Alfalfa(Medicago sativa L.) and Guyana stylosanthes(Stylosanthes guianensis(Aubl.) Sw.) are two important forage legumes with high feeding quality.Alfalfa is cold-tolerant,but heat-sensitive,while Guyana stylosanthes ...Alfalfa(Medicago sativa L.) and Guyana stylosanthes(Stylosanthes guianensis(Aubl.) Sw.) are two important forage legumes with high feeding quality.Alfalfa is cold-tolerant,but heat-sensitive,while Guyana stylosanthes is heat-tolerant,but cold-sensitive.In order to explore the adaptation mechanisms to temperature stress,the physiological responses of sugars metabolism,photosynthesis,and nitrogen reduction of these two forage legumes under low and high temperature stresses were comparatively studied by this paper.The seedlings of alfalfa(M.sativa cv.Longdong) and Guyana stylosanthes(S.guianensis cv.CIANT 184) were grown in glasshouse with temperature 20 to 30℃ under natural light condition.Six to seven-week seedlings of these two legumes were divided into three groups and transferred to growth chambers with three temperature treatments,respectively.The first group was subjected to temperature at 5 to 6℃ as cold treatment,the second group at 35 to 36℃ as heat treatment,and the third group at 25℃ as control.After a given time of treatment, the physiological indices were measured and the experimental results were summarized in the following sections.1 Sugars metabolism in response to temperature stressCompared to seedlings under room temperature,significant decrease of relative water content and increase of MDA content and ion leakage in alfalfa leaves occurred after 2 days of heat stress,while no significant changes were detected for those under cold stress for 5 days.The results indicate that alfalfa was easily injured by heat stress but tolerant to cold stress.On the contrary,Guyana stylosanthes maintained relatively high relative water content,low ion leakage,and low MDA content after 5 days of heat stress,while the MDA content and ion leakage were significantly increased under cold stress,indicating that Guyana stylosanthes was tolerant to heat stress,but sensitive to cold stress.The contents of total soluble sugars and sucrose and the activities of sucrose phosphate synthase(SPS) and sucrose synthase(SS) were enhanced in both legumes after cold or heat stress treatments in comparison to control,but more enhancements were detected under cold stress than heat stress.Cold stress had a more influence on sugars accumulation than heat stress.Starch contents decreased in both legumes after cold or heat stress with highest content under control and lowest under cold stress.2 Chlorophyll fluorescence parameters and photosynthetic rates in response to temperature stressPhotosynthetic rates in both legume forages decreased after heat or cold stress, but the responses were different between the two legumes.The photosynthetic rate of alfalfa decreased more under heat stress than cold stress,while it decreased more under cold stress than heat stress in Guyana stylosanthes.The chlorophyll fluorescence parameters(Fv/Fm,ФPSⅡ and qP) in alfalfa decreased greatly after heat stress,but decreased slightly under cold stress.In contrast,these parameters decreased greatly under cold stress and were not affected under heat stress in Guyana stylosanthes.The results indicate that photosystemⅡ activity in alfalfa was more sensitive to heat stress than Guyana stylosanthes,while that in Guyana stylosanthes was more sensitive to cold stress than alfalfa.ATP synthase activity and ATP content in chloroplasts of alfalfa were greatly decreased under heat stress,while those were little affected under cold stress.ATP content was also decreased in Guyana stylosanthes under cold stress,but it was not affected under heat stress.RuBPcase and FBPase activities in both legumes decreased under both heat and cold stress with lower activities in alfalfa and higher activities in Guyana stylosanthes under heat stress than cold stress,respectively.3 N assimilation in response to temperature stressActivities of nitrate reductase(NR),nitrite reductase(NiR),glutamine synthase(GS),and glutamate synthetase(GOGAT) were greatly decreased in leaves of alfalfa under heat stress,while they were little affected under cold stress.On the contrary,they were greatly decreased in Guyana stylosanthes under cold stress but little affected under heat stress.Temperature stresses induced the accumulation of NO-3-N and NH+4-N in leaves of both legumes,and higher levels were accumulated under heat stress in comparison to cold stress.The decreased activities of NR and NiR caused by temperature stresses resulted in the accumulation of NO3——N and NH4+-N in leaves. In summary,alfalfa exhibited tolerant to cold stress but sensitive to heat stress,while Guyana stylosanthes was tolerant to heat but sensitive to cold stress.Temperature stresses resulted in the injury of photosyntem Ⅱ and therefore inhibited photophosphorylation,which led to the limits of ATP supply in leaves,decreased photosynthesis,and N assimilation.展开更多
