Acclimation to high irradiance in temperate deciduous trees in the field: changes in xanthophyll cycle pool size and in photosynthetic capacity alonga canopy light gradient

To test the hypothesis that in temperate deciduous trees acclimation to pot entially damaging high irradiances occurs via long-term adjustments in foli ar photosynthetic capacity, and short-term changes in xanthophyll cycle poo l size in response to weather fluctuations, nitrogen concentration and pigm ent composition were examined along a canopy light gradient in three specie s - Betula pendula, Populus tremula and Tilia cordata (from most shade into lerant to tolerant), and foliage photosynthetic potentials in P. tremula an d T. cordata. Integrated quantum flux density (Q(i)) incident on leaves was estimated with a method combining hemispherical photography and light meas urements with quantum sensors made over the growing season. Long- and short -term light indices - average total seasonal daily integrated quantum flux density (T-s, mol m(-2) d(-1)) and that of the 3 d preceding foliage sampli ng (T-3d) - were calculated for each sampled leaf. In addition to total int egrated quantum flux density, the part of ei attributable to direct flux wa s also computed. Strong linear relationships between the capacity for photo synthetic electron transport per area (J(max)(a)), estimated from in situ m easurements of effective quantum yield of photosystem II (PS II), and ei av eraged over the season and over the preceding 3 d were found for all studie d species. However, the major determinant of J(max)(a), the product of elec tron transport capacity per leaf dry mass (J(max)(m)) and leaf dry mass per area (M-A), was M-A rather than J(max)(m), which was relatively constant a long the light gradient. There was evidence that J(max)(a) is more tightly related to T-s, which characterizes the light climate during foliar develop ment, than to short-term integrated light, possibly because there is little flexibility in adjustments in M-A after the completion of foliar growth, L eaf chlorophyll concentrations and the investment of leaf nitrogen in chlor ophyll (Chl/N) were negatively related to Q(i) - an investment pattern whic h improves light harvesting in low light. Xanthophyll cycle pool size (VAZ, violaxanthin + antheraxanthin + zeaxanthin) either expressed per unit chlo rophyll (VAZ/Chl) or as a fraction of total carotenoids (VAZ/Car) increased with increasing ei in all species. However, contrary to J(max)(a), if tend ed to correlate more strongly with short-term than with long-term average i ntegrated light. There were few interspecific differences in J(max)(a), Chl /N, VAZ/Chl and VAZ/Car when the variability in light level incident to the leaves was accounted for, indicating that the foliage of both shade-intole rant and -tolerant temperate tree species possesses considerable phenotypic flexibility, Collectively these results support the view that rapid adjust ment of the xanthophyll cycle pool size provides an important means for acc limation to light fluctuations in a time scale of days, during which the po tential for photosynthetic quenching of excitation energy is not likely to change appreciably.