Wilkinson Sally, Bacon Mark A, Davies William J
The Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster LA1 4YQ, UK.
J Exp Bot. 2007;58(7):1705-16. doi: 10.1093/jxb/erm021. Epub 2007 Mar 20.
Increasing the nitrate (N) concentration in the rooting substrate above deficiency decreased stomatal conductance and leaf growth rate compared with sufficient N in maize seedlings (Zea mays L.) growing in drying substrate. Novel effects were detected when N in the non-deficient range was supplied directly to the xylem of detached shoots: concentrations above 2.0 mol m-3 KNO3 reduced transpiration, and concentrations above 12 mol m-3 KNO3 reduced leaf growth rate. Evidence is provided that the novel effects of N on transpiration and growth were mediated by pH-based ABA redistribution. ABA at 0.05 mol m-3, whilst ineffective alone, sensitized leaf growth to increases in KNO3 concentration (from 3.0 mol m-3), and the capacity of higher concentrations of ABA to reduce growth was enhanced by KNO3. Transpiration was sensitively reduced by KNO3, ABA, or buffers adjusted to pH 6.7-7.0 (compared with buffers adjusted to pH 5.0) alone. Nevertheless, a synergistic effect of KNO3 and either ABA or buffers adjusted to pH 6.7-7.0 was observed. Buffers of pH 5.6 supplied to detached shoots alleviated the depression of transpiration caused by 12 mol m-3 KNO3. Buffers adjusted to pH 6.7 increased the sensitivity of growth to KNO3. Xylem sap extracted from intact seedlings growing in drying soil exhibited an initial increase in N concentration, followed by a decrease at progressively lower soil water potentials. The importance for novel N signalling above deficiency is discussed with reference to the generality of fluctuations in soil and xylem N concentration within this range.
与生长在干燥基质中的玉米幼苗（Zea mays L.）处于充足氮素供应状态相比，提高生根基质中硝酸盐（N）浓度至高于缺乏水平会降低气孔导度和叶片生长速率。当将非缺乏范围内的氮直接供应给离体枝条的木质部时，检测到了新的效应：KNO₃浓度高于2.0 mol m⁻³会降低蒸腾作用，而浓度高于12 mol m⁻³会降低叶片生长速率。有证据表明，氮对蒸腾作用和生长的新效应是由基于pH的脱落酸（ABA）重新分布介导的。0.05 mol m⁻³的ABA单独作用时无效，但会使叶片生长对KNO₃浓度增加（从3.0 mol m⁻³起）敏感，而较高浓度的ABA降低生长的能力会因KNO₃而增强。单独使用KNO₃、ABA或调节至pH 6.7 - 7.0的缓冲液（与调节至pH 5.0的缓冲液相比）会敏感地降低蒸腾作用。然而，观察到KNO₃与ABA或调节至pH 6.7 - 7.0的缓冲液之间存在协同效应。供应给离体枝条的pH 5.6缓冲液减轻了12 mol m⁻³ KNO₃引起的蒸腾作用抑制。调节至pH 6.7的缓冲液增加了生长对KNO₃的敏感性。从生长在干燥土壤中的完整幼苗中提取的木质部汁液显示，氮浓度最初增加，随后在土壤水势逐渐降低时下降。结合该范围内土壤和木质部氮浓度波动的普遍性，讨论了高于缺乏水平的新氮信号传导的重要性。
