| Alfalfa(Medicago sativa L.) is usually harvested 3~6 times annually. The biomass yields of first cut in the spring are generally highest in practice. The yields are then declining in the following cuts. This seasonal reduction in alfalfa biomass yields could be associated with prevailing changes in environmental factors such as rainfall and heat stress. And heat stress maybe is one of the main reasons reduce summer yields of alfalfa. The purpose of this study is to investigate the variation in cut-to-cut biomass yield of alfalfa grown under suitable conditions and the effects of heat stress on the yield and morphology of alfalfa, and develop a fast and accurate screening method for alfalfa heat tolerance evaluation.1.In order to clarify the reason of reduction in alfalfa summer yield, 8 alfalfa varieties were grown under the controlled greenhouse conditions with suitable temperature, light, water and nutrient supply to determine the driving force in cut-to-cut biomass yield variations of alfalfa genotypes. Another 4 alfalfa varieties were chosen to repeat the experiment in the second year. Our results showed that as the alfalfa plants develop, its biomass yield increased from cut 1 to cut 3. The yields after cut 3 were stabilized and each cut yielded similarly. Without high temperature, water deficit and nutrient limitations, biomass yields of alfalfa remained similar or even increased from the first to last cuts. Our data implies that decline in biomass yield after cut 2 under field conditions was unlikely due to the biological characterization of the crop itself, but the influence of environmental factors. Dry matter yield was positively correlated with plant height, leaf weight.2.In order to study the effects of high temperature on yield and morphology in alfalfa,6 alfalfa varieties were grown in growth chamber with different day/night temperatures of 25/20 ℃ and 35/30 ℃. The results showed that heat stress decreased plant height, yield and single leaf area, the yield decreased 54% and the leaf thickness increased 54%. The data implied that heat stress is one of the main reasons reduce summer yields and alfalfa is heat sensitive, it can adapt to heat stress through a series of morphological changes.3. In order to develop a fast and accurate screening method for alfalfa heat tolerance evaluation. A controlled growth chamber study was conducted to assess if some easily measurable traits such as chlorophyll fluorescence(F0, Fv/m), leaf chlorophyll content(SPAD value), relative water content(RWC), leaf weight(LW) and soluble sugar content(SSC) could be used as sensitive indicators for heat-tolerance of alfalfa varieties. In this study, 58 varieties were planted in an environment-controlled greenhouse. After an initial 30-d growth at 20/15 ℃ day/night temperature regime, the plants were moved to a growth chamber with 20/15℃ for one week, and then exposed to different levels of heat stress, 25/20℃, 30/25℃, 35/30℃, 40/35℃ and 43/40℃ for another week prior to the determinations of F0, Fv/m, SPAD, RWC, LW and SSC. Our data showed that as increasing temperature, leaves of all alfalfa plants increased F0, SPAD and SSC, while Fv/m, RWC and LW were decreased significantly. It appeared that heat tolerance of alfalfa is an integrated complex trait, it could not be characterized consistently by just one of these indicators. RWC is not suitable be used for heat tolerance evaluation. But, we can get more accurate result by F0, Fv/m, SSC, SPAD, LW and Yield. In general, biomass yield was positively correlated with SPAD or with LW under both normal and high temperature conditions. These parameters thus could be used as indicators for screening alfalfa genotypes for heat tolerance in normal temperature, and it is likely no need to test them under high temperature regimes. |
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