Strategies of leaf water uptake based on anatomical traits

Soil-plant-gall relationships: from gall development to ecological patterns

The adaptive nature of the galler habit has been tentatively explained by the nutrition, microenvironment, and enemy hypotheses. Soil attributes have direct relationships with these three hypotheses at the cellular and macroecological scales, but their influence has been restricted previously to effects on the nutritional status of the host plant on gall richness and abundance. Herein, we discuss the ionome patterns within gall tissues and their significance for gall development, physiology, structure, and for the nutrition of the gallers. Previous ecological and chemical quantification focused extensively on nitrogen and carbon contents, evoking the carbon-nutrient defence hypothesis as an explanation for establishing the plant-gall interaction. Different elements are involved in cell wall composition dynamics, antioxidant activity, and regulation of plant-gall water dynamics. An overview of the different soil-plant-gall relationships highlights the complexity of the nutritional requirements of gallers, which are strongly influenced by environmental soil traits. Soil and plant chemical profiles interact to determine the outcome of plant-herbivore interactions and need to be addressed by considering not only the soil features and galler nutrition but also the host plant's physiological traits. The quantitative and qualitative results for iron metabolism in gall tissues, as well as the roles of iron as an essential element in the physiology and reproduction of gallers suggest that it may represent a key nutritional resource, aligning with the nutrition hypothesis, and providing an integrative explanation for higher gall diversity in iron-rich soils.

Integrative leaf anatomy structure, physiology, and metabolome analyses revealed the response to drought stress in sainfoin at the seedling stage

INTRODUCTION

Sainfoin (Onobrychis viciaefolia) is a vital legume forage, and drought is the primary element impeding sainfoin growth.

OBJECTIVE

The anatomical structure, physiological indexes, and metabolites of the leaves of sainfoin seedlings with a drought-resistant line of P1 (DRL) and a drought-sensitive material of 2049 (DSM) were analyzed under drought (-1.0 MPa) with polyethylene glycol-6000 (PEG-6000).

METHODS

The leaf anatomy was studied by the paraffin section method. The related physiological indexes were measured by the hydroxylamine oxidation method, titanium sulfate colorimetric method, thiobarbituric acid method, acidic ninhydrin colorimetric method, and Coomassie brilliant blue method. The metabolomics analysis was composed of liquid chromatography tandem high-resolution mass spectrometry (LC-MS/MS).

RESULTS

The results revealed that the thickness of the epidermis, palisade tissue, and sponge tissue of DRL were significantly greater than those of DSM. The leaves of DRL exhibited lower levels of superoxide anion (O2 •-) production rate, hydrogen peroxide (H2O2) content, and malondialdehyde (MDA) content compared with DSM, while proline (Pro) content and soluble protein (SP) content were significantly higher than those of DSM. A total of 391 differential metabolites were identified in two samples. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed that the primary differential metabolites were concentrated into the tyrosine metabolism; isoquinoline alkaloid biosynthesis; ubiquinone and other terpenoid quinone biosynthesis; neomycin, kanamycin, and gentamicin biosynthesis; and anthocyanin biosynthesis metabolic pathways.

CONCLUSION

Compared with DSM, DRL had more complete anatomical structure, lower active oxygen content, and higher antioxidant level. The results improved our insights into the drought-resistant mechanisms in sainfoin.

The multiple roles of trichomes in two Croton species

Trichomes are common in plants from dry environments, and despite their recognized role in protection and defense, little is known about their role as absorptive structures and in other aspects of leaf ecophysiology. We combine anatomical and ecophysiological data to evaluate how trichomes affect leaf gas exchange and water balance during drought. We studied two congeneric species with pubescent leaves which co-occur in Brazilian Caatinga: Croton blanchetianus (dense trichomes) and Croton adenocalyx (sparse trichomes). We found a novel foliar water uptake (FWU) pathway in C. blanchetianus composed of stellate trichomes and underlying epidermal cells and sclereids that interconnect the trichomes from both leaf surfaces. The water absorbed by these trichomes is redistributed laterally by pectin protuberances on mesophyll cell walls. This mechanism enables C. blanchetianus leaves to absorb water more efficiently than C. adenocalyx. Consequently, the exposure of C. blanchetianus to dew during drought improved its leaf gas exchange and water status more than C. adenocalyx. C. blanchetianus trichomes also increase their leaf capacity to reflect light and maintain lower temperatures during drought. Our results emphasize the multiple roles that trichomes might have on plant functioning and the importance of FWU for the ecophysiology of Caatinga plants during drought.

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.

Towards multivariate functional trait syndromes: Predicting foliar water uptake in trees

Analysis of functional traits is a cornerstone of ecology, yet individual traits seldom explain useful amounts of variation in species distribution or climatic tolerance, and their functional significance is rarely validated experimentally. Multivariate suites of interacting traits could build an understanding of ecological processes and improve our ability to make sound predictions of species success in our rapidly changing world. We use foliar water uptake capacity as a case study because it is increasingly considered to be a key functional trait in plant ecology due to its importance for stress-tolerance physiology. However, the traits behind the trait, that is, the features of leaves that determine variation in foliar water uptake rates, have not been assembled into a widely applicable framework for uptake prediction. Focusing on trees, we investigated relationships among 25 structural traits, leaf osmotic potential (a source of free energy to draw water into leaves), and foliar water uptake in 10 diverse angiosperm and conifer species. We identified consistent, multitrait "uptake syndromes" for both angiosperm and conifer trees, with differences in key traits revealing suspected differences in the water entry route between these two clades and an evolutionarily significant divergence in the function of homologous structures. A literature review of uptake-associated functional traits, which largely documents similar univariate relationships, provides additional support for our proposed "uptake syndrome." Importantly, more than half of shared traits had opposite-direction influences on the capacity of leaves to absorb water in angiosperms and conifers. Taxonomically targeted multivariate trait syndromes provide a useful tool for trait selection in ecological research, while highlighting the importance of micro-traits and the physiological verification of their function for advancing trait-based ecology.

