Improvement of greenhouse microenvironment and sweetness of melon (Cucumis melo L.) fruits by greenhouse shading with a new kind of near-infrared ray-cutting net in mid-summer

- We developed new kind of near infrared ray (NIR)-cutting nets for cooling greenhouses.
- The NIR-cut nets combine high visible light transmittance with strong NIR absorption.
- Air temperature decreased by 4-5 °C when covering with the NIR-cut nets on a sunny day.
- The sweetness of melon fruits increased when covering with the NIR-cut nets.

In melon (Cucumis melo L.), low soluble solids content (SSC, °Brix) of midsummer-harvested fruits represents a problem for growers. We developed two new near infrared ray (NIR)-cutting nets that combine high visible light (VL) transmittance and strong absorption in the NIR region (700-2500 nm) to improve the sweetness of melon fruits harvested in mid-summer. The first type, NIR-cut net A, had a VL transmittance of 69.2% and a NIR absorbance of approximately 45%. The other type, NIR-cut net B, had a VL transmittance of 61.6% and a NIR absorbance of 55%. These NIR-cut nets decreased the temperature by approximately 9 °C compared to the control when using a metal halide lamp for artificial lightning in laboratory experiments. In greenhouse experiments, the air temperature of the control greenhouse reached 38 °C at noon on a sunny day in midsummer, while the temperature was 34 °C in the greenhouses shaded with NIR-cut nets in the same conditions. Thermal imaging indicated that our NIR-cut nets also reduced the leaf temperature of greenhouse-grown plants by approximately 5 °C on a sunny day in midsummer. The use of the NIR-cut nets during the fruit maturation period did not affect the size of fruits, while still increasing their sugar accumulation. The Brix values of fruits grown in greenhouses covered with NIR-cut net A or B were 13.8 and 14.0°, respectively, higher than that of control (13.4°). Sucrose concentration of fruits shaded with NIR-cut net A or was 81.3 mg l−1, significantly higher than that of the control (68.5 mg l−1), while it was 71.4 mg l−1 for fruits grown in the greenhouse shaded with NIR-cut net B. These results demonstrate that NIR-cut net A is a valuable tool for improving the sugar content of greenhouse-grown melon fruits in midsummer.