Response of groundnut to deficit irrigation during vegetative growth

P.C. Nautiyal, Y.C. Joshi and D. Dayal,
National Research Centre for Groundnut,
Gujarat, India

Summary

Studies were conducted to investigate patterns of field water use efficiency (WUE) by groundnut under moisture stress. Experiment 1 examined four cultivars of Spanish groundnut (Arachis hypogaea L. ssp. fastigiata var. vulgaris), Ak 12-24, J 11, GAUG 1 and GG 2, during the summer seasons of 1989 and 1990. The crops were subjected to soil moisture deficit stress by withholding irrigation for 30 d, starting at 20 d after sowing (treatment T 101), and for 25 d starting at 20 d (T 102). In treatment T 101, the stress period was followed by two relief irrigations with an interval of 5 d. In Experiment 2, stress was imposed on cv. GG 2 from 10 to 30 d after sowing, based on an irrigation water/cumulative pan evaporation (IW/CPE) ratio of 0.6. In one case, this period of stress was followed with irrigation to maintain IW/CPE at 1.0 until harvest (T 200, control); in the second case, the initial stress period was followed by maintenance of IW/CPE at 0.8 until harvest (treatment T 201); in the third case, IW/CPE of 0.6 was continued until harvest (treatment T 202). The results of Experiment 1 showed that leaf area indices of plants stressed during the vegetative phase were higher during the reproductive phase, i.e. from 80 d after sowing until harvest, than those of control (T 100) plants. In general, percent dry matter distribution to leaves remained higher in control plants; percent dry matter distribution to the leaves of stressed plants was lower both during and after stress. Dry matter distributions to pods, and thus the harvest indices, were higher in the stressed plants. Among the treatments, total biomass and economic yields were higher in T 102, followed by T 101. The results of Experiment 2 were consistent with those of Experiment 1. Where groundnut is cultivated with irrigation, it is possible to increase field WUE and dry matter production, including economic yield, by imposing a transient deficit in soil moisture during the vegetative phase.

Groundnut is an important source of oil (51 percent), protein (28 percent) and minerals (2.5 percent). India and China account for about 50 percent of global production, and developing countries in the semi-arid tropics contribute 60 percent. The average in-shell yield is 900 kg/ha (FAO, 1999). Groundnut is an important component of intercropping systems in the dry tropics, and the haulm provides fodder for cattle. High and stable groundnut productivity is an essential element in the improvement of efficiency of farming systems in the semi-arid tropics.

In India, farmers grow groundnuts on ustic alfisols, oxisols, and usterts (the dry vertic soils). The major groundnut-producing areas are in western and southern India. The crop is primarily rainfed, and moisture is a primary constraint on yield. As a result, the tendency is bring groundnut under irrigation for cultivation during the summer season (January/June). Water use by groundnut in different cropping seasons in different parts of the world varies between 250 mm under rainfed conditions (Angus et al. 1983) to 831 mm under irrigated conditions (irrigation at intervals of seven to ten days during winter months) (Nageswara Rao et al., 1985). The total water use of a groundnut crop may be affected by scheduling irrigations based on requirements at the various growth stages.

The average yields (1 400 kg/ha) of the summer season crop are almost double those obtained in the rainy season. Therefore, the contribution of summer season crops to total production is about 45 percent. The key factor affecting growth and yield is the availability of moisture during the cropping season. There is a need for strategies that maximize the efficiency of use of the limited amounts of water that are available. Therefore, a study attempted to examine the effects on water use efficiency (WUE) of scheduling irrigation to take advantage of the lower water requirements of groundnut during the early growth phase. Two experiments were conducted with two moisture-deficit regimes, the first for two years and the second for one year, to determine the effects of early-stage irrigation.

Materials and methods

The experimental site

The study site was the research farm of the National Research Centre for Groundnut at Junagadh (21°31'N, 70°36'E). The soil was a vertic ustochrept with low organic matter content and available nitrogen (N) and phosphorus (P) (Table 1). Soil temperatures were recorded in all plots daily at 0900, 1300 and 1500 hours, at depths of 15 and 30 cm. Soil moisture contents at two depths, i.e. 0-15 cm and 15-30 cm, were estimated gravimetrically at the end of each stress period.

