as per the above regression equation. As shown therein, the estimated number of flowers per inflorescence ranged from 304 to 393, with a mean of 344 for inflorescence length varying from 9.4 to 13.9 cm; average value for the latter was 11.47 ± 1.3 cm.

The analysis of Tables 1 and 2 shows that inflorescence length and number of flowers per inflorescence varied both within and between trees, with the wider variations occurring between trees.

TABLA 1
Average Inflorescence Length (IL) and Average Number of Flowers per Inflorescence (NFI)

    IL: Average inflorescence length.
 NFI: Number of flowers per inflorescence.

Figure 1. Distribution of the number of flowers/inflorescence as related to average inflorescence length.

No fruits were produced by the inflorescences stored in bags, confirming the expected predominance of allogamy.

Pollination efficiency and fruit production

Pollination efficiency, defined as the amount of flowers pollinated per inflorescence, and fruit production and characteristics per tree are shown in Table 2.

Taking as base 10 inflorescences per tree, it was found that the number of inflorescences pollinated varied from 0.00 to 10.0, i.e. pollination efficiency basing on the amount of pollinated inflorescences ranged from 0.00% to 100%, with a mean of 29% (Table 2). This shows high phenotypic variation among trees in terms of the amount of flowers pollinated, which may be due to pollinators, level of incompatibility, flower abortion or degree of kinship between trees, bearing in mind that Pires and Kageyama (1985) questioned the genetic base of those populations. Figure 2 illustrates the pollination efficiency at tree level, basing on the inflorescences pollinated and inflorescences producing ripe fruits.

It must be stressed that out of the pollinations taking place, on the average, only 42.3% formed fruit that held on until it reaching maturation, (Table 2). Taking as base the total number of flowers per inflorescence and the amount of fruits produced, a 1.48% pollination efficiency was found. Solbrig and Cantino (1975), working with Prosopis flexuosa and Prosopis chilensis, also found low fruit production for a high number of flowers per inflorescence. The authors suggest that the high number of flowers may have the sole function of attracting pollinator insects.

The definition of pollinating agents, as well as of the pollen release period and stigma receptivity, are fundamental to explain such low pollination efficiency. The lack of synchronization between pollen release and pollen reception period, added to the absence of pollinating agents at the moment of anthesis, may affect dramatically pollination efficiency (Haber and Frankie, 1982).

Figure 2. Percentage of pollinated inflorescences and inflorescences with ripe fruits per tree.

TABLE 2
Pollination Efficiency, Inflorescence and Fruit Characteristics Taking 10 Inflorescences per Tree as Base

Fruit average length, presented in Table 2, varied between 13 and 21.4 cm, evidencing the great phenotypic variation among trees.

As seen in Table 2, pollination efficiency varies greatly from individual to individual, which leads to hypotheses involving environmental influence, including pollinating insects, as well as genetic factors. The need for further and more detailed research is thus evident, at clone level, to verify the reasons for the low pollination efficiency in P. juliflora populations in the Brazilian Northeast.

Conclusions

1. Average length of P. juliflora inflorescences was 11.47 cm;
2. Average number of flowers per inflorescence was 344;
3. Pollination efficiency based on the number of inflorescences per tree was 29%; however, in relation with the number of flowers, efficiency dropped to a mere 1.48%;
4. High phenotypic variation exists among trees as regards pollination efficiency;
5. Further in-depth studies on floral biology are needed to identify the factors responsible for the low efficiency of pollination in P. juliflora populations in the Northeast.

