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Premise 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. Methods 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 (δ2H, δ18O, and Δ13C); (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. Results 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. Conclusions 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.

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Water is the main limiting factor for groundwater-dependent ecosystems (GDEs) in drylands. Predicted climate change (precipitation reductions and temperature increases) and anthropogenic activities such as groundwater drawdown jeopardize the structure and functioning of these ecosystems, presenting new challenges for their management. We developed a trait-based analysis to examine the spatiotemporal variability in the ecophysiology of Ziziphus lotus, a phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional and hydraulic traits along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the growing season of the species. Increasing DTGW and salinity negatively affected photosynthetic and transpiration rates, increasing plant water stress (lower predawn and midday water potential), and positively affected Huber value (sapwood cross-sectional area per leaf area), reducing leaf area and likely, plant hydraulic demand. However, higher atmospheric evaporative demand fostered higher transpiration rates and water stress. Differences in climatic conditions throughout the growing season drove temporal variability in Z. lotus responses along the DTGW gradient, with warmer and drier conditions promoting carbon assimilation and water loss more intensively at shallow water tables. This multiple-trait analysis allowed us to identify plant ecophysiological thresholds related to the increase in DTGW and evaporative demand during the growing season. These findings highlight the existence of tipping points in the ecophysiological functioning of phreatophytic plants in drylands, which contribute to disentangle the functional responses of the related GDEs under groundwater detriment because of climate change effects.

Water is the main limiting factor for groundwater-dependent ecosystems (GDEs) in drylands. Predicted climate change (precipitation reductions and temperature increases) and anthropogenic activities such as groundwater drawdown jeopardise the functioning of these ecosystems, presenting new challenges for their management. We developed a trait-based analysis to examine the spatiotemporal variability in the ecophysiology of Ziziphus lotus, a long-lived phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional traits and stem water potential along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the species-growing season. Increasing DTGW and salinity negatively affected photosynthetic and transpiration rates, increasing plant water stress (lower predawn and midday water potential), and positively affected Huber value (sapwood cross-sectional area per leaf area), reducing leaf area and likely, plant hydraulic demand. However, the species showed greater salt-tolerance at shallow depths. Despite groundwater characteristics, higher atmospheric evaporative demand in the study area, which occurred in summer, fostered higher transpiration rates and water stress, and promoted carbon assimilation and water loss more intensively at shallow water tables. This multiple-trait analysis allowed us to identify plant ecophysiological thresholds related to the increase in salinity, but mostly in DTGW (13 m), and in the evaporative demand during the growing season. These findings highlight the existence of tipping points in the functioning of a long-lived phreatophyte in drylands and can contribute to the sustainable management of GDEs in southern Europe, paving the way for further studies on phreatophytic species.

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Studies of pollen wall development produce a great deal of morphological data that supplies useful information regarding taxonomy and systematics. We present the exine development of Euptelea and Pteridophyllum, two taxa whose pollen wall development has never previously been studied using transmission electron microscopy. Both genera are representatives of the two earliest-diverging families of the order Ranunculales and their pollen data are important for the diagnosis of the ancestral pollen features in eudicots. Our observations show these genera are defined by having microechinate microreticulate exine ornamentation, perforate tectum, columellate morphology of the infratectum and the existence of a foot layer and endexine. The presence of lamellations is detected during the early stages of development in the nexine of both genera, especially in the apertures. Euptelea presents remains of the primexine layer during the whole maturation process, a very thin foot layer, and a laminate exinous oncus in the apertural region formed by ectexine and endexine elements. Pteridophyllum has a thicker tectum than Euptelea, a continuous foot layer and a thicker endexine. In the apertures, the exinous oncus is formed by islets and granules of endexine, in contrast to the Euptelea apertures. The secretory tapetum produces orbicules in both genera, but they have different morphology and electron-density. Comparisons with pollen data from related orders and families confirm the ancestral states for the pollen of eudicots proposed in previous studies: reticulate and echinate surfaces, columellate infractectum and a thin foot layer relative to the thickness of the ectexine. According to our observations, we propose considering the possibility of a polymorphic state for the aperture number in the ancestor of Ranunculales, and suggest the development of orbicules as the ancestral state in this order.

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