Source-sink patterns on coffee trees related to annual climate variability: an approach through isotopes, carbohydrates, and spectral analysis

Abstract

The coffee tree is subject throughout the year to adversities resulting from climatic events that can affect the development and productivity of the crop. One of the processes highly influenced by climate is the allocation and partitioning of carbon, with changes in the export/import of resources between source and sink tissues. Thus, unraveling the mechanisms of differential allocation as well as the source-sink relationship under different climatic conditions is of great relevance for management, agroecological and socioeconomic issues. A quick and straightforward approach to assessing a plant's carbon status is to measure the size of its overall non-structural carbon (NSC) stock in periods of variable demand. However, long-term studies with this approach are scarce, making it difficult to understand the physiological processes and controls involved. Thus, this thesis proposes the use of three analytical methods to address the spatio-temporal variation in source-sink relationship in coffee plants in field conditions: growth analysis, stable isotope analysis and carbohydrate profile analysis validated by near infrared spectroscopy (NIRS). The growth analysis involves and integrates the form and function of the plant in a holistic way and allowed us to know the main patterns of vegetative and reproductive growth depending on the phenology and climate of the region, as well as confirming the bienniality patterns linked to the negative correlation that exists between growth and reproduction. Stable isotopic analysis constitutes a useful tool to identify the processes that control the dynamics of carbon (C) and nitrogen (N) flux in the plant and led us to identify the null variation of the C isotopic ratio over time, as well as the isotopic differences between heterotrophic and autotrophic tissues and the isotopic differences within the tree canopy between the established thirds. Carbohydrate profile analysis is a tool that allows us to determine the stock of the main non-structural compounds (NSC) in plant organs, leading us to quantify the stocks of protein, amino acids, reducing sugars, total soluble sugars, sucrose, and starch throughout of time and in the different portions of the tree canopy. The values quantified in the laboratory were validated through near-infrared spectroscopy, which is a quick and easy technique to evaluate the composition of NSC in different tissues and plant organs. After obtaining the raw spectral information, several predictive models were used to scale the spectra in NSC content measurements, using measured values as a reference. These methods were used to achieve a "metabolic flux" vision allowing us to understand the carbon and nitrogen flows/allocation in response to the plant's phenology (alternating between vegetative / reproductive cycles) and especially in response to the climate.