Department of Biology and Geology, Universidad de Almería, Carretera de Sacramento s.n, La Cañada de San Urbano, Almería, 04120, Spain.
Andalusian Centre for the Monitoring and Assessment of Global Change (CAESCG), University of Almería, Spain.
Am J Bot. 2021 Feb;108(2):236-248. doi: 10.1002/ajb2.1606. Epub 2021 Feb 14.
Water is the most limiting factor in dryland ecosystems, and plants are adapted to cope with this constraint. Particularly vulnerable are phreatophytic plants from groundwater-dependent ecosystems (GDEs) in regions that have to face water regime alterations due to the impacts of climate and land-use changes.
We investigated two aspects related to the water-use strategy of a keystone species that dominates one of the few terrestrial GDEs in European drylands (Ziziphus lotus): where it obtains water and how it regulates its use. We (1) evaluated plants' water sources and use patterns using a multiple-isotope approach (δ H, δ O, and Δ C); (2) assessed the regulation of plant water potential by characterizing the species on an isohydric-anisohydric continuum; and (3) evaluated plants' response to increasing water stress along a depth-to-groundwater (DTGW) gradient by measuring foliar gas exchange and nutrient concentrations.
Ziziphus lotus behaves as a facultative or partial phreatophyte with extreme anisohydric stomatal regulation. However, as DTGW increased, Z. lotus (1) reduced the use of groundwater, (2) reduced total water uptake, and (3) limited transpiration water loss while increasing water-use efficiency. We also found a physiological threshold at 14 m depth to groundwater, which could indicate maximum rooting length beyond which optimal plant function could not be sustained.
Species such as Z. lotus survive by squandering water in drylands because of a substantial groundwater uptake. However, the identification of DTGW thresholds indicates that drawdowns in groundwater level would jeopardize the functioning of the GDE.
水是旱地生态系统中最具限制性的因素，而植物则适应于应对这种限制。在由于气候和土地利用变化的影响而必须面对水情改变的地区，来自地下水依赖型生态系统（GDE）的地下植物尤其脆弱。
我们调查了主导欧洲旱地中少数几个陆地 GDE 之一的关键物种（Ziziphus lotus）的水利用策略的两个方面：它从何处获取水以及如何调节其利用。我们（1）使用多种同位素方法（δ H、δ O 和 Δ C）评估了植物的水源和利用模式；（2）通过特征化物种在等水势-异水势连续体上的特性来评估植物水分势的调节；（3）通过测量叶片气体交换和养分浓度，评估植物对沿地下水深度（DTGW）梯度的增加的水分胁迫的响应。
Ziziphus lotus 表现出兼性或部分地下植物的特性，具有极端的异水势气孔调节。然而，随着 DTGW 的增加，Z. lotus（1）减少了对地下水的利用，（2）减少了总水分吸收，（3）在增加水分利用效率的同时限制了蒸腾水分损失。我们还在离地下水 14 米的深度处发现了一个生理阈值，这可能表明最大根系长度超过该长度就无法维持最佳植物功能。
像 Z. lotus 这样的物种由于大量地下水的吸收而在干旱地区浪费水得以生存。然而，DTGW 阈值的确定表明，地下水水位的下降将危及 GDE 的功能。
