Fruit, yield, and vegetative growth responses to photosynthetically active radiation during oil synthesis in olive trees

Maximizing productivity in super high density and intensive olive orchards requires proper management of illumination of the canopy walls and their interior. Currently, this is difficult to achieve due to the limited knowledge about the responses to incident photosynthetically active radiation (PAR) of yield determinants and components. We determined the response functions for PAR during the oil synthesis phase of yield components (fruit dry weight and oil concentration) of fruit at a height of 2m on the canopy periphery by applying several radiation levels (3, 20, 40, and 70% of incident PAR) to the north side (S hemisphere) of well-illuminated trees. The experiment was initiated after endocarp hardening as fruit number had already been established at that time. This avoided possible confounding effects due compensation between fruit number and size. Absence of differential fruit fall in response to treatments and of changes in (endocarp+seed) dry weight after application of treatment confirmed the achievement of this objective. Fruit dry weight, oil concentration, and, consequently, yield increased linearly with mean daily PAR receipt up to a threshold of 15mol PARm−2d−1 (i.e., 40% of PAR). In treatments with irradiance levels below this threshold the fruit became the priority sinks for assimilates, although their growth rate and oil concentration were reduced. Increments in length of non-fruiting branches and of trunk cross-sectional areas were substantially reduced in response to shading. We conclude that manipulation of PAR levels during the oil synthesis phase can reduce final fruit dry weight and oil concentration, confirms the existence of upper thresholds to PAR responses for these variables, and provides evidence that fruit growth has priority in the partitioning of photosynthate over vegetative growth under low to moderate levels of PAR.