References

alves, a.q., 1972: “Algaroba, uma experiencia válida,” João Pessoa-pb, Secretaria da Agricultura, Indústria e Comércio, Departamento da Produção, 20 p., illust.

augspurger, c.k., 1980: “Mass-flowering of a tropical shrub (Hybanthus prunifolius): Influence on pollinator attraction and movement,” Evolution, 34 (3): 475–88.

bawa, k.s. and webb, c.j., 1983: “Floral variation and sexual differentiation” in Mutingia calabura (Elaeocarpaceae), a species with hermaphrodite flowers,” Evolution, 37 (6): 1271–82.

habit, m.a., 1981: “Prosopis tamarugo: arbuste fourragens pour zones arides,” In: Programme regional pour la production d'aliments de base, Bureau Régional pour l'Amerique Latine, fao/danida, Rome, p. 116.

haber, e.a. and frankie, g.w., 1982: “Pollination of Luehea (Tiliaceae), in Costa Rica,” Ecology, 63 (6): 1740–50.

koptur, s., 1984: “Outcrossing and pollination limitation of fruit set: Breeding systems of neotropical Inga trees (Fabaceae: Mimosoideae),” Evolution, 38 (5): 1130–1143.

leakey, r.r.b. and last, f.t., 1980: “Biology and potential of Prosopis species in arid environments, with particular reference to Prosopis cineraria,” Journal of Arid Environments, 3: 9–24.

oliveira, v.s. and pires, i.e., 1985: “Eficiencia da polinização em Prosopis juliflora, (Sw) dc,” 4 p., cpatsa-embrapa, Research under way, 45.

pires, i.e. and kageyama, p.y., 1985: “Caracterização da base genética de uma população de algaroba, Prosopis juliflora (Sw) dc, existente na região de Soledade-pb,” Piracicaba, (30): 29–36.

solbrig, o.t. and cantino, p.d., 1975: “Reproductive adaptations in Prosopis (Leguminoseae, Mimosoideae),” Journal of the Arnold Arboretum, 56 (2): 185–209.

Characterization of the Genetic Base of a P. juliflora Population at Soledade, Paraíba

Ismael Eleotério Pires
embrapa, cpatsa, Petrolina, Pernambuco

Paulo Yoshio Kageyama
esalq-usp, Silviculture Department
Piracicaba, São Paulo

Introduction

Around 56% of the Brazilian Northeast area is covered with a vegetation known as “caatinga,” characterized by low wood productivity and small diversity of species as compared with the tropical rainforests. In this region, the prevailing climates are arid and semi-arid, with a dry season lasting some 9 months and annual rainfall ranging between 250 and 1,000 mm. Mean temperature is 25° C, with little variation. Soils are generally flat and of low fertility.

In light of the vast areas unsuitable for agriculture or pastures, forestry appears to be a recommendable option. Consideration must be given to the introduction of versatile species able to produce timber and fodder, as well as to control erosion, support apiculture, provide fuelwood and charcoal, etc. Consequently, research studies aimed at identifying the most suitable species and provenances are mandatory.

Being Prosopis juliflora (Sw) dc an allogamous species and considering the doubts regarding the effective size of the existing populations, it is possible that material is being used with a high degree of inbreeding, through crossbreeding of related individuals. Even if this situation may not yet be widespread, it is necessary to keep close attention on the implications of using this material in genetic improvement programs involving selection through successive generations.

This research work has the objective of determining genetic variability of a P. juliflora population at the Pendência Farm, Soledade, Paraíba, with the purpose of:

1. Making inferences as to the genetic base of the population;
2. producing improved seeds through the establishment of production areas and seed orchards.
3. genetic conservation of this material;
4. devising strategies for utilizing the P. juliflora populations existing at the Northeast in genetic improvement programs.

Review of Literature

The existence of genetic variability in a population and knowledge thereon are indispensable for the success of improvement programs.

The fact that the greatest majority of forest species are allogamous, together with the existence of self-incompatibility mechanisms, insures that these species have a great genetic variability. This variability, according to Sluder (1970), is due to the considerable pollen and seed flow normally taking place within and among forest populations.

