Fruit Shading Changed Photosynthate Partitioning, Sugar Metabolism and Accumulation in Developing Satsuma Mandarin (Citrus unshiu Marc.) Fruit

摘要

Photosynthesis occurs in citrus fruit. The objective of present study was to investigate char acteristics of fruit photosynthesis and the partitioning of fruit-derived photosynthates within various sinks of fruit. The physiological effects of reducing fruit photosynthesis by covering fruit with a two-layer thick opaque paper bag at the primary stage of fruit enlargement in developing Satsuma mandarin fruit were also studied. Change in fruit photosynthetic rate was some what related to the change in flavedo chlorophyll content. Fruit photosynthetic rate markedly declined as chlorophyll degradation occurred in the flavedo. Before the fruit mature, photosynthate produced by a 14C-fed leaf were mainly distributed to juice sacs even during periods when dry matter accumulation in the exocarp was more rapid than in juice sacs. When fruit were mature, peel photosynthetic rate approached zero and the exocarp became the major sink of leaf photosynthates. Most of the flavedo assimilates, however, remained in situ for up to 48h after fed 14CO2 to the fruit, with only a small portion being transported to other parts of the fruit. The percentage of fruit-produced photosynthates exported from the flavedo decreased with fruit development and maturation, but at the peak rate of export to juice sacs, the amount was as high as 12%. The total sugar content of the exocarp in shaded fruit was lower, but the relative content of sucrose in the exocarp was significantly higher than that in control fruit. Fruit shading also reduced the sucrose content of juice sacs in developing Satsuma mandarin fruit. Fruit shading increased the percentage of photosynthate partitioned to the exocarp, and correspondingly decreased the percentage of photosynthates partitioned to juice sacs. The activities of sucrose synthase (SS) , invertase and sucrose phosphate synthase (SPS) in the exocarp of shaded fruit were much higher than those of control fruit, but fruit shading had almost no effect on the activities of sucrose-metabolizing enzymes in the juice sacs. These results indicate that in the absence of fruit photosynthesis, the sink strength of the exocarp was increased by elevated activities of sucrose-metabolizing enzymes in this tissue, thus leading to the partitioning of more photosynthate to the exocarp and a corresponding decrease in the sugar content in the juice sacs of Satsuma mandarin.