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

[摘要] English: Grain legumes play a major role in low input agricultural systems by providing qualityprotein to the poor communities and improving the natural resource base used for theproduction of other rainfed cereal crops. The yield of the crops, however, is low mainly due towater shortage. This study had a major aim of comparing the resource use and productivity ofbeans, chickpea and cowpea under water stress and well-watered conditions in a semi-aridenvironment so as to facilitate crop choice and management practices in different legumeproducing environments.Resource utilization and productivity studies for a given crop or cropping system involve boththe crop and its growing environment. In this study, therefore, resource utilization andproductivity were studied through field experimentation with three grain legume species andanalysis of rainfall/water supply behaviour of ten representative grain legume growingregions in Ethiopia. The field experiments were conducted at Dire Dawa, Ethiopia. Thestation lies in the semi-arid belt of the eastern Rift Valley escarpment with a long-term meanannual rainfall of 612 mm and a soil dominated by Eutric Regosol. The field experimentswere conducted for three seasons in 200112002, 2002 and 2002/2003. The treatments werethree water regimes, viz., well-watered (C), mid-season (MS) and late season (LS) waterstress and three species arranged in a randomised split plot design using water regimes asmain plot and the species as sub-plot. The experiments involved measurements of importantvariables in the soil-plant-atmosphere continuum.Analysis of the long-term rainfall of 10stations in chapter 2 indicated the existence of majorregional 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 amajor bottleneck for crop production. Based on water supply, the regions were grouped asample water supply, intermediate water supply and poor water supply regions. The studyindicated the need to adjust crop choice and management practices based on site and seasonalconditions.The resource utilization and productivity of the three species was studied based on amicrometeorological approach involving phenology, growth and dry matter partitioning(Chapter 3), water use and water use efficiency (Chapter 4), radiation and radiation useefficiency (Chapter 5), water relations and carbon assimilation (Chapter 6) and yield and itscomponents (Chapter 7). Analysis of phenology and growth indicated a reduction of leaf areaand dry matter only in the MS treatment and a shortened growth period only in the LStreatment in all species. However, species differences were observed in that the reduction inleaf area due to MS stress was the least in cowpea compared to beans and chickpea. Both thetiming of water supply and species influenced dry matter allocation among abovegroundparts. The LS stress hastened dry matter allocation to the pod while the MS depressed it in allspecies. In the LS stress, beans allocated a higher percentage of the above ground dry matterto the seed than chickpea and cowpea during the mild temperature seasons while cowpeaallocated the highest percentage during the high temperature season. Such high dry matterallocation to the pod is important to maintain high harvest index (HI) under water-limitedenvironments.Water use varied across water regimes, the highest being in the C treatment followed by theMS and LS treatments in descending order in all species. However, the MS treatmentsresulted 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 treatmentshad similar WOE in all species indicating that some periods of water stress during the latestage of crop growth may increase WOE and improve water saving in water-limitedenvironments. 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 beused as a selection criterion for high WUE in the species. The MS treatment reducedextinction coefficient (K) and thereby reduced fractional radiation interception (F) in allspecies. Radiation use efficiency (RUE) was also negatively affected by the MS stress inbeans and chickpea whereas it was not affected by any of the water stress treatments incowpea.The relationship among soil water, leaf water potential, stomatal resistance, rate ofphotosynthesis (A) and transpiration (E), vapour pressure deficit and leaf temperature aredescribed in Chapter 6. Cowpea, followed by beans, closes its stomata at higher level of soilwater content and leaf water potential as compared to chickpea. Cowpea also has a capacity tophotosynthesise and transpire at a higher rate under favourable water supply and also tomaintain a slower rate of decline in A and E under low soil water status when compared withbeans and chickpea. The magnitude and rate of A decline was higher and faster in the MSthan 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 beestimated from a few weather and physiological parameters with reasonable accuracy in allthe three species.In contrast to cowpea, which is less and almost equally sensitive to both stress periods, thegrain yield of beans and chickpea was found to be more sensitive to the MS than the LS stressduring all seasons. The high sensitivity of beans and chickpea grain yield to the MS stress wasassociated with reductions in LAI, WUB, RUE and dry matter partitioning to the pod as aresult of the stress. The lower grain yield reduction of cowpea under water stress is attributedto the crop's ability to adjust its stomata promptly and maintain its LAl, photosynthesis andRUE at a higher level than beans and chickpea.Simulation of grain yield with CROPGRO in beans and chickpea gave a satisfactory resultwith some limitations in simulating yield components. The model has shown a promisingpotential to be used as a decision support tool in the semi-arid regions after further calibrationand testing.The results generally show that cowpea is more productive and resource efficient than beansand chickpea under water-limited conditions while beans is more productive and has higherresource efficiency than cowpea and chickpea under well-watered conditions. It is concludedthat better productivity and optimum resource utilization can be achieved through propercrop-environment matching. Moreover, crop management and breeding practices shouldfocus on increasing the WUB, RUE and HI of grain legumes to improve the yield of the crops in mid-season drought prone environments.

[发布机构] University of the Free State