Photosynthetic Rates and Solute Partitioning in Relation to Growth of Salt-Treated Pistachio Plants

Abstract

Seedlings of Pistacia vera cv. Kerman were grown in a porous medium under glasshouse conditions and treated for up to 5 weeks with 0.5 (control), 100 or 175 mol m^-3 Cl^- in irrigation water, with accompanying cations Na⁺ , Ca²⁺ and Mg²⁺ in the molar ratio 6 : 1 : 1. Salinity treatments were raised to the respective levels by daily increments of 25 mol m^-3 Cl^-.

Shoot growth was decreased by 100 mol m^-3 CI^- and ceased during treatment with 175 mol m^-3 CI^- . Turgor was maintained in expanding leaves during and after the period of incremental increase in salinity up to the maximum of 175 mol m^-3 CI^-. There was an increase in sucrose and reducing sugar concentrations on a tissue water basis in expanding leaves of plants after 2 days of treatment with 175 mol m^-3 CI^- but only a marginal increase in Cl^- and Na⁺. Extension rates and final lengths of successively emerging leaves decreased with duration of treatment with 175 mol m^-3 CI^-. Emergence of new leaves eventually ceased on these plants.

Bulk leaf Cl^- and Na⁺ concentrations of expanded 42-day-old leaves after treatment of plants for 35 days with 175 mol m^-3 CI^- were 255 and 142 mol m^-3, respectively, but despite these concentrations photosynthetic rates were not reduced. There was an increase in proline and a decrease in shikimic acid in these leaves. Rates of water vapour loss per unit leaf area of whole plants did not differ markedly between control and salt (175 mol m^-3 CI^-)-treated plants after 40 days of salt treatment. The salt- stressed plants accumulated higher sucrose and starch concentrations in the stem and higher concentrations of sucrose, reducing sugars and starch in the main root. Total carbohydrate concentrations were also marginally higher in the root tips of salt-treated plants.

These data indicate that photosynthetic rate of expanded leaves, photoassimilate supply to, water relations of, and Cl^- and Na⁺ concentrations of expanding leaves are unlikely to be responsible for the growth reduction of pistachio plants observed at relatively high salinities.