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

Abstract

Leaf-boundary-layer conductance (ga), 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 ga 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 ga, with the aid of numerous artificial leaves facilitating a continuous and multipoint evaluation of ga. In our testing and development of the method, the artificial leaves consisted of thin brass sheets that sandwiched constantan micro heaters, and ga 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 ga within the canopy to be determined. The artificial leaves successfully captured the fluctuations in ga affected by the light airflow within the canopy, although ga 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 ga 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 ga values were higher values than night-time stomatal conductance, which indicates the small impact of ga on heat and mass exchange via the stomata during the night-time. Spatiotemporal changes in ga substantially varied within the canopy, owing to the different environmental controls. Consequently, remarkable non-uniformities in ga 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 ga.