Effect of Contrasting Patterns of Nitrate Application on the Nitrate Uptake, N2-Fixation, Nodulation and Growth of White Clover

White clover (Trifolium repens L.) plants were grown from seedin perlite, inoculated with effective rhizobia and exposed tothe same ‘concentration x days’ of ¹⁵N-labellednitrate in four contrasting patterns of doses. Acetylene reductionwas measured at intervals using an open, continuous-flow sytem.Mean dry weight per nodule and rates of acetylene reductionfell rapidly (2–3 d) during periods of exposure to highnitrate concentrations (> 7 mM N) and rose again, equallyrapidly, when nitrate was withdrawn or substantially reduced.The fall in mean dry weight per nodule (50–66 per cent)was almost certainly too large to be accounted for by loss ofsoluble or storage carbohydrate only. No new nodules were formedduring periods of high nitrate availability. When nitrate wassupplied continuously at a moderate concentration (5.7 mM N)nodule numbers stabilised although existing nodules increasedin dry weight by almost four-fold over the 30 d measurementperiod. Treatment had no effect on the percentage nitrogen in planttissues although there were large differences in the proportionsderived from nitrate and N₂-fixation. Plants exposed continuouslyor frequently to small doses of nitrate took up more nitrate,and hence relied less heavily on N₂-fixation, than those exposedto larger doses less often. Increased reliance on nitrate broughtwith it increased total dry weight and shoot: root ratios. Possiblemechanisms involved in bringing about these differences in nitrogennutrition and growth are discussed. White clover, Trifolium repens, nitrate, N₂-fixation, nodule, acetylene reduction, ¹⁵N