Photochemistry and gas exchange in cold conditions in Zn-deficient red cabbage (Brassica oleracea L. var. capitata f. rubra) plants

The responses of red cabbage (Brassica oleracea L. var. capitata f. rubra) plants to a low Zn supply and cold conditions (10°/7°C day/night temperature) were investigated in a hydroponic growing medium. A low Zn supply caused a significant reduction of shoot and root dry weight - up to 55% and 45% for the control and 62% and 52% for cold-treated plants, respectively. The total soluble carbohydrates and starch declined in Zn-deficient plants. Exposure to low temperatures, however, led to a decline in starch but an increase in soluble sugars. In Zn-sufficient plants, low temperatures increased the excitation capture efficiency of open photosystem II (PS II) reaction centres (RCs) (F'_v/F'_m), the quantum yield of PS II (Φ_PSII), the electron transport rate (ETR) and the proportion of active chlorophyll associated with the RCs of PS II (F_v/F₀). Low temperatures did not affect net CO₂ uptake in Zn-sufficient plants, though a reduction of stomatal conductance occurred. The results demonstrated that although cold-treated plants were slightly more susceptible to Zn deficiency, cold treatment caused greater shoot biomass (up to 32%) in plants supplied with adequate Zn. The adaptation of red cabbage plants to cold conditions is attributable to improved photochemical events in the leaves, a maintenance of the net CO₂ assimilation rate, lower water loss and the accumulation of anthocyanins as antioxidants.