Table 1
Minerological, physical and chemical properties of
the soil of the experimental site at Junagadh

Experiment 1

Experiment 1 studied four short-duration (120-125 days) groundnut (Arachis hypogaea L. ssp. fastigiata var. vulgaris) cultivars, viz. Ak 12-24, J 11, GAUG 1 and GG 2, during the summer seasons of 1989 and 1990. Fertilizers were applied as urea (25 kg N/ha) and single superphosphate (40 kg/ha P₂O₅). The experiment had three replications in a randomized block design (RBD) with 5x3-m plots. The spacing was 30 cm between rows, and 10 cm between plants within rows. After sowing, the crop was irrigated twice at an interval of seven days to ensure good emergence. Recommended agronomic practices and plant protection measures, except irrigation, were followed to maintain crop health. The irrigations provided 50 mm of water, with treatments as follows:

• T 100: control crop; irrigations at intervals of 10 d from emergence until harvest.
• T 101: stress imposed during vegetative phase; irrigation withheld for 30 d starting at 20 d after sowing.
• T 102: stress imposed during vegetative phase; irrigation withheld for 25 d starting at 20 d after sowiing, followed by two irrigations at intervals of 5 d.

After the treatment periods, treatment and control plots received the same irrigation.

Data collection

Data on leaf area development, dry mass accumulation and its partitioning among various plant parts other than the roots were recorded every 10 d starting at 20 d after sowing. Dry matter distribution to various plant parts was calculated on a percentage basis. At each sampling date, 0.28 m² of each plot was sampled. Leaf area was measured with a LI-COR 3000 area meter, and the plant parts were then dried at 80°C to a constant weight. At the final harvest (125 d after sowing) pod yields were recorded. The harvest index (HI) and shelling outturn were calculated with the following formulae.

    (1)

    (2)

During the 1990 season, leaf transpiration rate (TR), diffusive resistance (DR), and relative water content (RWC) were measured on the second and the third leaves from the apex of the main axis at the end of each stress period at around 1400 hours. Transpiration and leaf diffusive resistance were measured on abaxial and adaxial sides of the leaves with a steady-state porometer (LI-COR 1600), and leaf RWC values were determined using the formula of Barrs and Weatherly (1962):

    (3)

Data were analysed statistically in a factorial set-up.

Experiment 2

During the summer of 1999, an experiment was conducted in an RBD with three replications in plots of 10x5 m with cv. GG 2.

The treatments were:

• T 200: control crop; deficit irrigation at an IW/CPE ratio of 0.6 imposed during vegetative phases from 10 d until 30 d after emergence; thereafter irrigations to maintain an IW/CPE ratio of 1.0 until final harvest (total application of 550 mm of water).
• Treatment T 201: deficit irrigation at an IW/CPE ratio of 0.6 imposed from 10 d until 30 d after emergence, after which an IW/CPE of 0.8 maintained until harvest (450 mm).
• Treatment T 202: deficit irrigation at IW/CPE ratio of 0.6 imposed from 10 d after emergence until harvest at 125 d (400 mm).

To ensure uniform emergence, each plot received two irrigations immediately after sowing and 5 d later. The irrigations provided 50 mm of water. Soil moisture at 15-cm intervals was recorded before each irrigation with the help of a neutron probe. Total pod yields and dry matter production were recorded at final harvest.

Results and discussion

Experiment 1

Characteristics of the growing season
The monthly mean maximum temperatures ranged between 32°C (February) and 42°C (April), while minimum temperatures ranged between 7°C (January) and 27°C (June). There was no rainfall during the growing period (last week of January to first week of June) in either year. Pan evaporation ranged between 5 and 10 mm/d, the highest being during April and May in both years. This is why summer crops in Junagadh are fully irrigated. During vegetative growth (January and February), evaporative demands (ET_m), and hence crop water requirements, are low. The requirements increase in the later stages of crop growth, especially during pegging and pod development, when evaporative demands also are high (April and May) (Figure 1). The soil needs to be moist for peg penetration and pod development.

