Photosynthetic acclimation of shade-grown red pine (Pinus resinosa Ait.) seedlings to a high light environment

Photosynthetic light acclimation in red pine (Pinus resinosa Ait.) seedling s was examined in a greenhouse study to better understand the physiological response of this species to increased light intensity following release fr om competition. Seedlings grown in a high (HL), medium (ML) or low (LL) lig ht environment for 12 weeks were transferred to high light. Gas exchange an d chlorophyll fluorescence of ML and LL seedlings were measured prior to an d following transfer and compared with the HL control treatment. Photosynth etic characteristics were related to initial light treatment and time after transfer. Acclimation of gas exchange features to high light in shade form ed ML and LL foliage was relatively rapid, with similar values among light treatments within 57 days of transfer. Acclimation of net photosynthetic ra te was similar in ML and LL seedlings, and was associated primarily with in creased mesophyll conductance to CO2. The ratio of variable to maximal chlo rophyll fluorescence (F-v/F-m) decreased initially after transfer, especial ly in LL seedlings, but recovered to normal values after 57 days. Red pine seedlings appear to be well adapted for photosynthetic acclimation to high light intensity, consistent with that reported for other early successional tree species.