Evaluation On Heat And Drought Tolerance Of Marigold (Ta Getes) And Study On It's Growth And Physiological Characteristics

Marigold (Tagetes) which belongs to the Compositae family is usually used as a beddingplant and cut flower. It has important ornamental, economic and ecological value. Thoughmarigold plants have good adaptability, their planting regions have along dry and heat periodwhich certainly depress plant growth and cause a limitation to the production of the plants.The effects of heat stress (day/night38/21℃)or continuous soil drought stress on thedifference of tolerance were studied in9marigold cultivars. The tolerances of the9cultivarswere evaluated through multiple statistics analysis. On the basis of the results, Tagetes patulaL. cv. Gate Gold (strong resistance) and Tagetes patula L. cv. Durango (weak resistance)were selected as experimental materials to study their mechanism of growth and physiologicalresponse differences under heat and drought stress at different growth stage (seeding and earlyflowering stage). We try to explore the changes of growth and the mechanism ofphysiological characters (osmotic adjustment, water status, defensive enzymes andphotosynthesis) as well as anatomical structures in different tolerance of two marigoldcultivars after they suffered stress. In addition, the main indexes were selected by correlativeanalysis to evaluate the differcence of the two cultivars under heat and drought stress. It willprovide value reference to instruct marigold breeding of marigold tolerance in regions aridand semi-arid areas.The main results were as follows:1. The growth and physiological responses of9cultivars were differently responded toheat stress (38/21℃). Heat tolerance of9cultivars was comprehensively evaluated with themethod of multiple statistics analysis, LT50and heat index. According to the results,'Giant','Gate Gold' and 'Chokdee' were higher tolerance to38/21℃heat stress,'Discovery' and'Janie' were moderate resistant intensity to heat stress, while 'Bonanza','Little Hero','GreatHero' and 'Durango'were sensitive to heat stress.2. Water stress has different influence on the growth and physiological indexes in9marigold cultivars. The drought resistance of9cultivars was evaluated by multiple statisticsanalysis and drought index. The results of the resistance to drought stress as follow:'Janie', 'Gate Gold','Bonanza' and 'Chokdee' were high-tolerant cultivars,'Giant' was next,'GreatHero','Discovery','Little Hero' and 'Durango' were weaker in drought tolerance.3. From the leaf anatomic stucture, the ability of tolerance in 'Gate Gold' with smallleaves and stomata, stomatal density of lower epidermis, leaf thickness and cutin layerthickness, as well as stockade tissue thickness were stronger than in 'Durango'. After4days38/21℃heat treatment, growth of two cultivars were inhibited to various degrees at seedingand early flowering stage, especially the relative growth rate and plant biomass of 'Durango'at seeding stage was obviously declined comparing with control, while the changes weremuch slighter in 'Gate Gold'. In addition, the tightness of leaf tissue structure, cutin layerthickness and stockade tissue thickness were decreased highly in 'Durango' after4days heatstress, but were not influenced significantly in 'Gate Gold'.4. There were differences in physiological responses of two cultivars under38/21℃stress. The contentes of osmotic adjustment in marigold were stimulated. Pro contentaccumulation was higher in'Durango' than in 'Gate Gold'. However, SPC and SSC contentesin leaves and petals increased more rapidly in 'Gate Gold' than that in 'Durango'. Meanwhile,leaves of 'Gate Gold' maintained more relative water content than 'Durango' during heattreatment. Additionally, heat stress caused MDA, RC, O₂.-and H₂O₂increased in the twocultivars, especially in heat-sensitive cultivar 'Durango'. SOD, POD, CAT, APX, ASA andGSH were increased initially, and then decreased. However, some of index of 'Durango'were reduced during the heat treatment. On day4, the protective enzymes and non-enzymaticantioxidants (GSH and ASA) of the leaves and petals in 'Gate Gold' were higher than in'Durango'. In addition to that, photosynihetic pigment, net photosynthesis rate, stomatalconductance and water use efficiency declined in 'Durango' with heat stress, while therewere little changes in photosynihetic pigment, net photosynthesis rate and stomatalconductance, water use efficiency increased as well as transpiration rate decreasedsignificantly in 'Gate Gold'.5. The results showed that plant biomass of 'Durango' were lesser than control after6days water stress. Furthermore, stomatal density of two cultivars increased under droughtstress, especially in upper epidermis of 'Durango'. Additionally, the tightness of leaf tissue,the upper epidermis and lower epidermis thickness and stockade tissue thickness weredecreased highly in 'Durango' after4days water stress, but not significantly in 'Gate Gold'.6. Under drought stress, the change tendencies of physiological index were similar toheat stress. There were also striking differences in two cultivars. The raise rate, prolongedtime and activity peaks of SOD, POD, CAT and APX as well as ASA and GSH contents inleaves and petals of 'Gate Gold' were higher than those of 'Durango', while MDA, RC, O₂.- and H₂O₂of 'Gate Gold' were lower than those of 'Durango'. Additionally, netphotosynthesis rate, stomatal conductance and intercellular CO₂concentration decreased from0d to2d stress. However, net photosynthesis rate, stomatal conductance continoued to declinewith stress time, while intercellular CO₂concentration showed increase at later stages of waterstress (4-6d). Therefore, the decrease of photosynthesis in marigold may be linked to stomatallimitation under light water stress, while that may be attributed to nonstomatal limitationunder heavy stress.7. The leaves and petals of marigold responded differently when suffering stress. Theantioxidant enzymes in leaves were higher than in petals, but conversely, the AsA and GSHwere lower in leaves than in petals. Further, MDA, RC, O₂.-and H₂O₂in petals were muchhigher than those of leaves.8. The path coefficient analysis showed a significant negatively correlation betweenRWC, SOD, POD, CAT, APX, Car and heat index. However, there was significant positivecorrelation between Pro, H₂O₂, O₂.-, MDA, RC and heat index of the two cultivars understress. These indexes which higher coefficient may be applied as identification of toleranceindexes in Tagetes other cultivars.9. The contents of H₂O₂, O₂.-, MDA and RC showed increased remarkably in 'Durango'in all process of high temperature and drought stress, while the enzyme activities in 'Durango'decreased notably on day4and were lower than that of control. It was different in single heatstress or drought stress. On the contrary, antioxidant enzyme activities in 'Gate Gold' wereraised at first and then reduced but still higher than those of control.'Gate Gold' maydecompose the active oxygen effectively under heat and drought stress, and perform strongerstress resistant adaptability.10. The high-tolerant marigold cultivars included 'Chokdee' and 'Gate Gold', while'Great Hero','Little Hero' and 'Durango' were weaker in tolerance.'Chokdee' and 'GateGold' are recommended for ornamental landscapes that receive high temperature in growthseason.