Effects of drought stress on assimilation and carbon allocation in a fruiting arabica coffee plant explained by 13C-CO2 pulse labelling

The effects of drought on coffee yield and quality during the flowering and fruiting stages are becoming a challenge in many coffee-producing regions. Nevertheless, coffee plants exhibit various adaptive mechanisms that mitigate the effects of short-term water scarcity during these phenological phases. Plants undergo numerous physiological and metabolic alterations in response to water deficits during their critical developmental stages, for example, during flowering, one of the stages that is related to yield. Although understanding these responses is essential for effective breeding and management strategies, they remain inadequately documented for coffee. This study employed a rapid and accurate method of pulse labelling utilising ¹³C-CO₂ on 32 four-year-old Venecia Arabica coffee plants from Costa Rica in a greenhouse. Carbon assimilation in young and old leaf pairs was assessed at 10, 11, 12, and 13 days post-stress initiation to determine the metabolic differences in leaf age and orientation. The allocation of assimilates to soluble sugars, starch, and cellulose in various structural components, such as fruits, stems, roots, and old and young leaves, was also measured at harvest (15 days of stress). These findings demonstrate a significant reduction (p< 0.05) in carbon assimilation and, consequently, photosynthesis under drought stress conditions, with a more pronounced decrease in older leaf pairs. This study revealed altered assimilate partitioning, with plants prioritising allocation to roots to presumably sustained soil water uptake. Conversely, under water stress, carbon allocation to young leaves diminished, whereas in fruit, a priority sink,  the assimilates remained constant for starch but increased for sugar (0.33±0.21%). Carbohydrate metabolism exhibited notable changes, including a significant (p< 0.05) decrease in foliar soluble sugars and enhanced starch allocation to stems and roots. Additionally, a significant (p< 0.0001) increase in cellulose production was observed, particularly in the older leaves (94%), stems (93%), and roots (89%), which suggests a physiological drought response with the upregulation of cellulose production, possibly providing structural support and protection against herbivory. In summary, this study revealed a response to short-term water deficit between the two leaf age categories and clarified the allocation of new assimilates in Coffea arabica. L. This study provides a foundation for improved breeding and management strategies to support the resilience and sustainability of coffee production.