Figure 1
Monthly mean temperature, relative humidity, and pan evaporation
during two summer cropping seasons, 1989-90, and 1999

Soil temperature and water content
In comparison with control plots, soil temperatures (means of the observations recorded at 0900, 1300 and 1500 hours during 1989 and 1990) at 15 cm were 3.2°C higher in treatment T101 and 1.2°C higher in treatment T 102 (Table 2). Soil water contents at both depths were always less in the irrigation-withheld plots than in the control plots. In control plots the water content generally remained around 18-20 percent at a depth of 0-15 cm, and 19-23 percent at 15-30 cm.

At the peak stress periods, i.e. 30 d without irrigation with T 101 and 25 d without irrigation with T 102, water contents in the surface layer (0-15 cm) were depleted to 9.5 percent in 1989 and to 10 percent in 1990 with T 101, and to 4.0 percent in 1989 and to 6.0 percent in 1990 with T 102 (Table 2). At 15-30 cm, the soil water content was depleted, but less so.

Table 2
Soil water content and temperature under various moisture stress treatments

Transpiration and leaf water content
In general, TR and RWC were significantly lower, and leaf DR was higher, in stressed plants than in their respective controls (Table 3). Although the cultivars differed significantly in terms of TR, DR and RWC, the trends of changes in these parameters due to stress were similar. The difference in the values of TR, DR and RWC compared to their respective controls were small. Leaf RWC under stress was also low. During the stress period, leaf temperatures of stressed plants were consistently higher than ambient (data not presented).

Table 3
Leaf diffusive resistance, transpiration and relative water
content of groundnut at peak stress, 1990

Leaf area index
As groundnut is an indeterminate crop, the leaf area index (LAI) values tended to increase rapidly from 40 d after sowing and continued to increase until 100 d. Genotype effects on LAI were significant. However, Figure 2 only trends in LAI that present stress influences. LAI values of control plants (T 100) were significantly higher during the 50-60-d period. However, the LAIs of plants stressed in the vegetative phase were higher during the reproductive phase, i.e. from 80 d until harvest, than the control plants. During the reproductive phase, the highest LAI was with treatment T102. Therefore, transient soil moisture stress during the vegetative phase increased LAI during the reproductive phase.

Figure 2
Leaf area index of groundnut with various treatments,
mean values over two years

Dry matter distribution
In general, transient soil moisture deficit stress during vegetative growth resulted in higher biomass at harvest (Table 4). During early growth (20 d after sowing) about 45-50 percent of dry matter was distributed to stems (Figure 3). Plants under stress distributed relatively more dry matter to the stems, not only during stress but also after it, than did control plants, although the differences were not statistically significant at final harvest. In contrast, percent dry matter distribution to leaves was significantly higher in the control plants between 40 and 50 d after sowing, when maximum percent dry matter was obtained. Stressed plants had significantly lower dry matter distribution to the leaves both during the stress period and after relief of stress (Figure 3). Accumulation of dry matter in the reproductive parts (pegs and pods) began at 50 d after sowing (Figure 3), and percent dry matter accumulation increased slowly from 50 to 80 d, and then rapidly until final harvest. Dry matter distribution throughout pod development period until maturity was higher in stressed plants, particularly treatment T 102, which resulted in higher pod yields in both years. In general, there were improvements also in shelling outturn due to imposition of stress during vegetative growth, except in the case of cultivar GG 2. In 1989, the shelling outturn in cv. GAUG 1, due to stress (T 102) was higher (75 percent) as compared to control plants (70 percent) (Table 4).

Table 4
Biomass, pod yield, harvest index and shelling outturn of groundnut
cultivars under stress treatments 1989 and 1990

Figure 3
Percent dry matter distribution to stem, leaves, and reproductive parts
of groundnut 1989 and 1990, mean values

Harvest index
In general, HIs increased due to the imposition of soil moisture deficit stress during the vegetative phase (Table 4). Cultivar GG 2 gave the highest HI values, irrespective of treatment, in both years. The 100-seed weight was highest (44.4 g) in GG 2 in treatment T101 in both years.

