Effect of fruit load and girdling on leaf photosynthesis in Mangifera indica L.

Abstract

Leaf nitrogen concentration (Nm), mass-to-area ratio, amount of nitrogen per unit leaf area (Na), non-structural carbohydrate concentration (TNCa), maximal rate of carboxylation (Vcmax), light-saturated rate of photosynthetic electron transport (Jmax), dark respira-tion (Rd), net photosynthetic assimilation (Anet), quan-tum yield of photosystem II (UPSII), and intercellular CO2 concentration (Ci) were measured in Mangifera indica L. leaves on three types of fruit-bearing branch-es (non-girdled, NG; girdled with high (HFL) and low (LFL) fruit load), experiencing similar light exposure. TNCa, Vcmax/Na, Jmax/Na, Rd/Na, Ci, and the initial quan-tum yield of photosynthetic electron transport (a) were similar in both HFL and NG treatments, but Nm, Na, and photosynthetic capacity parameters (Vcmax and Jmax) were lower in the HFL than in the NG treatment. The strong depressing effect of girdling on leaf nitrogen concentration cannot therefore be attributed to a change in TNCa. By contrast, Na and TNCa were lower and higher, respectively, in the LFL than in the HFL treatment, suggesting that carbohydrate content may become the driving force behind photosynthetic acclimation to changing source–sink relationships, like the ones resulting from the presence of developing fruits. Vcmax/Na and Jmax/Na were lower in the LFL than in the HFL treatment, while Rd/Na, Ci, and a were not affected by fruit load. It is concluded that girdling and high fruit load affect photosynthesis permanently by decreasing and increasing, respectively, leaf nitrogen concentration. Fruit load, moreover, may have an additional effect on photosynthetic capacity by affect-ing the relationship between Vcmax and Jmax, and Na. Key words: Fruit load, girdling, leaf nitrogen,Mangifera indicaL., non-structural carbohydrates, photosynthesis, photosyntheti