Response Of Root Growth Of Pea And Its Ability Of Symbiotic Nitrogen Fixation To Nitrogen Forms And Drought Stress

Both soil moisture and supply of exogenous nitrogen are the important factors that affect the growth and nitrogen fixation of leguminous crop. Pea is the main rotation crop in dry farming, which has good performance of nitrogen fixation. It also plays a critical role on nitrogen balance of agriculture ecosystem in dry land. Study on the influence of drought stress and nitrogen forms on pea root growth and nitrogen fixation is critical to provide theoretical basis for using water rationally and the optimal measures of nitrogen management. Pot experiment was conducted to study the effects of nitrogen forms and drought stress on root system growth, nodulation and nitrogen fixation of pea. 0.05 g NH4+-N or NO3--N per kg soil was used before planting and stress of 55% relative water content for 15 days was exerted in early branching, budding and flowering stages respectively. Results of the study would provide theoretical foundation for reasonable water controlling and N management measure of pea in northwest district of China. The main results were as follows:1. NO3--N and drought stress accelerated the growth process of pea at different level, NO3--N and drought stress in branch stage, budding stage were beneficial to promote pea advancing into reproductive stage, while pea matured earlier relatively under drought stress in budding stage and flowering stage. Drought stress inhibited stem elongation and leaves generation, and the earlier the stress, the greater of inhibition and the worse the compensative effect.2. The effect of nitrogen forms on root growth varied with soil water, NO3--N promoted root elongation under normal water conditions and also was favorable for the improvement of root length and volume as well as surface area, while NH4+-N improved root diameter under normal and water stress conditions. In addition, the influence of drought stress on root growth also varied with nitrogen forms, and root length and volume as well as surface area under three kinds of drought stress significantly decreased under NO3--N condition, whereas stress had less influence on root growth under N H4+-N nutrition. Furthermore, drought stress was beneficial to reducing root decay rate from full- flowering to maturity of pea, especially generated equivalent-compensation or over-compensatory effects on root growth in mature period, and the best compensatory growth of rewatering after drought stress was in budding stage.3. The influence of drought stress on biomass of pea also varied with stress stage and nitrogen forms, and the inhibition caused by stress in branch stage and budding stage had hysteresis effect, and the earlier the stress, the better the compensative effect on shoot dry matter accumulation. NO3--N was beneficial to promote root biomass accumulation at full- flowering stage and maturity under normal water conditions, while N H4+-N promote shoot dry weight at full- flowering stage under drought stress.4. Drought stress significantly reduced nodulation and nitrogen fixation ability, and the inhibition on nodule formation and its growth could not be compensated by re-watering; and the earlier the stress, the better the compensative effect on nitrogenase activity. The largest inhibition in the three drought stress was budding stage. Drought stress and NO3--N were beneficial to reducing nodule decay rate from full- flowering to maturity of pea, thus improving nodule number, weight and nitrogenase activity in mature period.The interaction between drought stress and NO3--N was beneficial to keep higher nitrogen fixation ab ility in full- flowering stage and maturity of pea.5. N accumulation in plant under drought stress in different stage during stress was inhibited, and the earlier the drought stress was exerted, the greater influence during stress, the smaller influence in full- flowering stage and maturity. The influence of nitrogen forms on N accumulation varied with growth stage and moisture content. NH4+-N was beneficial to improve N accumulation during earlier stage under normal water condition, NO3--N was conducive to increase N accumulation in mature period, and the interaction between water and N forms was not significant in full- flowering stage and maturity. Drought stress significantly reduced N distribution in pea shell, and the earlier the stress, the smaller influence. Drought stress in flowering stage significantly reduced nitrogen utilization efficiency in plant.6. Soil moisture content significantly affected grain yield and yield composition, but nitrogen form did not affect yield formation. The earlier the stress, the smaller of the influence on yield and yield composition. Yield under drought stress of flowering stage reduced by 16.16%~20.28% and 14.23~15.29% compared with that under branching and budding stage stress. Drought stress during branching stage and budding stage reduced grain number per plant, whereas drought stress in flowering stage inhibited the formation and growth of grain as well as that of pod.