Effect of leaf-to-fruit ratio on leaf nitrogen content and net photosynthesis in girdled branches of Mangifera indica L.

Variations in leaf nitrogen concentration (N_m), mass-to-area ratio (M_a), amount of leaf nitrogen per unit leaf area (N_a), and non-structural carbohydrates were measured in well-lit girdled branches of 11-year-old mango trees that were experiencing similar conditions of irradiance and gap fraction. The influence of source–sink relationships was studied by testing three levels of leaf-to-fruit ratio: 40, 70 and 150, during the period of linear fruit growth. N_a was negatively correlated to the leaf-to-fruit ratio. Differences in N_a were reflected in differences in net photosynthetic assimilation, A_net, although they could not fully account for them. All differences in N_a resulted exclusively from differences in N_m, not M_a. Starch and total non-structural carbohydrates accumulated in the leaves as the result of higher leaf-to-fruit ratio, which suggests that the leaf carbohydrate status may play a role in photosynthetic acclimation to fruit load in mango. These observations complement previous findings where photosynthetic acclimation to light was found to be driven by changes in M_a, while N_m remained almost constant over a large range of gap fractions. Observations about the effect of fruit load were also in contrast with previous observations, since no evidence was found that leaf carbohydrate status played any role in photosynthetic acclimation to light. This study demonstrates that acclimation to changing source–sink relationships does not follow the same pattern as acclimation to progressive shading; but these observations do suggest that there may be different mechanisms by which leaves acclimatise to changing conditions.