Salt Tolerance Characteristics Of Typical Coastal Plants And Suitability Of Coastal Vegetation Construction

China’s coastal zone is seriously affected by typhoons,storm surges and other natural disasters.While building a large number of seawalls and revetments to protect people’s lives and property in coastal areas,it also has caused the loss or fragmentation of coastal wetlands,the reduction of biodiversity,the weakening of tide absorption function,the weakening of water purification capacity and so on.On the one hand,China’s coastal zone renovation and restoration work started relatively late,coupled with the blind reference of land landscaping experience,resulting in the low survival rate of vegetation,the restoration effect is not ideal.On the other hand,coastal plants are an important component of coastal ecosystem,and marine salinity has become the main environmental factor affecting their growth mainly through seawater intrusion,spray spatter,marine droplets,salt spray and other ways to affect the growth of plant roots,stems and leaves,bringing many difficulties to the restoration of coastal vegetation.On the basis of a large number of field investigations,through the screening and comparative study of salt tolerance of common coastal plants,and selecting typical tree species such as Clerodendrum inerme and Euphorbia alluaudii subsp.oncoclada,aiming at the phenomena found in the field,A nested automatic water cycle of plant leaf salt tolerance experimental device and method for salt tolerance of plant branches and leaves was designed to study the adaptability of foliar salt stress and whole plant salt stress respectively.The collocation scheme of tree species was determined according to the results of salt tolerance research,and the application demonstration of coastal vegetation construction was carried out in Quanzhou Bay,Jinjiang.The main research contents and conclusions are as follows:（1）Salt tolerance of typical coastal plantsThrough the screening and comparative study on the salt tolerance of 18 species of typical coastal plants in 17 families and 18 genera,from the perspective of tree plants,the order of salt tolerance is Excoecaria agallocha>Talipariti tiliaceum,Hibiscus hamabo>Ficus microcarpa>Cinnamomum camphora;from the perspective of shrubs,the order of salt tolerance is Rhaphiolepis umbellata>Heptapleurum heptaphyllum>Dodonaea viscosa>Pentacoelium bontioides>Pluchea indica>Breynia fruticosa>Bougainvillea spectabilis>Caesalpinia bonduc.From the perspective of herbs,the order of salt tolerance from large to small is Arundo donax>Agave americana var.marginata>Gynura divaricata>Sphagneticola trilobata>Crinum’Menehune’.This provides a scientific reference for the subsequent coastal vegetation construction and tree species collocation.（2）Salt tolerance characteristics of C.inerme and E.alluaudii subsp.oncocladaDifferent salt stress can inhibit the growth and development of C.inerme and E.alluaudii subsp.oncoclada,and the higher the salt concentration is,the more obvious the inhibition can be obseverd.Morphologically,the number of C.inerme leaves decreased under salt stress,while the thickening of them was caused by the increase of succulent degree caused by salt,and the palisade tissue was elongated,which was an adaptive strategy to keep the salt concentration constant by steadily increasing the storage volume.Salt stress led to the osmotic imbalance of the highly succulent stem which was originally beneficial to water storage for E.alluaudii subsp.oncoclada,resulting in cell dehydration,growth rate decrease of plant height,and even death.As for physiological response of C.inerme,the activity of superoxide dismutase（SOD）in leaves decreased and the contents of malondialdehyde（MDA）and free proline（Pro）in leaves increased significantly under foliar salt stress.The contents of MDA,Pro and SOD in stems of E.alluaudii subsp.oncoclada increased under salt stress,especially in high salt concentration treatment groups.In general,in the environment of high salt concentration,the osmotic regulation of C.inerme and E.alluaudii subsp.oncoclada were enhanced,and the response of reactive oxygen species scavenging of E.alluaudii subsp.oncoclada was significant.Foliar salt stress significantly inhibited the photosynthesis of C.inerme leaves from stomatal and non-stomatal factors,respectively.Stomatal factors showed that net photosynthetic rate,stomatal conductance,transpiration rate and water use efficiency decreased with the increase of salt concentration,while intercellular CO₂ concentration and leaf saturated water vapor pressure deficit increased in different degrees.In terms of non-stomatal factors,the photosynthetic pigment content of C.inerme leaves decreased under salt invasion.After normal watering,all treatment groups of C.inerme maintained high survival rate and good recovery ability,while E.alluaudii subsp.oncoclada only recovered well in low salt treatment group.On the whole,C.inerme leaves can be well tolerant to foliar salt stress below 2.0%concentration,but can also tolerate 3.0%foliar salt stress.On the other hand,E.alluaudii subsp.oncoclada has better tolerance to salt stress below 1.0%concentration and poor tolerance to 3.0%salt stress.Therefore,when vegetation restoration is carried out in the coastal area which is greatly affected by sea spray and salt fog,C.inerme can be considered as the restoration tree species.For E.alluaudii subsp.oncoclada which is drought-tolerant and slightly salt-tolerant,it is suitable to plant on the upper part of the coastal slope with low soil moisture and salt content.（3）Application demonstration of seawall vegetation constructionThe application demonstration of coastal vegetation construction was carried out in Quanzhou Bay of Jinjiang,and on the whole,most of the plants grew well.Among the tree plants,T.tiliaceum could maintain a high survival rate whether there are mangroves shield in front or not,but L.glutinosa died.Among the shrub plants,the potted seedlings of C.inerme located on the highland of tidal flat and the bare root seedlings of C.inerme located at the bottom of the seawall survived best.On the seawall slope,V.rotundifolia grows well,but the coverage is not high,and the bare root seedlings of C.inerme grows better than the cuttings of it.P.bontioides,E.alluaudii subsp.oncoclada,Dodonaea viscosa,B.fruticosa and H.hamabo which are located in the middle and high position of the slope grow better.Among the herbs,the coverage of I.pes-caprae and S.trilobata is the highest,and the coverage and growth of S.trilobata in the high position is higher than that in the low position.In contrast,C.bonduc and G.divaricata at the bottom of the slope did not grow well.Field investigation found that C.bonduc mostly grows vigorously at the top of culture ponds,so in the future coastal vegetation construction should be planted in coastal areas with a certain elevation.Although P.tectorius and E.alluaudii subsp.oncoclada of the wave dissipation platform survived,they all showed salt injury symptoms in varying degrees.This paper has well realized the organic combination of theory and practice.On the basis of a lot of field investigations,through the combination of salt tolerance research and field planting demonstration of typical coastal plants,it provides scientific guidance for coastal vegetation restoration and ecological seawall construction.