Uptake and Assimilation of NO3− and NH4+ by Nitrogen-Deficient Perennial Ryegrass Turf

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Assimilation of NO3− and NH4+ by perennial ryegrass (Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH4+, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO3− had been reduced, whereas 97% of the NH4+ had been assimilated. During the early stages (0 to 8 hours) of NO3− uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO3−, NH4+ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO3− uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate.

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