Effects of light environments within Cinnamomum camphora canopy on leaf functional traits and photosynthetic...

To explore the adaptive mechanism of leaf photosynthetic capacity in different light environments within Cinnamomum camphora canopy and enhance carbon sequestration, we investigated morphological structures, nutritional and physiological traits and photosynthetic characteristics of leaves in different orientations of C. camphora canopy, southern side in the outer layer (100% full light), southern side in the inner layer (34% full light) and northern side (21% full light). We analyzed the main limitation resulting in down-regulation of photosynthetic capacity in low light environments. Results showed that specific leaf weight, the thickness of lower and upper epidermal cuticle, lower epidermis, palisade tissue as well as cell number and width of palisade tissue, the thickness ratio of palisade to spongy tissue, cell structure closely degree significantly decreased with decreasing light intensity within canopy, opposite to the responses of spongy tissue thickness, cell length-width ratio of palisade tissue, and cell structure loose degree. The contents of leaf carbon, soluble protein, soluble sugar and starch were significantly lower in two low light environments compared with full light, whereas nitrogen content was markedly higher in north side. Low light prominently reduced gas exchange parameters, i.e., net photosynthetic rate (P_n), dark respiration rate, stomatal conductance to CO₂(g_sc), mesophyll conductance to CO₂(g_m), total conductance to CO₂(g_tot), intercellular CO₂ concentration (C_i), CO₂ concentration at the chloroplast (C_c). P_n was positively correlated with g_sc, g_m, g_tot and C_c. There were no differences in maximum quantum photochemical efficiency, actual quantum photochemical efficiency, photochemical quenching coefficient, maximum rate of Rubisco carboxylation (V_{c max}) and maximum rate of electron transport (J_max) among light environments. V_{c max} and J_max were positively correlated to P_n. Of the shading-induced limitations to photosynthesis, g_m limitation was the most important, and g_sc limitation was enhanced with further weakened light intensity while biochemical limitation was rather limited. In summary, the results suggested that full light could improve leaf photosynthetic potential in C. camphora canopy leaves, reduce the effects of g_m and g_sc limitation on photosynthesis, and consequently enhance carbon assimilation capacity.