Diurnal and seasonal changes in Prunus amygdalus gas exchanges

Diurnal courses in net photosynthetic rate (P-N), Stomatal conductance (g(s )), leaf water potential (psi), internal CO2 concentration (c(i)), and wate r use efficiency (WUE) were studied as season progressed, in relation to en vironmental factors in field grown Prunus amygdalus. In sun leaves P-N reac hed maximum between 09:00 to 11:00 h and subsequently declined when high te mperature and low humidity occurred. An increase was observed late in the a fternoon. A decrease in g(s) and psi was found as season progressed in both years of measurements. In periods of high evaporative demand, psi was very low, however, it did not explain the reductions of P-N in all the three pe riods (spring, early and late summer). Midday depression of P-N and g(s) se emed to be related with leaf temperature (T-l) and high irradiance. Increas e in c(i) and F-0 and decrease in F-V/F-m found between 12:00 and 14:00 h c orresponded to the decrease in P-N. Therefore, a transient modification of photosynthetic machinery might be considered. WUE was negatively correlated with vapour pressure difference of leaf to air, that decreased during the day. The September values, higher than in the previous months, were due to the lower seasonal decreases in P-N than in g(s).