Crop load does not increase the photosynthetic rate in Citrus leaves under regular cropping conditions. A study throughout the year

• Photosynthesis was studied throughout the year in 40 years-old field grown citrus trees.
• The relative influence of environmental and physiological factors (crop load) was assessed.
• No significant seasonal variation in photosynthetic capacity was observed.
• Photosynthetic rate does not differ among high crop and low crop mature trees.
• Photosynthetic capacity declined by 20% during the second year of life of leaves.

The objective of this work was to study the influence of fruit load on CO2 assimilation in the leaves of citrus trees presenting alternate bearing habits, and the importance of this factor on photosynthetic rate variability throughout the year and under regular cropping conditions. The photosynthetic rate was measured on 60 days throughout the year on field-grown sweet orange plants under natural conditions in the Valencian Community, the most important citrus-producing area of Spain. The experiments were performed on the ‘on’ (high crop) and ‘off’ (low crop) bearing 40-year-old Salustiana sweet orange trees growing in the same orchard. Gas exchange and fluorescence parameters were measured during the year in young and old leaves on sun-exposed branches with and without fruit in the ‘on’ trees, and in fruitless branches of the ‘off’ trees. In non-manipulated Citrus trees, fruit load has no significant effect in any season on the photosynthetic rate in the leaves from branches without fruit. However, in high crop trees, the leaves of branches bearing fruit present a slightly lower photosynthetic rates (approx. 10%) than those of fruitless branches. Variations in mineral content (N, K and P) might explain not only these differences, but also the lower photosynthesis rate observed in old leaves (13–24 month-old leaves). Environmental conditions were the main factor for the variation of the photosynthetic rate, with variability of the monthly mean photosynthetic rate being much lower than that between days in the same month.