McDonald Evan P, Erickson John E, Kruger Eric L
Forestry Sciences Laboratory, US Department of Agriculture Forest Service, Rhinelander, WI 54501, USA.
Department of Forest Ecology and Management, University of Wisconsin - Madison, WI 53706, USA.
Funct Plant Biol. 2002 Aug;29(9):1115-1120. doi: 10.1071/FP02007.
N acquisition often lags behind accelerated C gain in plants exposed to CO2-enriched atmospheres. To help resolve the causes of this lag, we considered its possible link with stomatal closure, a common first-order response to elevated CO2 that can decrease transpiration. Specifically, we tested the hypothesis that declines in transpiration, and hence mass flow of soil solution, can decrease delivery of mobile N to the root and thereby limit plant N acquisition. We altered transpiration by manipulating relative humidity (RH) and atmospheric [CO2]. During a 7-d period, we grew potted cottonwood (Populus deltoides Bartr.) trees in humidified (76% RH) and non-humidified (43% RH) glasshouses ventilated with either CO2-enriched or non-enriched air (~1000 vs ~380μmol mol). We monitored effects of elevated humidity and/or CO2 on stomatal conductance, whole-plant transpiration, plant biomass gain, and N accumulation. To facilitate the latter, NO3 enriched in N (5 atom%) was added to all pots at the outset of the experiment. Transpiration and N accumulation decreased when either CO2 or humidity were elevated. The disparity between N accumulation and accelerated C gain in elevated CO2 led to a 19% decrease in shoot N concentration relative to ambient CO2. Across all treatments, N gain was positively correlated with root mass (P<0.0001), and a significant portion of the remaining variation (44%) was positively related to transpiration per unit root mass. At a given humidity, transpiration per unit leaf area was positively related to stomatal conductance. Thus, declines in plant N concentration and/or content under CO2 enrichment may be attributable in part to associated decreases in stomatal conductance and transpiration.
在暴露于高二氧化碳浓度大气中的植物中，氮素获取通常滞后于加速的碳积累。为了帮助找出这种滞后现象的原因，我们考虑了它与气孔关闭的可能联系，气孔关闭是植物对二氧化碳浓度升高的一种常见的一级反应，会降低蒸腾作用。具体而言，我们检验了这样一个假设：蒸腾作用的下降，以及因此土壤溶液质量流的减少，会降低可移动氮向根系的输送，从而限制植物对氮的获取。我们通过控制相对湿度（RH）和大气中[CO₂]来改变蒸腾作用。在为期7天的时间里，我们将盆栽的三角叶杨（Populus deltoides Bartr.）树种植在湿度为76%RH的加湿温室和湿度为43%RH的未加湿温室中，温室用富含二氧化碳或未富含二氧化碳的空气通风（约1000μmol/mol对约380μmol/mol）。我们监测了湿度升高和/或二氧化碳浓度升高对气孔导度、整株植物蒸腾作用、植物生物量增加和氮积累的影响。为了便于进行后者的研究，在实验开始时向所有花盆中添加了富含氮（5原子%）的硝酸盐。当二氧化碳浓度或湿度升高时，蒸腾作用和氮积累都会下降。相对于环境二氧化碳浓度，在高二氧化碳浓度下氮积累与加速的碳积累之间的差异导致地上部氮浓度下降了19%。在所有处理中，氮增加量与根质量呈正相关（P<0.0001），其余变异的很大一部分（44%）与单位根质量的蒸腾作用呈正相关。在给定湿度下，单位叶面积的蒸腾作用与气孔导度呈正相关。因此，在二氧化碳浓度升高条件下植物氮浓度和/或含量的下降可能部分归因于气孔导度和蒸腾作用的相应降低。
