Leaf Boundary Layer Conductance in a Tomato Canopy under the Convective Effect of Circulating Fans in a Greenhouse Heated by an Air Duct Heater

Abstract

In greenhouses, ventilation systems and circulating fans are generally employed to improve air currents in crop canopies and to produce spatial uniformity across crop environments. Low efficiency and inadequate management of these air control systems result in poor control over the crop microclimate, which significantly affects yield and the quality of crop production (Katsoulas et al., 2007). Therefore, a method to design the optimal air currents is required to facilitate optimal control over the crop microclimate, i.e., heat and mass exchange between the plant canopy and the environment. Leaves play a dominant role in heat and mass exchange between crop canopies and the environment as they comprise the majority of the plant surface (Defraeye et al., 2013). As the primary organs of photosynthesis and transpiration, leaves are considered the most important sources or sinks of heat and mass in the canopy (Schuepp, 1993). The balance of heat and mass on leaf surfaces is strongly influenced by the convective exchange between leaves and the environment through the leaf boundary layer. A key factor in the convective exchange is leaf boundary layer conductance (GA), which represents the transfer coefficient of convection on leaf surfaces. Thus, for optimal design of air currents in crop canopies, GA, which is regulated by the convective airflow adjacent to leaves, must be evaluated. Due to the difficulty of directly measuring the air currents adjacent to leaves (Boulard et al., 2002), GA is generally estimated using the semi-empirical formulae of forced convection (e.g., Monteith and Unsworth, 1990). However, such formulae cannot be applied under the lower air velocity in crop canopies within greenhouses, which requires consideration of both free and forced convection (i.e., mixed convection) (Kitano and Eguchi, 1989). Leaf boundary layer conductance can be measured using the heat balance of electrically heated artificial leaves, and many researches have evaluated GA by using various types of artificial leaves (Grace et al., 1980; Dixon and Grace, 1983; Kitano and Eguchi, 1990; Leuning and Foster, 1990; Brenner and Jarvis, 1995; Grantz and Vaughn, 1999; Stokes et al., 2006; Katsoulas et al., 2007; Kimura et al., 2016). Profiles of GA in a greenhouse under ventilated conditions have been reported by Katsoulas et al. (2007). However, there is little information on GA in closed conditions, which are typical of heated greenhouses during winter nights. Under such conditions, circulating fans are employed to maintain uniformity in air conditions and convective heat transfer in the crop canopies. In this study, continuous and multipoint measurements of GA in a tomato canopy were carried out using electrically heated artificial leaves. In addition, the vertical and horizontal distributions of GA in the canopy within the green-