Height-related decreases in mesophyll conductance, leaf photosynthesis and compensating adjustments associated with leaf nitrogen concentrations in Pinus densiflora

Abstract

Hydraulic limitations associated with increasing tree height result in reduced foliar stomatal conductance (g s) and light-saturated photosynthesis (A max). However, it is unclear whether the decline in A max is attributable to height-related modifications in foliar nitrogen concentration (N), to mesophyll conductance (gm) or to biochemical capacity for photosynthesis (maximum rate of carboxylation, V cmax). Simultaneous measurements of gas exchange and chlorophyll fluorescence were made to determine g mand V cmax in four height classes of Pinus densiflora Sieb. & Zucc. trees. A s the average height of growing trees increased from 3.1 to 13.7 m, g mdecreased from 0.250 to 0.107 mol m -2 s -1, and the CO 2 concentration from the intercellular space (C i) to the site of carboxylation (C c) decreased by an average of 74mol mol -1. Furthermore, V cmax estimated from C c increased from 68.4 to 112.0mol m -2 s -1 with the increase in height, but did not change when it was calculated based on C i. In contrast, A max decreased from 14.17 to 10.73mol m -2 s -1. Leaf dry mass per unit area (LMA) increased significantly with tree height as well as N on both a dry mass and an area basis. A ll of these parameters were significantly correlated with tree height. In addition, g mwas closely correlated with LMA and g s, indicating that increased diffusive resistance for CO 2 may be the inevitable consequence of morphological adaptation. Foliar N per unit area was positively correlated with V cmax based on C c but negatively with A max, suggesting that enhancement of photosynthetic capacity is achieved by allocating more N to foliage in order to minimize the declines in A max. Increases in the N cost associated with carbon gain because of the limited water available to taller trees lead to a trade-off between water use efficiency and photosynthetic nitrogen use efficiency. In conclusion, the height-related decrease in photosynthetic performance appears to result mainly from diffusive resistances rather than biochemical limitations. © 2011 The Author. Published by Oxford University Press. A ll rights reserved.