Differences in growth and nitrogen productivity between a stay-green genotype and a wild-type of Lolium perenne under limiting relative addition rates of nitrate supply.

The stay-green mutation of the nuclear gene sid inhibits chlorophyll degradation during leaf senescence in grasses. Decreased productivity is expected under conditions of limited external N availability, due to the higher retention of N in senescent tissues. However, this has not been reported when plants are grown at limiting low external concentrations of N. In the present study a different approach was taken, based on the relative addition rate method for defining limiting N supply. Wild-type and stay-green genotypes of Lolium perenne L. were grown for 30 days in flowing solution culture and then supplied with NO3- on an hourly basis over 35 days at relative addition rates (RARs) of 0.03, 0.06, 0.09 and 0.12 day-1, ranging from severe N limitation to optimal supply. Plants were steady-state labelled with 15NO3- prior to RAR treatments, then switched to 14NO3- to allow measurement of the re-distribution of N absorbed prior to RAR control. Following acclimation, relative growth rates (RGRs) approached the corresponding RAR, but were significantly lower for stay-green than wild-type at RARs of 0.03 and 0.06 day-1. Tiller numbers were lower in stay-green plants after 35 days at all RARs except 0.12 day-1. Concentrations of total N in senescent laminae of stay-green plants exceeded those in wild-type plants by a similar margin (4.8-6.8 mg g-1 DW) irrespective of RAR. Maximum nitrogen productivity (Pn) was 3.9 g DW g-1 N day-1 (Nmin = 7.1 mg g-1 DW) in wild-type plants, and 5.1 g DW g-1 N day-1 (Nmin = 10.7 mg g-1 DW) in stay-green plants. The higher N productivity of stay-green plants indicated these plants used a smaller pool of metabolically available N more efficiently in biosynthesis compared with wild-type plants. The retention of N, absorbed prior to RAR treatments, in senescent laminae was significantly higher in stay-green plants at RAR of 0.03 day-1 after day 21 (i.e. 20% versus 15% of the total N recovered). However, in terms of the whole N economy of the plant the margin represented only 1.7% of the total N content on day 35.

A simple model of feedback regulation for nitrate uptake and N2 fixation in contrasting phenotypes of white clover.

A simple three equation model is proposed for the feedback regulation of nitrate uptake and N2 fixation, based on the concentration of the organic N substrate pool within the plant and two parameters denoting the N substrate concentrations at which half-maximal inhibition occurs. This model simulated three contrasting phenotypes of white clover (Trifolium repens L.) inbred lines with (1) normal rates of nitrate uptake and N2 fixation (NNU); (2) low rates of nitrate uptake (LNU); and (3) very low rates of N2 fixation (VLF). The LNU phenotype was simulated by a decrease in the value of the inhibition parameter for nitrate uptake and the VLF phenotype was simulated by a decrease in the value of the N2 fixation inhibition parameter. The model was tested against nitrate uptake data obtained from white clover plants growing in flowing nutrient culture. There was an accurate prediction of the increase in nitrate uptake caused by N2 fixation activity of the NNU and LNU inbred lines being interrupted by a switch in gas phase from air to Ar : O2. The model was also tested against data for nitrate uptake, N2 fixation and %N from fixation for the three inbred clover lines grown in flowing nutrient culture at 0, 5 or 20 mmol m(-3) N(3-). Again there was accurate prediction of nitrate uptake, although simulated values for N2 fixation were more variable. The simple model has potential use as a sub-routine in larger models of legume growth under field conditions.

Effects of nitrate pulses on BnNRT1 and BnNRT2 genes: mRNA levels and nitrate influx rates in relation to the duration of N deprivation in Brassica napus L.

A de-repression mechanism based on the disappearance of 'signals' down-regulating N transporter activity has been proposed in the literature to explain the transient increase of NO(3)(-) uptake by the roots following N deprivation in higher plants. This hypothesis was investigated at the physiological and molecular levels by measuring NO(3)(-) influx into roots of Brassica napus L. grown under low or high external concentrations of KNO(3) following N deprivation. Parallel measurements were made of endogenous NO(3)(-), amino acid concentrations and abundance of mRNA for BnNRT1 and BnNRT2, genes encoding nitrate-inducible transport proteins. The effect of NO(3)(-) pulsing on NO(3)(-) transport components in N-deprived plants was also investigated by measuring influx of high- and low-affinity transport system (HATS and LATS) and assaying mRNA levels. Influx of NO(3)(-) via HATS and LATS, and transcript levels of BnNRT2 and BnNRT1 decreased with the duration of N deprivation. The results suggested that the absence of de-repression of NO(3)(-) influx and BnNRT2 gene expression following N starvation was related to a high amino acid status. Pulsing with NO(3)(-) induced a large increase in BnNRT2 mRNA level, but a comparatively small increase in NO(3)(-) influx via HATS. The level of BnNRT1 mRNA also increased, but there was no effect on LATS uptake activity. The absence of a strict correlation between the NO(3)(-) transport activity and the mRNA BnNRT1 and BnNRT2 levels is discussed in terms of possible post-transcriptional regulation by the amino acids.

A stay-green mutation of Lolium perenne affects NO3 - uptake and translocation of N during prolonged N starvation.

Apparent Km and Vmax for net NOS″ uptake and short-term translocation patterns of recently absorbed N were compared in a stay-green mutant and wild-type selection line of Lolium perenne L. by means of a series of depletion studies using 18 NO3 , performed over 12 d under conditions of progressively increasing N deprivation. In view of the greater retention of N in senescent leaves of the stay-green phenotype, it was predicted that NOS″ uptake would be up-regulated relative to the normal line, and that a proportionally higher fraction of recently absorbed N would be allocated to young leaves. It was shown that the stay-green trait had significant phenotypic consequences for plant N relations, with higher 'sink strength' of shoots for recently absorbed N, and higher Vmax for NO3 uptake compared with those of normal plants. The stay-green mutation had no effect on the Km of the nitrate uptake system. Although the N-use efficiency might he expected to be lower in stay-green than in normal plants, there were no differences in rates of dry matter production.