Fruit size in different plum species (genus Prunus L.) is determined by post-bloom developmental processes and not by ovary characteristics at anthesis

The mechanisms driving drupe differentiation have been investigated in several plant species. Often, bigger fruits derive from bigger ovaries; however, in other species, ovary size is similar and differences in fruit size are determined by differences in post-bloom tissue development. The aim of this work was to investigate whether, in three Prunus species (P. salicina, P. domestica and P. cerasifera), fruit size is related to ovary size or whether it depends on post-bloom developmental processes. This study used microscope image analyses to evaluate ovary wall thickness at anthesis and fruit mesocarp thickness at final fruit size. In the same tissues (i.e. ovary wall and fruit mesocarp), we also measured cell area and number of cellular layers. Ovary wall thickness did not differ among the genotypes tested (except for Mirabolano, the smallest-fruited genotype, having slightly bigger ovaries) and was not correlated to either cell size or to the numbers of cell layers. Fruit mesocarp thickness varied three fold among genotypes and was strongly and positively correlated to the number of cell layers, and weakly and negatively correlated to cell size. No correlation was found between fruit weight (or mesocarp thickness or number of cell layers) and the respective ovary characteristic: bigger fruits did not derive from bigger ovaries. Fruit weight was instead related to the fruit-to-ovary weight and cell layer number ratios, indicating that fruit size in these species is related not to ovary characteristics, but to processes that occur to a different extent after bloom, particularly to a greater cell division. In particular, P. salicina cv. TC Sun, which had the biggest fruit, had a longer post bloom cell division activity and/or a higher post bloom cell division rate. Indeed, the number of TC Sun cell layers increased four times, while in the other species the mesocarp had twice the ovary layers. TC Sun also had the lowest increase in mesocarp cell area. This study represents a starting point to understand the mechanisms of cell division and expansion in Prunus species and their implication for fruit size management.