| Cucumber (Cucumis Sativus. L.) is different from other high plants in translocating photoassimilate primarily in the form of stachyose rather than sucrose. From source to sink in cucumber, photoassimilate changes several times between sucrose, glucose, fructose and stachyose, but where these conversional reactions take place is poorly understand in detail. Comparing with sucrose-transporting plants, stachyose-transporting plants need an additional metabolic step, i.e. synthesizing stachyose from sucrose before photoassimilate loading. It is supposed that the diurnal pattern of photoassimilate translocation from source to sink is more complicated in stachyose-transporting plants, however, up to date little research has focus on this field. Therefore, carbohydrates content and related enzymes activity were investigated in mature leaf, phloem sap, mesocarp, seed, petiole, peduncle, root and central spike at 16:00 p.m. to clarify the position of sucrose-stachyose conversion in cucumber. In addition, the diurnal pattern of these carbohydrates and enzymes in mature leave, phloem sap, mesocarp and seed were also studied. The results showed that:I After forming sucrose by photosynthesis, stachyose is synthesized in leaf in cucumber leaf. Stachyose is catabolized rapidly to sucrose in peduncle after long-distance transportation, and the sucrose formed is continually transported into the fruit. Stachyose in seed is not transported from fruit directly but synthesized in situ by glucose , fructose or sucrose coming from fruit.II The ratio concentration of different soluble sugars in cucumber mature leaf of cucumber changes during a photoperiod. Glucose and fructose are the primary soluble sugars from night to 10:00 a.m., but at 13:00-16:00 p.m., stachyose becomes the most abundant oligosaccharides. This diurnal change is mostly due to the tempestuousfluctuation of stachyose concentration in mature leaf.III During photoassimilate transportation, the primary soluble sugar in phloem sap is stachyose. When transportation is finished, sucrose becomes the main sugar in phloem sap. An apoplastic phloem loading pathway may exist in cucumber during night.IV Cucumber leaf may store photoassimilate in the forms of starch, glucose, fructose, stachyose and sucrose at night. The concentrations of glucose, fructose and stachyose in phloem sap are much lower than in leaf. The mechanism maintaining concentration difference of these sugars between leaf and phloem is unknown. There may be some unique mechanisms modulating source-sink translocation by which some sugars (such as stachyose) in leaf were prevented from loading into the phloem further in night.V The proportion of different soluble sugars in cucumber mesocarp changes regularly in a diurnal cycle. Concentration of fructose and glucose remain low level during 10:00-13:00, increase to maximal values at 16 p.m., then decline slowly, but still maintain relatively high level during nocturnal time. Stachyose can not be detected in mesocarp, but a -galactosidase, a key enzyme catabolize stachyose maintain high activity in a photoperiod. The results indicate that at least a little stachyose has been transported into cucumber mesocarp ,but was catabolized by a -galactosidase rapidly.VI Stachyose level is lower than glucose, fructose and sucrose levels in cucumber seeds from 14-days-after-anthesis fruit. Stachyose showed very little accumulation during night, coupled with significant a -galactosidase activity at this time in seeds, indicating stachyose may be catabolized into other sugars to be substrates for respiration or embryogenesis at night in young cucumber seeds.
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