Beyond the boundary: a new road to improve photosynthesis via wind.

Plants interact with their microclimate, simultaneously responding to and influencing it. A key element in this interaction is the leaf boundary layer: a stagnant air layer enveloping the leaf, creating a resistance to heat and gas exchange. Its thickness, altered mainly by airflow and leaf morphology, determines the leaf-to-air interaction. Field crops experience wind speeds of 0-8 m s-1 at the canopy top, with wind gusts up to 20 m s-1, but wind speeds drop significantly within the canopy, creating localized low-airflow conditions. Conversely, indoor-grown crops always encounter low wind speeds (0-1 m s-1) and these, especially with larger leaves, restrict heat and gas exchange, impacting photosynthesis and transpiration. Although the effect of the leaf boundary layer on plant exchange processes has been defined, its magnitude remains poorly characterized and is frequently underestimated. This review re-examines its role, and underlying processes are further explained by using an existing modelling approach informed by published physiological parameters from relevant crops. This model suggests that in greenhouses, increases in wind speed typically smaller than 0.2 m s-1 could boost diurnal photosynthesis by 10-20%, although with possible detrimental side-effects such as growth reductions due to mechanical effects and excessive transpiration. In the field, leaves within the canopy often experience thick boundary layers (conductance <0.5 mol m-2 s-1). The role of the boundary layer needs to be re-evaluated, but this will require new tools, methods, and models to make a breakthrough in understanding this overlooked process in both field and controlled-environment agriculture crops.