Effect Of Low Temperature Treatment On Accumulation Of The Bitter Compounds-Cucurbitacin B In Oriental Melon(Cucumis Melo Var.Makuwa Makino)

Oriental melon(Cucumis melo var.makuwa Makino)is popular among consumers because of its sweetness and fragrance.However,in the production practice,the unpleasant bitter-tasting melon fruits are common,which reduce the merchandise value and edible value of melon fruits.The bitterness is caused by a group of triterpenoids named cucurbitacin B(Cu B).Previous studies have show that the bitterness of melon is related to both genetic conditions and environments.Oriental melon is a kind of thermophilic crops,the fruits are more likely to be bitter when melons are cultivated in the crop stubble out of season,such as winter and spring stubble or autumn and winter stubble.Therefore,in this study,relationship between low temperature and accumulation of Cu B in oriental melon was studied.On the one hand,‘YMR’,a melon verity which bitter fruit is common,is used as the experimental material in the low temperature test at fruit development stages and seedling stage,aim to study the effect of low temperature treasured at different fruit development stages on the accumulation of Cu B in the melon fruit and the effect of low temperature on the accumulation of Cu B in the melon root and leaf.On the other hand,4 melon varieties with different cold tolerance were used as the experimental material to explore the correlation between Cu B content and the cold tolerance in melon seedling.The main research results are as follows:1.The low temperature(18/8°C)was treasured on 10 or 20 days after pollination,respectively,with 28/18°C as control.After the treatment,the plants were transported to the solar greenhouse for room temperature recovery.The fruits were divided into 5 parts to be taken the samples: pedicel pulp,middle pulp,areola pulp,pericarp and inner pulp.The results show that the content of Cu B was different in different parts of the melon fruit.And with the development of fruit,the Cu B content in melon fruit decreased.In the group of treat on 10 days after pollination,the Cu B content was increased in pulp and inner pulp when 0d and 5d after the plants were transported to normal temperature.In the group of treat on 20 days after pollination,no significant change in Cu B content in different parts of the fruit at different time points.It indicated that the accumulation of Cu B in oriental melon fruit have a higher response to low temperature at 10 days after pollination.Besides,in the group of treat on 10 days after pollination,the fruit weight was significantly lower than that of control when 5d after the plants were transported to the solar greenhouse;while,in the group of treat on 10 days after pollination,low temperature treatment had no significant effect on the fruit weight.In both of the group,there are no significant effect on Cu B content,soluble solids,and total soluble sugar content in mature fruits.2.In order to eliminate the interference of non-treatment factors to the experimental results,10 and 20 days after pollination,the fruits were harvested for the disk incubation.The low temperature was set to 8°C for 24 h,with 25°C as the control.The results show that the content of Cu B in the fruit discs at 10 days after pollination increased highly significant 4h and 24 h after treatment.In addition,multiple genes related to Cu B biosynthesis wereup-regulated.Cu B content in the fruit discs at 10 days after pollination increased significantly 4h after treatment.These results indicates that the fruits at 10 days after pollination are more sensitive to low temperature treatment,which is consistent with the experimental results obtained by cold-treating the plants.3.Low temperature treatment was carried out when the ‘YMR’ seedlings grew to three-leaf stage,the low temperature is 15/6°C,with 28/18°C as control.After 5d,the seedlings were moved to the control temperature to recover 2d.The Cu B content in seedling roots and leaves were determined.The results show that the content of Cu B in root of melon seedling was higher than that in leaf.During the low temperature treatment,there was no significant change in the Cu B content in root and leaf,while,on the first day of the seedlings were removed to normal temperature,the Cu B content in leaf was significantly reduced,and on the second day,the Cu B content in root increased significantly.The difference of Cu B accumulation in root and leaf at low temperature may be due to the different response mechanism of root and leaf to low temperature.4.Two low-temperature tolerant melon varieties namely ‘IVF571’ and ‘L5283’ and two sensitive melon varieties ‘IVF004’ and ‘IVF601’ selected by predecessors of this research group were treated(the order of their low temperature resistance is‘IVF571’>‘L5283’>‘IVF004’>‘IVF601’)under 15/6°C low temperature treatment for 3d,with 28/18°C as control.After the treatment,leaves were taken to determined the content of Cu B.The results show that the sensitive varieties ‘IVF004’ have the highest content of Cu B content in its leaves.Almost no Cu B was detected in the leaves of ‘IVF571’ and ‘IVF601’,the varieties most and least resistant to low-temperature,respectively.Besides,the Cu B content of ‘IVF004’ increased significantly at low temperature,while that of the other three varieties did not change significantly at low temperature.Therefore,the Cu B content in melon seedlings depends more on the variety difference,and the relationship between its low temperature resistance and Cu B content needs to be further studied.