Nitrate Nutrition of Grasses from Steady-State Supplies in Flowing Solution Culture following Nitrate Deprivation and/or Defoliation

Abstract

Plants of two genotypes of Lolium perenne L. cv. S23 and a L. perenne × L. multiflorum Lam. hybrid cv. Augusta were grown in flowing solution culture. N was suppled in one treatment at 10 mmol m−3 NO−3 throughout (HN), and in another (LN) the N supply was terminated after 10 d for 11 d. When NO3- was re-supplied both LN and HN plants were left entire or defoliated. The two genotypes showed similar responses to all treatments. The concentration of N in shoot dry matter declined from 4.4% to 2.0% and in the root from 2.8% to 1.0% over the 11 d of N deprivation, with 95% of the NO3- initially present being assimilated during this period. LN plants assimilated 10% more of their total NO3- uptake than did HN plants. The in vitro nitrate reductase activity (NRA) was 10- to 50-fold higher in the youngest fully-expanded leaves than in roots and declined in the leaves during N deprivation. Between 2–6 d after defoliation, there was a large increase in NRA in leaves of HN (but not LN) plants. After defoliation of HN plants, net uptake from 10 mmol m−3NO3- declined to negligible levels within 15 h, but in defoliated LN plants it increased to levels similar to those of entire HN plants (15–20 μmol h−1 g−1 fr. wt. root) within 8 h. When NO3- was re-supplied to entire LN plants, uptake of NO3- increased to levels similar to those of entire HN plants within 2.3 h, but did not markedly exceed that of HN plants for at least 10 h. Net uptake of NO3- by LN plants during depletion of stirred static nutrient solutions containing 1.0 mol m−3NO3- lagged behind that by HN plants by 70–100 min, but the maximum unit absorption rate was similar for LN and HN plants (5–7 μmol h−1 g−1 fr. wt. root). The nature of the short-term demand for NO3- uptake following recovery from the stresses of defoliation and N starvation is discussed.