Diurnal Variations in Photosynthetic Gas Exchanges in Two Iranian Pistachio (Pistacia vera L.) Cultivars

Diurnal variations in net photosynthesis rate (A), stomatal conductance (gs) and transpiration rate (E) were assessed at the leaf surfaces of two pistachio (Pistacia vera L.) cultivars, in reference to diurnal fluctuations of leaf temperature (Tleaf), vapor pressure deficit (VPD) and photosynthetic photon flux density (PPFD). In this experiment, diurnal changes of net CO2 assimilation showed two-peaks pattern in which the first peak were appeared at 9:00 a.m. and 8:00 a.m. in ‘Badami’ and ‘Qazvini’ cultivars, respectively. The second peaks also emerged at 16:00, and the lowest assimilation rates were observed at 15:00 in both cultivars. The corresponded PPFD with the highest net assimilation rate were achieved at 1500 and 1150 µmol CO2 m-2 s-1 in ‘Badami’ and ‘Qazvini’ cultivars, respectively. It is interpreted that stomatal conductance in 'Badami' was primarily limited by VPD and leaf temperature higher than 3.5 kPa and 32 ° C, respectively. However, in ‘Qazvini’, the VPD higher than 3.8 kPa and leaf temperature above 33 ° C were the restricted factors. Quantum yield of CO2 fixation was decreased at leaf temperatures higher than 24 ° C. Generally, diurnal patterns of leaf gas exchange characteristics revealed that afternoon depression of net photosynthesis was mainly affected by stomatal factors, however, non-stomatal factors involved in this event, too.