Variation of leaf photosynthetic traits is mostly driven by changes in source-sink relationship rather than irradiance in pear tree canopies

Understanding the variation of leaf photosynthetic traits and their regulation is essential for accurately predicting canopy photosynthesis and gross primary production of fruit trees and for modelling carbon balance in agroforestry systems. However, this is underexplored in fruit trees in which management practices strongly impact plant morphology and sink: source ratios. We conducted a field experiment in pear with three levels of fruit loads (0%, 10%, and 100%) on two cultivars (Cuiyu and Hosui) with a Y-trellis trained architecture. Irradiance, leaf photosynthesis, and several leaf physiological traits were measured at different canopy heights and developmental stages. Both cultivars exhibited a very shallow vertical gradient of leaf photosynthetic capacity (Amax), despite a significant irradiance gradient. Considering the canopy Amax profile only had a marginal impact on modelling canopy photosynthesis in Y-trellis trained pear trees. In both cultivars, Amax was affected by source-sink variations, albeit in opposite directions: as fruit load increased, Amax increased in Hosui but decreased in Cuiyu. Increases of Amax in Hosui were related to lower carbon accumulation in the leaf and higher stomatal conductance, whereas decreases of Amax in Cuiyu correlated with reductions in leaf nitrogen. We conclude that under the conditions of this study and in Y-trellis trained pear trees, leaf photosynthetic variation was more strongly associated with internal factors than with irradiance gradients, and that the regulation of Amax is cultivar dependent. Considering photosynthetic responses to source-sink variation might be more relevant than irradiance profiles for accurately modelling canopy photosynthesis in fruit trees.