A MODEL SEPARATING LEAF STRUCTURAL AND PHYSIOLOGICAL-EFFECTS ON CARBON GAIN ALONG LIGHT GRADIENTS FOR THE SHADE-TOLERANT SPECIES ACER-SACCHARUM

A process-based leaf gas exchange model for C-3 plants was developed w hich specifically describes the effects observed along light gradients of shifting nitrogen investment in carboxylation and bioenergetics an d modified leaf thickness due to altered stacking of photosynthetic un its. The model was parametrized for the late-successional, shade-toler ant deciduous species Acer saccharum Marsh. The specific activity of r ibulose-1,5-bisphosphate carboxylase (Rubisco) and the maximum photosy nthetic electron transport rate per unit cytochrome f (cyt f) were use d as indices that vary proportionally with nitrogen investment in the capacities for carboxylation and electron transport. Rubisco and cyt f per unit leaf area are related in the model to leaf dry mass per area (M-A), leaf nitrogen content per unit leaf dry mass (N-m), and partit ioning coefficients for leaf nitrogen in Rubisco (P-R) and in bioenerg etics (P-B). These partitioning coefficients are estimated from charac teristic response curves of photosynthesis along with information an l eaf structure and composition. While P-R and P-B determine the light-s aturated value of photosynthesis, the fraction of leaf nitrogen in thy lakoid light-harvesting components (P-L) and the ratio of leaf chlorop hyll to leaf nitrogen invested in light harvesting (C-B), which is dep endent on thylakoid stoichiometry, determine the initial photosyntheti c light utilization efficiency in the model. Carbon loss due to mitoch ondrial respiration, which also changes along light gradients, was con sidered to vary in proportion with carboxylation capacity. Key model p arameters - N-m, P-R, P-B, P-L, C-B and stomatal sensitivity with resp ect to changes in net photosynthesis (Gf)- were examined as a function of M-A, which is linearly related to irradiance during growth of the leaves. The results of the analysis applied to A. saccharum indicate t hat P-B and P-R increase, and G(f), P-L and C-B decrease with increasi ng M-A. As a result of these effects of irradiance on nitrogen partiti oning, the slope of the light-saturated net photosynthesis rate per un it leaf dry mass (A(max)(m)) versus N-m relationship increased with in creasing growth irradiance in mid-season. Furthermore, the nitrogen pa rtitioning coefficients as well as the slopes of A(max)(m) versus N-m were independent of season, except during development of the leaf phot osynthetic apparatus. Simulations revealed that the acclimation to hig h light increased A(max)(m) by 40% with respect to the low light regim e. However, light-saturated photosynthesis per leaf area (A(max)(a)) v aried 3-fold between these habitats, suggesting that the acclimation t o high light was dominated by adjustments in leaf anatomy (A(max)(a) = A(max)(m) M-A) rather than in foliar biochemistry. This differed from adaptation to low light, where the alterations in foliar biochemistry were predicted to beat least as important as anatomical modifications . Due to the light-related accumulation of photosynthetic mass per uni t area, A(max)(a) depended on M-A and leaf nitrogen per unit area (N-a ). However, N-a conceals the variation in both M-A and N-m (N-a = N-m M-A), and prevents clear separation of anatomical adjustments in folia ge structure and biochemical modifications in foliar composition. Give n the large seasonal and site nutrient availability-related variation in N-m, and the influences of growth irradiance on nitrogen partitioni ng, the relationship between A(max)(a) and N-a is universal neither in time nor in space and in natural canopies at mid-season is mostly dri ven by variability in M-A. Thus, we conclude that analyses of the effe cts of nitrogen investments on potential carbon acquisition should use mass-based rather than area-based expressions.