Influence of Water Deficits on Gas-Exchange and the Leaf Area Development of Cassava Cultivars

Measurements of gas-exchange, leaf water potential and the leaf diffusive conductance of the abaxial leaf surface of six cassava cultivars, M Mex 59, M Ven 218, M Col 1684, M Col 72, M Col 22, and M Col 638, were made at 48 h intervals and between 1200–1500 h, on potted plants, grown outdoors during a 58 d period of withdrawal of irrigation. Rates of net-photosynthesis of about 28 mg CO2 dm−2 h −1 were reduced to zero within the first 5 d of the drying cycle, despite the small differences in leaf water potential of 0.15 MPa. Water shortage also caused a reduction in mean conductance to < 1.0 mm s−1 at which level the control of transpiration maintained leaf water potential at > —1.6 MPa. Cultivar differences in the response of net-photosynthesis and leaf diffusive conductance to water shortage were seen within 2 d of the dry cycle and the leaf water potential was commonly 0.15 MPa lower than in the wet controls. The most vigorous cultivars (M Mex 59, M Ven 218 and M Col 1684) reduced their rates of net-photosynthesis to zero by day 5 of the dry cycle when the soil water content was depleted by 65%. Less vigorous cultivars (M Col 72, M Col 22 and M Col 638) reduced their rates of net-photosynthesis to zero by day 30, when the soil water content was depleted by 75%. Measurements are also reported of the leaf production per apex and leaf extension for leaves produced during the drying cycle.