Field comparison of resource utilization and productivity of three grain legume species under water stress

English: Grain legumes play a major role in low input agricultural systems by providing quality protein to the poor communities and improving the natural resource base used for the production of other rainfed cereal crops. The yield of the crops, however, is low mainly due to water shortage. This study had a major aim of comparing the resource use and productivity of beans, chickpea and cowpea under water stress and well-watered conditions in a semi-arid environment so as to facilitate crop choice and management practices in different legume producing environments. Resource utilization and productivity studies for a given crop or cropping system involve both the crop and its growing environment. In this study, therefore, resource utilization and productivity were studied through field experimentation with three grain legume species and analysis of rainfall/water supply behaviour of ten representative grain legume growing regions in Ethiopia. The field experiments were conducted at Dire Dawa, Ethiopia. The station lies in the semi-arid belt of the eastern Rift Valley escarpment with a long-term mean annual rainfall of 612 mm and a soil dominated by Eutric Regosol. The field experiments were conducted for three seasons in 200112002, 2002 and 2002/2003. The treatments were three water regimes, viz., well-watered (C), mid-season (MS) and late season (LS) water stress and three species arranged in a randomised split plot design using water regimes as main plot and the species as sub-plot. The experiments involved measurements of important variables in the soil-plant-atmosphere continuum. Analysis of the long-term rainfall of 10stations in chapter 2 indicated the existence of major regional differences in water supply. In some of the regions (e.g. Bahir Dar, Bako and Bole) excess water is a problem while in other areas (e.g. Dire Dawa and Jijiga) water shortage is a major bottleneck for crop production. Based on water supply, the regions were grouped as ample water supply, intermediate water supply and poor water supply regions. The study indicated the need to adjust crop choice and management practices based on site and seasonal conditions. The resource utilization and productivity of the three species was studied based on a micrometeorological approach involving phenology, growth and dry matter partitioning (Chapter 3), water use and water use efficiency (Chapter 4), radiation and radiation use efficiency (Chapter 5), water relations and carbon assimilation (Chapter 6) and yield and its components (Chapter 7). Analysis of phenology and growth indicated a reduction of leaf area and dry matter only in the MS treatment and a shortened growth period only in the LS treatment in all species. However, species differences were observed in that the reduction in leaf area due to MS stress was the least in cowpea compared to beans and chickpea. Both the timing of water supply and species influenced dry matter allocation among aboveground parts. The LS stress hastened dry matter allocation to the pod while the MS depressed it in all species. In the LS stress, beans allocated a higher percentage of the above ground dry matter to the seed than chickpea and cowpea during the mild temperature seasons while cowpea allocated the highest percentage during the high temperature season. Such high dry matter allocation to the pod is important to maintain high harvest index (HI) under water-limited environments. Water use varied across water regimes, the highest being in the C treatment followed by the MS and LS treatments in descending order in all species. However, the MS treatments resulted in the lowest water use efficiency (WOE) in all species due to low leaf area index (LAI) and high soil evaporation. Despite differences in water use, the C and LS treatments had similar WOE in all species indicating that some periods of water stress during the late stage of crop growth may increase WOE and improve water saving in water-limited environments. WOE was also strongly negatively correlated with specific leaf area (SLA) under well-watered conditions in all species and in both seasons suggesting that it could be used as a selection criterion for high WUE in the species. The MS treatment reduced extinction coefficient (K) and thereby reduced fractional radiation interception (F) in all species. Radiation use efficiency (RUE) was also negatively affected by the MS stress in beans and chickpea whereas it was not affected by any of the water stress treatments in cowpea. The relationship among soil water, leaf water potential, stomatal resistance, rate of photosynthesis (A) and transpiration (E), vapour pressure deficit and leaf temperature are described in Chapter 6. Cowpea, followed by beans, closes its stomata at higher level of soil water content and leaf water potential as compared to chickpea. Cowpea also has a capacity to photosynthesise and transpire at a higher rate under favourable water supply and also to maintain a slower rate of decline in A and E under low soil water status when compared with beans and chickpea. The magnitude and rate of A decline was higher and faster in the MS than in the LS stress, and among species, it was faster in chickpea than in beans and cowpea. Stepwise regressions of data indicate that, unlike transpiration, photosynthesis could be estimated from a few weather and physiological parameters with reasonable accuracy in all the three species. In contrast to cowpea, which is less and almost equally sensitive to both stress periods, the grain yield of beans and chickpea was found to be more sensitive to the MS than the LS stress during all seasons. The high sensitivity of beans and chickpea grain yield to the MS stress was associated with reductions in LAI, WUB, RUE and dry matter partitioning to the pod as a result of the stress. The lower grain yield reduction of cowpea under water stress is attributed to the crop's ability to adjust its stomata promptly and maintain its LAl, photosynthesis and RUE at a higher level than beans and chickpea. Simulation of grain yield with CROPGRO in beans and chickpea gave a satisfactory result with some limitations in simulating yield components. The model has shown a promising potential to be used as a decision support tool in the semi-arid regions after further calibration and testing. The results generally show that cowpea is more productive and resource efficient than beans and chickpea under water-limited conditions while beans is more productive and has higher resource efficiency than cowpea and chickpea under well-watered conditions. It is concluded that better productivity and optimum resource utilization can be achieved through proper crop-environment matching. Moreover, crop management and breeding practices should focus on increasing the WUB, RUE and HI of grain legumes to improve the yield of the crops in mid-season drought prone environments.