Humidity Controls the Timing and Persistence of Ozone Injury in Citrus: Linking Leaf Physiology and Regional Canopy Responses

Tropospheric ozone (O3) is a major air pollutant that threatens crop productivity, yet its effects depend strongly on environmental conditions that regulate plant O3 uptake. Here, we explore how citrus, an O3-sensitive perennial crop, responds to O3 exposure under humid subtropical (Florida) and semi-arid (California) climates. In controlled chamber experiments, Meyer lemon trees exposed to moderate O3 concentrations (80 ppb for 4 h d-1 over four days) showed a faster and more persistent decline in the maximum photosystem II efficiency (Fv/Fm) under humid air, while under dry air the response was delayed by one day and reversible. This humidity-dependent behavior reflects differences in stomatal conductance (gs) where high humidity maintains open stomata and accelerates O3 flux and dry air limits uptake but enhances slower non-stomatal injury pathways. At regional scale, satellite solar-induced chlorophyll fluorescence (SIF) from Sentinel-5P TROPOMI revealed similar patterns. In Florida, SIF decreased significantly during O3-episode weeks and remained low for up to three weeks, while in California it showed a brief rebound before a delayed decline - mirroring the timing observed in the chamber experiment. Analysis of the SIF and gross primary productivity (GPP) relationship further showed that O3 decoupled canopy fluorescence from productivity in the dry region, whereas drought stress weakened this coupling in the humid region, indicating a climate-specific shift in the dominant stressor. We demonstrate and argue that humidity governs both the timing and persistence of O3 injury, linking leaf-level physiology to regional canopy responses. These findings emphasize that effective O3-risk assessments for perennial crops must incorporate local humidity and vapor pressure deficit conditions and both stomatal and non-stomatal deposition pathways.