Foliar water uptake via cork warts in mangroves of the Sonneratia genus

Anatomical adaptations of mangroves to the intertidal environment and their dynamic responses to various stresses

Mangroves are intertidal plants that survive extreme environmental conditions through unique adaptations. Various reviews on diverse physiological and biochemical stress responses of mangroves have been published recently. However, a review of how mangroves respond anatomically to stresses is lacking. This review presents major mangrove anatomical adaptations and their modifications in response to dynamic environmental stresses such as high salinity, flooding, extreme temperatures, varying light intensities, and pollution. The available research shows that plasticity of Casparian strips and suberin lamellae, variations in vessel architecture, formation of aerenchyma, thickening of the cuticle, and changes in the size and structure of salt glands occur in response to various stresses. Mangrove species show different responses correlated with the diversity and intensity of the stresses they face. The flexibility of these anatomical adaptations represents a key feature that determines the survival and fitness of mangroves. However, studies demonstrating these mechanisms in detail are relatively scarce, highlighting the need for further research. An in-depth understanding of the structural adaptations of individual mangrove species could contribute to appropriate species selection in mangrove conservation and restoration activities.

Vegetation vulnerability is driven by either higher drought sensitivity or lower fog exposure in tropical cloud ecosystems

Both reduced precipitation and reduced fog uplift increase drought-driven plant mortality. However, it is still unclear how plant vulnerability to drought in cloud ecosystems depends on the role of fog in relieving water stress via foliar water uptake (FWU). To investigate how plants in contrasting montane vegetation rely on fog to alleviate drought impacts, we measured 11 morpho-physiological traits in 10 phylogenetic pairs of plants in a montane grassland (~2000 m a.s.l.) and in a submontane forest (~700 m a.s.l.), both in southeast Brazil. Forest species are more sensitive to drought (i.e., lower conservative trait values, lower resistance to embolism, and lower FWU) than grassland species. Nonetheless, decreased frequency of fog events in the montane grassland may expose these species to a higher risk of dehydration, despite higher FWU capacity. Both forest and grassland vegetation are vulnerable to drought, but the vulnerability is attributable to different causes: higher sensitivity to drought in forests and lower fog exposure in grasslands. Therefore, for a more accurate description of plant responses to drought, we recommend introduction of theoretical-experimental models to assess drought vulnerability to changes in both atmospheric and soil water availability.

Foliar Water Uptake Supports Water Potential Recovery but Does Not Affect Xylem Sap Composition in Two Salt-Secreting Mangroves

Mangroves are highly salt-tolerant species, which live in saline intertidal environments, but rely on alternative, less saline water to maintain hydraulic integrity and plant productivity. Foliar water uptake (FWU) is thought to assist in hydration of mangroves, particularly during periods of acute water deficit. We investigated the dynamics of FWU in Avicennia marina and Aegiceras corniculatum by submerging and spraying excised branches and measuring leaf water potential (Ψ) at different time intervals. Daily changes in xylem sap composition (ionic concentrations, pH and surface tension) were monitored during 2 days characterised by the presence of morning dew and difference in tides. In both species, FWU occurred over relatively short times, with leaf Ψ recovering from -4.5 MPa to about -1.5 MPa in 120-150 min. At predawn, Ψ was higher (-1.5 MPa) than sea water Ψ, indicating that leaves had been partially rehydrated by absorbed dew. Tides did not affect Ψ, but high tides increased the overall ionic content of xylem sap. The results indicated mangroves are extremely efficient in absorbing non-saline water via the leaves and restoring the water balance to Ψ higher than seawater. Changes in xylem sap composition, which were strongly influenced by tides, were not affected by observed FWU.

