| Normally the flag leaf which is developed later than all other leaves of wheat turns yellow and undergoes senescence at the latest,and that is called the sequential senescence.However,some wheat cultivars perform differently,as the flag leaf turns yellow and senesces earlier than the second leaf.Such process is named non-sequential senescence.During the development stage of grains,the senescence of the flag leaf which is closed to the spike is essential to wheat grain,because carbon and nitrogen nutrient sources in those aging leaves are once again removed and transported to the seeds.Study suggests the grain production in wheat of non-sequential senescence is higher than that in ones of sequential senescence.Hence,understanding the carbon and nitrogen metabolism in non-sequential senescence wheat is important for both plant physiological research and relative agricultural application.In this study,we put two non-sequential senescence wheat cultivars(“Wenmai19” and “Lankaoaizao8”)and two sequential senescence wheat cultivars(“Xinong88” and “NR9405”)under natural and drought conditions.Carbon and nitrogen metabolism related physical indexes and enzyme activities were measured to analyze the differences between these two types.Possible forming mechanism of non-sequential senescence and the response to drought condition of both non-sequential and sequential senescence wheat cultivars were discussed.The value of non-sequential senescence cultivars in wheat breeding was also evaluated.1.Under natural and drought conditions,the chlorophyll of the flag leaves in “Wenmai19” and “Lankaoaizao 8” were significantly lower than that of the second leaves at anthesis 25 d,indicating that the senescence of the flag leaves were much earlier than the second leaves,and they were non-sequential senescence wheat cultivars.However,the chlorophyll of the flag leaves in “Xinong88”and “NR9405” were significantly higher than that of the second leaves at the same sampling point,indicating that the flag leaves senesced later than the second leaves,and they were sequential senescence wheat cultivars.2.Under natural and drought conditions,the Pn(Net Photosynthesis Rate)in the flag leaves and the second leaves of non-sequential senescence wheat cultivars were higher than those of sequential senescence wheat cultivars,especially the Pn in the second leaves(P < 0.05).The Pn in the flag leaves and the second leaves of both non-sequential and sequential senescence wheat cultivars decreased in drought,but such decrease in the former was less than the latter.These results suggest the adaptation to drought in non-sequential senescence wheat cultivars was better than that in sequential senescence ones.3.Under natural and drought conditions,the soluble sugar accumulation in the flag leaves,the second leaves,the flag sheaths,the second sheaths,the peduncles and the penultimates of non-sequential senescence wheat cultivars were significantly higher than those of sequential senescence wheat cultivars.Given that a positive correlation was detected between tissue soluble sugar concentration and net photosynthesis rate(in leaves),the higher photosynthesis rate and soluble sugar concentration in tissues of the non-sequential senescence wheat cultivars could lead to larger amount of soluble sugar transportation from leaves,sheaths and stems to the grains,which could be advantageous in grain forming and drought resistance.4.Under natural and drought conditions,the sucrose content,SS(Sucrose Synthase)(synthesis direction)and SPS(Sucrose Phosphate Synthase)activity of the the flag leaves,the second leaves,the flag sheaths,the second sheaths,the peduncles and the penultimates in non-sequential senescence wheat cultivars were higher than those in the sequential senescence ones.Given that positive correlations were detected between the sucrose content and SS activity as well as the sucrose content and SPS activity in leaves,sheaths,and stems of wheat,the higher SS and SPS activity of leaves,sheaths and stems in non-sequential senescence wheat cultivars were beneficial to sucrose synthesis and accumulation,which may lead to more photo assimilates transportation to grains and result in higher grain and spike weight.5.Under natural and drought conditions,the leaves senesced rapidly after anthesis from 20 to 25 d,the SS(decomposition direction)activity in grains of non-sequential senescence wheat cultivars were significantly higher than that of sequential senescence ones,which might be beneficial for the non-sequential senescence wheat to produce more substance for grain forming.6.After anthesis from 0 to 40 d,the growth of spike weight and grains weight per spike in non-sequential senescence wheat cultivars were faster than thoes in sequential senescence ones.In mature period,the spike weight,the grains weight per spike and hundred-grain weight of non-sequential senescence wheat cultivars were significantly higher than those of sequential senescence ones.These results may be due to the higher photosynthetic capacity,more soluble sugar accumulation,stronger SS and SPS activity,and more