Effects of Shading by NIR-Excluding Materials on the Photosynthesis and Transpiration of Tomato Plants

NIR-excluding materials have high PAR transmittance and low NIR transmittance by virtue of absorbing or reflecting NIR selectively. Therefore, compared with standard shading materials, shading with NIR-excluding materials leads to a reduction in air temperature inside the greenhouse without a reduction in PAR. Several studies have demonstrated the positive effects of shading using NIR-excluding materials on plant growth and crop yield; however, the mechanism of the effect is still unclear. In this study, we investigated the effects of shading by NIR-excluding materials on the growth, photosynthesis, and transpiration of tomato plants grown in a greenhouse. Two types of NIR-excluding films with different average PAR and NIR transmittances (PAR/NIR = 70%/66%, 80%/6%) were used as coverings above the canopy. Leaf temperatures under NIR-excluding materials were lower than those under non-shading material. This was because the NIR-excluding materials cut off both NIR and incident PAR to the plant canopy compared with non-shading material. It appears that the photosynthetic rate under conditions of high leaf temperatures decreased with an increase in respiration rate; however, there was no significant difference within the range 27-33 degrees C. The transpiration rate of tomato plants increased as solar radiation over the canopy increased. When solar radiation above the canopy was the same, it was assumed that the amount of heat absorbed by leaves and the transpiration rate both increased because the ratio of NIR to total solar radiation decreased and the ratio of PAR to total solar radiation increased under NIR-excluding materials compared with non-shading materials. However, there was no significant difference in the transpiration rate in these experiments. It is suggested that the photosynthesis and transpiration of tomato plants were more strongly influenced by the decrease in air temperature or decrease in incident solar radiation than by the change in the amount of NIR.