Modelling temporal variation of parameters used in two photosynthesis models: Influence of fruit load and girdling in fruit bearing branches of apple trees

A large number of studies have found seasonal and temporal variability in leaf photosynthesis parameters in different crops. This variability depends upon the environment, the developmental stage of the plant and the presence or absence of sinks. Girdling removes the bark and phloem down to the youngest xylem all around the stem and prevents export of photoassimilates out of the stem. This technique is used to manipulate tree development and growth, as well as fruit growth, in a variety of fruit species. It has been shown to decrease net photosynthesis rate (Pn) in several tree species. Developing fruits and crop load have often been reported to influence individual leaf photosynthesis rate in tree crops. In the literature, numerous studies propose models to simulate photosynthesis. In this “preliminary” study, we have chosen to compare three models: The rectangular and non-rectangular hyperbola model by Thornley, as well as the model by Marshall and Biscoe. Ten-year old apple trees (Malus domestica L. (Borkh.), cv. `Elstar') were used to study and model the seasonal variation of photosynthetic parameters in leaves of long shoots and rosettes, as a function of global fruit load (at the branch level), with or without girdling, during the growing season of 2015. Three 6-years-old fruit bearing branches were selected in each tree and treated independently. Gas exchange measurements were carried out using a LI-6400XT portable photosynthesis system. For each fruit bearing branch, light response curves of Pn for 2 leaves of a rosette were measured at two different positions, proximal and distal. Our comparison of models indicates that the model of Marshall and Biscoe is the most accurate model for the simulation of photosynthesis rate in bearing branches of apple tree with time (season) and the three treatments applied.