文摘Alfalfa(Medicago sativa L.) and Guyana stylosanthes(Stylosanthes guianensis(Aubl.) Sw.) are two important forage legumes with high feeding quality.Alfalfa is cold-tolerant,but heat-sensitive,while Guyana stylosanthes is heat-tolerant,but cold-sensitive.In order to explore the adaptation mechanisms to temperature stress,the physiological responses of sugars metabolism,photosynthesis,and nitrogen reduction of these two forage legumes under low and high temperature stresses were comparatively studied by this paper.The seedlings of alfalfa(M.sativa cv.Longdong) and Guyana stylosanthes(S.guianensis cv.CIANT 184) were grown in glasshouse with temperature 20 to 30℃ under natural light condition.Six to seven-week seedlings of these two legumes were divided into three groups and transferred to growth chambers with three temperature treatments,respectively.The first group was subjected to temperature at 5 to 6℃ as cold treatment,the second group at 35 to 36℃ as heat treatment,and the third group at 25℃ as control.After a given time of treatment, the physiological indices were measured and the experimental results were summarized in the following sections.1 Sugars metabolism in response to temperature stressCompared to seedlings under room temperature,significant decrease of relative water content and increase of MDA content and ion leakage in alfalfa leaves occurred after 2 days of heat stress,while no significant changes were detected for those under cold stress for 5 days.The results indicate that alfalfa was easily injured by heat stress but tolerant to cold stress.On the contrary,Guyana stylosanthes maintained relatively high relative water content,low ion leakage,and low MDA content after 5 days of heat stress,while the MDA content and ion leakage were significantly increased under cold stress,indicating that Guyana stylosanthes was tolerant to heat stress,but sensitive to cold stress.The contents of total soluble sugars and sucrose and the activities of sucrose phosphate synthase(SPS) and sucrose synthase(SS) were enhanced in both legumes after cold or heat stress treatments in comparison to control,but more enhancements were detected under cold stress than heat stress.Cold stress had a more influence on sugars accumulation than heat stress.Starch contents decreased in both legumes after cold or heat stress with highest content under control and lowest under cold stress.2 Chlorophyll fluorescence parameters and photosynthetic rates in response to temperature stressPhotosynthetic rates in both legume forages decreased after heat or cold stress, but the responses were different between the two legumes.The photosynthetic rate of alfalfa decreased more under heat stress than cold stress,while it decreased more under cold stress than heat stress in Guyana stylosanthes.The chlorophyll fluorescence parameters(Fv/Fm,ФPSⅡ and qP) in alfalfa decreased greatly after heat stress,but decreased slightly under cold stress.In contrast,these parameters decreased greatly under cold stress and were not affected under heat stress in Guyana stylosanthes.The results indicate that photosystemⅡ activity in alfalfa was more sensitive to heat stress than Guyana stylosanthes,while that in Guyana stylosanthes was more sensitive to cold stress than alfalfa.ATP synthase activity and ATP content in chloroplasts of alfalfa were greatly decreased under heat stress,while those were little affected under cold stress.ATP content was also decreased in Guyana stylosanthes under cold stress,but it was not affected under heat stress.RuBPcase and FBPase activities in both legumes decreased under both heat and cold stress with lower activities in alfalfa and higher activities in Guyana stylosanthes under heat stress than cold stress,respectively.3 N assimilation in response to temperature stressActivities of nitrate reductase(NR),nitrite reductase(NiR),glutamine synthase(GS),and glutamate synthetase(GOGAT) were greatly decreased in leaves of alfalfa under heat stress,while they were little affected under cold stress.On the contrary,they were greatly decreased in Guyana stylosanthes under cold stress but little affected under heat stress.Temperature stresses induced the accumulation of NO-3-N and NH+4-N in leaves of both legumes,and higher levels were accumulated under heat stress in comparison to cold stress.The decreased activities of NR and NiR caused by temperature stresses resulted in the accumulation of NO3——N and NH4+-N in leaves. In summary,alfalfa exhibited tolerant to cold stress but sensitive to heat stress,while Guyana stylosanthes was tolerant to heat but sensitive to cold stress.Temperature stresses resulted in the injury of photosyntem Ⅱ and therefore inhibited photophosphorylation,which led to the limits of ATP supply in leaves,decreased photosynthesis,and N assimilation.
