Fruit set is inversely related to flower and fruit weight in olive (Olea europaea L.)

Fruit weight is strongly correlated to ovary weight in olive as big-fruited cultivars have larger ovaries and flowers at bloom. We tested the hypothesis that larger ovaries imply stronger sinks and therefore reduce fruit set, expressed as number of fruits. Flower/fruit load per centimeter of shoot was assessed every month from flowering to harvest, on sample shoots of many different olive cultivars differing in fruit size. Data were taken for two years: 2007 and 2008. Additionally, in 2008, a fruit thinning experiment was carried out by leaving a single fruit per shoot, to assess the maximum potential fruit weight achievable by each cultivar. Results indicated that fruit size was mostly genetically determined as thinned fruits of small-fruited cultivars, although bigger than control fruits, never got as big as fruits from large-fruited cultivars, confirming that large ovaries are a prerequisite for large fruits. Among all cultivars, the number of flowers per inflorescence, the number of inflorescences, and the number of flowers per centimeter of shoot were not correlated to average flower weight while total flower mass per centimeter of shoot was positively correlated to average flower weight (i.e. large flowered/fruited cultivars had greater total flower mass). However, one month after bloom and thereafter, all parameters were negatively and exponentially correlated with fruit average weight across cultivars, except fruit mass per centimeter of shoot which was generally not correlated to fruit average weight, except in one year, at harvest, when fruit mass per centimeter of shoot was positively correlated to fruit average weight. These results suggest that fruit set is a consequence of, and inversely proportional to flower/fruit size in olive.

Research highlights▶ Fruit set is a poorly understood phenomenon and it is not clear whether fruit set is related to fruit size and, if so, which is the cause and which the consequence. ▶ In this paper it is shown that fruit set differs greatly among olive cultivars with varying fruit size, and is inversely related to average fruit weight. ▶ This negative relationship appears already one month after bloom and persists until harvest. ▶ Fruit size is shown to be relatively independent of fruit load since heavy thinning of fruit, though increasing fruit size slightly, does not allow cultivar differences to disappear. ▶ Therefore, in olive fruit set (i.e. number of fruits) appears as a consequence of fruit size, and a mechanism that allows the fruit load (i.e. mass of fruit) to be modulated in function of fruit size. ▶ These results support the hypothesis that competition among fruits occurs already at bloom in olive.