Relationships between fruit growth and oil accumulation with simulated seasonal dynamics of leaf gas exchange in the olive tree

Carbohydrates availability, which are directly related to photosynthesis (AN), and turgor are the main determinants of fruit growth. Since stomatal conductance (gs) is the main limiting factor of AN in fruit trees in most environments, and is strongly regulated by turgor, its measurement is pivotal to understanding fruit growth dynamics. Despite its relevance, the use of gs to estimate AN faces the major limitation of being difficult to measure in an automated and continuous manner. Based on these observations, and considering the control that the stomata exert on transpiration, and thus on sap flux density (Js) under conditions of high coupling to the atmosphere, we conducted a multi-faceted experiment in olive trees. The main objective was to assess the use of continuously modelled AN, derived using a simulated gs, as a tool to study fruit growth and oil accumulation and other components of vegetative above-ground growth (leaf area and number of shoot internodes) in a super-high-density olive orchard under different irrigation levels. Sixteen olive trees under four different irrigation treatments (two control and two deficit irrigated, with one and two drip lines each) were continuously monitored with Js sensors from May to November 2016. Gas exchange, fruit growth, number of shoot internodes and leaf area were measured periodically. Stomatal conductance was empirically simulated through Js, and AN was modelled using previously simulated gs and a biochemical model of photosynthesis. Results showed that AN can be accurately modelled from simulated gs, obtained in turn from Js measurements divided by pressure deficit. Moreover, the approach was shown to be sensitive enough to infer the response of gs and AN to soil water content and vapour pressure deficit. Interestingly, accumulated AN was significantly related to fruit growth and oil content for all the irrigation treatments which determine the slope of these relations. In contrast, the relationship with leaf area was only significant for the control irrigation treatments, where the number of shoot internodes increased significantly more than in the water-stressed trees. Our results show that under water stress conditions trees prioritize fruit growth and oil content accumulation over vegetative growth, suggesting a higher sink strength for reproductive growth than for vegetative growth. We conclude that the use of sap flow and the proposed approach provides reliable gs and AN data, and allows the modelling of the relations between carbon assimilation and allocation, which are helpful to estimate fruit growth.