Responses of seven diverse rice cultivars to water deficits II. Osmotic adjustment, leaf elasticity, leaf extension, leaf death, stomatal conductance and photosynthesis

Turgid osmotic pressures, leaf extension rates and the bulk modulus of elasticity of the leaves were followed in seven rice (Oryza sativa L.) cultivars before, during and after the imposition for 10 days of differential irrigation treatments by a line source sprinkler system imposed pre-flowering. Additionally, leaf death was monitored in three cultivars and leaf conductance and the rate of net photosynthesis measured in two cultivars. The seven rice cultivars, Azucena, Tachiminori, IAC 25, IRAT 13, BPI 76, IR 20 and IR36, represented a wide range of germplasm from traditional dryland types to improved wetland types. With the development of water deficits, apparent differences in osmotic adjustment were observed among cultivars, but no differences among cultivars or with treatments were observed in the bulk modulus of elasticity. The degree of osmotic adjustment was shown to be correlated with the cumulative stress days above a threshold cumulative leaf water potential of -16 to -17 MPa days. As no differences among cultivars in the threshold stress or rate of decrease of osmotic potential below this threshold were observed, we conclude that the apparent differences in osmotic adjustment among the seven cultivars arose from differences in the development of stress. The daily rate of leaf extension was more sensitive to water deficits than leaf death, but was no more sensitive to midday leaf water potentials than was leaf conductance. Both leaf extension and leaf conductance had maximum values at midday water potentials of -0.8 MPa and minimum values at -2 MPa and below. At high water potentials, the rate of net photosynthesis decreased between 10.00 h and 14.00 h even though the abaxial stomatal conductances remained high.