Photosynthetic acclimation in eastern hemlock [Tsuga canadensis (L.) Carr.] seedlings following transfer of shade-grown seedlings to high light

We studied photosynthetic acclimation of eastern hemlock [Tsuga canadensis (L.) Carr.] seedlings in the first month after sudden exposure of shade-gro wn seedlings to full sunlight. In a greenhouse experiment, seedlings were g rown under full sun or 80% shade, and after 7 months, a sample of the shade d trees was transferred to full sun in the greenhouse. Photosynthetic respo nses of shaded, transferred, and sun trees were followed over the course of 26 days to track short to medium-term acclimation responses. A partial acc limation of photosynthesis at high light occurred in pre-existing (formed i n the previous environment) and new foliage of transferred seedlings. This was associated with non-stomatal limitations to photosynthesis. Pre-existin g foliage of transferred plants had a prolonged reduction in the ratio of v ariable to maximal fluorescence, and a limited capacity to adjust photochem ical quenching or photosystem ii quantum yield in the Light to increasing l ight intensity compared to sun foliage, and apparently had some difficulty sustaining non-photochemical quenching. Seedling survival was only 58% amon g transferred seedlings, compared to 80% and 100% in the shade or sun group s, respectively. Photosystem II quantum yield in the light, and photochemic al and non-photochemical quenching were similar between newly formed foliag e of transferred and sun plants. These findings indicate that eastern hemlo ck depends strongly on the production of new foliage for photosynthetic adj ustments to high light, and that development of photosynthetic competence m ay be a gradual process that occurs over successive foliar production cycle s.