Response Mechanisms Of Sedum Spp. To Waterlogging Stress

Time unsuitable and unpredictable rainfall or soil compaction,excessive irrigation and poor drainage often cause waterlogging or flooding,which can inhibit plant growth,even cause death.Sedum(Crassulaceae)has characteristics of effective aesthetic values,wide range adaptation,and low-maintenance and is especially famous for its drought resistance.However,the death of Sedum plantscaused by waterlogging is common in nurseries and garden applications,seriously affecting the normal growth and ornamental effect of plants.At present,most of researches have been focused on the physiological responses of drought stress in Sedum,but few reports have been reported on the waterlogging tolerance,growth,physiological,biochemical and molecular aspects of Sedum,and the response mechanisms of Sedum under water logging stress is still unclear.Therefore,17 of Sedum plants were treated to identify the sources of waterlogging tolerance by simulating waterlogging.Two cultivars with different waterlogging tolerance were selected to explore the morphological,physiological and molecular response mechanisms of waterlogging,to provide theoretical references for application and innovation.The main results are as follows:1.The growth of 17 sedums was inhibited with varying degrees after 36 days waterlogging.Plant height,crown width,coverage,biomass,root-shoot ratio and root length were significantly lower than those of control.The major symptoms of waterlogging damage included leaf chlorosis,yellowing,defoliation and rot.Except for the early deaths of 'Purple Emperor' and S.sexangulare,other sedums survived to varying degrees under 36d waterlogging.After 12days recovery,their growth were restored with different degree,indicating that most tested sedums could endure waterlogging stress for a long time.On the basis of morphological observation,an evaluation system for waterlogging tolerance of sedums was established,and 8 materials showing with strong waterlogging tolerance were preliminarily screened out.2.Photosynthetic physiological indexes were all affected differently by waterlogging stress.With the extension of waterlogging,the content and proportion of photosynthetic pigments showed a downward trend.Although the light energy capture of leaf was affected by waterlogging,materials can improve photochemical quenching coefficient(qP)to a certain extent in the early stage of waterlogging to promote photosynthetic electron transfer,and enhance non-photochemical quenching coefficient(qN)to regulate their energy metabolism,consume excess light energy by heat dissipation,so as to protect PS II reaction center from destruction.After 12 days drainage,the photosynthetic physiological parameters of all materials tended to the control level.3.With the prolongation of waterlogging,the accumulation of MDA and relative membrane permeability increased significantly,and the activities of SOD,CAT and APX generally increased first and then declined.The change trends and degrees of soluble sugar,soluble protein and proline content varied from each other.All different changes indicated that the sensitivities of tested-material to water stress were obviously different and they can aviod damages caused by waterlogging by regulating antioxidant enzyme defense system and osmotic regulation system.After 12 days of drainage,the physiological indexes generally tended to be the control level.A comprehensive evaluation of waterlogging tolerance was carried out on 17 Sedum resources,and the results could be divided into 5 categories.(1)high waterlogging tolerance,including 'Carl','Brilliant','Autumn Joy',S.aizoon,'Immergrunchen','Spirit';(2)medium waterlogging tolerance,obtaining'Joice Henderson' and 'Gold Mound';(3)medium sensitive,including S.spectabile,'Vera Jameson',S.sarmentosum,'Blue Spruce';(4)sensitive,including 'Rose nteller','Coccineum','Fuldaglut';(5)most sensitive,including S.sexangulare and 'Purple Emperor'.4.A tolerant sedum cultivar('Carl')and a susceptible cultivar('Purple Emperor')were selected to compare with the response to waterlogging.The major symptoms of waterlogging injury including wilting,leaf chlorosis and water-soaked of base stem during waterlogging was observed earlier and serious in sensitive 'Purple Emperor' within 4 days of stress,while no obvious symptoms occurred in 'Carl' after six days of treatment.Moreover,no adventitious roots had emerged from 'Purple Emperor',but vigorous adventitious roots had emerged above the water level and internodes of 'Carl' just only 2 days of treatment,indicating that the formation of adventitious roots is one of the most important mechanisms of waterlogging tolerance.The stomatal opening rate,carbon assimilation ability,antioxidant enzyme activity and anaerobic respiration metabolism of both materials were significantly increased by waterlogging.Only after 3 days of waterlogging,the stomatal opening rate of the lower epidermis was over 99.00%during dark period.Except for the PPDK activity of 'Purple Emperor' decreased significantly with the prolongation of waterlogging,the total titratable acid content,tricarboxylic acid cycle and the activities of key CAM pathway enzymes(Rubisco,PEPC),antioxidant enzymes(SOD,POD,CAT)increased significantly in both cultivars,and which were significantly higer in 'Carl' than those of 'Purple Emperor'.The key enzyme activities of glycolysis/gluconeogenesis(PDC,ADH and LDH)were enhanced in roots of both materials following the imposition of stress,indicated that glycolysis/gluconeogenesis pathway played an important role in response to short-term waterlogging.In'Carl',the increase of ADH activity was more pronounced,whereas 'Purple Emperor' exhibited a significantly higher LDH activity.All these data suggested that the greater waterlogging tolerance of'Carl'depend on a combination of morpho-anatomical and physiological responses,such as formation of adventitious roots,stomatal opening,photosynthetic system,antioxidant enzyme defense system,glycolysis/gluconeogenesis pathway while 'Purple Emperor' regulated the same process with poor effect and large accumulation of the toxic fermentation by-products lactic acid is one of the main reasons for the significant difference between the two cultivars..Ethylene production was enhanced by waterlogging in both tested materials,appears to signal an acceleration of adventitius roots to alleviate hypoxia in tolerant cultivar.5.We compared the differences between the transcriptomes of two sedum cultivars,the waterlogging-tolerant cultivar 'Carl' and the waterlogging-sensitive cultivar 'Purple Emperor',by performing RNA-seq to elucidate the possible mechanism of waterlogging and reoxygenation.Our results showed that 168,413 unigenes were obtained and 131,531 unigenes were annotated,which was significantly enriched the current situation of Sedum.These unigenes were mainly classified into 52 functional groups and were enriched to gene ontology(GO)accessions of translation,protein folding,sorting and degradation,carbohydrate metabolism,and energy metabolism.These DEGs were riched to KEGG pathways of plant hormone signal transduction,photosynthesis,plant and pathogen interaction,carbon synthesis and energy maintenance which indicate that these pathways were the main response mechanism of Sedum under waterlogging stress.In summary,it is a complex multi-gene participation,and multiple biological processes co-work to regulate.