On the other hand, human action by applying recurrent selection, reducing in each generation the effective number of individuals of the population, as well as through environmental alteration, tends to reduce the populations' genetic variability (Namkoong et al., 1983), jeopardizing future genetic improvement. This process is more acute in those species with precocious flowering and fruiting, as they permit many cycles in a short period of time.

As Lindgren (1969) points out, populations with a narrow genetic base can show variability as a result of the different levels of inbreeding exhibited by the individuals making them up. This variability tends to disappear in adult populations, due to the effects of competition and of other selecting forces, as well as in advanced generations due to the trend to crossbreeding among increasingly related individuals. This same author stresses that the degree of inbreeding becomes more accentuated when intensive selection is promoted within families.

The behavior of the variances of a population depends on the type of crossbreeding and on the population's effective size (Crow and Kimura, 1970; Allard 1971; Falconer, 1981), concluding that the manipulation of such variances can be used for making inferences on the genetic base and on the reproductive system of forest species.

Fonseca (1982) suggested the ratio between the variance within families and variance between families [var(d)/var(p)], for plant height, as an indicator of the type of crossbreeding, stating that in allogamous species this ratio would always tend to values higher than ten. Therefore, basing on this relation, as well as on the behavior of the variances of populations of narrow genetic base and of the autogamous populations, the possibility arises of using the ratio between variances to infer on the genetic base and reproductive system of forest species populations.

P. juliflora (Sw) dc exhibits great phenotypic variability in its natural range in terms of plant shape, biomass production, and fruit production and characteristics (National Academy of Sciences, 1979; Felker, 1982). However, no studies have been reported on the genetic variability presented by this species regarding its various economically important features.

The species introduced in the Brazilian Northeast is P. juliflora (Sw) dc, according to the classification of Professor Arturo Burkart, from the Darwinian Institute, Argentina, quoted by Azevedo (1982). However, Ffolliott and Thames (1983) state that this species belongs to the algaroba group, which is extremely variable and requires further in-depth studies.

As regards the P. juliflora populations existing in the Northeast, phenotypic variations were also observed in stem shape, presence of thorns, and fruit output and size (Gomes, 1961; Azevedo, 1982); no data is available, however, regarding the genetic structure of said populations.

Material and Methods

Seeds were collected from 36 parent trees from a population located at the Pendência Farm, Soledade, Paraíba. Two trials were established, one at the same locality and another in Caicó, Rio Grande do Norte, each one involving 30 open pollinated progenies, 24 of which were common to both trials.

The experimental layout was triple rectangular lattice (5×6) with linear plots of 10 trees, at 3 × 3 m spacing, with a simple general fence around the whole trial.

The trials were established during the months of March and April 1982, respectively, in Caicó and Soledade.

The height of each plant was measured at 6 and 12 months of age in Caicó. At 18 months of age, the following characteristics were measured: height, mean crown diameter, diameter at base and survival rate. Characteristic height plus mean crown diameter were considered for overall assessment, assuming that the association of both features would reproduce the real plant height, basing on plant architecture and on Halle et al. (1978).

The analysis of individual variance at 6 and 12 months of age for plant height, in Caicó, and at 18 months for all characteristics at both locations, was performed according to the triple rectangular lattice layout proposed by Cochran and Cox (1981).

In those cases where lattice efficiency, corresponding to the ratio between mean square of effective error for the lattice analysis and error mean square for randomized block analysis, was lower than 110% (Miranda Filho, 1978), a randomized block layout analysis was performed, as proposed by Ditlevsen (1980).

Estimates of genetic variances, environmental variances and variance within plots were obtained by breaking down mean squares of the randomized block analyses.

Heritability coefficients and variation coefficients within the plots were estimated as per Venkovsky (1978).

The genetic variation coefficients (CVg), environmental variation coefficients (CVe) and variation coefficient within (CVd), used by Kageyama (1983), are presented as percentage in relation to the mean value (x).

