Nitrate Assimilation during Vegetative Regrowth of Alfalfa

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Abstract
Dry matter accumulation, nitrate reductase activity of various organs, nitrate accumulation, N derived from nitrate and N content were studied during 17 days of vegetative regrowth of harvested (detopped) alfalfa (M. sativa L.). Seedlings were grown in the glasshouse and treated with 0, 40 and 80 kg N/ha applied as K15NO3 to determine whether reduced nitrogenase activity after shoot harvest limited vegetative regrowth. The role of nodules [inoculated by Rhizobium meliloti] in reducing NO3- during this period of low nitrogenase activity was also investigated. Applied N had no effect on shoot dry matter accumulation during the regrowth cycle. Reduced nitrogenase activity after shoot removal may not limit vegetative regrowth. Nitrate reductase activity was present in all tissues and ranked leaves > nodules > stems > roots. Root nitrate reductase did not respond to applied K15NO3, while leaf, stem and nodule nitrate reductase increased significantly. Significant increases in total plant nitrate reductase activity were highly correlated with shoot regrowth. Nodules incorporated N from 15NO3-; 1.5% of the total plant N formed during regrowth could have been derived through nodule nitrate reductase. Nitrate accumulated in tissues of all plants, including those grown in the absence of added K15NO3. Accumulation of NO3- in plant tissues ranked leaves > stems > roots. Analysis of the percentage of 15N showed that plants grown in either 40 or 80 kg N/ha derived 65-75% of the N that accumulated during regrowth from NO3-. Approximately 23 mg of N/plant accumulated during the regrowth period. Leaves accounted for 80% of this net increase, while stems accounted for 20%. There was no net change in root N. Although alfalfa will efficiently utilize low levels of applied N in lieu of N2 fixation, N fertilization of seedling alfalfa after shoot removal will be of little value to increase yields. Nodules reduce NO3-, but their contribution to total plant N during regrowth is relatively small.