DEVELOPMENTAL HISTORY AFFECTS THE SUSCEPTIBILITY OF SPINACH LEAVES TO INVIVO LOW-TEMPERATURE PHOTOINHIBITION

Room temperature chlorophyll a fluorescence was used to determine the effects of developmental history, developmental stage, and leaf age on susceptibility of spinach to in vivo low temperature (5-degrees-C) induced photoinhibition. Spinach (Spinacia oleracea cv Savoy) leaves expanded at cold hardening temperatures (5-degrees-C day/night), an irradiance of 250 micromoles per square meter per second of photosynthetic proton flux density, and a photoperiod of 16 hours were less sensitive than leaves expanded at nonhardening temperatures (16 or 25-degrees-C day/night) and the same irradiance and photoperiod. This differential sensitivity to low-temperature photoinhibition was observed at high (1200) but not lower (500 or 800 micromoles per square meter per second) irradiance treatment. In spite of a differential sensitivity to photoinhibition, both cold-hardened and nonhardened spinach exhibited similar recovery kinetics at either 20 or 5-degrees-C. Shifting plants grown at 16-degrees-C (day/night) to 5-degrees-C (day/night) for 12 days after full leaf expansion did not alter the sensitivity to photoinhibition at 5-degrees-C. Conversely, shifting plants grown at 5-degrees-C (day/night) to 16-degrees-C (day/night) for 12 days produced a sensitivity to photoinhibition at 5-degrees-C similar to control plants grown at 16-degrees-C. Thus, any resistance to low-temperature photoinhibition acquired during growth at 5-degrees-C was lost in 12 days at 16-degrees-C. We conclude that leaf developmental history, developmental stage, and leaf age contribute significantly to the in vivo photoinhibitory response of spinach. Thus, these characteristics must be defined clearly in studies of plant susceptibility to photoinhibition.