MORPHOLOGICAL RESPONSES OF PULSE ( Vigna spp . ) CROPS TO SOIL WATER DEFICIT

The present experiment was conducted with two common pulse crops namely black gram (Vigna mungo.L) and green gram (Vigna radiata.L) with the objective to study the morpho-physiological changes that took place in response to low moisture stress. Parameters such as plant height, leaf number, leaf area and pod number were studied under moisture stress condition as well as subsequent recovery stages. At harvest, yields of these two crops were recorded and various yield indexes like drought susceptibility index, drought tolerance index, mean and productivity rate were calculated. The study revealed that moisture stress has a significant impact on all these parameters in both crops. The effect was more significant in green gram compared to black gram. From the findings it is observed that moisture stress during flowering stage is detrimental for yield of the pulse crops and re-watering does not have a significant impact on yield improvement. Black gram variety T9 and green gram variety Pratap were identified as drought-tolerant varieties.


Introduction
Drought is one of the most universal and significant environmental stress affecting plant growth and productivity worldwide.Therefore, understanding crop response to this stress is the basis for regulating crops appropriately and achieving agricultural water savings.There are significant differences in the tolerance of plants to drought stress depending upon intensity and duration of stress, plant species and the stage of development (Sing et al., 2012).The response of a crop to water stress varies with crop species, crop growth stage, soil type, environment and season.Drought stress causes a series of physiological, biochemical and morphological responses of crops, which finally results in low yield of green gram (Malik et al., 2006).However, questions remain regarding effects of the stress relief on subsequent crop growth and whether the adverse effects of the stress on the crops can be eliminated.Till date, most of the research works mainly concentrated on crop responses during the period of stress, but relatively little research has been conducted regarding crop response to post-stress of re-watering, and knowledge on this is very limited.
Green gram and black gram are two important sources of high quality protein and amino acids for mankind.They are the basic ingredients in the diets of a vast majority of Indian population.The abrupt climatic change, particularly the erratic rainfall is one of the major causes of reduced pulse production in India (Ali and Gupta, 2012).In this study, four pulse varieties (two each of black gram and green gram) commonly grown in Assam, India were selected as the experimental material to evaluate the various morphological changes taking place during drought stress and subsequent recovery period and on the basis of these changes we tried to identify the resistant varieties.This study aimed to provide a theoretical basis for improving water use efficiency in pulse cultivation by analyzing the relationship between various physiological factors during drought and subsequent re-watering.

Description of the experimental site
The experiment was conducted during September to December 2011 at the experimental field of Tezpur University campus which is located at north bank plain zone of Assam (26º14´ N and 92º50´ E) at Tezpur, India.The maximum and minimum average temperature recorded during the experimental period ranges from 20.5 o C to 32.8 o C and the average rainfall recorded was 2.48 mm with a relative humidity of 50 -80%.The experimental site is characterized by silt loam textured soil being slightly acidic in nature.

Experimental design
The experimental site was ploughed with the help of a tractor.Fertilizers were applied according to the package of practice of Assam Agricultural University @ 32 and 220kg/ha of urea and SSP respectively.A temporary rain shed was constructed in the field with PVC (polyvinyl chloride) film (of about 0.15 mm thickness and 85% of transmittance) to avoid rainfall.The plot was divided into 24 subplots, consisting of 6 rows and 4 columns.The varieties taken for black gram were T9, PU-39 and for green gram were Pratap, TMB-37 and they were denoted as C 1 , C 2 , C 3 and C 4 respectively.All the varieties were collected from Regional Agricultural Research Station (RARS), Shillongoni, Nagaon (Assam), India.Seeds were soaked in running tap water and then sown in the field on the 4 th of September 2011 maintaining requisite gap of 10 and 30 cm between plants and rows respectively.
Two treatments were arranged for each variety: T 1 -control plants with regular watering.T 2 -treatment plants where watering was withdrawn for three weeks (21 days) during reproductive stage (from 37 DAS).
The experiment was conducted in randomized block design with three replications.

