Soluble sugars mediate sink feedback down-regulation of leaf photosynthesis in field-grown Coffea arabica.

Source-sink relationships of field-grown plants of Coffea arabica L. cultivar 'Caturra' were manipulated to analyze the contribution of soluble sugars to sink feedback down-regulation of maximal leaf net CO2 assimilation rate (Amax). Total soluble sugar concentration (SSCm) and Amax were measured in the morning and afternoon on mature leaves of girdled branches bearing either high or low fruit loads. Leaf Amax was negatively correlated to SSCm, increased with fruit load and decreased during the day, indicating that limiting sink demand for carbohydrates caused SSCm to accumulate in the leaf tissue which results in down-regulation of Amax. To further analyze source-sink feedback on Amax, we compared Amax of mature, non-sink-limited coffee leaves fed with water or sucrose for 5, 10 or 30 min with that of non-fed control leaves. Sucrose-feeding reduced Amax compared with the control and water-feeding treatments, indicating that down-regulation of Amax is related to phloem sucrose concentration in coffee source leaves, independent of SSCm concentration in other leaf tissues. Although sucrose appeared to be more closely related to the mechanism underlying sink feedback down-regulation of Amax in coffee leaves than SSCm, Amax was closely related to SSCm by a nonlinear equation that may be useful for integrating sink limitations in coffee leaf photosynthetic models.