Photosynthesis and the Xanthophyll Cycle-mediated Photoprotection in Leaves of Quercus Rubra and Q. Alba Seedlings of Different Light Environments

Two and three years after the outplanting of 1-O northern red oak (Quercus rubra, NRO) and white oak (Q. ulba, WO) nursery stocks, the highest net photosynthetic rates (A-) were observed from seedlings growing on a clearcut site followed by those under a pine stand. Both NRO and WO seedlings under a hardwood stand had A, less than 10% of the full sun seedling A,,. Oaks grown under hardwoods increased their A,, more in response to sunflecks than those under a pine stand. Besides NRO and WO seedIings, leaves of several hardwood seedlings and shrubs grown under different light environments were analyzed for pigments and the operation of the xanthophyll cycle. All species investigated shared the following characteristics: higher contents of chlorophyll a+b, a-carotene, lutein, and neoxanthin; smaller xanthophyll cycle pool (sum of violaxanthin V, antheraxanthin A, and zeaxanthin Z); and lower ratios of Z+A to Z+A+V in leaves of understory plants than leaves of the same species growing in full sun. The diurnal xanthophyll cycle (i.e., high Z+A/Z+A+V ratio in midday and low Z+A/Z+A+V ratio near dawn and dusk) was present in leaves of NRO and WO seedlings on the clearcut site. Almost no xanthophyll cycle was operating in understory leaves except that upon sunflecking NRO and WO under hardwoods increased their Z+A/Z+A+V ratio. For every unit of the xanthophyll cycle pool, twice as many chlorophylis must be protected in shade-grown leaves as in sun-grown leaves. The potential use of leaf a-carotene levels in silviculture is discussed.