Humidity Controls the Timing and Persistence of Ozone Injury in Citrus: Linking Leaf Physiology and Regional Canopy Responses
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Tropospheric ozone (O 3 ) is a major air pollutant that threatens crop productivity, yet its effects depend strongly on environmental conditions that regulate plant O 3 uptake. Here, we explore how citrus, an O 3 -sensitive perennial crop, responds to O 3 exposure under humid subtropical (Florida) and semi-arid (California) climates. In controlled chamber experiments, Meyer lemon trees exposed to moderate O 3 concentrations (80 ppb for 4 h d −1 over four days) showed a faster and more persistent decline in the maximum photosystem II efficiency (F v /F m ) 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 (g s ) where high humidity maintains open stomata and accelerates O 3 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 O 3 -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 O 3 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 O 3 injury, linking leaf-level physiology to regional canopy responses. These findings emphasize that effective O 3 -risk assessments for perennial crops must incorporate local humidity and vapor pressure deficit conditions and both stomatal and non-stomatal deposition pathways.
