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 downregulation of maximal leaf net CO2 assimilation rate (A(max)). Total soluble sugar concentration (SSCm) and A(max) were measured in the morning and afternoon on mature leaves of girdled branches bearing either high or low fruit loads. Leaf A(max) 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 A(max). To further analyze source-sink feedback on A(max) we compared A(max) 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 A(max) compared with the control and water-feeding treatments, indicating that down-regulation of A(max) 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 A(max) in coffee leaves than SSCm, A(max) was closely related to SSCm by a non-linear equation that may be useful for integrating sink limitations in coffee leaf photosynthetic models.