Leaf-age effects on temperature responses of photosynthesis and respiration of an alpine oak, Quercus aquifolioides, in southwestern China.

Temperature responses and sensitivity of photosynthesis (A(n)_T) and respiration for leaves at different ages are crucial to modeling ecosystem carbon (C) cycles and productivity of evergreen forests. Understanding the mechanisms and processes of temperature sensitivity may further shed lights on temperature acclimation of photosynthesis and respiration with leaf aging. The current study examined temperature responses of photosynthesis and respiration of young leaves (YLs) (fully expanded in current growth season) and old leaves (OLs) (fully expanded in last growth season) of Quercus aquifolioides Rehder and E.H. Wilson in an alpine oak forest, southwestern China. Temperature responses of dark respiration (R-dark), net assimilation (A(n)), maximal velocity of carboxylation (V-cmax) and maximum rate of electron transport (J(max)) were significantly different between the two leaf ages. Those differences implied different temperature response parameters should be used for leaves of different ages in modeling vegetation productivity and ecosystem C cycles in Q. aquifolioides forests and other evergreen forests. We found that RuBP carboxylation determined the downward shift of A(n)_T in OLs, while RuBP regeneration and the balance between Rubisco carboxylation and RuBP regeneration made little contribution. Sensitivity of stomatal conductance to vapor pressure deficit changed in OLs and compensated part of the downward shift. We also found that OLs of Q. aquifolioides had lower A(n) due to lower stomatal conductance, higher stomatal conductance limitation and deactivation of the biochemical processes. In addition, the balance between R-dark and A(n) changed between OLs and YLs, which was represented by a higher R-dark/A(n) ratio for OLs.