Starch accumulation does not lead to feedback photosynthetic downregulation in girdled coffee branches under varying source-to-sink ratios

Key message Leaves were forced to accumulate starch as much as possible but maintained relatively low soluble sugar levels with no evidence of photosynthetic feedback downregulation. Here, we examined whether the regulation of photosynthesis in coffee depends on sink activity or carbohydrate build-up in source leaves and how the coffee tree adjusts its photosynthetic performance and primary metabolism to varying source-to-sink ratios. We designed a field experiment by girdling coffee branches that were further manipulated by controlled defoliation and/or defruiting such that three highly varying source-to-sink ratios were created. Under markedly high source-to-sink ratios, photosynthesis rates were primarily limited by diffusional factors, and feedback downregulation contributed a maximum of 7% of the photosynthetic reduction. Such responses were accompanied by a relatively enormous capacity for starch accumulation coupled with the maintenance of low levels of soluble sugars. Chronic photoinhibition and photodamage could be avoided by increases in nonphotochemical energy dissipation, photorespiration and respiration rates, thus diminishing the excitation pressure on photosystems. Overall, the activities of key enzymes (and their transcript abundance) associated with carbon metabolism varied only marginally across treatments. Leaf metabolic adjustments were more evident under high sink demand conditions, and nitrogen metabolism was more affected than carbon metabolism. In conclusion, our results offer novel insights into the high coordination between the source supply and sink demand in coffee, with a minor role of photosynthetic downregulation even under dramatically low sink conditions.