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.