Study On Cross Adapatation Of Drought Induced To Chilling Stress In Cucumber Seedlings

To research the mechanism of cross adaptation to drought-low temperature in cucumber (Cucumis sativa L) seedlings, the effects of pretreatment by drought with 10 % polyethylene glycol (PEG) on osmoregulation, photosynthesis, antioxidase system, cell structure and endogenous hormones content under low temperature were investigated with'Jinyou No.3'as material. The main results were as follows:1. Low temperature stress significantly decreased the relative water content, water potential and osmotic potential, while increased proline, soluble sugar and soluble protein contents. Pretreated with 10% PEG before exposure to stress condition, cucumber seedlings showed a significant low decrease degree in relative water content, water potential and osmotic potential, while high increase degree in proline, soluble sugar and soluble protein contents, in comparison with non-pretreated seedlings. At the end of the experiment (7 d), the relative water content of seedlings pretreated with PEG increased by 24.7 percentage points, the water potential and osmotic potential increased by 0.35 MPa and 0.13 MPa, respectively, and the proline, soluble sugar and soluble protein contents increased by 41.4 %, 80.8 % and 260.7 %, respectively, in comparison with non-pretreated seedlings.2. Low temperature stress let to decrease in chlorophyll content, photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr), whereas increase in intercellular CO₂ concentration (Ci) in cucumber leaves. The actual photochemical efficiency (ΦPSâ…¡) and maximal photochemical efficiency of PSâ…¡in darkness (Fv/Fm) in cucumber seedlings also declined during chilling stress days. The initial fluorescence (Fo) increased significantly over three days of chilling stress. PEG treatment showed higher Pn, Gs, Tr,ΦPSâ…¡, Fv'/Fm'and Fv/Fm, while lower Ci and Fo, compared with the control. These data suggests that non-stomatal limitation is the main cause of the Pn decrease in cucumber seedlings during chilling stress days. Three days'chilling stress caused obviously injury in reactive center of PSâ…¡. PEG pretreatment alleviates the chilling damage degree in photosynthetic apparatus of cucumber seedlings.3. Low temperature stress resulted in a notable increase in electrolyte leakage, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) contents, a significant decrease in activities of superoxidedismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). The activitiy of peroxidase (POD) of the PEG treatment maintained a rising trend during the chilling days, that of the control increased on the early days, while decreased on the late days of chilling stress. Compared to the control, PEG pretreatment showed a lower electrolyte leakage rate, MDA and H₂O₂ contents, a higher activities of SOD, POD, CAT, APX and higher contents of AsA and GSH. The data indicates that PEG pretreatment alleviates the chilling damage degree of cell membrane in cucumber seedlings.4. Chilling stress for 7 d let to injury in the cell membrane and chloroplast membrane the control leaves, vanish in grana lamella, derease in size and number of starch grains, accumulate in osmiophilic grains, dissolve in mitochondrial membrane. However, the PEG pretreatment showed complete membranes of mitochondria and chloroplasts approximately, distincted grana lamella, excepted for injury in a part of the plasma membranes, and alveolation in mitochondria. This reveals that PEG pretreatment plays an important role in stabilization of cell structure in cucumber leaves.5. PEG pretreatment showed significant higher ABA content, lower GA₃ content and the ratio of GA₃/ ABA in cucumber seedlings before and after chilling stress. Higher ABA content and lower GA₃/ ABA ratio is one of the internal causes in increase of cold tolerance in PEG pretreated seedlings.6. The results of the experiment in soil cultivation showed that the electrolyte leakage and chilling injury index of drought pretreated seedlings were lower obviously than those of the control after chilling stress for three days. Recovery in optimal condition for three days, the drought pretreatment was close to normal, whereas the control seedlings showed significant higher electrolyte leakage than before chilling stress days.