LOW-TEMPERATURE STRESS AND PHOTOPERIOD AFFECT AN INCREASED TOLERANCE TO PHOTOINHIBITION IN PINUS-BANKSIANA SEEDLINGS

The capacity to develop tolerance to photoinhibition of photosynthesis was assessed in jack pine seedlings (Pinus banksiana Lamb.). Photoinh ibition induced at 5 degrees C in control jack pine seedlings grown at 20 degrees C was saturated above an irradiance of 1000 mu mol . m(-2) . s(-1) but was detectable at an irradiance as low as 25 mu mol . m(-2 ). s(-1). However, 20 degrees C seedlings shifted to 5 degrees C were 2-fold more tolerant to photoinhibition than 20 degrees C unshifted co ntrol seedlings, as detected by either the light-dependent decrease in photochemical efficiency or the apparent quantum yield of O-2 evoluti on. The extent of this tolerance of photoinhibition was dependent upon time, photoperiod, and irradiance during exposure to the low-temperat ure shift. Furthermore, the tolerance of photoinhibition was correlate d with anthocyanin accumulation in 20 degrees C grown seedlings shifte d to 5 degrees C. In addition, seedlings shifted to 5 degrees C and an 8-h photoperiod exhibited a 2-fold higher yield of photosystem II ele ctron transport, which was associated with an increased capacity to ke ep Q(A), the first stable quinone electron acceptor of photosystem II, oxidized at high irradiance. This was consistent with a 2-fold higher rate of photosynthesis on a chlorophyll basis. We propose that the co mbination of light attenuation by anthocyanin in the epidermis and enh anced rates of photosynthesis may, in part, account for the reduced se nsitivity of jack pine to photoinhibition at low temperature.