Departament de Biologia Vegetal, Unitat Fisiologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Diagonal 643, E-08028 Barcelona, Spain.
Tree Physiol. 2012 Jul;32(7):829-38. doi: 10.1093/treephys/tps035. Epub 2012 Apr 26.
The evergreen holm oak Quercus ilex L. is the most representative tree in Mediterranean forests. Accurate estimation of the limiting factors of photosynthesis for Q. ilex and the prediction of ecosystem water-use efficiency by mechanistic models can be achieved only by establishing whether this species shows heterogenic stomatal aperture, and, if so, the circumstances in which this occurs. Here, we collected gas-exchange and chlorophyll fluorescence data in Q. ilex leaves from a nursery to measure the effects of stomatal oscillations on PSII quantum yield (Φ(PSII)) under water stress. Stomatal conductance (g(s)) was used as an integrative indicator of the degree of water stress. Images of chlorophyll fluorescence showed heterogeneous Φ(PSII) when g(s) was <50 mmol H(2)O m(-2) s(-1), representative of severe drought and corresponding to a container capacity <45%. Stomatal patchiness was related to a coefficient of variation (CV) of Φ(PSII) values >2.5%. A parallel study in the forest confirmed heterogeneous Φ(PSII) values in leaves in response to declining water availability. Three kinds of Q. ilex individuals were distinguished: those resprouting after a clear-cut (resprouts, R); intact individuals growing in the same clear-cut area as resprouts (controls, C); and intact individuals in a nearby, undisturbed area (forest controls, CF). Patchiness increased in C and CF in response to increasing drought from early May to late July, whereas in R, Φ(PSII) values were maintained as a result of their improved water relations since the pre-existing roots were associated with a smaller aerial biomass. Patchiness was related to a % CV of Φ(PSII) values >4 and associated in the summer with mean g(s) values of 30 mmol H(2)O m(-2) s(-1). Under milder drought in spring, Φ(PSII) patchiness was less strictly related to g(s) variations, pointing to biochemical limitants of photosynthesis. The occurrence of heterogenic photosynthesis caused by patchy stomatal closure in Q. ilex during severe drought should be taken into account in ecosystem modelling in which harsher water stress conditions associated with climate change are predicted.
常绿的油橄榄 Quercus ilex L. 是地中海森林中最具代表性的树种。只有确定该物种是否具有异质的气孔开度，如果是，在什么情况下会出现这种情况，才能通过建立机制模型来准确估计油橄榄光合作用的限制因素，并预测生态系统的水分利用效率。在这里，我们从苗圃中收集了油橄榄叶片的气体交换和叶绿素荧光数据，以测量气孔振荡对 PSII 量子产量（Φ（PSII））在水分胁迫下的影响。气孔导度（g(s)）被用作水分胁迫程度的综合指标。当 g(s) < 50 mmol H(2)O m(-2) s(-1)时，叶绿素荧光图像显示出异质的 Φ(PSII)，这代表严重干旱，相当于容器容量 < 45%。气孔镶嵌与 Φ(PSII)值的变异系数（CV）> 2.5%有关。在森林中的平行研究证实，随着水分供应的减少，叶片中的 Φ(PSII)值呈现异质性。区分了三种油橄榄个体：经过砍伐后重新萌发的个体（重新萌发的个体，R）；在与重新萌发的个体相同的砍伐区域中生长的完整个体（对照，C）；以及在附近未受干扰的区域中的完整个体（森林对照，CF）。从 5 月初到 7 月底，随着干旱的加剧，C 和 CF 中的镶嵌性增加，而 R 中的 Φ(PSII)值保持不变，这是由于其水分关系得到改善，因为原有的根系与较小的地上生物量有关。镶嵌性与 Φ(PSII)值的 %CV > 4 有关，在夏季与平均 g(s)值 30 mmol H(2)O m(-2) s(-1)有关。在春季轻度干旱下，Φ(PSII)的镶嵌性与 g(s)变化的关系不那么严格，这表明光合作用存在生化限制因素。在生态系统建模中，应该考虑到油橄榄在严重干旱期间由于气孔不均匀关闭而导致的异质光合作用的发生，这种情况在预测与气候变化相关的更恶劣水分胁迫条件时会出现。
