Effects Of High Ammonium Stress On Seedling Growth And Its Physiological Basis In Wheat

NH4+is one of the important nitrogen forms for crops absorbed directly, and a proper amount of NH4+concentration in soil can improve nitrogen use efficiency and reduce the nitrogen losses which brought from the excessive use of nitrate nitrogen. However, higher NH4+will lead to adverse effects on crop growth and development. Wheat as an upland crop is more sensitive to high ammonium with lowland crops. During early growth stage of wheat, soil ammonium is higher due to excessive use of basal nitrogen fertilizer, which results in high ammonium stress for growth and development of root-established processing. Therefore, it is crucial to value the influences of high ammonium stress on wheat seedling growth and its physiological mechanism for screening high ammonium resistant varieties and cultivation management. Hydroponic experiments were conducted at Pailou Experiment Station to investage the biological and physiological responses of wheat cultivars to high ammonium stress from2010to2012. The main results were as follows:1. Thirty four wheat cultivars had significant genotypic difference in seedling growth under high ammonium stress. Compared with nitrate treatment, leaf area, tiller number, lateral root number, total root length, root surface area, root volume, shoot dry weight, root dry weight and plant dry weight significantly reduced under high ammonium stress, and tolerant indexes to high ammonium in total root length, root surface area, root volume, shoot dry weight, root dry weight and plant dry weight had significant genotypic difference among cultivars. The principal component and correlation analysis showed that the tolerance indexes in shoot dry weight and root surface area could be used as main screening indexes, and34wheat cultivars were devided into three groups by cluster analysis:tolerant type, moderate type and sensitive type.2.Two wheat genotypes differential responses to high ammonium stress, Yannongl9(tolerant type) and Zhengyinl (sensitive type), were used to investigate the effects of high ammonium stress on wheat growth and photosynthetic charcteristics. The suppression degrees of leaf area, tiller number, lateral root number, total root length, root surface area, root volume, shoot dry weight, root dry weight and plant dry weight in Yannong19were less than that of Zhengyinl. High ammonium stress significantly reduced photosynthetic rate, stomatal conductance, actual light quantum efficiency (F PSII) and maximum energy conversion efficiency (Fv/Fm), but increase initial fluorescence (F0). Compared with Zhengyin1, the influence on Yannong19was significantly lower than Zhengyinl. O2.-content in leaves of Zhengyinl was significantly higher than Yannongl9after13days, indicated that the membrane permeability damaged degree of Yannong19was smaller than Zhengyinl. In addition, Yannongl9had higher POD and SOD enzyme activity than Zhengyinl, indicated that Yannongl9had higher light energy utilization ability and the ability to defense the photoinhibition.3. Under high ammonium stress, nitrogen content, soluble protein, free amino acid and leaf GS activity of wheat seedling increased. The increased amplitude of free amino acid and leaf GS activity of Yannong19had significant higher than Zhengyinl, but the increased amplitude of nitrogen content had lower. That indicated tolerant type cultivars had stronger ammonium assimilation ability. NH4+content in wheat seedling increased significantly under high ammonium stress, especially in the root. But the NH4+content in leaves and roots of Yannong19(tolerant type cultivar) was significantly lower than that of Zhengyinl (sensitive type cultivar). That indicated Yannongl9could reduce the content of NH4+in vivo by enhanced the ammonium assimilation capacity in order to adapt to the high ammonium stress. Potassium absorption capacity of the wheat seedling was restrained at under high ammonium stress. But potassium nitrogen ratio of Yannongl9more balanced than that of Zhengyinl, may alleviate the high ammonium stress. Under high ammonium stress, the absorption of phosphrous in wheat seedling increased but there was no significantly difference between varieties.In summary, high ammonium stress significantly inhibited shoot and root growth of sensitive genotype zhengyin1, but had less negative effects on tolerant genotypeYannong19. Furthemore, high ammonium stress significantly reduced net photosynthetic rate(Pn), actual light quantum efficiency (FPSII) and maximum energy conversion efficiency (Fv/Fm) of two wheat genotypes, but the inhibition to Yannong19was much lower than that of Zhengyin1due to higher SOD and POD enzyme activity and lower ROS content, which could effectively weaken the photoinhibition caused by the reactive oxygen species. In addition, lower NH4+content but higher ratio of potassium to nitrogen in roots of Yannong19under high ammonuium stress indicated that tolerant genotype had stronger potassium uptake and ammonium assimilation ability to balance nutrition requirement, hence lighten the inhibition of high ammonium stress.