Soil Analysis
Before sowing, soils were collected from the experimental field, processed and analyzed for various physico-chemical parameters (e.g.bulk density, soil pH, water holding capacity, soil available nitrogen and organic matter content).Gravimetric method was used to determine the soil moisture content at regular interval throughout the crop growing period.

Plant morphological parameters
Changes in various plant morphological parameters like plant height, leaf number and leaf area, were recorded at 7-day interval from 23 DAS till harvest.Leaf area was recorded by using laser leaf area meter (CI-203, USA).

Yield and yield attributing parameters
On maturity pods were harvested and various yield and yield attributing parameters like number of pod per plant, number of seeds per pod, and finally the weight of seeds per plant were recorded.From these parameters the following yield indexes were calculated: Mean

Statistical analysis
The mean values of plant height, leaf number, leaf area and yield were taken from the measurements of three replicates and the 'Standard Error' of the means was calculated.The correlation study was also done for different growth parameters and soil moisture content with yield.Two-way ANOVA was applied to determine the significance of the results between different treatments.All the statistical analyses were done using the Statistical Package for Social Sciences (SPSS) for Windows (version 16.0).

Results and Discussion
Effect of drought stress on plant height Plant height was recorded for each cultivar during treatment period and at recovery stages.Drought stress has been found to decline the linear growth of shoots in all the cultivars as compared to those of controlled plants.Green gram variety TMB-37 (C 4 ) was more affected by water deficit compared to variety Pratap (C 3 ).Under water deficit, variety T9 (C 4 ) of black gram maintained greater plant height compared to all the varieties of two pulse cultivars under study.Upon re-watering, both black gram and green gram plants recovered in terms of height but did not reach the control level (Figure 1).Under drought stress, declination in plant growth in terms of height is due to the loss of cell turgor which greatly suppresses cell expansion and cell growth thereby inhibiting the linear growth of shoot.Hence it can be inferred that the slow decline in plant height might be due to lack of adequate moisture (Figure 3) in plant root zone.The reduction in plant height is generally associated with a decline in the cell enlargement under water deficit which is greatly hampered due to low moisture as concluded by Bhatt and Srinivasa Rao (2005).

Effect of drought stress on leaf number
During initial stages of growth, an increasing rate of leaf number was recorded in all the varieties of black gram and green gram.When water was withdrawn at reproductive stage, more defoliation was observed in PU-39 (C 2 ) variety of black gram and TMB 37 (C 4 ) variety of green gram compared to other varieties.On rewatering better recovery was not seen in these two varieties.On the other hand, the T9 (C 1 ) variety of black gram was maintaining a good number of leaves and was almost at par with the control T 1 plants (Figure 2).These results are consistent with the findings of Mwale et al. (2007).This decreased leaf number is the result of reduction and termination of new leaf production and also increased leaf abscission.Water deficit stimulates leaf abscission as drought stress has been reported to induce production of ethylene in a variety of species (Kacperska and Kubacka-Zebalska, 1989).Though the resulting decrease in leaf area is one of the mechanisms of moderating water loss from the crop canopy and averting excessive drought induced injury to the plant; this may lead to a decrease in total dry matter production and yield due to reduction in photosynthetically active leaf area (Vurayai et al., 2011).

Effect of drought stress on leaf area
Control plants of all the varieties maintained a gradual increase in leaf area up to 72 DAS.A reduced leaf area was observed in the treated plants from 65 DAS due to leaf senescence.The black gram variety T9 (C 1 ) and the green gram variety Pratap maintained higher leaf area throughout the stress period than rest of the varieties (PU-39, TMB 37).The negative effect of low moisture on leaf area development was evident from a steady increment under low moisture environment which is in accordance with the findings of Zhang et al. (2004).The reduced leaf area observed in the stressed plants in the present study is primarily from a mitotic sensitivity to water stress as given by Wullschleger et al. (2005).Reduced leaf area also decreases carbon assimilation as it is positively related to leaf area and ultimately the yield of the crop as the yield integrates many of the plant morphological processes in a complex way (Graca et al., 2010).Under water deficit conditions, plants first show reduction in cell division resulting in reduced cell number and stop cell elongation inhibiting leaf expansion.This modification in leaf anatomy is one of the basic causes which lead to a reduction in average leaf size under water limiting situation (Baroowa and Gogoi, 2012).

