Roots and drought resistance

Abstract

The influence of roots on the yield of water-limited crops is analysed with the help of the identity: yield = water used X water-use efficiency X harvest index. Despite being severely water-stressed, many droughted crops leave substantial amounts of apparently available water in the subsoil at maturity. The factors influencing this amount are outlined, particularly those concerning the morphology of the root system. Prospects for improving yield by extracting the residual water are discussed. Because roots are difficult to harvest, water-use efficiency is usually defined as above-ground-biomass/water-used. It follows that the more assimilate a plant transfers to its roots the lower will be its water-use efficiency. There is presumably an optimal root/shoot ratio (in terms of water relations) at which above-ground biomass is maximal for a given water supply. This ratio appears to exceed the optimum in many cases. For a given biomass, the yield of a grain crop depends in part on the pattern of water use during the season, because harvest index is often related to the proportion of the total water supply that is used after anthesis. For crops relying on a limited supply of stored water, a high axial resistance to flow in the roots may ensure that water in the subsoil is not used so quickly that too little remains at anthesis for the plants to set and fill an adequate number of grains. A breeding program aimed at changing this resistance in wheat roots is described. Finally, the principles are discussed on which physiological research can be useful in improving drought resistance. The need to dissect water-limited yield into largely independent components is emphasised, for such a dissection greatly improves the focus of the research. © 1983.