Climate dependent variations in leaf respiration

植物在调节大气二氧化碳（CO2，一种重要的温室气体）浓度方面起着至关重要的作用。决定大气中二氧化碳浓度和全球变暖幅度的关键是植物的呼吸速率;全球每年有近60 Gt C通过植物呼吸释放到大气中。与化石燃料燃烧产生的相对较小的CO2释放量（每年< 6 Gt C）相比，这是一个很大的通量。为了预测未来的大气CO2浓度和全球表面温度，我们需要准确地模拟植物呼吸对气候的响应。目前，大多数气候模型假设植物呼吸作用以一种简单的、可预测的方式对气候作出反应。例如，假设温度每上升10 ℃，呼吸速率加倍。此外，叶呼吸速率被假定为是相同的，在黑暗中，在相同的温度下测量时，在光明中。然而，越来越多的证据表明，这两种假设都不正确。例如，我们知道呼吸的温度响应是高度动态的，并适应温度的长期变化（即呼吸“适应”）。此外，我们知道光抑制叶片呼吸，特别是在白天高温下。不考虑这种动态响应的呼吸结果在预测的净生态系统C交换率大的错误，这种错误可能是特别重要的低生产力的生态系统，植物呼吸代表了整体C交换的一个大比例。因此，在预测未来气候变化对生物圈的影响的生态系统气体交换模型中，必须考虑到呼吸作用的这种动态变化。要做到这一点，我们首先需要仔细量化植物呼吸如何响应气候变化（无论是在黑暗中还是在光照下）。然后，这些数据可以用来开发数学方程，允许植物呼吸变化的影响被纳入碳交换模型，准确预测当前和未来的植物碳交换率，无论是在个别生态系统和全球。我们的研究将集中在气候对全球广泛分布的三个低生产力森林生态系统的叶片气体交换的影响上（地中海旱地、北方万年青针叶林和北方落叶针叶林），夏季白天的叶温往往很高，接近正生长的临界值;叶片呼吸作用的微小变化可导致这些森林从大气碳的净吸收者转变为碳的净排放者。目前，我们的了解如何依赖于气候变化的叶片呼吸的影响率的净CO2交换低生产力的森林是有限的。特别是，作用光抑制叶呼吸在确定率净CO2交换在炎热的夏天天是未知的。我们的研究将量化叶呼吸变化的重要性，由于季节性驯化和抑制在这些低生产力生态系统中生长的树木的整体植物碳平衡。因此，除了为提高气候模型的预测能力提供必要的数据外，我们的研究还将首次建立叶呼吸在确定碳经济方面的定量重要性，从而确定这些全球广泛分布的低生产力生态系统的增长。由于所获得的知识将基于一般原则，我们也将能够将研究结果应用于其他生态系统，其中叶呼吸可能是生态系统CO2净交换的重要决定因素（例如热带森林中的光限制下层植物）。在这样的环境中，整个植物碳平衡的微小变化可能对幼苗的生长和存活产生重要影响。

Plants play a vital role in regulating the concentration of atmospheric carbon dioxide (CO2, an important greenhouse gas). Critical in determining atmospheric CO2 concentrations and magnitude of global warming is the rate of respiration exhibited by plants; globally, near 60 Gt C per year is released into the atmosphere by plant respiration. This is a large flux compared with the relatively small release of CO2 from the combustion of fossil fuels (< 6 Gt C per year). To predict future atmospheric CO2 concentrations and global surface temperatures, we need to accurately model how plant respiration responds to climate. At present, most climate models assume that plant respiration responds to climate in a simplistic, predictable manner. For example, respiration is assumed to double in rate for every 10oC rise in temperature. Moreover, rates of leaf respiration are assumed to be the same in the light as in darkness, when measured at the same temperature. There is, however, growing evidence that neither assumption is correct. For example, we know that the temperature response of respiration is highly dynamic and adjusts to long-term changes in temperature (i.e. respiration 'acclimates'). Moreover, we know that light inhibits leaf respiration, particularly at high daytime temperatures. Failure to account for such dynamic responses of respiration results in large errors in predicted rates of net ecosystem C exchange; such errors are likely particularly important in low productivity ecosystems where plant respiration represents a large proportion of overall C exchange. It is vital, therefore, that such dynamic variations in respiration be accounted for in ecosystem gas exchange models predicting the impacts of future climate change on the biosphere. To do this, we need to first carefully quantify how plant respiration responds to variations in climate (both in darkness and in the light). This data can then be used to develop mathematical equations that allow the effects of variations in plant respiration to be incorporated into C exchange models that accurately predict current and future rates of plant C exchange, both in individual ecosystems and globally. Our research will focus on the effects of climate on leaf gas exchange of three low productivity forest ecosystems that are globally widespread (Mediterranean dryland, boreal evergreen conifer and boreal deciduous conifer forests), often experience high day-time leaf temperatures during the summer and operate close to the threshold of positive growth; minor changes in leaf respiration can result in such forests switching from net absorbers of atmospheric C to net emitters of C. At present, our understanding of how climate dependent variations in leaf respiration impact on rates of net CO2 exchange of low productivity forests is limited. In particular, the role light inhibition of leaf respiration in determining rates net CO2 exchange on hot summer days is unknown. Our research will quantify the importance of variations in leaf respiration due to seasonal acclimation and inhibition in the light on overall plant carbon balances of trees growing in these low productivity ecosystems. Thus, in addition to providing the data necessary for improving the predictive power of climate models, our research will establish for the first time the quantitative importance of leaf respiration in determining the C economy, and thus growth, of these globally widespread low productivity ecosystems. Because the knowledge obtained will be based on general principles, we will also be able to apply the findings to other ecosystems where leaf respiration potentially represents an important determinant of net ecosystem CO2 exchange (e.g. light-limited under-storey plants in tropical forests). In such environments, small changes in whole plant carbon balances may have important consequences for seedling growth and survival.