Vapor pressure deficit, not temperature per se, limits coffee photosynthesis under irrigation

Coffee (Coffea arabica and Coffea canephora) is among the world's most valuable agricultural commodities. It is cultivated across tropical regions where rainfall, temperature, and irradiance strongly influence phenology, yield, and bean quality (DaMatta et al., 2025). However, coffee production is increasingly threatened by climate-related stresses, particularly heat waves and droughts, with models projecting substantial yield losses under future warming (DaMatta et al., 2018; Rahn et al., 2025). In response, irrigation has expanded steadily, especially in Brazil, the world's leading producer (Ronchi & DaMatta, 2025). Elevated air temperature has long been regarded as the main constraint to coffee growth under irrigation, mainly by affecting flower production and, particularly, photosynthesis-driven fruit filling (DaMatta & Ramalho, 2006). This perception originates from an outdated notion, still widely cited in technical circles, that C. arabica photosynthesis decreases by 10% for each 1°C rise above 24°C and ceases at 34°C (Nunes et al., 1968). Controlled experiments, however, show that both C. arabica and C. canephora maintain high photosynthetic rates up to 37°C (Dubberstein et al., 2020; Rodrigues et al., 2016), indicating that temperature alone cannot explain the reduction in carbon assimilation under warm conditions. The true limiting factor is vapor pressure deficit (VPD), which increases exponentially with temperature and declining relative humidity (Kath et al., 2022). Elevated VPD enhances evaporative demand, triggering stomatal closure to reduce water loss. Although protective, this response restricts CO2 diffusion into the leaf and limits photosynthesis, the process underpinning 90%–95% of plant biomass accumulation (Khanna-Chopra, 2000). In coffee, stomatal conductance declines sharply with increasing VPD, especially during warm, dry afternoons, leading to strong depressions in photosynthesis even when soil water is adequate (DaMatta et al., 2025). Conversely, experimentally lowering VPD helps preserve stomatal aperture and photosynthesis (Batista et al., 2012). Collectively, these findings indicate that coffee photosynthesis is resilient to moderately high leaf temperatures and that carbon assimilation declines above ≈30°C mainly reflect VPD-induced stomatal limitation (DaMatta et al., 2025). Supporting this view, coffee yields correlate more strongly with VPD than with temperature or humidity, with productivity dropping sharply above ≈0.8 kPa—a threshold that, under projected global warming exceeding 2°C, will be surpassed frequently, jeopardizing yields across producing regions (Kath et al., 2022). Recognizing VPD as the principal constraint thus opens new avenues for adaptation: Breeding genotypes with reduced stomatal sensitivity to VPD (which would respond better to irrigation) and adopting agronomic practices that mitigate evaporative demand (e.g., agroforestry shading, windbreaks, or optimized planting density) could enhance photosynthesis and, consequently, fruit filling, a major determinant of yield and cup quality (DaMatta et al., 2018, 2025). In irrigated systems, higher transpiration is not detrimental; rather, it promotes canopy cooling, lowers leaf temperature, and sustains CO2 supply for photosynthesis (DaMatta, 2004). In conclusion, elevated temperatures (≈30°C–35°C) constrain coffee growth not by directly impairing photosynthesis but by increasing VPD, inducing stomatal closure, and reducing the efficient use of water. Distinguishing between these mechanisms is crucial to guide breeding and management strategies for sustaining coffee production under global warming. Fábio M. DaMatta: Conceptualization; writing—original draft; writing—review and editing. The author thanks the Foundation for Research Assistance of Minas Gerais State, Brazil (FAPEMIG, Project CRA-RED-00060-23), and the National Institute of Science and Technology in Plant Physiology under Stress Conditions (INCT, Plant Stress Physiology, grant 406455/2022-8). The author declares no conflicts of interest.