Adaptive Variations For Nitrogen Deficiency In Photosynthetic Characteristics,Ion Balance And Nitrogen Metabolism In Young And Old Leaves Of Different Ecotypes Wild Soybean

It is a hot issue in botany and agronomy to improve cultivation crops using existing wild plant resources.Nitrogen（N）is the major factor of plant growth and development.In order to meet the needs of crops,there is an overuse of nitrogen fertilizer in agricultural production.In order to solve this problem and ensure the nitrogen demand of crops,it is urgent to improve the ability of crops to tolerate low nitrogen under low nitrogen（LN）conditions.Wild soybean is not only the wild ancestor of cultivated soybean,but also an excellent wild germplasm resource with rich stress resistance.The research on stress physiology of wild soybean will provide an important theoretical basis for breeding,utilization and protection of wild soybean.In this study,two wild soybean genotypes were used as experimental materials.Three LN stress environments with different intensities were set up.By measuring the growth parameters,total nitrogen and carbon content of young and old leaves,photosynthetic parameters of young and old leaves,ion content and nitrogen metabolism enzyme activities of young and old leaves,the changes of common wild soybean and LN stress-tolerant wild soybean under LN stress were compared and analyzed,in order to reveal the physiological adaptation mechanism of LN stress-tolerant wild soybean to low nitrogen stress.The results showed that:LN stress inhibited the growth and total N content in both genotypes,and common wild soybean was inhibited to a greater extent.In addition,under the high intensity of LN stress,the root length in common wild soybean and LN stress-tolerant wild soybean inceased by 0.74 and0.86,respectively（p<0.01）.Under different intensities LN stress,the young leaves of LN stress-tolerant wild soybean maintained a stable net photosynthetic rate（p _N）and chlorophyll content,especially under high intensity LN stress.Compared with the control,the p _N and chl（a+b）contents in young leaves of common wild soybean were decreased by 0.64 and 0.45,and these were decreased by 0.96 and 0.74 in the old leaves;the p _N and chl（a+b）contents in young leaves of LN stress-tolerant wild soybean were decreased by 0.26 and 0.16,and these were decreased by 0.74 and 0.60 in the old leaves.Meanwhile,carotenoids（Car）were also accumulated in the young leaves,under medium and high intensities LN stress,and Car content increased by 0.60and 0.40 compared with the control（p<0.05）,respectively.The results showed that the young leaves of LN tolerant wild soybean maintained stable photosynthetic function under LN stress.However,the old leaves,in with photosynthetic capacity was significantly inhibited,migrated beneficial ions such as Mg²⁺and K⁺to young leaves by accelerating self consumption.Through the transport of beneficial ions,the nutrition and osmotic balance of young leaves were regulated,and the normal metabolic activities of young tissues were maintained.Compared with control groups,under three intensities LN stress,glutamate synthase（GOGAT）activities in old leaves of LN tolerant wild soybean increased by 0.72,0.20 and 0.38（p>0.05）,respectively;and glutamate dehydrogenase（GDH）activities in old leaves of LN tolerant wild soybean increased by 1.56,0.70 and 0.72（p<0.01）,respectively.While the activity of glutamic acid synthase（GS）in young leaves of LN tolerant wild soybean was higher than that in control group,and the aspartate aminotransferase（GOT）activity in young leaves of LN tolerant wild soybean increased by 0.46,0.08 and 0.25 under different intensities LN stress（p<0.05）.The results showed that the young leaves and old leaves of LN stress-tolerant wild soybean maintained certain nitrogen metabolism capacity,and the N metabolism ability of old leaves was more stable.The transport of beneficial nutrients in old leaves was helpful to enhance carbon metabolism of young leaves,and stable N metabolism in old leaves could provide N metabolites for carbon metabolism in young leaves.Meanwhile,the stable N metabolism in old leaves can provide the products of N metabolism,such as free amino acids,for the carbon metabolism of young leaves.Under LN stress,the adaptability of LN tolerant wild soybean was higher than that of common wild soybean,and the coupling of carbon and nitrogen metabolism between young leaves and old leaves played a key role.Under LN stress,the adaptability of LN tolerant wild soybean was better than that of common wild soybean.The resistance of LN tolerant wild soybean to LN stress depends on stable photosynthetic capacity,special accumulation of carotenoids,beneficial ion accumulation to maintain nutrient balance and compensatory accumulation of K⁺in young leaves,while old leaves promote metabolites to regulate intracellular osmotic balance by improving nitrogen metabolism.The coupling of carbon and N metabolism between young and old leaves plays a key role.Our study provides a theoretical basis for the adaptation mechanism of LN stress-tolerance wild soybean,and a theoretical foundation for the protection and utilization of wild soybean resources.Meanwhile,it also provides new ideas for the sustainable development,utilization and research of wild plant resources.