Effects of crown development on leaf irradiance, leaf morphology and photosynthetic capacity in a peach tree

The three-dimensional (3-D) architecture of a peach tree (Prunus persica L. Batsch) growing in an orchard near Avignon, France, was digitized in April 1999 and again four weeks later in May 1999 to quantify increases in leaf area and crown volume as shoots developed. A 3-D model of radiation transfer was used to determine effects of changes in leaf area density and canopy volume on the spatial distribution of absorbed quantum irradiance (PAR(a)). Effects of changes in PAR(a) on leaf morphological and physiological properties were determined. Leaf mass per unit area (M(a)) and leaf nitrogen concentration per unit leaf area (N(a)) were both nonlinearly related to PAR(a), and there was a weak linear relationship between leaf nitrogen concentration per unit leaf mass (N(m)) and PAR(a). Photosynthetic capacity, defined as maximal rates of ribulose-1,5-bisphosphate carboxylase (Rubisco) carboxylation (V(cmax)) and electron transport (J(max)), was measured on leaf samples representing sunlit and shaded micro-environments at the same time that the tree crown was digitized. Both V(cmax) and J(max) were linearly related to N(a) during May, but not in April when the range of N(a) was low. Photosynthetic capacity per unit N(a) appeared to decline between April and May. Variability in leaf nitrogen partitioning between Rubisco carboxylation and electron transport was small, and the partitioning coefficients were unrelated to N(a). Spatial variability in photosynthetic capacity resulted from acclimation to varying PAR(a) as the crown developed, and acclimation was driven principally by changes in M(a) rather than the amount or partitioning of leaf nitrogen.