Effects of Fruit Load and Intra-Inflorescence Competition of Fruits on Apple Growth during Fruit Development
Résumé
Ecophysiological sink-strength models, developed to explain/predict fruit growth at the fruit/shoot level, considered each fruit as one sink. Models have been developed for several fruits but not for apple. The variable number and size of apple fruit, developing within each inflorescence, needed therefore some attention. It is assumed in this study that inflorescence sink strength, which we supposed depends on its own global attributes, could interact with the growth of each individual fruit by modulating global influx into the inflorescence and by regulating the nutrient fraction available for each fruit. The aim of this study was to validate these hypotheses using the average daily growth rate of each inflorescence (AGR) as a quantitative measure of inflorescence growth rate and the temporal dynamic of fruit drop as the factor of variability. In an experimental orchard, anthesis dates and fruit diameters were collected for 'Elstar' apple from early growth stage to maturity. From our data and the literature, we defined five growth periods: the first characterized by cell division, the next three by cell expansion and the last one by fruit maturation. Without fruit drop, there was no significant difference between the AGR of single-fruit and two-fruit inflorescences. However, single-fruit exhibited a greater AGR from the first stage of cell expansion known to be critical for nutrient availability. Besides, after an early drop or hand-thinning of fruit per inflorescence in periods 2 and 3, the AGR values attained those of inflorescences which always maintained one or two fruits and for some cases were significantly greater than two-fruit. Such a positive tendency in growth rate induced by unloading was sustained during the following periods. It seemed therefore that the regulation process depended mainly on the history of the previous developmental events.