Effects of Water, Aeration, and Salt Regime on Nitrogen Fixation in a Nodulated Legume--Definition of an Optimum Root Environment

Nitrogen assimilation of pea plants (Pisum sativum cv. Meteor)was studied in growth cabinets tinder a range of water, salt,and aeration regimes in the rooting medium. Treatments wereimposed in the period 14–30 d after germination usingseedlings already nodulated in an optimum root environment. Highest nitrogen fixation in water culture required a strengthof culture solution one fifth of that optimal for fixation insand culture. Fixation in water culture of optimum strengthwas significantly improved by continuous bubble aeration orby lowering the level of culture solution below the main zoneof nodulation. However, if supra-optimal concentrations of solutionwere used, fixation was markedly inhibited by lowering the solutionlevel, this being associated with an accumulation of high levelsof salts on exposed root and nodule surfaces. In – N (minus nitrogen) sand culture continuous waterloggingreduced nitrogen content to 40 per cent of that of non-stressedplants. In nitrate-fed plants waterlogging effects were lesssevere. Waterlogging decreased nodule tissue production anddecreased the specific activity of nitrogenase, as assayed byacetylene reduction. These effects were most marked three ormore cm below the sand surface. Watering on alternate days with free drainage at all times yieldedmaximum fixation in – N sand culture. Regimes increasingthe extent of waterlogging or drying out in comparison withthis optimum produced increasingly great decreases in nitrogenfixation. For equivalent reductions in total fixation, percentageN in dry matter was consistently lower in waterlogged than indroughted plants suggesting that excess water had the more specificeffect on symbiotic activity. Both forms of stress affectedparticularly the transport of nitrogen from root to shoot.