| Carbohydrate source-sink relationships between photosynthetic source leaves and vegetative/reproductive growth sinks determine tree growth, fruit yield and fruit quality. However, in order to investigate the effects of source-sink relationships on photosynthetic of fruit trees, many of the published studies were limited to a comparison between fruiting and defruiting trees. For example, changes of photosynthesis of source leaves in response to source-sink manipulations by deflowering, debranch or defruiting, retaining fruit at different times on 1-year-old shoots and the limbs were studied during different phenological stages of fruit development, and opinions are not completely consistent. There is little information on the leaf photosynthetic response to different natural source-sink ratios. Therefore, we designed this experiment to examine the effects of different natural source-sink ratios on citrus. leaf photosynthesis, leaf growth and fruit quality.30-year-old 'Newhall'navel orange(Citrus sinensis (L.)Osbeck) and 8-year-old Jiang-jin 78-1 jincheng{Citrus sinensis (L.)Osbeck) were used as materials, and mature leaves on three types of shoots, including no fruit-bearing shoots, one fruit-bearing shoots and two fruit-bearing shoots, during fruit development period in 2010 were studited. The results are as the following;1. Effect of source-sink ratio on photosynthetic characteristicsThere were significant differences in leaf photosynthetic capacity among the three types of shoots. The net photosynthetic rates (Pn) of leaves on the top 2nd and 4th nodes on no fruit-bearing shoots were significantly or very significantly higher than those of the same position leaves on fruit-bearing shoots, and so were the average Pn of the 2nd, 4th and 6th leaves. As to the leaf Pn of the two types of fruit-bearing shoots, the one fruit-bearing shoots was slightly higher. With the fruits bearing increased on a shoot, the average leaf stomatal conductance (Gs) decreased along with the intercellular CO2 concentration (Ci) increased. For the Pn of leaves located on different shoot positions, they were the 2nd leaves>the 4th leaves>the 6th leaves for no fruit-bearing shoots, and the 4th leaves on one fruit-bearing shoot and two fruit-bearing shoots were the main function leaves.2. Effect of source-sink ratio on nitrogen, relative chlorophyll and physical characteristics of leavesLeaves on shoots with no fruit-bearing were larger, thicker, higher nitrogen concentration and relative chlorophyll than leaves on fruit-bearing shoots of'Newhall' navel orange and Jiang-jin 78-1 jincheng, as to the leaf characteristics of the two types of fruit-bearing shoots, the one fruit-bearing shoots was slightly higher.For the area, thickness and relative chlorophyll of leaves located on different shoot positions, they were the 2nd leaves>the 4th leaves>the 6th leaves for no fruit-bearing shoots, and the 4th leaves> 2nd leaves>the 6th leaves for one fruit-bearing shoots and two fruit-bearing shoots of'Newhall'navel orange. However, for Jiang-jin 78-1 jincheng, they were the 2nd leaves>the 4th leaves>the 6th for one fruit-bearing shoots, the 4th leaves> the 2nd leaves> the 6th leaves for two fruit-bearing shoots.3. Effect of source-sink ratio on fruit qualityFruit from the one fruit-bearing shoots was larger, had significantly higher (P< 0.05) Vc content than fruit from the two fruit-bearing shoots, but there were no other significant effects of source-sink ratio on peel thickness, Vertical diameter/Horizontal diameter ratio, Brix, acid or Brix/acid ratio, and peel color at harvest.Therefore, the relatively poor differentiation, growth and development of leaves on fruit-bearing shoots resulted in their lower Pn. Compared with the leaf differentiation, growth and development; source-sink ratio had slighter effects on leaf Pn.
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