Canopy shade in agroforestry regulates source–sink balance and green bean quality of Coffea arabica L. as an adaptive strategy to climate extremes

Coffee production is increasingly threatened by extreme weather events. Agroforestry systems integrating coffee with shade trees are regarded as a climate-adaptive strategy for buffering environmental stress. Understanding the physiological responses of Coffea arabica L. under heterogeneous shade conditions is key to optimizing shade management. During a historical drought in Taiwan, we examined leaf physiology, source–sink balance, and seed composition of coffee trees across four distinct natural shade environments: no shade (NS, 100%), moderate shade (MS, 60%), severe shade (SS, 14.3%), and changing shade (CS, 17.5%), which had annual light availability similar to SS but a distinct short-term high-light period during fruit development. Under NS, increased phenylalanine ammonia-lyase activity and phenolic accumulation in leaves reflected photoprotective adjustments to high irradiance. In contrast, shade promoted chlorophyll accumulation, improving light-harvesting efficiency and mitigating photoinhibition by reducing excess light energy input, as indicated by higher PSII quantum yield. Leaf non-reducing sugars declined during fruit development, then rebounded after harvest to ~ 60 mg g −1 DW in NS, representing a 2.2-fold increase over SS. Seed carbohydrate profiles showed no significant differences; however, SS reduced seed yield to 42.8 g per tree, 72–81% lower than in the other shade environments, suggesting source–sink regulation to maintain seed biochemical integrity under the conditions of the study year. In CS, seasonal canopy opening increased light availability during fruit development, accelerated berry maturation, and was associated with higher seed yield (150.8 g per tree) without major changes in bean biochemical composition, despite similarly low annual light availability to SS. These findings provide physiological insight into coffee responses to contrasting shade environments during a historically dry year and indicate that dynamic canopy light may help buffer environmental constraints while balancing yield retention and bean quality.