SEASONAL-VARIATION IN STOMATAL RESPONSES OF 2 CULTIVARS OF LYCHEE (LITCHI-CHINENSIS SONN)

The effect of cultivar and environment on the stomatal conductance and plant water relations of lychee (Litchi chinensis Sonn.) was investigated. Diurnal changes in stomatal conductance (g(s)) and leaf water potentials (psi(l)) were determined for leaves of irrigated trees of cv. 'Bengal' and cv. 'Kwai May Pink' between July and December 1990. Leaves of Bengal always had much higher g(s) than Kwai May Pink in winter, but as summer approached, this difference became less. Psi(l) values at midday for Bengal were always much lower than for Kwai May Pink. The linear model, E = G(psi(l) - psi(soil), where E is transpiration rate and G is whole plant conductance, was found to be valid for nearly all the data sets collected. The values of G for Kwai May Pink were higher than for Bengal, especially in summer, and the average values of G (Bengal 4.1 and Kwai May Pink 6.3 mmol H2O m-2 s-1 MPa-1) indicate that the lychee has a relatively efficient water transport system compared with other fruit tree species. Laboratory measurements of the responses of these cultivars confirmed observed differences in g(s) in the field. The responses of each cultivar to irradiance (I), leaf temperature (T(l)) and leaf-air water vapour pressure deficit (D) were obtained and used to model the orchard data. The equation g(s) = [g(dark) + I/(k(I).T(opt)/T(l)+I][1- k(T) square-root T(opt)-T(l)] / k(d) square-root D where T(opt) is the temperature for maximum g(s), g(dark) is the basal g(s) in the dark at given T(l) and D, and k(l), k(T) and k(D) are constants fitted by non-linear least squares, provided an acceptable fit for both cultivars (R2 = 0.68 for Bengal and 0.55 for Kwai May Pink). The fit was not improved by including psi(l) in the model. There was a significant difference between cultivars in k(T), the temperature sensitivity coefficient. Possible implications of inter-cultivar differences in temperature sensitivity are discussed.