| Drought stress caused by soil water deficit is not only a major obstacle to plant growth and artificial vegetation construction in arid areas,but also affects global ecological security and socio-economic sustainable development.With the global climate change and the increase of greenhouse effect,the problem is becoming more and more serious.However,the effect of the important phenomenon of"drying-wetting"on the growth,morphology and physiology of woody plants at different growth stages in different regions caused by precipitation fluctuation is not well elucidated.For this reason,the seedlings of Platycladus orientalis and Amorpha fruticosa were selected as the experimental materials to carry out the pot experiment under rain-protected shed.The soil relative water content?SWC?was maintained at 100%as the control.The effects of different drought degrees?87.84%SWC,70%SWC,52.16%SWC and40%SWC?and drought duration?15d,30d,45d and 60d?on the growth and physiology of the seedlings of P.orientalis and A.fruticosa at different growth stages,and their recovery performances under the condition of rehydration after drought stress were studied.The main results are as follows:1.The response of the growth of P.orientalis and A.fruticosa seedlings at three growth stages were different under drought stress and rewatering conditions,and the compensatory growth effect on the seedlings of two tree species after rewatering was in the order of Initial growth stage?IGS?>Fast growth stage?FGS?>Later growth stage?LGS?.In IGS and FGS,the compensation effect of rehydration after drought on plant height,basal diameter,leaf and root growth,and biomass accumulation of two tree species seedlings was higher in the treatments with 87.84%SWC and 70%SWC than that of 52.16%SWC and 40%SWC.In LGS of two tree species seedlings,the growth indicators were lower than the control level,and the growth compensation effect of rehydration after drought was not obvious.In IGS of P.orientalis seedlings,the average biomass of 70%SWC treatment was the highest under four drought durations,which was 27.52%higher than that of the control;In FGS of P.orientalis seedlings,the average biomass of 87.84%SWC treatment was the highest under four drought durations,which was 15.32%higher than that of the control.In IGS and FGS of A.fruticosa seedlings,the average biomass of 87.84%SWC treatment reached the highest level under four drought durations,which increased by 25.59%and 11.04%compared with control,respectively.2.Leaf water potential of P.orientalis and A.fruticosa seedlings decreased gradually with the aggravation of drought stress.52.16%SWC and 40%SWC treatments had significant effects on leaf water potential at different growth stages of seedlings of two tree species,indicating that the decrease of leaf water potential at different growth stages of seedlings of two tree species was closely related to soil moisture content.The leaf water potential of40%SWC treatment at 15d reached the lowest level in IGS,FGS and LGS of P.orientalis seedlings,and in IGS and FGS of A.fruticosa seedlings,which decreased by 1.46MPa,1.10MPa,1.84MPa,0.71MPa and 1.12MPa,respectively.Contrarily,the leaf water potential of 40%SWC treatment at 30d reached the lowest level in LGS of A.fruticosa seedlings,which decreased by 0.58MPa as compared with control.With the extension of time after rewatering,the leaf water potential of the seedlings of two tree species gradually increased to close to the control level.The compensation effect on leaf water potential of two tree species seedlings stimulated after rewatering.After 72 h of rewatering,the leaf water potential of seedlings of both the tree species at different growth stages recovered to near normal level.3.The photosynthesis of P.orientalis and A.fruticosa seedlings at different growth stages was significantly affected by soil moisture content.With the increase of drought stress,the seedlings of two tree species could enhance their adaptability and resistance to drought stress by decreasing the photosynthetic parameters?the net photosynthetic rate,transpiration rate,stomatal conductance and intercellular CO2 concentration?,and improving water use efficiency.Under drought stress conditions,both stomatal and non-stomatal factors inhibited the photosynthesis of two tree species seedlings.The net photosynthetic rate?Pn?and transpiration rate?Tr?in IGS,FGS and LGS of two tree species treated with 40%SWC treatment decreased to the lowest level of the average values under 4 drought durations,where Pn and Tr of P.orientalis seedlings in IGS decreased by 61.72%and 80.97%,respectively.After rewatering,the photosynthetic parameters of all the three growth stages of two tree species seedlings gradually recovered,and rewatering compensated the photosynthetic physiology of plants.After 72h of rewatering,the photosynthetic capacity of seedlings of two tree species improved to some extent.The results showed that photosynthesis of two tree species seedlings had strong sensitivity and adaptability to drought stress.4.Under drought stress conditions,the soluble sugar and proline contents in leaves of P.orientalis and A.fruticosa seedlings showed an increasing trend,and the content of soluble sugar and proline with 52.16%SWC and 40%SWC treatments increased significantly.At40%SWC treatment,the content of soluble sugar and proline in IGS,FGS and LGS of P.orientalis and A.fruticosa seedlings reached the highest level of the average soluble sugar and proline contents under 4 drought durations,where the soluble sugar content of A.fruticosa seedlings in LGS was 1.63 times of the control,and the proline content of A.fruticosa seedlings in IGS was 22.65 times of the control,respectively.After rewatering,the soluble sugar and proline content of two tree species seedlings gradually recovered to close to normal level.After 72h of rewatering,the soluble sugar content at all the three growth stages of P.orientalis seedlings and the soluble sugar content in IGS of A.fruticosa seedlings gradually recovered to normal level,and the proline content in IGS of P.orientalis seedlings recovered to close to normal level.The results indicated that P.orientalis and A.fruticosa seedlings could regulate the osmotic potential by accumulating soluble osmoregulation substances under water deficit conditions,and enhanced the ability of the adaptability and resistance to drought stress conditions.5.Under drought stress and rewatering conditions,the response of antioxidant enzyme system in three growth stages of P.orientalis and A.fruticosa seedlings was different,and the degree of influence varied with the degree of drought,drought duration,growth stage and species.With the decrease of soil moisture content,the activity of antioxidant enzymes and MDA content in the leaves of the two species were higher than those of the control level.The CAT activity in three growth stages of P.orientalis seedlings,SOD activity in LGS of P.orientalis seedlings treated with 40%SWC,the SOD and POD activity in three growth stages of A.fruticosa seedlings,CAT activity in IGS and FGS of A.fruticosa seedlings treated with40%SWC were lower than those treated with 52.16%SWC treatment,respectively.The results showed that the function of cell membrane of two tree species seedlings were gradually damaged at 40%SWC treatment.At 52.16%SWC treatment,SOD activity in LGS of P.orientalis seedlings,SOD and POD activity in IGS,FGS and LGS of A.fruticosa seedlings,CAT activity in IGS,FGS and LGS of P.orientalis seedlings,and CAT activity in IGS and FGS of A.fruticosa seedlings reached the highest level under 4 drought durations,where SOD activity was 1.36,1.36,1.45 and 1.25 times of the control,POD activity was 1.82,1.69 and1.82 times of the control,CAT activity was 2.18,2.26,2.68,2.84 and 2.52 times of the control,respectively.After rewatering,the antioxidant enzyme activities and MDA content of P.orientalis and A.fruticosa seedlings in three growth stages gradually recovered.The antioxidant enzyme activities and MDA content of the two afforestation plants in IGS and FGS were restored to close to the control level after 72h of rewatering;In LGS,CAT activity and MDA content recovered to close to the control level,indicating the seedlings of the two tree species had certain recovery ability after rewatering under drought stress. |
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