Distribution of N, Rubisco and photosynthesis in Pinus pinaster and acclimation to light

Abstract

AbstractThe light–nitrogen hypothesis suggests canopy photosynthesis is maximized when there is a positive relationship between irradiance received by foliage, its nitrogen content (per unit area Narea), and maximum rate of photosynthesis (Amax). Relationships among relative irradiance and Narea, allocation of nitrogen within the photosynthetic apparatus to Rubisco and chlorophyll, and Amax were examined in Pinus pinaster Ait. needles up to 6 years of age. Measurements were made before bud break in August 1998, and in May 1999 after the first ‘winter’ rains. In August, Narea in P. pinaster needles decreased from 5·1 to 5·7 g m−2 in sunlit 1‐year‐old needles to 2·3 g m−2 in shaded 6‐year‐old needles. In May, Narea was 5–40% less but spatial trends were the same. At both sampling dates, Amax was less in old shaded needles compared with young sunlit needles, and was thus consistent with the light–nitrogen hypothesis. Relationships between Narea and Amax were positive at both dates yet varied in strength and form. Allocation of nitrogen within the photosynthetic apparatus was qualitatively consistent with acclimation to light (i.e. Rubisco/Chl decreased with shading), but quantitatively suboptimal with respect to photosynthesis owing to consistent over‐investment in Rubisco. This over‐investment increased with height in the canopy and was greater in May than in August.