Effects of different color shading nets on preventing sunburn of Newhall navel orange during high temperature season

Abstract:【Objective】In the early autumn, high temperature and strong light often inhibit the photosynthesis of citrus leaves and fruit development, and cause sunburn on leaves and fruits in severe cases, leading to the decline of fruit yield and quality. Covering shading net above tree canopy is one of the main and effective measures to reduce the harm of high temperature and high light intensity in citrus production, which has good effects on increasing fruit production and improving quality. Color shading nets maybe a new choice for preventing citrus sunburn, and it not only has the effect of shading and cooling, but also plays a role in regulating the light quality under the net. Previous studies have shown that the light quality has a significant role in regulating plant growth, development and physiological metabolism. Therefore, in order to provide a basis for the selection of the appropriate shade net color, we studied the effects of different color shading net coverings on light intensity, light quality and temperature under the net, as well as the photosynthesis and fruit growth and quality of Newhall navel orange.【Methods】In high temperature season (Aug.) in Chongqing, weft knitting 6 stitches of color shading nets (yellow, red, green, blue) and black sunshade nets were selected for tree canopy sunshade treatments for Newhall navel orange. The diurnal changes of light quality, light intensity, leaf and fruit surface temperature, leaf photosynthesis and relative chlorophyll content in the leaves, and the increase of the fruit diameter were measured during the net covering period. The fruit growth increment and quality indexes, including peel color, contents of vitamin C (Vc), titratable acids (TA) and total soluble solids (TSS) were measured just post high temperature season and during the harvest time.【Results】Different color shading nets had different shading rates on tree canopy and the shading rate was 59.9%-75.4%. However, yellow, red, green and blue nets basically had the same shading rate, but black net had 75.4% shading rate, which was 12.2%-15.5% higher than those of color nets (59.9%-63.2%). Compared with the control (no shading net covering), the cooling effect of all color nets was basically the same, and the temperature decreased on the leaves and fruits by 7.4-10.0 ℃ and 7.0-8.3 ℃, respectively. All the treatments had no sun burning damage on the leaves and fruits, and significantly increased the relative chlorophyll contents and photosynthesis. The color shading nets covering treatments, especially the blue net, yellow net, black net also incresed the fruit diameter and fruit weight in just after the high temperature season. In terms of internal quality, the color shading nets decreased the fruit TSS and increased the fruit TA, although the difference between the control and each treatment was not significant . The color shading nets increased the fruit Vc and the increase of the fruit Vc by the green and blue shading net was significant compared with the control. In harvest time, the color shading nets increased the weight of fruit and transverse diameter and the increase by the blue and black nets was significant compared with the control. There were no significant differences in the longitudinal diameter, the fruit Index, and the fruit surface color between the various color shading net treatments and the control. In terms of internal quality, the contents of TSS and Vc treated with the various color shading- net treatments were higher than those of the control, while the content of TA was lower than that of the control. The fruit TSS content treated with the blue shading net treatment was significantly higher than that of the control, and the fruit Vc content of the green and blue shading net was significantly higher than that of control. Comparing the changes of fruit quality at the end of high temperature and ripening stage, the single fruit weight and transverse diameter treated with the blue and black shading net treatments was significantly the highest in both periods. The fruit Vc content treated with the blue and green shading net treatments was significantly the highest in both periods. The fruit TSS of all the treatments increased rapidly after removing the shading nets, and finally higher than that of control, in which blue net was significantly higher than that of the other treatments and the control. The TA content of fruits treated with the color shading-nets was slightly higher than that of the control in just after high temperature season, but after removing the shading nets, it decreased as the fruits entered the mature stage, and finally it was slightly lower than that of the control.【Conclusion】The covering with black, yellow, red, green or blue weft knitting 6 stitches shading nets could completely control sunburn damage, increase the net photosynthetic rate and improve fruit growth and quality of Newhall navel orange, and blue or black shading net covering increased the leaf net photosynthetic rate, promoted the fruit enlargement and increased the fruit weight significantly. Generally, the blue shading net had the best effects on improving fruit internal quality, including increasing the fruit TSS and Vc, reducing the fruit TA. Therefore, blue shading net would be suitable application for navel orange in high temperature season for controlling sunburn.