Effect of drought stress on yield and yield attributing parameters
Pod numbers for all varieties were recorded and highest pod number was obtained in variety T9 of black gram.Variety TMB 37 of green gram recorded the lowest pod number (data not presented).The reduced soil moisture caused a general reduction in pod number in all the varieties.From the analysis of variance (ANOVA) it was observed that the treatment has a significant impact on number of pods.
Drought stress during pre-anthesis stage for 21 days greatly reduced the grain yield of both pulses.A large decline in leaf area under water stress is disadvantageous for crop yield as it leads to reduced nutrient uptake (Cabuslay et al., 2002).This coincides with the findings of earlier workers in wheat (Edward and Wright, 2008).The yield of both green gram and black gram were calculated in terms of DSI, DTI, TDS, MP and RP, values of which are presented in Tables 1  and 2. The yields of the pulses in the non-stress environment clearly out yielded in the stress environment (Table 1).The percentage reduction in the yields was higher in PU-39 (26.26%) of black gram varieties while in green gram variety TMB-37 showed a larger percentage reduction (45.77%) in yield.A positive correlation was obtained between soil moisture content and yield of all the varieties and was found to be significant for black gram variety PU-39 (C 2 ) and green gram variety TMB-37 (C 4 ) at the level 0.05.Variety T9 (C 1 ) of black gram showed a significant positive correlation between yield and two important growth parameters (leaf number and leaf area) at the level 0.05 (Table 3).Other yield indexes like DSI, DTI, MP and RP clearly indicate that black gram variety T9 is more resistant than PU-39 and in case of green gram; Pratap is more resistant than TMB-37 against drought stress.Most of the study reveals that drought stress greatly reduces the grain yield, which is dependent on the level of defoliation due to the water stress during early reproductive growth (Monneveux et al., 2006).Reduced photosynthesis and decreased translocation of assimilates to the grain during drought result in lower grain weight and produce more empty grains (Liu et al., 2008).In spite of high drought susceptibility index (DSI), the drought tolerance indices (DTI) of T9 and Pratap were much higher than other two varieties of both pulse cultivars.In addition, high values for tolerance to drought stress (TDS) of PU-39 (black gram) and TMB-37(green gram) could be revealed as favourable for only adequate moisture environment.Using the analysis of variance (ANOVA) it has been found that there is a significant difference between the treatments and varieties.

Conclusion
In the present experiment, it has been observed that both pulse cultivars were significantly affected by drought stress leading to reduced productivity and yield when applied during reproductive stage, the most crucial stage of crop growth.The findings of this experiment suggested that black gram is more resistant than green gram against drought stress particularly at reproductive stage.Variety T9 of black gram and Pratap of green gram are identified as resistant varieties against the applied moisture stress in comparison to the other varieties.Based on this study, plant breeders can consider this information while going for breeding the drought tolerant varieties of these two pulses.

Figure 1 .
Figure 1.Effect of drought stress on plant height of (a) black gram and (b) green gram (DAS -days after sowing).

Figure 2 .
Figure 2. Effect of drought stress on leaf number of (a) black gram and (b) green gram (DAS-days after sowing).

Figure 3 .
Figure 3.Effect of drought stress on leaf area (cm 2 ) of (a) black gram and (b) green gram (MC-moisture content of soil, DAS-days after sowing).

Table 1 .
Crop yield of each variety under control and stressed conditions.

Table 2 .
Crop yield in terms of DSI, DTI, TDS, MP, and RP of black gram and green gram.

Table 3 .
The result of correlation analysis between yield, soil moisture content and growth parameters.