Mechanisms of Ozone Effects on Plant Stress in Soybean Across Growing Season: From Leaf to Regional Perspective

Ground-level ozone (O) is a major constraint on agricultural productivity, yet most knowledge comes from controlled fumigation experiments using chronic exposures that differ from the episodic conditions crops experience in the field. Here, we combine a five-week chamber experiment with multi-year satellite observations (2018-2021, Arkansas, U.S.) to investigate how O affects photosynthesis, efficiency, and growth across scales of soybean plants (Glycine max). At the leaf level, initial O fumigation (80 ppb for 4 h) caused the strongest suppression of CO2 assimilation (A), stomatal conductance (Gs), and photosystem II efficiency ({Phi}PSII), indicating entry into a physiological strain phase. Recovery between exposures was incomplete, leading to sustained growth reductions despite moderate O levels. At the regional scale, analysis of solar-induced fluorescence (SIF) and MODIS productivity metrics revealed parallel patterns. Early-season O episodes produced greater suppression of SIF, GPP, and Gs compared to equivalent late-season events, and recovery lagged for several weeks. Seasonal yield proxies were best explained not by total O accumulation, but by early- and peak-season exposures, which accounted for up to 98% of variance across four growing seasons. Our findings highlight that the timing of O episodes is more consequential than cumulative dose, and that functional indicators such as SIF can detect strain-phase stress before structural indices diverge. By linking controlled experiments with regional-scale satellite monitoring, this study advances mechanistic understanding of O impacts on soybean and supports the development of remote sensing-based early warning tools for crop management.