High-temperature-induced Alteration of ABA and Polyamine Contents in Leaves and its Implication in Thermal Acclimation of Photosynthesis in Cucumber (Cucumis sativus L.)

The possible involvement of ABA and polyamines in the thermal acclimation process of photosynthesis in cucumber (Cucumis sativus L., cv. Nankyoku No. 2) was investigated. Photosynthetic activity was measured in the photosynthetic oxygen evolution rate and photosystem (PS) II chlorophyll fluorescence yield (Fv/Fm). Raising the growth temperature from 25/25°C to 38/38°C (day/night) caused gradual reduction in heat damage to photosynthesis from the prior exposure of intact leaves to 45°C. Thylakoids isolated from acclimated leaves were more thermostable than those from non-acclimated leaves, as judged by a lesser degree of reduction in PS II and PS I electron transport activity and the loss of 33-kDa polypeptides and manganese after exposure of isolated thylakoids to 40°C. The enhanced thermostability of the photosynthetic apparatus is probably attributable to the decrease in lipid unsaturation of thylakoid membranes. Exposing the plants to 38°C caused a rapid decrease in ABA content in leaves, which may exclude the possible involvement of ABA in increased thermostability of photosynthesis during the acclimation treatment. However, acclimation treatment caused a gradual increase in spermidine and spermine titers in leaves. Application of 5 mM spermidine or spermine to non-acclimated leaves made the photosynthetic apparatus more thermostable. Spermine treatment to isolated thylakoids from non-acclimated leaves also alleviated the heat-inactivation of photosystems. The results suggest that polyamines play a role in thermal acclimation of photosynthesis in cucumber.