Steppe Kathy, Saveyn An, McGuire Mary Anne, Lemeur Raoul, Teskey Robert O
Laboratory of Plant Ecology, Department of Applied Ecology and Environmental Biology, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, B-9000 Ghent, Belgium.
Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602, USA.
Funct Plant Biol. 2007 Sep;34(9):785-792. doi: 10.1071/FP07077.
Rates of CO efflux of stems and branches are highly variable among and within trees and across stands. Scaling factors have only partially succeeded in accounting for the observed variations. In this study, the resistance to radial CO diffusion was quantified for tree stems of an eastern cottonwood (Populus deltoides Bartr. ex Marsh.) clone by direct manipulation of the CO concentration ([CO]) of xylem sap under controlled conditions. Tree-specific linear relationships between rates of stem CO efflux (J) and xylem [CO] were found. The resistance to radial CO diffusion differed 6-fold among the trees and influenced the balance between the amount of CO retained in the xylem v. that which diffused to the atmosphere. Therefore, we hypothesised that variability in the resistance to radial CO diffusion might be an overlooked cause for the inconsistencies and large variations in woody tissue CO efflux. It was found that transition from light to dark conditions caused a rapid increase in J and xylem [CO], both in manipulated trees and in an intact tree with no sap manipulation. This resulted in an increased resistance to radial CO diffusion during the dark, at least for trees with smaller daytime resistances. Stem diameter changes measured in the intact tree supported the idea that higher actual respiration rates occurred at night owing to higher metabolism in relation to an improved water status and higher turgor pressure.
树木的茎和枝的CO外流速率在树木之间、树木内部以及林分之间存在很大差异。比例因子仅部分成功地解释了观察到的变化。在本研究中，通过在受控条件下直接操纵木质部汁液的CO浓度（[CO]），对东部杨属三角叶杨（Populus deltoides Bartr. ex Marsh.）无性系的树干径向CO扩散阻力进行了量化。发现了树干CO外流速率（J）与木质部[CO]之间的树木特异性线性关系。树木之间的径向CO扩散阻力相差6倍，并且影响了木质部中保留的CO量与扩散到大气中的CO量之间的平衡。因此，我们推测径向CO扩散阻力的变异性可能是木本组织CO外流不一致和变化较大的一个被忽视的原因。结果发现，从光照条件转变为黑暗条件会导致操纵树木和未进行汁液操纵的完整树木的J和木质部[CO]迅速增加。这导致在黑暗期间径向CO扩散阻力增加，至少对于白天阻力较小的树木是如此。在完整树木中测量的茎直径变化支持了这样的观点，即由于与改善的水分状况和更高的膨压相关的更高代谢，夜间实际呼吸速率更高。