Yield and water use efficiency
Genotypic variation in pod yield was significant. Cultivar GG 2, known for its moisture stress tolerance gave the highest pod yields under control and treatments in both years (Table 4). Consequently, the percent increase in pod yield due to stress was lowest in cv. GG 2 (8.4 percent); it was highest in cv. Ak 12-24 (25 percent). Total biomass and pod yield values at the final harvest were highest with treatment T 102 in both years. The higher reproductive efficiency of these plants was due to improved synchrony in flowering, higher conversion rates of flowers to pegs, and of pegs to pods, e.g. the high peg-to-pod conversion rate observed with T 102 (Nautiyal et al. 1999). Water stress in the vegetative stage stimulated the growth of reproductive and vegetative parts.

Field WUE among treatments and cultivars varied widely (Table 5). Watering regime and variety had significant effects on field WUE. Although the highest field WUE (6.2) was for cv. Ak12-24 in 1989, GG2 provided the highest overall values, particularly in 1989. Overall, T 101 values were higher than T 102 values, which were higher than the control values. Thus, withholding irrigation during vegetative growth improved WUE.

Table 5
Field water use efficiency for groundnut pod production in 1989 and 1990

Experiment 2

This experiment built on findings that mild stress during vegetative growth was beneficial to biomass accumulation and yield production. Control plants gave the highest biomass accumulation and pod yield (Table 6). Reductions in biomass and pod yield were greater with T 202 than with T 201. The significant reduction in HI with T 201 suggests that the reduction in economic yield due to prolonged moisture deficit stress was more than the reduction in biological yield. Stress during the vegetative phase was beneficial in terms of economic yield. The available soil moisture at 0-15 cm soil depth was consistently higher in T 200 than T 201 (Figure 4). These results tend to support those obtained in Experiment 1, indicating that mild stress during vegetative growth may be beneficial to groundnut biomass accumulation and pod yield; the trend might have been clearer in Experiment 2 if the control plants had not been exposed to stress. Prolonged stress (T 202), even when mild, was detrimental to crop development and final yield production. Thus, the water requirement of a groundnut crop during the vegetative phase is relatively low, and higher during the flowering, pegging and initial pod development periods.

Table 6
Effect of soil moisture deficit stress on groundnut cultivar GG 2, summer 1999

Figure 4
Available soil moisture at 0-15 cm depth, Experiment 2, in 1999

Conclusion

Climate, agronomic and variatal facators determine total water use by a crop. The data presented here show that by water deficit stress during the vegetative phase of development, can increase WUE significantly. Soil moisture deficit stress during vegetative growth increased total biomass accumulation and pod yield. These increases were due mainly to increases in leaf area during reproduction, and partitioning of more dry matter to the reproductive parts. In addition, the yield advantage due to water stress in the vegetative phase was due to improved synchrony in flowering and the increased peg-to-pod conversion. Moreover, stress during vegetative growth may have promoted root growth, an area which requires further study.

The results presented here show it is possible to increase field WUE and dry matter production, including the economic yield of groundnut crops cultivated under irrigated conditions by the imposition of transient soil moisture deficit stress during the vegetative phase. However, exact scheduling may differ in different environments.

References

Angus, J.F., Hasegawa, S., Hsiao, T.C., Liboon, S.P. & Zandstra, H.G. 1983. The water balance of post-monsoonal dryland crops. Journal of Agricultural Science (UK) 101: 699-710.

Barrs, H.D. & Weatherly, P.E. 1962. A re-examination of the relative turgidity technique for estimating water deficits in leaves. Australian Journal of Biological Science 15: 413-428.

FAO. 1999. FAO production yearbook. Rome.

Nageswara Rao, R.C., Singh, S., Sivakumar, M.V.K., Srivastava, K.L. & Williams, J.H. 1985. Effect of water deficit at different growth phases of peanut. I. Yield response. Agronomy Journal 77: 782-786.

Nautiyal, P.C., Ravindra, V., Zala, P.V. & Joshi Y.C. 1999. Enhancement of yield in groundnut following the imposition of transient soil-moisture-deficit stress during the vegetative phase. Experimental Agriculture 35: 371-485.