Increasing leaf glutathione through stem feeding does not acclimate Eucalyptus camaldulensis seedlings towards high-light stress

Elevated foliar concentrations of glutathione (GSH) are a common stress response and potentially crucial in conferring increased stress tolerance. The present study addressed the following questions: can increased foliar GSH levels be achieved in the short term by applying a stem feeding technique to tree seedlings? If yes, will elevated GSH concentrations provide improved tolerance to the adverse effects of high-light stress? To this end Eucalyptus camaldulensis seedlings were stem fed a 5 mM GSH solution for 6–7 h before subjecting them to high-light exposure designed to induce photoinhibition. GSH in leaves was measured using a standard photometric method, and the effect of the high-light treatment was evaluated by the decrease in the optimum quantum efficiency of photosystem II (PSII) measured by chlorophyll fluorescence (F _v/F _m). Stem feeding GSH significantly increased GSH concentrations in the leaves up to 40% above control plants. Exposure to artificial high-light intensity for 3 h induced significant photoinhibition in leaves, measured by a 15% decrease in F _v/F _m. At the same time, photosynthesis and stomatal conductance measurements indicated that leaf physiology was not disrupted as a result of the stem feeding technique. However, we have no indication that elevated GSH increased tolerance; neither did it increase sensitivity of plants to high light-induced photoinhibition. This result was accompanied by maintained rates of photochemistry before and after light stress. Unlike previous GSH-related experiments increased tolerance by increasing the rate of photochemistry was not achieved in the present experiment.