Effects Of Water Stress On Seedlings Of Two Kinds Of Landscape Shrubs

Water stress is the most common abiotic stress during plant growth and development.In southern China,due to the high temperature and heavy rain in summer,the drought caused by the green space in the gardens and the waterlogging caused by urbanization have occurred over the time.The alternate environment of waterlogging,drought and“flood-drought”caused by the change of moisture content in the short term has restricted the growth of landscape plants.According to such characteristics of water,studying the adaptability of plants to water stress is of great significance to the development of sponge cities and low-impact green infrastructure.In this study,Senna alata Roxb.and Hibiscus mutabilis L.were selected as the objects of study in southern China.The biological characteristics and germination characteristics of seedlings,the seedling growth and the water stress of seedlings of the two shrubs were studied.The response was tested.The seedling growth pattern was measured once a month for a total of 5 months.The water stress was set with four treatments:continuous drought,continuous flooding,alternating floods and droughts,and control.The test lasted 28 days,once every 7 days.Biomass,morphology,photosynthetic parameters,and chlorophyll fluorescence parameters were measured.The results of the study are as follows:?1?The seed size,germination rate,and vigor index of S.alata were significantly higher than those of hibiscus?P<0.01?.During the five-month growth period,the growth patterns of the two shrubs generally showed a"fast-slow"trend.The increase in plant height and ground diameter of S.alata was significantly higher than that of H.mutabilis?P<0.05?;the leaf number and leaf area growth rate of H.mutabilis was significantly higher than that of S.alata?P<0.05?;the root morphology of the two shrubs have no siginificant difference.?2?The morphological growth and biomass of the two shrubs showed a“U”trend under control,continuous drought,continuous flooding,and alternating floods and droughts.The control showed the best performance and continued drought.The performance of flooding was poor,droughts and floods alternated stronger than continuous droughts and flooding,with persistent droughts performing the worst.Under continuous drought stress,both the aboveground and underground morphological growth of both shrubs were severely inhibited.In the aerial part,the plant height and ground diameter of both shrubs decreased significantly?P<0.05?,and the leaf morphology was severely inhibited.In the underground part,the root length,root surface area,specific root length and specific surface area of the two shrubs decreased significantly?P<0.05?,and the grading of different indexes of the roots,namely LG2,SG2,VG2,and TG2,showed significant differences under continuous drought.The length of root system connection and the number of root bifurcations showed a clear trend.The trade-off relationship showed the most obvious.The biomass of both plants decreased significantly?P<0.05?,and the biomass allocation was shifted from leaf to root and stem.?3?The net photosynthetic rate,stomatal conductance and transpiration rate of the two shrubs decreased significantly under three water stresses?P<0.05?.The intercellular CO₂ concentration of S.alata increased significantly under continuous drought stress?P<0.05?.The intercellular CO₂ concentration of H.mutabilis increased in varying degrees under three kinds of water stress,and the change direction of net photosynthesis rate was different.On the contrary,the reasons for the photosynthetic restriction of the two plants under water conditions at this time were non-stomatal factors.The minimum fluorescence parameter Fo and the non-photochemical quenching coefficient NPQ of the two shrubs increased in different degrees under different water stress,and the increase was most significant under drought stress?P<0.05?.The Yiield,maximum photochemical efficiency Fv/Fm,electron transfer efficiency ETR and photochemical quenching coefficient qP of both shrubs decreased to some extent under each stress treatment,and the continuous drought and flooding treatment decreased most significantly?P<0.05?.?4?Principal component analysis was performed on 19 indicators.The 4principal components can reflect 97.16%of the original data.Using fuzzy membership functions,the stress responses of two shrubs to water stress were evaluated.The results showed that the adaptability of S.alata was stronger than that of the H.mutabilis.The two shrubs had the strongest adaptability to the drought-flood alternation,followed by the continuous flooding stress,and the continuous drought had the most severe inhibitory effect on the two shrubs.S.alata can grow under all water conditions,while H.mutabilis is not suitable for drought environment.The two shrubs are more suitable for continuous flooding and flooding with alternate moisture conditions,such as waterfront green areas,rain gardens and other environments.