Results and Discussion

The analyses of individual variances for both locations, Caicó-rn and Soledade-pb, at the ages considered, did not reveal equally significant differences for all characteristics. As Table 1 shows the differences were significant between the mean values of height-plus-mean-crown-diameter for progenies, while at Caicó significant differences were found only for survival rates.

The different results obtained for both locations can be attributed not only to local effects, but, mainly, to having established the Soledade trial with a delay of some thirty days with respect to Caicó. This delay caused the trial to be intensely affected by drought, as may be verified in loco, compromising the findings. The comparison of mean values for the growth characteristics obtained for both locations evidences the limitations of the Soledade trial.

In Caicó, the analysis of plant height at 6, 12, and 18 months of age shows the influence of nursery on the field establishment stage, as evidenced by the increase in the experimental variation coefficient with age.

The low values of F, without significance for most characteristics, were not due to experimental variation coefficients, which were variable and whose values were considered medium to low (Gomes, 1976), revealing low genetic variability between progenies at the ages considered.

TABLE 1
Progeny Mean Values and Results of Individual Variance Analyses, for all characteristics, at the ages indicated, in Caicó and Soledade

Basing on the 110% limit (Miranda Filho, 1978) for the efficiency of lattice layout as compared with randomized block layout, only plant height and diameter at base were analyzed according to this scheme, at 18 months of age, in Caicó.

It must be pointed out that the results obtained with the analysis of plant height plus mean crown diameter did not exhibit the efficiency expected, when compared with the values shown by both characteristics individually.

In view of the restrictions affecting the Soledade trial, genetic parameter estimates were only considered for the data from Caicó. This was so decided considering that these factors demand the greatest possible accuracy to permit correct inferences regarding the population's genetic base.

Genetic and non-genetic variance estimates, for the growth characteristics at the ages considered, in Caicó, are shown in Table 2. Genetic variance for plant height among progenies was positive at 6 months of age, turning negative at both subsequent evaluations, probably as a result of possible nursery effects at that age. Negative variance occurred also for plant height plus mean crown diameter. These negative variances, however, may be considered equal to zero, as demonstrated by Searle (1971).

Heritabilities in the restricted sense at plant level exhibited values ranging from 0.00% to 7.00% for all characteristics (Table 3). At the level of mean values for progenies, heritabilities showed values ranging from 0.00% to 17.00%, considering negligible the heritability obtained for height at 6 months of age (Table 3).

At heritability estimates, progenies were considered as being from half-siblings (Vencovsky, 1978; Falconer, 1981), considering negligible the occurrence of inbreeding, despite its importance, as pointed out by Namkoong (1979) and Lindgren (1976).

Genetic variation coefficients, variation coefficients between plants within plots and among plots, as well as the ratio between variance within plots, and genetic and environmental variances are presented in Table 4.

TABLE 2
Genetic Variance Estimates among Progenies [vcr(p)] Environmental Variances among Plots [var (D)], Caicó

TABLE 3
Restricted Heritability Coefficient Estimates for all Characteristics, Except Survival, Caicó

h² = Heritability in restricted sense at plant level.
h²m = Heritability in restricted sense at progeny level.

TABLE 4
Estimation of Genetic Variation Coefficients (CVg), Variation Within (CVd) and Environmental Variation (CVe), of the Ratio Between Variance Within and Variance Among Progenies [var(d)/var(p)], and Environmental Variance Between Plots [var(d)/var(e)] for the Different Characteristics, Caicó

The genetic variation coefficient, expressed as percentage of the general mean for each characteristic, is an extraordinarily important parameter to understand a population's genetic structure, as it shows the amount of variation existing among families and, obviously, because it enables an estimate on genetic improvement (Kageyama, 1980). The values obtained in this research for the various characteristics ranged from 0.00% to 5.14%, not taking into consideration the 6-month age. Kageyama (1983), for height of E. grandis plants at 12 months of age, found a 6.85% genetic variation coefficient, and Fonseca (1982) found 9,24% for Mimosa scabrella at three months of age under nursery conditions.

