ABA-induced Increase in Thermostability of Photosynthetic Electron Transport of Thylakoids in Leaves of Cucumber (Cucumis sativus L.).

Abstract

The effect of ABA on thermostability of the photosynthetic apparatus in leaves of cucumber (Cucumis sativus L.) was investigated. Cucumber plants grown in soil at 25°C (day and night) were administered foliar sprays of 1 mM ABA when the third leaves were fully expanded. Compared to untreated control leaves, ABA-treated leaves had faster rates of photosynthetic O2 evolution and a higher photosystem (PS) II chlorophyll fluorescence yield following exposure to 45°C in the dark for 10 min. These effects of ABA were attenuated over time after ABA treatment. Heating the isolated thylakoids from treated and control leaves at 40 °C in the dark for 5 min revealed that heat-induced loss of PS II- and PS I-mediated electron transport rates was alleviated by the ABA treatment. The PS II reaction center was not heat-damaged, regardless of the source of thylakoids. On the other hand, a reduction in the loss of proteins and manganese during heat treatment occurred in thylakoids from ABA-treated leaves compared to those from control leaves. This reduction indicates that ABA increased the thermostability of the O2 evolving complex. Direct treatment of isolated thylakoids with ABA was ineffective in preventing the heat inactivation of the photosystems. Thus, it seems that high foliar levels of ABA induce a mechanism associated with thermostability of the O2 evolving complex. However, these effects of ABA on isolated thylakoids were smaller than were those on intact leaves, indicating that ABA can also induce thermostability in other components of the photosynthetic apparatus besides the O2 evolving complex.