GAS-EXCHANGE, WATER-LOSS AND BIOMASS PRODUCTION IN RICE AND WILD ORYZA SPECIES IN WELL-WATERED AND WATER-LIMITING GROWTH-CONDITIONS

A major reason for the drought susceptibility of cultivated rice is th e inability to regulate water loss as effectively as other cereals. We report studies of growth and gas exchange under well-watered and wate r-limiting conditions on selected species of Oryza and seek to relate this physiological information to the ecology of the genus. The immedi ate ancestor of the Oryza sativa cultigen is native to swamps and mars hes, and wet habitats are typical for the genus as a whole. However, t he ecological range of some species does extend to habitats that dry o ut seasonally. In all species studied growth was reduced by water defi cit. In all species studied leaves had small absolute water contents a nd began to roll at a relative water content above 90%. However, there were species differences in leaf rolling and the response of stomatal conductance to an increase in vapour pressure deficit. Following re-w atering, there were persistent reductions in stomatal conductance in m ost of the species tested, but the assimilation rate was not reduced i n all of these cases. Where there was a persistant reduction in assimi lation rate, there was also a reduction in carboxylation efficiency. I t was a frequent observation that plants had a stomatal conductance gr eater than expected for their carbon assimilation rate; that is in the range where substantial changes in conductance have little effect upo n photosynthesis. It is suggested that a reason for this may be the co oling of leaves which have a small thermal capacity in environments wh ich often combine high temperature, humidity and irradiance. Large con ductances combined with small water contents may be no disadvantage in the natural habitats of Oryza, but provide some reasons for the poor regulation of water loss in cultivated rice. Although there were signi ficant differences in gas exchange amongst species the advantages that were observed over O. sativa were not of a magnitude likely to justif y wide hybridisation. This implies that improvement in the drought res istance of rice is more likely to come from increasing water acquisiti on than from decreasing water loss.