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 trait has not been successfully exploited through breeding. Selection for high WUE often resulted in decrease in crop growth rates (CGR) and this was perhaps the most significant setback for further improvement in this trait. Variations in WUE are brought about by stomatal diffusive 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 production is strongly associated with total transpiration (T), any reduction in T results in reduced CGR, Since g(s) is associated both with T and 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 growth rates. Determination of g(m) is difficult unlike estimation of g(s). We propose a novel approach to estimate the time averaged mesophyll efficiency based on the carbon and oxygen isotope discrimination that occurs during photosynthesis and transpiration, And we discuss the importance of g(m) types (capacity types) in improving WUE as well as total dry matter (TDM).