A major reason for the drought susceptibility of cultivated rice is the inability to regulate water loss as effectively as other cereals. We report studies of growth and gas exchange under well-watered and water-limiting conditions on selected species of Oryza and seek to relate this physiological information to the ecology of the genus. The immediate ancestor of the Oryza sativa cultigen is native to swamps and marshes, and wet habitats are typical for the genus as a whole. However, the ecological range of some species does extend to habitats that dry out seasonally. In all species studied growth was reduced by water deficit. In all species studied leaves had small absolute water contents and 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-watering, there were persistent reductions in stomatal conductance in most of the species tested, but the assimilation rate was not reduced in all of these cases. Where there was a persistant reduction in assimilation rate, there was also a reduction in carboxylation efficiency. It was a frequent observation that plants had a stomatal conductance greater than expected for their carbon assimilation rate; that is in the range where substantial changes in conductance have little effect upon photosynthesis. It is suggested that a reason for this may be the cooling of leaves which have a small thermal capacity in environments which often combine high temperature, humidity and irradiance. Large conductances 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 significant differences in gas exchange amongst species the advantages that were observed over O. sativa were not of a magnitude likely to justify wide hybridisation. This implies that improvement in the drought resistance of rice is more likely to come from increasing water acquisition than from decreasing water loss.
Yeo, M.E.; Cuartero, J.; Flowers, T.J.; Yeo, A.R. Gas exchange, water loss and biomass production in rice and wild Oryza species in well-watered and water-limiting growth conditions. Botanica Acta (1997) 110 (1) 21-42.