Revisiting an ecophysiological oddity: Hydathode-mediated foliar water uptake in Crassula species from southern Africa

Hydathodes are usually associated with water exudation in plants. However, foliar water uptake (FWU) through the hydathodes has long been suspected in the leaf-succulent genus Crassula (Crassulaceae), a highly diverse group in southern Africa, and, to our knowledge, no empirical observations exist in the literature that unequivocally link FWU to hydathodes in this genus. FWU is expected to be particularly beneficial on the arid western side of southern Africa, where up to 50% of Crassula species occur and where periodically high air humidity leads to fog and/or dew formation. To investigate if hydathode-mediated FWU is operational in different Crassula species, we used the apoplastic fluorescent tracer Lucifer Yellow in combination with different imaging techniques. Our images of dye-treated leaves confirm that hydathode-mediated FWU does indeed occur in Crassula and that it might be widespread across the genus. Hydathodes in Crassula serve as moisture-harvesting structures, besides their more common purpose of guttation, an adaptation that has likely played an important role in the evolutionary history of the genus. Our observations suggest that ability for FWU is independent of geographical distribution and not restricted to arid environments under fog influence, as FWU is also operational in Crassula species from the rather humid eastern side of southern Africa. Our observations point towards no apparent link between FWU ability and overall leaf surface wettability in Crassula. Instead, the hierarchically sculptured leaf surfaces of several Crassula species may facilitate FWU due to hydrophilic leaf surface microdomains, even in seemingly hydrophobic species. Overall, these results confirm the ecophysiological relevance of hydathode-mediated FWU in Crassula and reassert the importance of atmospheric humidity for some arid-adapted plant groups.

Bark water uptake through lenticels increases stem hydration and contributes to stem swelling

Foliar water uptake can recharge water storage tissue and enable greater hydration than through access to soil water alone; however, few studies have explored the role of the bark in facilitating water uptake. We investigated pathways and dynamics of bark water uptake (BWU) in stems of the mangrove Avicennia marina. We provide novel evidence that specific entry points control dynamics of water uptake through the outer bark surface. Furthermore, using a fluorescent symplastic tracer dye we provide the first evidence that lenticels on the outer bark surface facilitate BWU, thus increasing stem water content by up to 3.7%. X-ray micro-computed tomography showed that BWU was sufficient to cause measurable swelling of stem tissue layers increasing whole stem cross-sectional area by 0.83 mm2 or 2.8%, implicating it as a contributor to the diel patterns of water storage recharge that buffer xylem water potential and maintain hydration of living tissue.

Foliar Water Uptake Capacity in Six Mangrove Species

Foliar water uptake (FWU) is a mechanism that enables plants to acquire water from the atmosphere through their leaves. As mangroves live in a saline sediment water environment, the mechanism of FWU might be of vital importance to acquire freshwater and grow. The goal of this study was to assess the FWU capacity of six different mangrove species belonging to four genera using a series of submersion experiments in which the leaf mass increase was measured and expressed per unit leaf area. The foliar water uptake capacity differed between species with the highest and lowest average water uptake in Avicennia marina (Forssk.) Vierh. (1.52 ± 0.48 mg H2O cm−2) and Bruguiera gymnorhiza (L.) Lam. (0.13 ± 0.06 mg H2O cm−2), respectively. Salt-excreting species showed a higher FWU capacity than non-excreting species. Moreover, A. marina, a salt-excreting species, showed a distinct leaf anatomical trait, i.e., trichomes, which were not observed in the other species and might be involved in the water absorption process. The storage of leaves in moist Ziplock bags prior to measurement caused leaf water uptake to already occur during transport to the field station, which proportionately increased the leaf water potential (A. marina: −0.31 ± 0.13 MPa and B. gymnorhiza: −2.70 ± 0.27 MPa). This increase should be considered when performing best practice leaf water potential measurements but did not affect the quantification of FWU capacity because of the water potential gradient between a leaf and the surrounding water during submersion. Our results highlight the differences that exist in FWU capacity between species residing in the same area and growing under the same environmental conditions. This comparative study therefore enhances our understanding of mangrove species’ functioning.