Melatonin Mitigates the Impacts of Recurrent Water Deficit in Robusta Coffee Plants by Modulating Photosynthesis and Biomass Partitioning

Melatonin (N-acetyl-5-methoxytryptamine) is a regulatory molecule with potential to increase plant tolerance to water stress by modulating stomatal function, redox balance, and photosynthetic efficiency. However, its physiological effects depend on dose, species, and stress intensity, and studies on Coffea canephora under recurrent drought are scarce. This study evaluated the effects of exogenous melatonin (0, 100, 200, 300, and 400 µM) on young plants of conilon coffee genotype 02 (Clone V12) subjected to three consecutive cycles of water deficit and rehydration. The experiment was conducted in a greenhouse using a randomized block design, with evaluations of gas exchange, water potential, and biomass allocation. Melatonin improved physiological recovery after rehydration, particularly at 100 and 300 µM during the second stress cycle, when photosynthesis reached values similar to the irrigated control. The 400 µM treatment maintained root dry mass close to the control under prolonged deficit, suggesting preferential biomass allocation to the root system. Overall, melatonin effects were more pronounced during recovery than stress, indicating a possible priming action. These results highlight the potential of melatonin as a phytoprotective agent in C. canephora, although responses depend on dose and stress cycle.