Osmotic stress affects physiological responses and growth characteristics of three pistachio cultivars

Pistachio (Pistacia vera L.) has a high tolerance to drought and soil salinity. Although adult pistachio trees are well known to be drought tolerant, the studies on physiological adaptation of pistachio cultivars to drought are limited. Therefore, three pistachio cultivars, i.e., Akbari, Kaleghochi, and Ohadi were subjected to three osmotic drought stress treatments: control (-0.1 MPa), moderate (-0.75 MPa) and severe drought (-1.5 MPa) stress using PEG 6000 for a 14-day period. All drought stress treatments decreased net photosynthesis (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci), and transpiration rate (E), but Ohadi maintained better its photosynthetic capacity compared to Akbari and Kaleghochi. Maximum quantum yield of PSII photochemistry (Fv/Fm), effective PSII quantum yield (UPSII) and photochemical quenching (qP) were also reduced. The chlorophyll fluorescence parameters indicated that Akbari was more susceptible to the applied drought stress. Drought stress levels decreased chlorophyll pigments, fresh weight, stem elongation, leaf nitrogen content (N), leaf water potential and increased water use efficiency (WUE). Proline increased strongly under drought stress for Akbari. After 2 weeks of stress a recovery of 2 weeks was applied. This period was insufficient to fully restore the negative effects of the applied stress on the studied cultivars. Based on the reduction of photosynthesis and the increase of the proline content Akbari seems more sensitive to the applied drought stress.