Absorption of nitrate and ammonium-ions by lolium-perenne from flowing solution cultures at low root temperatures

Lolium perenne L. cv. 23 (perennial ryegrass) plants were grown in flowing solution culture and acclimatized over 49d to low root temperature (5-degrees-C) prior to treatment at root temperatures of 3, 5, 7 and 9-degrees-C for 41d with common air temperature of 20/15-degrees-C day/night and solution pH 5.0. The effects of root temperature on growth, uptake and assimilation of N were compared with N supplied as either NH4+ or NO3- at 10 mmol m-3. At any given temperature, the relative growth rate (RGR) of roots exceeded that of shoots, thus the root fraction (Rf) increased with time. These effects were found in plants grown with the two N sources. Plants grown at 3 and 5-degrees-C had very high dry matter contents as reflected by the fresh weight: freeze-dried weight ratio. This ratio increased sharply, especially in roots at 7 and 9-degrees-C. Expressed on a fresh weight basis, there was no major effect of root temperature on the [N] of plants receiving NH4+ but at any given temperature, the [N] in plants grown with NH4+ was significantly greater than in those grown with NO3. The specific absorption rate (SAR) of NH4+ was greater at all temperatures than SAR-NO3. In plants grown with NH4+, < 3.5% of the total N was recovered as NH4+, whereas in those grown with NO3- the unassimilated NO3 rose sharply between 7 and 9-degrees-C to become 14 and 28% of the total N in shoots and roots, respectively. The greater assimilation of NH4+ lead to concentrations of insoluble reduced N (= protein) which were 125 and 20% greater, in roots and shoots, respectively, than in NO3-grown plants. Plants grown with NH4+ had very much greater glutamine and asparagine concentrations in both roots and shoots, although other amino acids were more similar in concentration to those in NO3- grown plants. It is concluded that slow growth at low root temperature is not caused by restriction of the absorption or assimilation of either NH4+ or NO3-. The additional residual N (protein) in NH4+ grown plants may serve as a labile store of N which could support growth when external N supply becomes deficient.