Effects of thermal insulation projection on indoor light environment and tomato growth performance in plastic greenhouse

Abstract: A thermal insulation material has widely been used to cover the roof of a greenhouse at night for an energy-saving production of indoor heat. The current mainstream winding of thermal insulation is fixed at the top of a roof, while movable at the bottom. However, there is often a shadow zone indoors during the daytime, when the thermal insulation staying on the top of the greenhouse. This study aims to clarify the changing characteristics of the shadow zone during the year, thereby determining the influence of the shadow zone on the growth of tomatoes. An asymmetric thermal insulation plastic greenhouse was utilized as the test object with a span of 18.0 m (inner span of 16.6 m), the length of 70.0 m, and the ridge height of 6.0 m (inner ridge height of 5.2 m) in Yangling, Shaanxi Province of China. In the east-west direction of the greenhouse, the area of horizontal projection on the south roof (SA) was set as the 11.26 m projection width of the inner south roof (the projection width of the outer south roof was 12.0 m), while, the area of horizontal projection on the north roof (NA) was 5.35 m projection width of the inner north roof (the projection width of the outer north roof was 6.0 m). The variation of shadow zone was calculated in the indoor cultivation area during the year, according to the trigonometric relationship between the direct sunlight and the thermal blankets on the roof. A solar radiation sensor was used to capture the solar radiation of NA and SA cultivation areas. The plant height and stem thickness of tomatoes in different cultivation areas were measured using tape measure and vernier caliper, respectively. A LI 6800 system was also utilized to measure leaf photosynthetic parameters, including the net photosynthetic rate, stomatal conductance, transpiration rate, and water use efficiency of tomato leaves. Finally, the yield and quantity of tomato fruit were also measured to compare. The results showed that: 1) Compared with the SA area, 38.9% and 27.9% of solar radiation in the NA area were blocked on sunny and cloudy days, respectively. 2) The shadow zone was formed in the indoor cultivation area from February 26. The area of the shadow zone reached the maximum of 146.8 m2 on March 29, accounting for 13.2% of the total cultivated area. Furthermore, the area of the shadow zone began to decrease on September 12 until it reached zero on October 14. The number of days with the shadow zone in the cultivation area was 231 d, accounting for 63.3% of the total number of days in the year. 3) Plant height and stem thickness of tomatoes in the NA area were significantly lower than those in the SA area from 33 days after planting. 4) The net photosynthetic rate, stomatal conductance, transpiration rate, and water use efficiency of tomatoes in the NA area were significantly lower than those in the SA area. 5) The average fruit weight per plant, the fruit number per plant, and the average fruit yield per plant in the NA area were 10.5%, 21.3%, and 29.5% lower than those in the SA area, respectively.