Inhibition of N₂ fixation by white clover (Trifolium repens L.) at low concentrations of NO ^sup−_inf3 in flowing solution culture

Abstract

The impact of sustained low external concentrations of NO ⁻₃ (0, 10, 100 and 1000 mmol m⁻³) on plant growth and the relative acquisition of N through N₂ fixation and NO ⁻₃ uptake by established, nodulated white clover (Trifolium repens L. cv. Blanca) was studied over 28 days in flowing solution culture. Nitrogen fixation was measured by N difference and ¹⁵N dilution methods. Plants supplied with NO ⁻₃ achieved higher relative growth rates (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3702!\[\bar x\]=0.091 d⁻¹) compared with ‘control’ plants dependent on N₂ fixation (0.073 d⁻¹). Nitrate plants showed progressive increases in shoot: root d.w. ratios from 4 to 6.5–7.6 between days 0–28, compared with 5.1 on day 28 for control plants. Increases in both nodule d.w. and numbers per plant were inhibited after day seven at all concentrations of NO ⁻₃ . The severity of inhibition of N₂ fixation increased with increasing NO ⁻₃ concentration and with time. The total amounts of N₂ fixed per plant between days 0–7 after supplying 10, 100 and 1000 mmol m⁻³ NO ⁻₃ , respectively, were 37–39, 28–30 and 0–13%, of the total N acquired. Between days 7–28 the proportional contributions of N₂ fixation to total N acquisition declined to 3, 0.5 and 0%, respectively, in these treatments. The corresponding mean specific rates of N₂ fixation between days 0–7 were, respectively, 5.4, 3.2, and 2.0 mmol N d⁻¹ g⁻¹ nodule d.w., compared with 7.9 mmol N d⁻¹ g⁻¹ nodule d.w. for zero NO ⁻₃ plants. There was no evidence of a transitory increase in N₂ fixation following the addition of NO ⁻₃ , even at the lowest supply concentration.