Effects of supplementary artificial light on growth of the tomato (Solanum lycopersicum) in a Chinese solar greenhouse

Light is one of the most limiting factors affecting plant growth in greenhouses. Recently, supplementary artificial light has been widely used in greenhouse production in winter or on overcast days when solar radiation is weak. However, plant responses to supplementary artificial light in a Chinese Solar Greenhouse (CSG) and the economic feasibility of the lighting strategy are poorly understood. The tomato, one of the most widely produced and consumed vegetables in the world, was tested in this study. A CSG was divided into three sections, and three treatments were arranged. They were top lighting with HPS lamps (TL), TL plus inter-lighting with LEDs (TL + IL), and control treatment without artificial light, respectively. We showed that supplementary artificial light improved the uniformity of light distribution from the south to north end of the CSG, and increased the average greenhouse temperature by ～1-1.4℃. Tomato yield was increased by 21.8% and 32.9% in TL and TL + IL, respectively, compared with that of the control. The higher yield under supplementary artificial light resulted from a larger number of ripe fruits rather than bigger fruit size. Moreover, fruit quality was improved by supplementary artificial light as indicated by a higher total soluble sugar content. There were no significant differences in total biomass production or total soluble sugar content in leaves, as well as fruits, between TL and TL + IL treatments, but TL + IL further stimulated fruit maturation compared with TL. Although supplementary artificial light confers an advantage in terms of tomato growth, it is not economically feasible as 28.8 kWh electricity was required to increase the tomato yield by 1 kg in the CSG. Therefore, it is important to find solutions to increase the efficiency of supplementary artificial light in the CSG.