Changes in total leaf nitrogen and partitioning, of leaf nitrogen drive photosynthetic acclimation to light in fully developed walnut leaves

Comprehensive studies on the processes involved in photosynthetic acclimation after a sudden change in light regime are scarce, particularly for trees. We tested (i) the ability of photosynthetic acclimation in the foliage of walnut trees growing outdoors after low-to-high and high-to-low light transfers made early or late in the vegetation cycle, and (ii) the relative importance of changes in total leaf nitrogen versus changes in the partitioning of leaf nitrogen between the different photosynthetic functions during a 2 month period after transfer. Changes in maximum carboxylation rate, light-saturated electron transport rate, respiration rate, total leaf nitrogen, ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) and total chlorophylls were surveyed before and after the change in light regime. Respiration rate acclimated fully within 1 week of transfer, and full acclimation was observed 1 month after transfer for the amount of Rubisco. In contrast, total nitrogen and photosynthetic capacity acclimated only partially during the 2 mouth period. Changes in photosynthetic capacity i were driven by changes in both total leaf nitrogen and leaf nitrogen partitioning. The extent of acclimation also depended strongly on leaf age at the time of the change in light regime.