Spatial examination of leaf-boundary-layer conductance using artificial leaves for assessment of light airflow within a plant canopy under different controlled greenhouse conditions

Leaf-boundary-layer conductance ( g a ), which is mainly affected by airflow near the leaf surface, is an important limiting factor on energy budgets, transpiration and photosynthesis, especially under very light-wind conditions. However, little research has been done, with a focus on the direct evaluation of g a under such conditions, because of the difficulty in measuring slower wind speeds that continuously vary in space and time. Here we propose a reasonable airflow assessment using spatiotemporal analysis of g a , with the aid of numerous artificial leaves facilitating a continuous and multipoint evaluation of g a . In our testing and development of the method, the artificial leaves consisted of thin brass sheets that sandwiched constantan micro heaters, and g a was evaluated on the basis of energy balance of the electrically heated leaves. The analysis was performed within a tomato canopy in a greenhouse under different regimes of environmental controls (air circulation, forced and natural ventilation, heating and air ductwork), thereby allowing the spatiotemporal distributions of g a within the canopy to be determined. The artificial leaves successfully captured the fluctuations in g a affected by the light airflow within the canopy, although g a was overestimated by only 2% (at most 5%) as compared to that of actual leaves owing to the buoyancy effect of electrical heating of the artificial leaves. Thus, without excessive electrical heating, the artificial leaves are considered reliable tools for airflow assessment in the greenhouse. Daytime g a values were small and equivalent to daytime stomatal conductance, even under environmental controls, thus limiting the heat transfer, transpiration and photosynthesis of the leaf. In contrast, night-time g a values were higher values than night-time stomatal conductance, which indicates the small impact of g a on heat and mass exchange via the stomata during the night-time. Spatiotemporal changes in g a substantially varied within the canopy, owing to the different environmental controls. Consequently, remarkable non-uniformities in g a appeared within the canopy, with implications for variable heat and mass exchange. These results indicate that airflow management in the greenhouse can still be improved by spatiotemporal analysis of g a .