Metabolomics study of Leymus chinensis
The CP content of forage is often different due to different species, harvest stage and other factors[12]. According to the chemical properties of forage, CP are mainly divided into two types[13]: true protein nitrogen and non-protein nitrogen. Non-protein nitrogen is mainly composed of free amino acids, amides, purines, pyrimidines and alkaloids, accounting for about 30% of the total nitrogen content of forage[14].Protein composition in foragemainly includes 20 amino acids, such as alanine, lysine, glutamic acid and aspartic acid. Amino acids, as the components of proteins, play a very important role in the nitrogen metabolism pathway in forage, and they are the preconditions for the composition of amino acids, purines and pyrimidines[15]. Studies have shown that glutamate plays a significant or extremely significant role in protein synthesis in wheat[16]. Study[17] showed that the content of glutamate in alfalfa in the early flowering stage was significantly higher than that in the mid-flowering stage(P<0.05), and the L-glutamic acid was the main aimo acid which affect the CP changed. The above studies indicate that amino acids are related to protein synthesis, and the amount of amino acid content will affect the amount of protein synthesis, according to the results of this study alanine content in jointing stage is lower than that in solid stage. In addition, the levels of purine and pyrimidineare was lower in solid stage. Alanine, purines and pyrimidines content was reduced before the protein synthesis, which could lead to protein synthesis decreased, which was the reason of CP content in solid stage is lower than the jointing stage. The content of phenylalanine increased, but the protein content did not increase in the related metabolic pathway[18]. It can be considered that phenylalanine mainly comes from the hydrolysis of protein, which may also be one of the reasons for the decrease of protein content. Ttyrosine, after hydroxylation of phenylalanine, is involved in the process of glucose metabolism of plants was together with phenylalanine to promote the synthesis of carbohydrate[19]. The results of this study showed that the expression levels of sugars and other substances in solid stage were significantly up-regulated. Forage signaling figure of this study show that cysteine metabolic pathways in the generated EIN3, under the action of EBF1/2 and u, products are mainly used for ubiquitin protein degradation mediated, this is the only one in the plant in the metabolic pathway of protein degradation pathway, which may be CP in Leymus chinensis in jointing stage processed to the solid is the main reason, which will be our main research direction in the future.
The growth and development process of Leymus chinensis is mainly dominated by amino acid metabolism[20]. Amino acid, as the main mode of existence and transportation of nitrogen compounds in Leymus chinensis, is also regarded as the principle of maintaining the balance of nitrogen and the synthesis of enzymes and hormones in vivo[21]. The nutritional quality of Leymus chinensis relies mainly on the protein content, amino acid composition and its equilibrium state, amino acid content that in the condition with higher and lower which will affect the amount of protein synthesis, and the essential amino acid was considered as one of Leymus chinensis nutrition qualitative decision factors, on how to improve protein content at the same time, and the amino acid related research is a top priority in the current scholars research [22–24]. Studies have shown that the improvement of crop quality cannot rely solely on the simple accumulation of protein and amino acid content, but it should be comprehensively evaluated in combination with the yield of crops, especially the proportion of essential amino acids[25]. In the nitrogen metabolism pathway, the first is the assimilation of ammonia, followed by the absorption of amino acid nitrogen to participate in the nitrogen metabolism[26]. The assimilation of ammonia first forms glutamine and glutamic acid, and then forms other amino acids and proteins[27]. Glutamic acid plays an important role in the nitrogen metabolism pathway, because glutamic acid is the initial precursor of protein synthesis, which is mainly co-synthesized by alpha-ketoglutaric acid and amino acid transaminase[28]. In this study, the reduced content of glutamic acid and protein synthesis precursor decreased, which reduces the amount of protein synthesis in Leymus chinensis, performance for protein content is reduced, the main reason is that glutamate content directly to reduce the influence to the Leymus chinensis metabolism, influenced the late protein synthesis, and made an impact on the quality of the Leymus chinensis.
Nutrition changes
Changes in nutrition (CP,ADF and NDF) contents changes
In this study, harvest time affected CP of Leymus chinensis. Significant (P< 0.05), CP of Leymus chinensis decreased with the delay of harvest time. The decrease of CP in Leymus chinensismay was related to its growth and development, with the delay of harvest time, the ratio of stem and leaf of Leymus chinensis increased, while a large amount of CP in Leymus chinensis is stored in leaves, so the CP decreases. On the other hand, harvest time delayed will cause Leymus chinensis consume a large amount of CP for its own growth and the formation of awn needles, leading to the reduction of CP[29]. The content of ADF and NDF has an important influence on its nutritional quality, among which ADF affects the digestibility[30]. NDF affects grazing rate of livestock, while high NDF content leads to low grazing rate[31]. In this study, harvest stage had significant effect on ADF and NDF of Leymus chinensis (P< 0.05). The content of ADF and NDF in Leymus chinensis increased with the delay of harvest stage, which may lead to the decrease of quality of Leymus chinensis with the delay of harvest stage.
In recent years, the proper harvesting of forage has been paid more and more attention by researchers at home and abroad. At present, the researches mainly focuses on the influence of harvest time on grass yield, quality and regeneration rate [32]. Some scholars pointed out that the proper harvest of forage should meet the following basic conditions[33]: first, the harvest stage should be conducive to the growth of forage; Secondly, it is necessary to maintain high nutritional quality of forage after harvest. Finally, harvesting is beneficial to the enrichment of nutrients in the roots of forage, so as to ensure the safe wintering of forage. Some scholars found that dry matter yield increased by 18% when alfalfa was cut at the peak of flowering, but dry matter digestibility and CP content decreased by 5.4% and 8.0% respectively[34]. With the prolongation of alfalfa growth period, the grass yield and dry matter content have been increasing, but the nutrients show a change trend of first increasing and then decreasing, and the budding stage is the critical point for the rise and fall of nutrients in forage[35]. When the forage was harvested at the flowering stage, the CP content remained at the highest level, and a higher yield could be obtained[36]. The above research results indicate that the harvest stage has an important influence on the preservation of forage’s nutritional quality. During the whole growth period of native grass, with the continuous extension of the growth period of forage, the forage yield gradually increases, but the quality gradually decreases. The longer the harvest period, the higher fiber. Therefore, grass yield and nutrient quality should be taken into account in the harvest of forage. Harvest stage has a great impact on the yield, quality and rejuvenation of forage, and proper harvest can ensure the yield and nutritional value of forage.
Establishment of forage nutrition evaluation methods
At present, the CP, NDF and ADF are the common indicators to evaluate nutritive value while the CP is the main index, but the studies about the quality evaluation of forage from the level of metabolism has not mentioned. Due to the physiological characteristics of forage heterogeneity, the different harvest stage will cause the different CP. In order to get a higher CP, we need to choose the suitable harvest stage[37]. CP was mainly expressed on the physiological and biochemical reaction synthesis of amino acids, and alanine as an important precursor of protein synthesis, which has important influence on the formation of a protein, and we can say alanine content on the forage quality has important influence, and this experiment showed alanine was important on the influence of CP, to explained the amino acids from molecular level for the importance of forage quality evaluation[38]. Therefore, based on the analysis results of this experiment, it is necessary to continue to clarify the changes of forage quality through amino acid molecules such as glutamic acid, strengthen the research on the relationship between protein synthesis precursor and forage quality, and discuss the evaluation index of hay quality from the molecular level.