Plant tolerance to water deficit: physical limits and possibilities for progress

Available strategies for an improved tolerance to water deficit are discussed. It is proposed that the 'water for CO2' trade off cannot be completely overridden by any genetic strategy. Genes conferring an ability to survive cell desiccation will probably not confer a better ability to accumulate biomass under agriculturally compatible drought scenarios. In the same way, reengineering of stomatal control or of turgor maintenance may face theoretical problems. Most striking 'success stories' until now are based either on an increased water use efficiency in very dry scenarios, or on growth maintenance in milder drought scenarios. They are based on a long-term analysis and exploitation of the existing genetic variability and have brought limited progress in yield, but still important in economic terms. Future work will probably consist in developing genotypes appropriate to precise drought scenarios, rather than in finding a unique genetic solution for drought resistance. This view would place the modelling of virtual genotypes at the centre of the research for drought tolerance, by enabling one to test the adequacy of a combination of alleles in each drought scenario. Only the best combinations would then be obtained genetically and tested in the field. (C) 2004 Published by Elsevier SAS on behalf of Academie des sciences.