Ecophysiology Adaptation Of Dominant Grasses And Their Response To Simulated Climatic Changes In The Songnen Grassland Of China

Songnen grassland known as the transition region of dry and wet area in China is one ofthe most sensitive zones to global climate changes. In this work we tested theecophysiological adaptations characteristic of dominant plants grown on the naturalgrasslands and expose to the simulated climate changes including CO₂ concentration, watercondition and temperature to evaluate the simultaneous effects of environmental changes toplants. We tended to explore the initial response signal of plants to the global climate changesbased on the results of three levels including modular, individual and community respectively.It is of major predictive importance.1. Researches on the rate and traits of carbon allocation in different dominat grasses approvedthat: (1) A two-humped tendency of net photosynthetic rate (P_N) is a universal phenomena inboth individual and community levels. The noon-break of P_N due to the non-stomatallimitation resulted form simultaneous effects of intense light, high temperature and lowrelative humidity at noon.The numerical differences of P_N characteristic curve mainlydepended on plant photosynthetic pathway types, and the characteristic differences dependedon plants habitat in stead. The diurnal variation in P_N was more fluctuant in the individuallevel than that in community, which explored a buffer againest to environmental changes. (2)Responses of P_N to increasing PAR all show the saturated exponential tencency in differentspecies or different modulars within species. Photosynthesis was down-regulated in both planttypes in response to intense light. C₄ plants were more benefit from light utilization both inrate and scope. Convergent adaptation appeared in plants growth in the similar habitat. (3)Plants photosynthesis can be promoted both in rate and in scope of light utilization. C₃ plantswas more profitted in P_N than C₄ and the difference was rather in photosynthetic pathwaytypes than in water ecotype species。(4) Significant interactions were found between doublingCO₂ concentrations and droughts for P_N (P<0.05). Elevated CO₂ can make up the inhibition ofdrought to P_N in a certain range of dought (0-7.5%PEG), however, the drought weaken theacceleration of CO₂ doubling. (5) CO₂ doubling can enhanced the optimum temperature forphotosynthesis with adequate moisture, and it was on the contrary with drought stress, whichwould intensify the inhabitation from high temperature and drought.2. Researches on the rate and traits of carbon release in different dominat grasses approvedthat: (1) Dark respiraton rate (Rd) of Stipa baicalensis was higher at vegetative tillers than atrespective tillers;Rd of C₄ plant was much higher than C₃ plants;the weakest Rd was found inmesophyte in all the water ecotype species. (2) The response of Rd to CO₂ doubling was moredifferent in photosynthetic types than in water ecotype species. Rd of C4 plants decreased withCO2 doubling, whereas C3 plants were independent of it due to the larger plasticity. Thevariation of CO2 compensation point consisted with the ratio of light respiration rate (Rl) tocarboxylation efficiency instead of Rl. (3) Rd of Stipa baicalensis was strengthened at feeblyarid and inhibited avobe drought as 4.84±1.00% PEG concentration. Elevated pick up theinhabitation of drought to S. baicalensis.With the increase of drought, the growth demand ofplant enhanced in terms of the ratio of Rl to PN. (4) Dark respiration process of plant isstrongly influenced by temperature, and it rised linearly with the temperature enhancing. Thepromotation of temperature to Rd was reduced by CO2 doubling.3. Researches on the rate and traits of water use efficiency (WUE) in different dominatgrasses approved that: (1) The WUE of stipa baicalensis was in accordance with the order ofthe backside of flag leaf (RTF-), the leaf of vegetative tiller (VT), the leaf from lower half ofreproductive tiller (RTL) and the face side of flage leaf (RTF+). The higher WUE of RTF-was minly result from the lower E and that of VT due to higher PN;C4 plants were better atwater use than C3 plants, and the mesophyte have the highest WUE in different water ecotypespecies. WUE was highest at the earlier growth season with high PN and low E;in contrast,WUE was lowest at the end of growth season with low PN and high E. WUE was a trade-offbetween the carbon allocation and the water loss. (2) Elevated CO2 can result in the increaseof WUE, but the methods to realize it were different between species. WUE of C3 increased asa result of PN enhanceing and that of C4 plant was due to the reduction of water loss. (3) WUEof Stipa baicalensis was promoted at feebly arid and inhibited avobe drought as 2.85±0.96%PEG concentration;the critical point of drought resistance to S. baicalensis was 7.5%PEG,above which S. baicalensis approached to the tolerance limit of drgought. (4) Elevated CO2could help to buffer the drought stress to WUE. The promotation of elevated CO2 to WUEwas strengthened at feebly arid and inhibited at severe drought. (5) Elevated CO2 doublingenhanced the optimum temperature for WUE, which was in favor of plants drought tolerance.Significant interaction was found between temperature and drought for water pressure.4. Stomata served to illustrate the plants sensitity to environmental changes. Apart from thedirect inducement, light also bring two important negative feedback loops regulating stomatalconductance for CO2 and water. (1) Light was the key limiting factor for RTL;Stomata of VTwas mostly regulated by CO2 feedback system , so as to the plants growth in the forest border,and that of RTF regulated by H2O system;which is also the major factor for meadow plainplants by closing stomata. Stomata of C3 plants were sensitive to light and that of C4 plantsmainly regulated by the two feedback system. The action of CO2 feedback system contributedmore effect on stomata behaviors than that of H2O system.For hygrophyte and xerophyte, themain trigger for opening stomata is the direct effect of light, wherease, mesophyte were moreaffected by the two feedback system. (2) The major control for stomata was CO2 feedbacksystem with CO2 increasing, and that was H2O feedback system under drought stress. Thefinal actions of stomata was decided by the trade-off between two sustem with temperaturerising.Survival pressure is the determinant for stomatal behavior, which is always adapt to theambient condition and the function of plants. Stomatal behaviors were always more affectedby the CO2 feedback regulation and that of H2O system can only act well as the severedrought occurred.