Altitudinal variation in photosynthetic capacity, diffusional conductance and δ13C of butterfly bush (Buddleja davidii) plants growing at high elevations

In this study, we have examined several physiological, biochemical and morphological features of Buddleja davidii plants growing at 1300 m above sea level (a.s.l.) and 3400 m a.s.l., respectively, to identify coordinated changes in leaf properties in response to reduced CO2 partial pressure (P-a). Our results confirmed previous findings that foliar delta C-13, photosynthetic capacity and foliar N concentration on a leaf area basis increased, whereas stomatal conductance (g(s)) decreased with elevation. The net CO2 assimilation rate (A(max)), maximum rate of electron transport (J(max)) and respiration increased significantly with elevation, although no differences were found in carboxylation efficiency of Rubisco (V-cmax). Consequently, also the J(max) to V-cmax ratio was significantly increased by elevation, indicating that the functional balance between Ribulose- 1,5-biphosphate (RuBP) consumption and RuBP regeneration changes as elevation increases. Our results also indicated a homeostatic response Of CO2 transfer conductance inside the leaf (mesophyll conductance, g(m)) to increasing elevation. In fact, with elevation, g(m) also increased compensating for the strong decrease in g(m) and, thus, in the P-i (intercellular partial pressure Of CO2) to P-a ratio, leading to similar chloroplast partial pressure of CO2 (P-c) to P-a ratio at different elevations. Because there were no differences in V-cmax, also A measured at similar PPFD and leaf temperature did not differ statistically with elevation. As a consequence, a clear relationship was found between A and g(m), and between A and the sum of g(s) and g(m). These data suggest that the higher dry mass delta C-13 of leaves at the higher elevation, indicative of lower long-term P-c/P-a ratio, cannot be attributed to changes either in diffusional resistances or in carboxylation efficiency. We speculate that because temperature significantly decreases as the elevation increases, it dramatically affects CO2 diffusion and hence P-c/P-a and, consequently, is the primary factor influencing C-13 discrimination at high elevation.