Studies On Physiological Mechanism Of Sugar Accumulationin Citrus Fruit

Ghina was the largest coamry of satsuma orange (Citrus reIiculata), which occupiedmore than 40 percent of total yield in the world. But the poor quality of citrus reduced theeconomic benefit considerably. With increasing life level and abundance of various tYuits,the higher quality of citrus fruit was needed at present. The quality of citrus fluits wasmainly determined by sugar coniellts because it not only affected the tastes and co1or offruit. But acted as a prerequisite for synthesis of organic acid, vitamin and carotenoids.So it was very urgent to Study the mechanism of sugar accumulation fOr improving thequality of citrus fruit.The experiments were cAned out by using satsuma mandarin as main material froml 999 tO 2000, The transport, partition of photosynthae, accumulation of sugars, the rolesof sucrose metabolizing enZymes on accumulation of sugar and changes of ABA indeveloping ffoits.were systemically studied. ll1 addition, the effects of cultivation onaccumulation of sugar in fruits tvere also evaluated. The main results were as fOllowst(l) "CO, tracing results showed that 14C- photosynthate partitioning to the juice sacsaccotaled for more than 50 percent of the total l'C- photosynthate that accumulatedin fruit during fruit enlargement, afterwards, the percentage of photos}inthatetransported to peel rose sharply whereas the percentage to juice sacs obviouslydropped. With fruit development, the ratio of "C radioactivity in segment epidermisto that in juice sacs increased gradually. indicating that postphloem photosynthatetrAnsport was limited sugar accumulatiol1 in juice sacs at the late stage of fruitdevelopmellt.(2) Juice sacs mainly accumulated soluble sugar frOm onset of fruit enlargement to fullffoit ripe. The contribution of juice sacs to the whole dry weight became greater withfruit development, which accounted fOr n1ore than 85 percent of fruit dry weightincrease at Stage of fruit full ripe.(3) Difference in respiratory intensity was existed fOr each developing stages and varioustissues in fruits. At the late stage of fl-uit development, with the reduction ofphotosynthate import into juice sacs the declining respiratory rate in edible tissue wasconstant, which might played an important role in promoting accumulation of drymatter accumulation in juice sacs of satsun1a mandalln ffoit.(4) Sucrose metabolizing enzymes intluenced fruit growth and sugar accumu1ation .Atthe early stage of fruit development; the sucrose cleavage enzymes (acid int'ertase,neutral invertase and sucrose synthase in cleavage direction) behaved more activitiesthan sucrose synthetic enZymes (sucrose phosphate synthase and sucrose synthase insynthetic direction) did. Owing to flourishing metabolism in young fruit and largerpart of translocated photosynthate was exploited, sugar accumulated slowly. Afterthis stage the activities of sucrose metabolizing enzymes decreased x'ith fruitdevelopment, especially for the sucrose cleavage enZymes. At onset of fruit co1oring,the net activities of sucrose metabolizing enzymes (sucrose synthetic enzymes minussucrose cleavage enzymes) aPpeared to be positive, and sugar accumulationaccelerated obviously. At full fruit ripening stage, the net activities of enzyn2es waslower but sharp drop of the speed in sugar acctunulation was not obserxred. \vhichsuggested that increase of sugar contents in edible tissue was the possible results ofphotosynthate direct enter in part. The acti\.ities of sucrose cleavage enzymes (mainlyinclude acid invertase) in peel generally u:as high than that in edib1e tissue at eachfruit development especially at late stage- whiCh possibly led to accumulated lnorehexose in peel tissue.(5) Further Studies was carried out with SaPpy orange, Poonen citrus, Meyer citrus andMiyagawa wase varieties, the results sho\\-ed that SaPpy orange and Miyaga\'a wasemainly accurnulated sucrose in juice sacs of hoit, the main sugars in Meyer citrus wa...