Response And Adaptation Of Protected Cucumber To High Temperature Stress

Cucumber（Cucumis sativus L.）is native to the tropical rain forest in the south of the Himalayas and the northern of India,therefore it is considered one of the best-adapted species to warm,but not resistant to heat environments.Cucumbers are cultivated widely in solargreenhouse or plastic greenhouse,where abiotic stresses due to high temperature and high light intensity are frequent.It usually shows a decrease in photosynthesis that limits growth and final yield.Therefore,it has great significance to study the response and adaptation mechanisms of protected cucumber to high temperature stress.In this study,we researched the response and defense mechanisms of photosynthetic apparatus in cucumber seedlings to high temperature and high light intensity with the ’Jinyou 35’ as material.Simultaneously,the effect of high temperature environment of plastic greenhouse in summer on growth and development,yield and quality in cucumber were investigated.The main results are as follows.1.The growth of cucumber seedlings was dramatically inhibited by high and sub-high temperature stresses.The photosynthetic rate（Pn）was gradually reduced,while intercellular CO₂ concentration（Ci）was increased as heat stress lasted.Under heat stress,stomatal conductance（Gs）,transpiration rate（Tr）,photorespiration rate（Pr）and dark respiration rate（Dr）showed a trend from rise to decline in cucumber seedlings.Compared with the shingle high temperature stress,cross-stress of high temperature and high light intensity increased the reduction rate of Pn in cucumber seedlings.Under high temperature and high light intensity,cucumber seedlings showed a trend from rise to decline in Gs and Ci,while significantly decrease in Pr and Dr.These results implied that heat-and subhigh-temperature stress induced decline of photosynthesis was mainly due to non-stomatal limitation,however,the decrease of Pn caused by the cross-stress of high temperature and high light intensity was mainly due to stomatal limitation.2.Maximal photochemical efficiency of PS Ⅱ in darkness（Fv/Fm）,actual photochemical efficiency（ΦPSⅡ）,photochemical quenching（qP）and electron transport rate（ETR）were severely hampered,while initial fluorescence（F0）and non-chemical quenching（NPQ）were increased as a result of high and sub-high temperature stresses.The greater the intensity of the light（PFD）,the greater reduction rate of Fv/Fm,ФPSⅡand qP,and ascending rate of F0 and NPQ in cucumber seedlings.These data suggested that short-term sub-high temperature did not cause photoinhibition under optimal light conditions,however,high temperature led to severe damage to PSⅡreaction center in cucumber seedlings.Under high temperature,the higher the PFD,the more serious dagmag to PS Ⅱ reaction center in cucumber seedlings.3.Under extended high temperature stress,the activities of RuBP carboxylase（RuBPCase）and Rubisco activase（RCA）as well as the mRN A abundance of Rubisco and RCA were on the trend of decrease,while they were reduced 3 days following sub-high temperature treatment.The activities and mRN A expressions of sedoheptulose-1,7-bisphosphatase（SBPase）and fructose 1,6-bisphosphate aldolase（FBA）increased initially,but decreased afterwards under heat stress.The activities and mRNA expression of RuBPCase,RCA,SBPase and FBA all decreased with extending of stress time under cross-stress of high temperature and high light intensity,and the higher the PFD,the greater the reduction rate of them in cucumber seedlings.These data suggested that the photosynthetic enzymes were induced by high temperature stress and the induction was affected by temperature and stress duration.High light intensity can offset the inductive effect of high temperature,to some extent,on the activity and mRNA expression in photosynthetic enzymes.4.HT and SHT treatments all showed an increase in malondialdehyde（MDA）and H₂O₂ contents,and the superoxide anion （O（?）） production rate in cucumber seedlings,but the increase in SHT treatment was significantly less than that in SH treatment.During the early stress days,the activities and mRN A expression level of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,ascorbate peroxidase（APX）and glutathione reductase（GR）,as well as the ascorbic acid（As A）and glutanthione（GSH）contents in HT and SHT cucumber seedlings were increased.1³ days later,most of the above parameters were decreased（As A content kept increasing）.Compared with the shingle high temperature stress,cross-stress of high temperature and high light intensity increased the MDA content,whereas decreased the activities and mRNA expression of antioxidant enzyme and contents of As A and GSH.These results indicate that cucumber seedlings can defense against the injury due to short-time stress of high temperature or sub-high temperature by up-regulate the activity and gene expression of antioxidant enzymes.However,the seedlings would show a decrease in the activity and gene expression of antioxidant enzymes,and an aggravation in the lipid peroxide,if the stress duration exceeds 1³ d.High light intensity can accelerate the accumulation of reactive oxygen caused by high temperature stress,and the higher the light intensity,the more serious damage to the membrane system.5.The diurnal variation of Pn in most cucumber seedling leaves were one-peak pattern curve,whereas the midday depression of photosynthesis appeared when the temperature was more than 42 ℃ at noon（10 August）.During the growing season of cucumber,the activities of SOD,POD,C AT,APX and GR increased at first,then decreased,and showed a similar variation tendency with air temperature and PFD.This indicated that cucumber has the active adaptation mechanism to the high temperature and high light intensity stress.It can alleviate the damage due to high temperature and high light intensity by enhance the activity of antioxidant enzymes6.The yield of cucumber is closely related to temperature and light.Under high temperature and high light intensity,cucumber showed a decreased in the ratio of fruits to leaves,fruits per plants,weight of single fruit and total yield.High temperature and high light intensity also led to a decrease in the contents of soluble sugar and Vc,while caused to an accumulation in tannin,and therefore resulted in a decrease in the nutritional and flavor quality.