Low wind, high impact: sustained low airflow alters water use efficiency and shapes plant acclimation.

Minor differences in airflow are often assumed to have negligible physiological effects, yet they directly modify heat and gas exchange by altering the leaf boundary layer. We investigated how sustained low airflow impacts mechanisms regulating leaf-microclimate exchange, and how these responses shape acclimation in Lactuca sativa and Solanum lycopersicum. Lettuce was grown under two low-velocity airflow regimes (0.22 m s-1 and 0.65 m s-1), and changes in the leaf surface microclimate, plant transpiration, stomatal regulation, leaf photosynthesis, and growth were quantified. Increased airflow elevated transpirational demand, to which stomata responded dynamically by closing, increasing intrinsic water use efficiency. This closure did not fully offset enhanced water loss and was constrained by the need to maintain CO2 assimilation, which revealed that airflow alters a functional trade-off between carbon gain and water loss. This trade-off varied spatially, as photosynthetic limitation emerged specifically on the adaxial leaf side, indicating humidity-driven, side-specific diffusion limitations. At the whole-plant scale, increased airflow reduced fresh weight and leaf length by ∼13%, associated with hydraulic constraints and thermal regulation. We propose a unifying theory in which physiological regulation to airflow constrains structural and leaf-level acclimation, setting limits for plant growth. This framework provides a mechanistic understanding of how airflow affects short-term feedback and acclimation, with important consequences for experimental interpretation and reproducibility.