Response of cassava to water shortage III. Stomatal control of plant water status

Diurnal measurements of leaf water potential and the diffusive conductance of the abaxial surface of two cassava cultivars, M Col 22 and M Mex 59, were made on three occasions on field grown plants during a 10-week period of rainfall exclusion. Conductances of about 10 mm s^-1 were observed in the rainfed plots but generally the mean conductance was in the range 3-5 mm s^-1. The minimum water potential of -1.8 MPa was observed in the rainfed plots. Water shortage caused reduction in mean conductance to <1 mm s^-1 at which level the control of water loss maintained leaf water potential > -1.5 MPa at all times. Stress plots recovered more slowly during the late afternoon but during the day had higher leaf water potentials than the controls. At the same levels of leaf water potential the conductance of M Mex 59 was less than that of M Col 22 in both control and stress plots. Measurements are also reported of the stomatal distribution, density and pore size for both fully expanded leaves and those whose expansion was seriously restricted by the water shortage.