Studies On The Aegiceras Corniculatum Seedlings In Response To Simulated Tidal Flooding Stress

Mangrove forests are one of the most productive ecosystems in the world. They enrichcoastal waters, yield commercial forest products, protect coastlines, and support coastalfisheries. Mangroves are woody plants that grow at the interface between land and sea intropical and sub-tropical latitudes where they are well adapted to deal with natural stressors(e.g., salinity, temperature, anoxia, muddy and strong wind). However, because theenvironmental conditions mangroves live in are close to their tolerance limits, mangroves maybe particularly sensitive to tide. Tide that gives rise to coastal sediment flooding, or morecomplete submergence, results in oxygen deprivation for plants. Tide depth and duration mayplay an important role to the zonation pattern of mangroves.Aegiceras corniculatum, one of mangrove species, is tolerant to flooding. In order toreveal the response of flooding stress of A. corniculatum and to propose the optimum floodingtime, seedlings of A. corniculatum were planted in tanks with simulated semidiurnal (twicedaily) tide for3months to assess their responses to flooding stress. Effects of flooding stresson growth and physiological changes of seedlings of A. corniculatum along a gradient offlooding time from2h to8h were examined. Addition to the experimental study, fieldobservation was established to assess the growth pattern of A. corniculatum with8year age indifferent tide height. The results were showed as following.1.The growth and biomass were significantly influenced by the flooding duration. Theseedlings obtained highest biomass, leaf area, leaf quantity, shoot elongation with6h treatment.With the increase in the duration of tide submergence, the proportion of root biomass andbiomass ratios of root/shoot decreased, resulting in the biomass movement from root to shoot.The optimal time for the growth of seedlings was6h for the treatment. In conclusion, similarlylike other flood-tolerant plants, shoot elongation and biomass allocation of seedling two ofpositive adaption under flooded condition. 2.Flooding increased water use efficiency of seedlings of A. corniculatum whichcontributed to carbohydrate accumulation and mineral nutrients absorption. Flooding stress hassignificant effect on mineral nutrients in seedlings. Flooding increased leaf nutrient contents ofN, K and those of P, K, Na, and Fe in root, while flooding decreased P, Ca, Fe, Mn, and Cuuptake in leaf and N, Ca, Mg, Mn, and Cu absorption in root.3.Maximum photosynthesis(Pn), and stomatal conductance(Gs), intercellular CO2concentration(Ci), transpiration rate(Tr), water use efficiency(WUE) of seedling of A.corniculatum had great correlation with flooding duration. Pn, Gs, Ci and Tr increasedsignificantly from2to6h for tide submergence, while these indices were inhibited whenflooding time prolonged to8h. Compared with other treatments, the6h treatment reached thehighest Pn, Gs, Ci and Tr, suggesting the optimal time of simulated tidal submergence wasabout6h in every tide period. With the prolongation of flooding time, contents of chlorophyll a,chlorophyll b and total chlorophyll increased significantly, but chlorophyll a/chlorophyll bdecreased. Response of non-structural carbohydrate to the flooding stress was examined. Thesoluble sugar content in leaves showed a significant decrease in3months treatment. There wasno great difference in starch content in leaf among the gradient of duration of2h to6htreatment. But it decreased sharply in the8h treatment, showing that the seedlings wereadapted to the flooding environment in an economical way. It was supposed that the supply ofcarbohydrate would not enough and the sugar would be depleted in the longer flooding time.4.The effect of stress on the nitrogen metabolism of A. corniculatum seedlings wasstudied. Nitrate reductase activity was related to the stress. Under2-6h inundation, nitratereductase activity was positive correlated to the nitrate nitrogen content in leaves. However, the8h flooding for3months caused reductions in accumulation of nitrogen, nitrate nitrogencontent, ammonium nitrogen content but a increase in nitrate reductase activity. Compared withother treatments, the6h treatment obtained the highest proline content in leaves and roots. Withthe prolongation of flooding time, proline content decreased, which indicated that long termflooding may caused serious depression in seedlings. 5.Tannin is a kind of secondary metabolism production. In this study, the relation oftannin and nutrients in seedling of A. corniculatum was discussed. Both total phenolics (TP)and extractable condensed tannins (ECT) contents decreased with prolonged flooding time.This pattern lends to support source-sink hypotheses such as the carbon-nutrient balance (CNB)hypothesis. There was a significant positive correlation between TP content and ratios of C/N,which demonstrated CNB hypothesis. Based on ratios of N/P (below14), was A. corniculatumN-limited. Under the flooding stress, TP contents were negatively related to ratios of N/P.6.Principal component analysis of29growth and physiological indices was to reveal theoptimal treatment in this study. Two principal components cumulative variance87.17%wereextracted. According to the values of principal components in every treatment, flooding for6hin every tide cycle is optimal for seedling of A. corniculatum.7.In a field study, the growth of A. corniculatum in tide height in Quanzhou Bay wasinvestigated. With the tide height increased, the height, diameter of breast height(DBH),crownand biomass of8-year-old A. corniculatum decreased significantly. There was no greatdifference of C,N,P contents in leaves of A. corniculatum in tide heights. The tide height of1.7m and flooding time of6-7h in every tide cycle were optimal to the growth of A.corniculatum, which coincided with that of controlled experiment in the greenhouse.