Ellsworth D S, Reich P B
Department of Forestry, University of Wisconsin-Madison, 120 Russell Laboratories, Madison, Wisconsin 53706, USA.
Tree Physiol. 1992 Jan;10(1):1-20. doi: 10.1093/treephys/10.1.1.
Field measurements were made of leaf photosynthesis (A), stomatal conductance (g) and leaf water relations for sugar maple (Acer saccharum Marsh.) seedlings growing in a forest understory, small gap or large clearing habitat in southwestern Wisconsin, USA. Predawn water status, leaf gas exchange and plasticity in field and laboratory water relations characteristics were compared among contrasting light environments in a wet year (1987) and a dry year (1988) to evaluate possible interactions between light and water availability in these habitats. Leaf water potentials (Psi(leaf)) at predawn and midday were lower for clearing than gap or understory seedlings. Acclimation of tissue osmotic potentials to light environment was observed among habitats but did not occur within any of the habitats in response to prolonged drought. During a summer drought in 1988, decreases in daily maximum g (g(max)) and maximum A (A(max)) in clearing seedlings were correlated with predawn Psi(leaf), which reached a seasonal minimum of -2.0 MPa. Under well-watered conditions, diurnal fluctuations in Psi(leaf) of up to 2.0 MPa in clearing seedlings occurred along with large midday depressions of A and g. In a wet year, strong stomatal responses to leaf-to-air vapor pressure difference (VPD) in sunny habitats were observed over nine diurnal courses of gas exchange measurements on seedlings in a gap and a clearing. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in A at high VPD for clearing seedlings. In understory seedlings, however, low water-use efficiency and development of leaf water deficits in sunflecks was related to reduced stomatal limitations to photosynthesis relative to seedlings in sunny habitats. Predawn Psi(leaf) and VPD appear to be important factors limiting carbon assimilation in sugar maple seedlings in light-saturating irradiances, primarily through stomatal closure. The overall results are consistent with the idea that sugar maple seedlings exhibit "conservative" water use patterns and have low drought tolerance. Leaf water relations and patterns of water use should be considered in studies of acclimation and species photosynthetic performance in contrasting light environments.
对生长在美国威斯康星州西南部森林林下、小间隙或大开阔地生境中的糖枫（Acer saccharum Marsh.）幼苗进行了叶片光合作用（A）、气孔导度（g）和叶片水分关系的实地测量。在湿润年份（1987年）和干旱年份（1988年），比较了不同光照环境下黎明前水分状况、叶片气体交换以及田间和实验室水分关系特征的可塑性，以评估这些生境中光照和水分有效性之间可能的相互作用。开阔地幼苗黎明前和中午的叶片水势（Ψ(leaf)）低于间隙或林下幼苗。在不同生境中观察到组织渗透势对光照环境的适应性，但在任何生境中，组织渗透势都未因长期干旱而发生变化。在1988年的夏季干旱期间，开阔地幼苗的日最大气孔导度（g(max)）和最大光合速率（A(max)）的降低与黎明前的Ψ(leaf)相关，黎明前的Ψ(leaf)达到季节性最小值-2.0 MPa。在水分充足的条件下，开阔地幼苗的Ψ(leaf)日波动高达2.0 MPa，同时中午A和g大幅下降。在湿润年份，通过对间隙和开阔地幼苗进行九个昼夜的气体交换测量，观察到在阳光充足的生境中，气孔对叶-气蒸汽压差（VPD）有强烈反应。对于开阔地幼苗，在高VPD下光合速率降低似乎是由于气孔对光合作用的限制增加所致。然而，对于林下幼苗，水分利用效率低以及光斑中叶片水分亏缺的发展与相对于阳光充足生境中的幼苗而言，气孔对光合作用的限制降低有关。黎明前的Ψ(leaf)和VPD似乎是限制光饱和辐照下糖枫幼苗碳同化的重要因素，主要是通过气孔关闭来实现。总体结果与糖枫幼苗表现出“保守”的水分利用模式且耐旱性低的观点一致。在研究不同光照环境下的适应性和物种光合性能时，应考虑叶片水分关系和水分利用模式。
