Global Change and Nitrate Assimilation

全球变化和硝酸盐同化大气中与全球变暖有关的二氧化碳浓度在过去两个世纪上升了30%以上，预计在接下来的两个世纪将翻一番。一些科学家认为，二氧化碳水平的上升将有利于植物，因为二氧化碳是光合作用的基本成分之一，光合作用是绿色植物利用阳光制造它们所需的化学能的过程。事实上，许多植物最初对大气二氧化碳水平翻倍的反应是将30%以上的二氧化碳同化为碳水化合物。然而，进一步的研究发现，二氧化碳同化的加速速度并不是持续的：在接触二氧化碳增加的几天或几周内，二氧化碳同化下降到仅比正常水平高12%，这一现象被称为二氧化碳适应。布鲁姆和他的同事发现，二氧化碳适应是由于缺氮造成的，因为二氧化碳增加会抑制硝酸盐同化为叶片中的氨基酸，而硝酸盐是植物从土壤中获得的主要氮素形式。有三种机制对此负有责任。首先，植物比氮素更重视吸收二氧化碳，因此当二氧化碳水平上升时，吸收硝酸盐所需的一些高能化合物已经被二氧化碳同化所束缚。第二，为了利用硝酸盐，植物首先将硝酸盐转化为细胞质中的亚硝酸盐，然后将亚硝酸盐转移到叶绿体中，转化为铵，然后是氨基酸。布鲁姆的研究表明，二氧化碳水平的升高阻止了亚硝酸盐向叶绿体的这种至关重要的转移。第三，在目前的二氧化碳和氧气水平下，大多数植物失去了大约四分之一的碳水化合物，这些碳水化合物可以通过一个称为光呼吸的过程积累起来。这一过程被认为是浪费的，是大多数工厂用来产生碳水合物的不可避免的后果。布鲁姆的最新研究结果表明，这些植物必须进行光呼吸才能将硝酸盐转化为氨基酸。这些研究表明，在自然生态系统中，依赖于叶片中硝酸盐转化为氨基酸的植物和树木物种，与那些能够在根部将硝酸盐转化为氨基酸或以氨为主要氮源的物种相比，可能处于竞争劣势。因此，随着大气二氧化碳水平的持续上升，野生植物的分布可能会发生重大变化。一氧化二氮是另一种导致全球变暖的主要温室气体。布鲁姆和他的同事表明，小麦植株在硝酸盐同化过程中会排放大量的一氧化二氮。因此，植物所使用的氮素形式可能会影响其氧化亚氮的排放。拟议中的研究将确定光呼吸、二氧化碳同化、一氧化二氮产生和硝酸盐同化之间的相互依赖关系。它将利用具有不同同化硝酸盐能力的拟南芥和一系列不同光呼吸程度的黄花属植物。实验室和田间研究还将检验铵和硝酸盐作为植物氮源的相对重要性。最后，在一个监测二氧化碳升高对沙漠植物影响的国家设施中，实验将在现场评估二氧化碳抑制植物硝酸盐同化的程度。

Global Change and Nitrate AssimilationCarbon dioxide concentrations in the atmosphere that are associated withglobal warming have risen by more than 30 percent during the past twocenturies and are expected to double during the next. Some scientistsbelieve these rising levels of carbon dioxide will benefit plants becausecarbon dioxide is one of the essential ingredients in photosynthesis, theprocess by which green plants use sunlight to manufacture the chemicalenergy they need. Indeed, many plants initially respond to a doubling ofatmospheric carbon dioxide levels by assimilating 30 percent more carbondioxide into carbohydrate. Further study, however, reveals that theaccelerated rate of carbon dioxide assimilation is not sustained: within afew days or weeks of exposure to elevated carbon dioxide, carbon dioxideassimilation drops back to just 12 percent greater than normal, a phenomenonknown a carbon dioxide acclimation.Bloom and colleagues discovered that carbon dioxide acclimation derives fromnitrogen deprivation because elevated carbon dioxide inhibits theassimilation of nitrate into amino acids in leaves and nitrate is a majorform of nitrogen that plants obtain from the soil. Three mechanisms appearto be responsible.First, plants place a higher priority on assimilating carbon dioxide thanthey do nitrogen, so when carbon dioxide levels rise, some of the highenergy compounds needed to assimilate nitrate are already tied up inassimilating carbon dioxide.Second, to make use of nitrate, the plants initially convert nitrate intonitrite in the cytoplasm and move the nitrite into the chloroplast forconversion into ammonium and then amino acids. Bloom's research indicatedthat elevated levels of carbon dioxide blocked this vital transfer ofnitrite into the chloroplasts.Third, under current levels of carbon dioxide and oxygen, most plants loseabout one-quarter of the carbohydrate that they could accumulate from aprocess known as photorespiration. This process was thought to be wasteful,unavoidable consequence of the method that most plants use to generatecarbohydrate. Bloom's latest results show that these plants mustphotorespire in order to convert nitrate into amino acids.These studies suggest that plant and tree species in natural ecosystems thatdepend on nitrate conversion into amino acids in their leaves are likely tobe at a competitive disadvantage to those species that are either able toconvert nitrate into amino acids in their roots or use ammonium as theirpredominant nitrogen source. As a result the distribution of plants in thewild may change significantly as atmospheric carbon dioxide levels continueto rise.Nitrous oxide is another major greenhouse gas that contributes to globalwarming. Bloom and colleagues showed that wheat plants emit a significantamount of nitrous oxide as part of nitrate assimilation. The form ofnitrogen that a plant uses, therefore, may influence its nitrous oxideemissions.The proposed research will determine the interdependence amongphotorespiration, carbon dioxide assimilation, nitrous oxide production, andnitrate assimilation. It will employ Arabidopsis genotypes with alteredcapacities to assimilate nitrate and a series of Flaveria species that varyin their extent of photorespiration. Laboratory and field studies will alsoexamine the relative importance of ammonium and nitrate as plant nitrogensources. Lastly, at a national facility for monitoring the influence ofelevated carbon dioxide on desert flora, experiments will assess in situ theextent to which carbon dioxide inhibits plant nitrate assimilation.