WHY HAS BREEDING FOR WATER-USE EFFICIENCY NOT BEEN SUCCESSFUL - AN ANALYSIS AND ALTERNATE APPROACH TO EXPLOIT THIS TRAIT FOR CROP IMPROVEMENT

Despite the realization about the importance of water use efficiency ( WUE) in crop improvement, the available genetic variability in this tr ait has not been successfully exploited through breeding. Selection fo r high WUE often resulted in decrease in crop growth rates (CGR) and t his was perhaps the most significant setback for further improvement i n this trait. Variations in WUE are brought about by stomatal diffusiv e characteristics (g(g)) and/or intrinsic photosynthetic capacity (g(m )), Most often plants have evolved to maximize WUE through a reduction in transpiration that is linked with the g(s). Since dry matter produ ction is strongly associated with total transpiration (T), any reducti on in T results in reduced CGR, Since g(s) is associated both with T a nd internal CO, partial pressure (Pi), WUE and T become strongly inter -dependent. However, if the variations in Pi and hence WUE are brought about by g(m), the inter dependency between T and WUE will be lesser and selection for WUE from such types will result in higher crop growt h rates. Determination of g(m) is difficult unlike estimation of g(s). We propose a novel approach to estimate the time averaged mesophyll e fficiency based on the carbon and oxygen isotope discrimination that o ccurs during photosynthesis and transpiration, And we discuss the impo rtance of g(m) types (capacity types) in improving WUE as well as tota l dry matter (TDM).