Mitigating Effect Of Spraying Exogenous Substances On Oriental Melon Seedlings Under Low Temperature Stress

Oriental melon(Cucumis melo var.makuwa Makino),as the one of the thermophilous vegetable crops,commonly occurrs the low temperature stress and dramatically limits the growth and development results in lower yield and quality,which has become the one of the most critical problem to solve in melon production.Previous studies have revealed that spraying exogenous substance could alleviate injury under low temperature stress on vegetable crops,whereas the effects of different substance are distinctive.Here,the cold-sensitive IVF004 and cold-resistance IVF571 seedlings as materials.Abscisic acid(ABA),salicylic acid(SA),melatonin(MT),calcium nitrate(Ca(NO3)2),proline(Pro)and spermidine(Spd)were sprayed separately.Meanwhile,photosynthetic,relative conductivity,malondialdehyde content,osmoregulation substance content and antioxidant enzymes activity were measured under low temperature,with two optimal exogenous substances being found finally.Next,in order to investigate the optimal mixed formula,two exogenous substances were used together and the effects of low temperature stress on physiological indexes were identified.This could provide more information on improving the resistance to low temperature and promoting growth on oriental melon seedling.The main results were as follows: 1.In the day/night 15?/6? under low temperature,0.2 mg·L-1ABA?3 mmol·L-1 SA?100 ?mol·L-1 MT?10 mmol·L-1 Ca(NO3)2?0.2 mmol·L-1 Pro?1 mmol·L-1 Spd were sprayed,separately.In 3-5 days after low temperature treatment,the increase of relative conductivity and MDA content was the largest and reached the maximum at the 5th day.Under the low temperature,especially at the 5th day,temperature stress was the most severe,whereas it still was significantly lower than the control group.The results showed that spraying these substances could relatively reduce REC and MDA contents in melon leaves.Among these substances,the effect of MT and Spd were better than others.Spraying these exogenous substances can improve Pn,SPAD and Fv/Fm.Although under the low temperature treatment,Pn,SPAD and Fv/Fm have decreased and reached the lowest at the 5th day,each treatment was significantly higher than the control group.Among them,MT,Spd and Ca(NO3)2 had the best effect in alleviating low temperature stress on oriental melon,and the Pn,SPAD and Fv/Fm became normal growth level after 5 days of recovery.Comparing with exogenous substances treatments,we found that the antioxidant enzyme activity increased at first and then decreased,and got to the highest level at the 3th day under low temperature.Among them,the antioxidant enzyme activity of melon leaves in MT and Spd treatments were significantly higher than that of the control under low temperature,which further indicated that these two exogenous substances treatments had greater alleviation of low temperature stress.The contents of soluble protein,soluble sugar and proline were significantly higher than control under low temperature after spraying MT and Spd exogenous substances,indicating that they had the best relieving effect.In addition,comparing cold-sensitive IVF004,the relieving effect of cold-resistance IVF571 was significantly greater when spraying exogenous substances under low temperature stress.2.Based on the research above,different contents of MT and Spd have been sprayed respectively,and then MT and Spd were used together(MT:A1 100 ?mol·L-1,A2 150 ?mol·L-1;Spd : B1 1 mmol·L-1,B2 0.5 mmol·L-1).The net photosynthetic rate and chlorophyll content have significantly increased in A1B1 and A2B1 treatments,and MDA and REC have significantly declined,compared to the control group.However,there were no significant difference in MDA content and Fv/Fm in cold-sensitive IVF004 and cold-resistance IVF571 when treated with A1B1 and A2B1.Therefore,there might be no difference under low temperature,which the combination improved the cold resistance of cold-sensitive IVF004.Photosynthetic,physiological and biochemical indexes have been analyzed treating with A1B1 and A2B1,which suggested that A1B1 was better than A2B1.Thus,we speculated that A1 was greater than A2,and it was possible that A1 was the optimal concentration lower than A2,and the higher content of A2 has the inhibitory effect.There is synergy between A and B,and A1B1 treatment had better relieving effect.It was also found that the treatment of composite B1 was better than the treatment of composite B2(it means that,A1B1,A2B1 is better than A2B1,A2B2),B1 was the optimal concentration and higher than B2.Meanwhile,in the appropriate range,higher content of B1 had better relieving effect.So A1B1 and A2B1 were greater than A1B2 and A2B2.The results proved that A1B1 was the optimal combination to alleviate injury under the low temperature stress.