Physiological Mechanism Of Waterlogging Tolerance In Watermelon

Waterlogging is an important factor that restricts the production of watermelon.In the coastal areas of East and South China Sea such as Zhejiang Province,waterlogging has led to rotted melons and crop failure frequently,which has seriously affected the yield of watermelon.In the present study,we exposed two watermelon cultivars with different tolerances('YL' and 'Zaojia 8424')to waterlogging conditions for 9 d.And then we investigated the changes of photosynthesis,respiration and reactive oxygen species metabolism.The main results are as follows:(1)The inhibition of watermelon growth by waterlogging was obvious,which was mainly manifested as root rot,leaf shrinkage and early flowering.The growth of 'YL' was significantly better than 'Zaojia 8424'.(2)The root growth and respiration were servely inhibited by waterlogging.During this period,the activities of succinic acid dehydrogenase(SDH),malic acid dehydrogenase(MDH)and cytochrome c oxidase(COX)decreased.The activity of ethanol dehydrogenase(ADH)and lactate dehydrogenase(LDH)increased,and ADH responded faster than LDH.The AOX acitivity of 'YL' gradually decreased after rising in the early stage of waterlogging,while 'Zaojia 8424' showed a decreasing trend.The above phenomenon was also reflected in gene expression.The expressions of MDH,SDH and COX were down-regulated by waterlogging,while those of ADH and AOX were up-regulated.The expression of MDH,COX,AOX of 'YL' was significantly higher than those of 'Zaojia 8424' at 48 h,48h and 24 h,respectively.The more tolerance to resistance in 'YL' is mainly associated with the higher activity of AOX.(3)The chlorophyll content,net photosynthetic rate(Pn),stomatal conductance(Gs)and transpiration rate(Tr)of watermelon was decreased by waterlogging.However,the intercellular CO2 concentration(Ci)was increased.The chlorophyll content,Pn,Tr and Ci in 'YL' were significantly higher than those in 'Zaojia 8424' after 3 d.Intrinsic photochemical efficiency(Fv/Fm),the efficiency of excitation energy capture by open PSII reaction center(Fv'/Fm'),quantum efficiency of PSII electron transport(?PSII)and photochemical quenching coefficient(q P)declined with the waterlogging,while non-photochemical quenching(NPQ)was enhanced.In comparasion with 'Zaojia 8424',the chlorophyll content,photosynthesis and PSII activity of 'YL' were less affected by hypoxic stress.(4)During a short period of flooding(0-3 d),reactive oxygen species(ROS)was accumulated,the activity of antioxidant enzymes and their related expression levels were up-regulated.However,it did not show significant differences between the two varieties in terms of antioxidant enzyme activity except for POD.The content of ascorbic acid(ASA)was increased by waterlogging,while glutathione was decreased.As the waterlogging continued(3-9 d),ROS accumulated and the content of malondialdehyde(MDA)increased led to the enzymes inhibited.On the whole,there were no significant differences in antioxidant enzyme activity and related gene expression between 'YL' and 'Zaojia 8424'.