Horton J L, Kolb T E, Hart S C
School of Forestry, College of Ecosystem Science and Management, Northern Arizona University, Flagstaff 86011-5018, USA.
Tree Physiol. 2001 Mar;21(4):233-41. doi: 10.1093/treephys/21.4.233.
We investigated leaf gas exchange responses to leaf temperature, leaf-to-air vapor pressure deficit (VPD), and predawn and midday shoot water potential (psipd and psimd, respectively) of two native Sonoran Desert riparian tree species, Fremont cottonwood (Populus fremontii S. Wats.) and Goodding willow (Salix gooddingii Ball), and one exotic riparian tree species, saltcedar (Tamarix chinensis Lour. and related species). Measurements were made at two sites over 2 years that differed climatically. Because multiple linear regression models explained less than 29% of the variation in stomatal conductance (gs) and less than 48% of the variation in net photosynthetic rate (Pn) of all species, we used boundary-line analysis to compare gas exchange responses among species. Gas exchange rates were high in all species. The hyperbolic relationship between Pn and gs suggested that initial reductions in gs at high gs did not inhibit Pn. Reductions in gs of cottonwood and willow occurred at psimd values at or below previously reported xylem cavitation thresholds (-1.6 and -1.4 MPa, respectively), indicating tight stomatal regulation of water loss and a narrow cavitation safety margin. In contrast, reductions in gs of saltcedar occurred at psimd values well above the cavitation threshold (-7.0 MPa), but at much lower psimd values than in cottonwood and willow, suggesting a wider cavitation safety margin and less tight regulation of water loss in saltcedar. High VPD had a smaller effect on leaf gas exchange in willow than in cottonwood. In contrast, willow had a less negative psipd threshold for stomatal closure than cottonwood. Compared with cottonwood and willow, leaf gas exchange of saltcedar was more tolerant of high VPD and low psipd. These physiological characteristics of saltcedar explain its widespread success as an invader of riparian ecosystems containing native Fremont cottonwood and Goodding willow in the Sonoran Desert.
我们研究了索诺兰沙漠原生河岸带两种树种（弗里蒙特杨（Populus fremontii S. Wats.）和古丁柳（Salix gooddingii Ball））以及一种外来河岸带树种（柽柳（Tamarix chinensis Lour.及相关物种））的叶片气体交换对叶片温度、叶-气蒸汽压亏缺（VPD）以及黎明前和中午嫩枝水势（分别为ψpd和ψmd）的响应。在气候条件不同的两个地点进行了为期两年的测量。由于多元线性回归模型对所有物种气孔导度（gs）变化的解释率低于29%，对净光合速率（Pn）变化的解释率低于48%，因此我们使用边界线分析来比较物种间的气体交换响应。所有物种的气体交换速率都很高。Pn与gs之间的双曲线关系表明，在高gs时gs的初始降低并未抑制Pn。杨树和柳树的gs降低发生在ψmd值等于或低于先前报道的木质部空化阈值时（分别为-1.6和-1.4 MPa），这表明气孔对水分损失的调控严格，空化安全边际较窄。相比之下，柽柳的gs降低发生在ψmd值远高于空化阈值（-7.0 MPa）时，但比杨树和柳树的ψmd值低得多，这表明柽柳有空化安全边际更宽且对水分损失的调控不那么严格。高VPD对柳树叶片气体交换的影响比对杨树的影响小。相比之下，柳树气孔关闭的ψpd阈值比杨树的负性更小。与杨树和柳树相比，柽柳的叶片气体交换对高VPD和低ψpd更具耐受性。柽柳的这些生理特征解释了其作为索诺兰沙漠中含有原生弗里蒙特杨和古丁柳的河岸生态系统入侵者广泛成功的原因。
