Alterations in nitrogen assimilation and partitioning in nitrogen stressed plants

Although nutrient stress is known to alter partitioning between shoots and roots, the physiological basis for the phenomenon is unresolved. Experiments were conducted to examine assimilation of 15NO3 by N‐stressed plants and to determine whether apparent changes in assimilation in the root contributed to alterations in whole‐plant partitioning of reduced‐N. Tobacco plants (Nicotiana tabacum L. cv. NC 2326) were exposed to a low concentration of NO3− in solution (80 μM) for 9 days to effect a N‐stress response. Exposure of plants to 1000 μM15NO3− for 12 h on selected days revealed that roots of N‐stressed plants developed an increased capacity to absorb NO3−, and accumulation of reduced‐15N in the root increased to an even greater extent. When plants were exposed to 80 or 1000 μM15NO3− in steady‐state, 15NO3− uptake over a 12 h period was noticeably restricted at the lower concentration, but a larger proportion of the absorbed 15N still accumulated as reduced‐15N in the root. The alteration in reduced‐15N partitioning was maintained in N‐stressed plants during the subsequent 3‐day “chase” period when formation of insoluble reduced‐15N in the root was quantitatively related to the disappearance of 15NO3− and soluble reduced‐15N. The results indicate that increased assimilation of absorbed NO3−, in the root may contribute significantly to the altered reduced‐N partitioning which occurs in N‐stressed plants.