Water use efficiency under stress environments

Abstract

Water supply is a major constraint to crop production particularly for grain legumes more than 80% of which are grown under rainfed conditions. Global warming is increasing the problem by creating serious water shortages and large inter-seasonal fluctuations. Water use efficiency (WUE) provides a quick and simple measure of how well rain or irrigation water can be converted into grain. It represents a given level of biomass or grain yield per unit of water used by the crop. Improving crop productivity therefore requires answering the following question. How can WUE be improved and how can cropping systems be modified to result in a more efficient use of water? A review of the literature reveals a large variation in WUE across a range of climate, soils and crops. Typically low and unstable yields of grain legumes are due to receding soil moisture at the time of sowing and terminal drought. Numerous studies have shown that grain yield is positively correlated with post-flowering water use. The actual crop water requirement usually varies with genotype, its growing duration and agronomic and soil management methods used. Differences in water requirements are mainly due to differences in canopy structure, root system and growth periods. Water stress adversely affects biological nitrogen fixation, plant growth, grain yield and WUE. Crop adaptation to drought is a function of the interaction of phenology with the pattern of water use. Plant architecture and root systems play a key role in the use of available soil moisture. Development of early flowering and pod set, rapid seed fill and early maturity varieties seems to be quite advantageous for increasing drought tolerance and improving WUE. It is possible to increase WUE, by 25-40% through soil management that involves deep tillage, crop residues, mulches, etc. Among agronomic practice options for increasing WUE, various experiments reveal about 12% increase in WUE associated with timely sowing. Supplemental irrigation at the reproductive phase may result in approximately 75% increase in grain yield and 35% increase in WUE. The drilling of phosphorus and zinc in deficient soils gives substantially higher grain yields and enhances water use. Conservation practices associated with harvesting the maximum amount of rain water and a watershed based water-harvesting and recycling system may be established on a large scale for providing life-saving irrigation. Sprinkling irrigation, drip irrigation or film hole irrigation practices save more than 50% water and increase the grain yield significantly. However, these advanced and costly techniques have very limited scope in grain legumes due to their low yields. Improved technology such as land leveling, crop rotation, intercropping, integrated use of farm resources and timely socio-economic interventions can help greatly in improving crop productivity and water use of cool season grain legumes.