Rffect Mechanism Of Molybdenum On The Chilling Tolerance In Cucumber Seedlings

In order to elucidate the regulating machenism of molybdenum (Mo) on chilling tolerance of cucumber seedlings, changes of the growth, antioxidant systems, osmoregulation, photosynthesis and ABA content in molybdenum deficiency (-Mo), molybdenum application (+Mo) and control (CK) cucumber (’Jinyou3’) plants subjected to low temperature stress (day/night temperature8℃/5℃) were investigated. The main results are as follows:1.-Mo treatment showed lower leaf area and dry weight, while highter electrolyte leakage (EL) and chilling injury index during low temperature stress, compared with the control.+Mo treatment increased the leaf area and dry weight significantly, but decreased the EL and chilling injury index. The results indicate that molybdenum deficiency aggravates chilling damage to cucumber seedlings. However when a proper molybdenum is applied, the cold tolerance of cucumber plants could be enhanced.2. Low temperature led to an increase in malondialdehyde (MDA) content, a decrease after an initial increase in actvities of superoxide dismutase (SOD), peroxidase (POD), aseorbate peroxidase (APX), and in contents of reduced glutathione (GSH) and ascorbic acid (AsA), a decrease in catalase (CAT) activity in cucumber leaves. Compared with the control,-Mo treatment showed higher MDA content, lower antioxidant enzyme activities, whereas,+Mo treatments revealed lower MDA content, higher antioxidant enzyme activities. These data suggest that-Mo aggravates chilling damage to the cell membrane, and reduces the cold tolerance of cucumber seedlings. This may be caused by reduction in antioxidant abilities of-Mo treatment seedlings.+Mo increases the scavenging activity of reactivie oxygens, and consequently alleviates the membrane lipid peroxidation, and enhances the cold tolerance.3. Low temperature led to a decrease in chlorophyll content, photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), carboxylation effiency (CE), apparent quatum yield (AQY), actual photochemical efficiency of PS II in illumination (Φ PS Ⅱ), maximal photochemical efficiency of PS II in darkness (Fv/Fm) of cucumber seedlings; whereas, an increase in intercellular CO2concentration (Ci) and initial fluorescence (Fo). Compared with the control,-Mo significantly decreased Pn, Gs, Tr, CE, ΦPS Ⅱ and Fv/Fm (P <0.05), but obviously increase Ci and Fo (P<0.05). On the contrary,+Mo elevated Pn, Gs, Tr, CE, OPS Ⅱ and Fv/Fm (P<0.05), while declined Ci and Fo (P<0.05). These data suggest that the reaction center of PS Ⅱ in cucumber plants have been damaged distinctly by low tempareature (8℃/15℃).-Mo can aggravate the chilling damage on photosynthetic apparatus in cucumber seedlings, however, the result of+Mo is on the contrary.4. The nitrate reductase (NR) activity decreased significantly, while the contents of soluble protein and free amino acid increased in all treatments under low temperature stress, compared with the control (P<0.05).-Mo treatment remarkably decreased the NR activity and the contents of soluble protein and free amino acid during low temperature stress, compared with the control (P<0.05). However,+Mo treatment was on the contrary. These results reveals that-Mo causes to decline in nitrogen metabolism in cucumber seedlings, while+Mo promotes the nitrogen metabolism in cucumber plants.5. Low temperature stress significantly increased the contents of soluble sugar and proline in cucumber seedlings, but the increase extent were obviouse lower in-Mo treatment, higher in+Mo treatment, compared with the control. This indicate that the decrease in cold tolarance in-Mo cucumber seedlings is involved in the reduction of osmotic regulation capacity. Similarly, enhance in ability of osmotic regulation in+Mo treatment is one of the important reasons of the increase in cold tolerance of cucumber.