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

To test the hypothesis that in temperate deciduous trees acclimation to potentially damaging high irradiances occurs via long‐term adjustments in foliar photosynthetic capacity, and short‐term changes in xanthophyll cycle pool size in response to weather fluctuations, nitrogen concentration and pigment composition were examined along a canopy light gradient in three species – Betula pendula, Populus tremula and Tilia cordata (from most shade intolerant to tolerant), and foliage photosynthetic potentials in P. tremula and T. cordata . Integrated quantum flux density ( Q i ) incident on leaves was estimated with a method combining hemispherical photography and light measurements 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 sampling ( T 3d ) – were calculated for each sampled leaf. In addition to total integrated quantum flux density, the part of Q i attributable to direct flux was also computed. Strong linear relationships between the capacity for photosynthetic electron transport per area ( J a max ), estimated from in situ measurements of effective quantum yield of photosystem II (PS II), and Q i averaged over the season and over the preceding 3 d were found for all studied species. However, the major determinant of J a max , the product of electron transport capacity per leaf dry mass ( J m max ) and leaf dry mass per area ( M A ), was M A rather than J m max , which was relatively constant along the light gradient. There was evidence that J a max is more tightly related to T s , which characterizes the light climate during foliar development, than to short‐term integrated light, possibly because there is little flexibility in adjustments in M A after the completion of foliar growth. Leaf chlorophyll concentrations and the investment of leaf nitrogen in chlorophyll (Chl/N) were negatively related to Q i – an investment pattern which improves light harvesting in low light. Xanthophyll cycle pool size (VAZ, violaxanthin + antheraxanthin + zeaxanthin) either expressed per unit chlorophyll (VAZ/Chl) or as a fraction of total carotenoids (VAZ/Car) increased with increasing Q i in all species. However, contrary to J a max , it tended to correlate more strongly with short‐term than with long‐term average integrated light. There were few interspecific differences in J a max , 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‐intolerant and ‐tolerant temperate tree species possesses considerable phenotypic flexibility. Collectively these results support the view that rapid adjustment of the xanthophyll cycle pool size provides an important means for acclimation to light fluctuations in a time scale of days, during which the potential for photosynthetic quenching of excitation energy is not likely to change appreciably.