Basing on these results, it may be concluded that the P. juliflora population under study exhibits null genetic variability for plant height among progenies, while for diameter at base and mean crown diameter, the coefficients were intermediate. However, in view of the low heritabilities observed, it would not be possible to obtain significant genetic improvements for the characteristics considered.

The coefficient for plant variation within plots ranged from 29.19% to 33.74%, thus exhibiting a very low variation amplitude. Kageyama (1983), for 12-month-old E. grandis plants, found a variation coefficient within plots of 17.09%, practically half of that found in this study.

The ratio between genetic and experimental variation coefficients showed very low values, except for plant height at 6 months of age. Basing on the argumentation of Venkovsky (1978), that the situation is favorable for obtaining improvement through selection in corn plants, when the ratio between the genetic and experimental variation coefficients (CVg/CVexp) tends to values greater than 1, the population in question does not offer prospects of genetic improvement for the characteristics studied.

The ratio between variances within and variances among progenies [var(d)/var(p)] exhibited different values for all the characteristics studied. The ratio between variances within plots and environmental variances [var(d)/var(e)] also offered different values; however, with less variation amplitude (Table 4).

The study of these ratios showed low amplitude for the variation of the var(d)/var(e) ratio when compared with the values for the ratio var(d)/var(p). This is due mainly to the fact that, in the second case, both variances suffered fluctuations as a function of crossbreeding and of the genetic base. Considering the breakdown of var(d)/var(e) and an intragenetic component [var(dg)/var(de)] and an enviromental component among plants [var(da)/var(e)], that ratio could be expressed by var(d)/var(e) - var(dg)/var(e) + var(da)/var(e).

Assuming the existence of said equivalences, it could be admitted that the ratio var(da)/var(e) is equal to 1, in a given environment and at any age of a population on square spacings, in full competition, leaving the ratio var(d)/var(e) influenced solely by the genetic component [var(dg)/var(e)]. Thus, in the first inbred generations, where variance within tends to increase (Crow and Kimura, 1970; Falconer, 1981), the ratio var(d)/var(e) would present higher values, tending in later generations to values proportional to var(da)/var(e), plus a possible effect of variance in dominance, with values around 5 for the growth characteristics, as found in this research study. For populations of autogamous species, variance between families will have to be proportional to var(da)/var(e), exhibiting a value approximately equal to 1.

Basing on these considerations, it may be established that, for square spacings, in allogamous species of narrow genetic base, the ratio will be var(d)/var(e) = var(da)/var(e) plus the dominance effect, if it exists. In autogamous species, the ratio var(d)/var(da)/var(e) will have a value equal to or close to 1. However, in allogamous species with adequate genetic base, the ratio var(d)/var(e) will show higher values, as a result of the presence of additive and dominant genetic variance within [var(dg)].

From the above it may be inferred, basing on the findings presented in Table 4, that the population in question has a narrow genetic base. Further studies with different species are recommended, using also other techniques, such as isoenzimes and tracer genes, to define more clearly the limits of those ratios.

Taking into consideration the background of this population, it may be assumed that: a) the trees surviving introduction and which gave rise to the population under study were half-siblings (HB); and, b) the surviving trees did not show any degree of kinship. As the progenies in this trial probably constitute a fourth generation of that introduction, the low genetic variation coefficients associated to the high coefficients of variation within progenies suggest alternative (a) as more likely, considering the behavior of the genetic variances in the inbreeding, according to Crow and Kimura (1979), and to Falconer (1981).

As successive expansions of the existing populations take place, it is possible that the degree of inbreeding increases gradually with each generation, something which Lindgren (1976) labels “soft inbreeding.” This author points out that the inbreeding depression, expressed by the inbreeding coefficient (f), does not correspond to a linear model, varying from individual to individual of the same generation.

