Effects Of Light Intensity On Fruit Size And Sugar Accumulation Of Melon

Light intensity is a major limiting factor for cold season production of melon. Photosynthetic pigment contents, photosynthetic properties, fruit development and flesh sugar accumulation of melon, cv. Yujinxiang, Yindi and Huanghemi, under three levels of light intensities (26%, 52% and100% of natural light respectively) were studied. The results showed that:1. The volume and fresh weight increases of melon fruits during development were "S"-typed curves, the fruit diameter growth concentrated in three weeks after anthesis, while fruit fresh weight growth was mainly concentrated in two to four weeks after anthesis; the size of melon fruits were mainly determined by the number of cells. The large-fruited Huanghemi have significantly more fruit cells than middle-fruited Yindi and small-fruited Yujinxiang. Shading treatments significantly reduced the number of fruit cells at anthesis stage, final volume of cells was slightly affected by shading treatments. No significant differences in cell volume increase after anthesis were found among cultivars or lighting treatments.2. The glucose and fructose of melon increased quickly at early stages of fruit development, while sucrose rapid accumulation took place at later stage. Under shading conditions, the soluble sugars accumulated in melon fruits decreased, and the rapid accumulation of sucrose was delayed. The soluble sugar content of Yujinxiang decreased only slightly after shading, the sugar content difference between control and slight shading (52%of natural sun light) of Yindi was not significant, while it was remarkably significant between control and heavy shading (26% of natural light ). The soluble sugar content of Huanghemi decreased significantly after shading, and significant difference was also found between two shading treatments. When mature, the order of sugar contents among three cultivars was Yujinxiang>Yindi>Huanghemi. The sugar content order of three parts in ripen melon fruits was pulp flesh> central flesh> pedicle flesh. In fruit development, the starch content of melon flesh increased then decreased, but increased after shading. When mature, the order of starch content was Huanghemi> Yindi>Yujinxiang.3. Total chlorophyll of mature leaves decreased after anthesis. Shading treatment increased total chlorophyll, Chlb and carotenoid contents of all three cultivars investigated, resulting in a decreased Chl a/b ratio.4. Under full light conditions, the diurnal change of net photosynthetic rate (Pn) of three melon cultivars showed clear midday depression, but only a single peak under shading conditions, and the photosynthetic peak was also delayed. Under natural sunlight, the Pn of Huanghemi was higher than those of Yindi and Yujinxiang, but less than Yindi in the condition of 52% of natural light, and remarkably lower than Yindi and Yujinxiang in the condition of 26% of natural light. The transpiration and photosynthesis of melon leaves changed almost simultaneously; the transpiration rates under shading were significantly lower than those of controls, but there was no significant difference between two shading treatments. The WUEs of Huanghemi and Yindi were significantly higher than that of Yujinxiang, shading treatments led to lower WUE. The differences of the SLW among melon cultivars were not significant, but the SLW under 26% of natural light treatment dropped significantly.