Effects of Spermidine Pretreatment through the Roots on Growth and Photosynthesis of Chilled Cucumber Plants (Cucumis sativus L.)

Effects of endogenous polyamine levels on the growth of chilled cucumber plants (Cucumis sativus L.) were investigated, with a focus on susceptibility of photosynthesis to photoinhibition. Plants, at the second leaf stage, were administered 0.5 mM spermidine (Spd) through the roots before they were exposed to mild chilling stress at 10/7°C (day/night) for 8 days under a photosynthetically active photon flux density of 240 μmol·m-2·sec-1. The Spd-pretreated plants had a high Spd content in both leaves and thylakoid membranes, compared to the control. The pretreated plants had higher growth rates and leaf chlorophyll content than had the control during chilling as well as after being transferred to 28/22°C. The control plants showed a marked decline in photosynthetic rates during chilling, which was less pronounced in Spd-pretreated plants. Spd pretreatment did not affect stomatal conductance of chilled leaves, but alleviated the decline of the chlorophyll fluorescence yield (Fv/Fm), photosynthetic electron transport activity of thylakoids, and activity of enzymes in carbon metabolism, as well as chill-induced increase of lipid peroxidation in the thylakoid membranes. In addition, preloading the isolated thylakoids with Spd resulted in decreased susceptibility to low-temperature photoinhibition of photochemistry. The results indicate that the high levels of Spd in leaves and/or chloroplasts contribute to the improvement of chilling tolerance of the photosynthetic apparatus in cucumber leaves.