Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J(max;A)), and maximum carboxylase activity of Rubisco (V-cmax;A) is proportional to average integrated daily quantum flux density (Q(int)) in a mixed deciduous forest dominated by the shade-intolerant species Populus tremula L., and the shade-tolerant species Tilia cordata Mill. We distinguished between the age-dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M-A), foliar nitrogen content per unit dry mass (N-M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F-B), Rubisco (F-R) and in light-harvesting chlorophyll-protein complexes (V-cmax;A proportional to MANMFR; J(max;A) proportional to MANMFB). In both species, increases in J(max;A) and V-cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M-A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M-A. There was only small within-canopy variation in the initial rate of biomass accumulation per unit Q(int) (slope of M-A versus leaf age relationship per unit Q(int)), suggesting that canopy differences in the rate of development of J(max;A) and V-cmax;A are directly proportional to Q(int). Nevertheless, M-A, nitrogen, J(max;A) and V-cmax;A of mature leaves were not proportional to Q(int) because of a finite M-A in leaves immediately after bud-burst (light-independent component of M-A). M-A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day-to-day fluctuations in Q(int). However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non-senescent leaves on the order of 15-20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light-dependent variation in the rate of developmental changes in M-A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.