translocation of photoassimilates in non-sequential senescence wheat cultivars.Moreover,the decrease in grains weight per spike and hundred-grain weight of non-sequential senescence wheat cultivars in drought condition were smaller than those of sequential senescence ones,which showed a relatively stronger drought resistance capacity of non-sequential senescence wheat cultivars.7.Under natural and drought conditions,the nitrogen content in the flag leaves and the second leaves of both non-sequential and sequential senescence wheat cultivars decreased gradually since 6 d after anthesis.From anthesis to harvest stage,the nitrogen content of the flag leaves were significantly higher than that of the second leaves in the sequential senescence wheat cultivars.However,the nitrogen content in the flag leaves of non-sequential senescence wheat cultivars were significantly lower than that in the second leaves after anthesis 24 d.8.Under natural or drought condition,the activity of GS(Glutamine Synthetase)and GOGAT(Glutamate Synthase)in the flag leaves of sequential senescence wheat cultivars were significantly higher than those in the second leaves from anthesis to maturity,indicating the flag leaves were the main source of amino acid and the dominating supplier of nitrogen to grains.However,the activity of GS in the second leaves of non-sequential senescence wheat cultivars was either approximated to or higher than that in the flag leaves;the GOGAT activity of the second leaves was lower than that of the flag leaves before non-sequential senescing,but then was higher after non-sequential senescing.These results suggest that the flag leaves and the second leaves both played an important role in amino acid synthesis and nitrogen transportation in non-sequential senescence wheat.9.Under natural and drought conditions,the activity of EP(Endopeptidase)in the flag leaves and second leaves showed a firstly increased then decreased change from anthesis to harvest stage.Drought aggravated the increase and decrease of EP activity,and facilitated the degradation and transportation of protein.In grain filling stage,the EP activity in the second leaves of the sequential senescence wheat cultivars were higher than that in the flag leaves,indicating that the degradation of protein in the second leaves were faster than that in the flag leaves;after anthesis 18 d,the EP activity in the flag leaves of the non-sequential senescence wheat cultivars were higher than that in the second leaves,indicating that the degradation of protein in the flag leaves was faster than that in the second leaves.During the wheat senescence,the EP activity in the flag leaves of the non-sequential senescence wheat cultivars was higher than that of sequential senescence wheat cultivars,which may led to more nitrogen transportation from leaves to grains.10.Under natural and drought conditions,the nitrogen content in grain per spike and per grain increased with the continuous growth of wheat.Under natural condition,there was no significant difference between non-sequential senescence and sequential senescence wheat cultivars in the nitrogen content of per grain,while the nitrogen content in grain per spike of the non-sequential senescence wheat cultivars was significantly higher than that of sequential senescence wheat cultivars at later filling stage.Under drought condition,the nitrogen content in grain per spike and per grain of the non-sequential senescence wheat cultivars were significantly higher than that of sequential senescence wheat cultivars at later filling stage.In conclusion,the photosynthetic capacity of the flag leaves and the second leaves,and the soluble sugar accumulation in leaves,sheaths and stems in non-sequential senescence wheat cultivars were higher than those in sequential senescence wheat,which might be the cause of non-sequential senescence.The flag leaves senesced earlier than the second leaves in non-sequential senescence wheat,which is beneficial for sugar transportation from leaves,sheaths and stems to the grains and is helpful for later grain development.Furthermore,the high accumulative soluble sugar in leaves,sheaths and stems of non-sequential senescence wheat could help enhance its drought resistant capacity,hence reduce the negative impact of drought induced decrease in grain weight.The flag leaves serves as the key source of amino acid synthesis after anthesis in sequential senescence wheat cultivars.However,both the flag leaves and the second leaves played important roles in the same process in non-sequential senescence wheat.In addition,the EP activity of leaves in the non-sequential senescence wheat cultivars was higher than that in the sequential senescence wheat during leaf senescence period.Those abovementioned nitrogen metabolism characteristics could facilitate nitrogen transportation to grains,thusly improve the grain quality.Therefore,the non-sequential senescence wheat cultivars were useful and possessed practical value in stable,high yield,high quality and drought resistant wheat breeding. |
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