Source-sink patterns on coffee trees related to annual climate variability: An approach through stable isotopes analysis

Stable isotopic determination constitutes a useful tool to identify the processes that control the dynamics of the carbon and nitrogen flow in plants, unravelling the mechanisms of their differential investment under different environments. This work aimed to evaluate the spatiotemporal variation of source-sink patterns of coffee trees under field conditions in response to climatic conditions through the assessment of stable isotopes. For this purpose, stems, leaves, and fruit samples from coffee trees were collected following a temporal pattern based on the region's climatic characteristics and the plant's phenology and a spatial pattern considering different parts of the canopy. The carbon and nitrogen percentage content, the C/N ratio, and the carbon and nitrogen isotopic compositions (δ¹³C and δ¹⁵N) were determined for all samples. The basal portion of the orthotropic branch was also considered for the isotopic analysis of the tree's growth rings. The results obtained were correlated with the climatic variables of the region through a Pearson correlation analysis (p < .05). Coffee plants showed traditional δ¹³C values of C3 plants. Temporal δ¹³C variation was associated with the different growth rates between phenological stages and the use of substrates produced at different times under different environmental conditions leading to differences in photosynthetic discrimination. Spatial δ¹³C variation was observed with heterotrophic tissues isotopically heavier than leaves, with a significant decrease trend in δ¹³C values from the top (upper third) to the bottom (lower third), associated with ecophysiological differences between the canopy, isotopic fractionation processes downstream of photosynthetic carbon discrimination, and the fixation of C from other pools. Temporal δ¹⁵N variation was associated with the precipitation rates in the region and the fertilization distribution across the tree, while the spatial variation was with the plant's nitrogen assimilation and translocation patterns. The tree growth rings isotopic analyses showed isotopic differences between growth rings of the same plant addressed by the climatic conditions, with precipitation being the primary climatic determinant influencing the fixation and discrimination against ¹³C. Our results highlight the importance of using stable isotope analysis as a reference point for coffee ecophysiological studies to characterize how the temporal and spatial patterns of δ¹³C and δ¹⁵N emerge and signal the influence of climate on the source-sink relationship of coffee trees under field conditions.