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To question how widespread plant species adapt to temperature variability, we used the cosmopolitan genus <i>Phragmites</i> as a model to assess the gene-based traits variance–covariance matrix pertaining to intraspecific photosynthetic acclimation across a thermal gradient encompassing low, moderate and high temperature regimes. Our findings reveal that gene-based clinal variations corresponding to the latitudinal origins of genotypes, manifest more prominently in cooler environments. Additionally, the coordination of integrated physiological traits, aimed at preserving whole-plant fitness, exhibits a heightened response under cold stress. Moreover, the observed nonconformity in clinal plasticity patterns, where high seasonality does not correlate with elevated plasticity, prompting a reevaluation of phenotypic plasticity. Our findings enhance comprehension of intrageneric variation in physiological acclimation strategies among widespread species, refining predictions of species responses, survival, and distribution amidst global climate change.