Effects of Daytime Heating on Growth of Tomato and Heating Loads of Greenhouse under Low Light Conditions

Abstract

Experiments were performed to investigate the influence of daytime heating temperature on dry matter production and photosynthesis of tomato plants grown under low light conditions. The light use efficiency and dry weight of tomato plants increased significantly with a high daytime temperature when the light intensity was between 10 and 25 W・m^–2PAR. In actual cultivation, it was considered that the recommended daytime heating was a solar radiation intensity of 125 W・m^–2 or more. When the daytime heating temperature setpoint was raised from 20°C to 24°C in May with higher light conditions, the growth rate of tomato increased to develop more leaves and shorten the intervals of trusses. The photosynthetic rate of tomato plants grown at 24°C in the daytime was significantly higher than in those grown at 20°C, when examined at a light intensity not less than 200 μmol・m^–2・s^–1 (50 W・m^–2PAR). In addition, the number of leaves increased and intervals between anthesis were improved. In order to clarify the heat balance of daytime heating, the daytime heating load of the pipe framed greenhouse was determined in a region with low winter light conditions. The daytime heating load in a single span greenhouse located in Fukuoka Prefecture increased with a higher temperature setting, with heat loss from walls and roofs accounting for 82 to 89% of the total heat loss from the greenhouse. In addition, it was considered that the heating load for daytime heating could be calculated by subtracting the value of the untreated house from heat loss and soil heat flux of the heated house. The values of the monthly heating load were similarly high from December to February and relatively low in March in forcing culture of tomato. The total fuel consumption for daytime heating (heating temperature setpoint: 20°C) from December to March was estimated to be 1.20 L・m^–2. The amount of fuel consumption was the largest between 9 to 10 o’clock when heating was started, accounting for 20 to 30% of the total.