Responses And Physiological Mechanism Of Sugar Accumulation In Melon (Cucumis Melo L.) Fruits To Low Light Stress

In off-season protected cultivation of melon?Cucumis melo L.?,low light stress is a major limiting factor for melon to reach its full potential of fruit yield and quality by influencing physiological metabolic process containing source organ growth,photosynthesis production,assimilates translocation,sugar metabolism in sink organ.Understanding the responses and physiological mechanism of sugar accumulation in melon fruits to low light stress may provide theoretical basis for improving off-season cultivation techniques and breeding low light-tolerant varieties.Two cultivars of melons?low light-tolerant‘Yujinxiang'and low light-sensitive‘Yuxue 3'?were used to study the capacity of photosynthesis,assimilate synthesis and stachyose loading in the leaf,stachyose unloading and sugar accumulation ability in the fruit,to analyze the responses and mechanism of sugar accumulation to low light stress in melon fruit.The main results are as follows:1.Under low light stress,absolute growth rate of leaf diameter developed slowly in two cultivars,leaf area and specific leaf weight were significantly smaller than those in the control;the reduction of leaf diameter,leaf area and specific leaf weight in low light-sensitive‘Yuxue 3'?7.8%,15.0%,6.7%respectively?were significantly bigger than those of the low light-tolerant‘Yujinxiang'?5.0%,9.6%,5.4%respectively?,suggesting that low light stress decreased light intercepting capacity in melon leaves,and‘Yuxue 3'was more sensitive to low light stress.Leaf chlorophyll content obviously increased,but chlorophyll a/b significantly decreased after shading in two cultivars.The increment of chlorophyll b and reduction of chlorophyll a/b in‘Yujinxiang'?12.3%,10.0%respectively?were significantly bigger compared with‘Yuxue3',indicating that low light-tolerant cultivar may be more adaptable to the weak light environment by increasing the proportion of light-harvesting pigments.2.Net photosynthetic rate and stomatal conductance significantly decreased in two cultivars after shading,while concentration of intercellular CO₂ was obviously increased,stomatal limits value significantly decreased,suggesting that the decrease of net photosynthetic rate under low light stress was mainly caused by non-stomatal factors.The reduction of net photosynthetic rate in‘Yujinxiang'?30.3%?was significantly smaller than that of‘Yuxue 3'?45.2%?after shading.Under low light environment,activities of sucrose synthesising enzymes and contents of fructose,glucose and sucrose were significantly lower than those in the control,the reduction of sucrose synthesising enzymes in activity and sucrose content in low light-tolerant‘Yujinxiang'were distinctly smaller than those in low light-sensitive‘Yuxue 3'.During earlier stage of shading treatment,leaf starch content was distinctly increased in two cultivars,indicating that when sucrose synthesis was blocked,the leaf assimilate was accumulated temporarily in chloroplast in the form of starch.3.Activity of galactinol synthase and galactinol content distinctly decreased after shading,but no significant difference was observed between two cultivars.The contents of raffinose,stachyose and the activity of stachyose synthase also distinctly decreased after shading,the reduction of stachyose synthase activity in‘Yuxue 3'?32.4%?was significantly bigger than that in‘Yujinxiang'?23.8%?,suggesting that the capacity of assimilate export was inhibited under low light condition,and the assimilate export capacity in low light-sensitive cultivar was inhibited more seriously.The difference of loading capacity between two cultivars may be decided by stachyose synthesis ability.Stachyose content in melon fruit significantly decreased in two cultivars after shading,suggesting that the imported assimilates reduced in melon fruit after shading,the reduction of stachyose content in‘Yujinxiang'?27.9%?was distinctly smaller than that in‘Yuxue 3'?37.9%?.Under low light stress,activities of acid and alkaline?-galactosidases also distinctly decreased,but raffinose content was higher during earlier stage,indicating that assimilate unloading in fruits was inhibited.The stachyose content was lower in‘Yuxue 3',but raffinose content was higher compared with‘Yujinxiang'suggesting that the inhibition of assimilate unloading was more serious in low light-sensitive cultivar.4.Under low light stress,fruit cell number and size in fruits significantly decreased in two cultivars,the reduction of cell number,length and width in‘Yujinxiang'?16.3%,5.1%and 9.0%respectively?were smaller than those in‘Yuxue3'?34.1%,10.4%and 16.2%?,resulted in smaller reduction of fruit volume and weight in‘Yujinxiang'?23.3%,24.4%respectively?than those in‘Yuxue 3'?54.9%,55.8%respectively?,indicating that low light stress inhibite the development of sink size by inhibiting cell division and growth.During fruit developing period,low light stress resulted in significant decrease of acid invertase,neutral invertase and sucrose synthase in cleavage direction in activity,sink activity decreased,activities of sucrose degrading enzymes in‘Yujinxiang'were significantly higher than those in‘Yuxue3'.5.Sucrose rapidly accumulated in melon fruit after 25d,activities of SPS decreased gradually and no peak appeared in two cultivars after shading,SS-s activities were also lower than that in the control during 30³5d,sucrose accumulate was inhibited,no significant difference was observed between cultivars.In mature melon fruits,low light stress significantly decreased total soluble sugar content,and the reduction of sucrose content were bigger than those of glucose and fructose contents,the reduction of total soluble sugar content in‘Yujinxiang'?24.8%?was significantly smaller than that in‘Yuxue 3'?32.2%?,suggesting that low light stress mainly affect the accumulation of sucrose,and sugar accumulation in‘Yujinxiang'was less sensitive to low light stress.In conclusion,under low light stress,synthesis and loading of photosynthetic assimilate inhibited,stachyose unloading inhibited in melon fruits and total sugar content reduced.The export assimilate reduced more in low light-sensitive‘Yuxue3'leaves was mainly due to the bigger reduction of light-harvesting area,Pn and activities of sucrose and stachyose synthesising enzymes;In fruits,the bigger reduction of assimilate unloading capacity in‘Yuxue3'was mainly due to the more serious inhibition of raffinose breakdown,these results provide scientific basis for improving cultivation measures,breeding valuable variety and increase the yield and quality of melon under low light conditions.