The type of management to which the species is subjected can affect its genetic characteristics, as shown by Namkoong et al. (1983) and Namkoong (1984), concerning populations for genetic improvement. Wide spacings, such as that used for P. juliflora (10 × 10 m), can permit the survival of highly endogamous species up to reproductive stage, permitting the exchange of pollen among individuals of different levels of inbreeding, which leads to a more accentuated depression. This depression can be influenced, also, by the effective size of the population, type of crossbreeding, presence of pollinators and environmental conditions, among other factors.

When attempting selection in a population of narrow genetic base, there is a great risk that phenotypically superior plants will be selected, which stand out not because of their genetic potential but for the fact that they are competing with inbred plants (Lindgren, 1976). On the other hand, in a population where the individuals have different levels of inbreeding, selection could be favoring those presenting lower coefficients of endogamy.

In light of the above considerations, prompted by the findings of this study, it is not advisable to use the P. juliflora populations existing at present in the Northeast for medium or long-term genetic improvement programs.

For the establishment of medium or long term genetic improvement programs, it would be convenient to expand the genetic base of these populations with the introduction of new material from the natural range of the species, taking advantage of the adaptation of the populations already established. Even the formation of local strains or races can be considered, once the species is spread throughout a vast portion of the semi-arid Northeast.

In the short term, provenance/progeny trials must be considered, either through seeds or through vegetative propagation involving parent trees from populations at various locations in the region, which would act as provenances. This procedure would provide seeds for immediate use in plantations, as well as more detailed information on the genetic structure and variations within and among said populations. It is even possible to make genetic improvements in the process, considering the possibility of occurrence of favorable combinations between individuals from different populations. Namkoong (1984) points out that the populations established in dissimilar environments segregate differently, even when they come from a narrow genetic base, and the grouping of material coming from these populations maximizes the favorable genetic combinations. which could be exploited in short-term genetic improvement programs.

Considering that the above was based on a rather short genetic trial, together with the fact that many of the inferences are based on theoretical formulations concerning the behavior of restricted populations, the need is evident of carrying out further studies to confirm the findings here presented. with the possibility of using controlled breeding techniques with isoenzymes, tracer genes, etc.

Conclusions

The findings of this study support the following conclusions:

The development of P. juliflora in the Caicó trial, in general terms, occurred within normal growth patterns for this species in the region.

The F values obtained from the variance analyses, for all characteristics, indicate low genetic variability among the progenies and, consequently, in the population studied.

The low heritability coefficients and the low genetic variation coefficients between progenies, together with the high variation coefficients within progenies for the growth characteristics, suggest that the population studied has a narrow genetic base, which leads to assume the presence of inbreeding.

The small apical dominance and the multiple uses of the species call for a definition of the characteristics most adequate to express its genetic variability, as well as for the development of suitable methods for assessing them.

The ratio between variance within families and environmental variance between plots [var(d)/var(e)] seems appropriate for defining the reproductive system and the genetic base of forest species.

The results obtained from these trials and the widespread occurrence of the species in the Northeast, together with its relatively short reproductive cycle, suggest a strong increase in the degree of kinship of each successive generation.

In the short term, it would be desirable to establish a clonal orchard with material from different populations, which would be tested later by means of provenance/progeny trials.

Acknowledgments

The authors express their gratitude to the Financiadora de Estudos e Projetos (finep) and to the Brazilian Institute of Forest Development (idbf) for the financial support which permitted the execution of this project through the National Forest Research Program (pnpf)/Brazilian Agricultural and Livestock Research Agency (embrapa); to the Paraíba State Agricultural and Livestock Research Agency (emepa) and to the Rio Grande do Norte Agriculture and Livestock Research Agency (emparn) for their support in the establishment and conduction of the field trials; to our colleagues Sérvulo Heber Lopes Vasconcelos, Guilherme de Castro Andrade and Manuel de Souza Araújo for their collaboration in the conduction of the trials and data collection.

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