Foliar water uptake in Pinus species depends on needle age and stomatal wax structures

BACKGROUND AND AIMS

Foliar water uptake (FWU) has been documented in many species and is increasingly recognized as a non-trivial factor in plant-water relationships. However, it remains unknown whether FWU is a widespread phenomenon in Pinus species, and how it may relate to needle traits such as the form and structure of stomatal wax plugs. In this contribution, these questions were addressed by studying FWU in current-year and 1-year-old needles of seven Pinus species.

METHODS

We monitored FWU gravimetrically and analysed the needle surface via cryo-scanning electron microscopy. Additionally, we considered the effect of artificial wax erosion by application of the surfactant Triton X-100, which is able to alter wax crystals.

KEY RESULTS

The results show for all species that (1) FWU occurred, (2) FWU is higher in old needles compared to young needles and (3) there is substantial erosion of stomatal wax plugs in old needles. FWU was highest in Pinus canariensis, which has a thin stomatal wax plug. Surfactant treatment enhanced FWU.

CONCLUSIONS

The results of this study provide evidence for (1) widespread FWU in Pinus, (2) the influence of stomatal wax plugs on FWU and (3) age-related needle surface erosion.

Do photosynthetic metabolism and habitat influence foliar water uptake in orchids?

Epiphytic and rupicolous plants inhabit environments with limited water resources. Such plants commonly use Crassulacean Acid Metabolism (CAM), a photosynthetic pathway that accumulates organic acids in cell vacuoles at night, so reducing their leaf water potential and favouring water absorption. Foliar water uptake (FWU) aids plant survival during drought events in environments with high water deficits. We hypothesized that FWU represents a strategy employed by epiphytic and rupicolous orchids for water acquisition and that CAM will favour increased water absorption. We examined 6 epiphyte, 4 terrestrial and 6 rupicolous orchids that use C3 (n = 9) or CAM (n = 7) pathways. Five individuals per species were used to evaluate FWU, structural characteristics and leaf water balance. Rupicolous species with C3 metabolism had higher FWU than other species. FWU (C_max and k) could be related to succulence, SLM and leaf RWC. The results indicated that high orchid leaf densities favoured FWU, as area available for water storage increases with leaf density. Structural characteristics linked to water storage (e.g. high RWC, succulence), on the other hand, could limit leaf water absorption by favouring high internal leaf water potentials. Epiphytic, rupicolous and terrestrial orchids showed FWU. Rupicolous species had high levels of FWU, probably through absorption from mist. However, succulence in plants with CAM appears to mitigate FWU.

Leaf plasticity across wet and dry seasons in Croton blanchetianus (Euphorbiaceae) at a tropical dry forest

Plant species of the Brazilian Caatinga experience seasonal wet and dry extremes, requiring seasonally different leaf characteristics for optimizing water availability. We investigated if Croton blanchetianus Baill exhibits leaf morphoanatomical traits across seasons and positioning in sunlight/natural shade. Leaves of ten 1-3 m tall plants in full sunlight and ten in natural shade were assessed in May, July (wet season), October and December (dry season) 2015 for gas exchange, leaf size, lamina and midrib cross sections (14 parameters), and chloroplast structure (5 parameters). Net photosynthesis was greater during the wet season (21.6 µm⁻² s⁻¹) compared to the dry season (5.8 µm⁻² s⁻¹) and was strongly correlated with almost all measured parameters (p < 0.01). Shaded leaves in the wet season had higher specific leaf area (19.9 m² kg⁻¹ in full-sun and 23.1 m² kg⁻¹ in shade), but in the dry season they did not differ from those in full sun (7.5 m² kg⁻¹ and 7.2 m² kg⁻¹). In the wet season, the expansion of the adaxial epidermis and mesophyll lead to larger and thicker photosynthetic area of leaves. Furthermore, chloroplast thickness, length and area were also significantly larger in full sunlight (2.1 μm, 5.1 μm, 15.2 μm²; respectively) and shaded plants (2.0 μm, 5.2 μm, 14.8 μm²; respectively) during wetter months. Croton blanchetianus exhibits seasonal plasticity in leaf structure, presumably to optimize water use efficiency during seasons of water abundance and deficit. These results suggest that the species is adaptable to the increased drought stress projected by climate change scenarios.

Cuticular wax composition contributes to different strategies of foliar water uptake in six plant species from foggy rupestrian grassland in tropical mountains

The cuticle is the outermost region of the epidermal cell wall of plant aerial organs. The cuticle acts as a two-way lipid barrier for water diffusion; therefore, it plays a vital role in foliar water uptake (FWU). We hypothesised that the chemical composition of the cuticular waxes influences the FWU strategy that plants adopt in a foggy tropical ecosystem. We analysed the leaf cuticular waxes of six plant species known by their different FWU strategies, in both qualitative and quantitative approaches, to test this hypothesis. We also investigated the fine structure of the plant cuticle by scanning electron microscopy. Neither the total wax loads nor the amounts of single wax compound classes correlated to the FWU. In contrast, the qualitative chemical composition of the cuticular waxes was related to the water absorption speed but not to the maximum water absorbed. The presence of wax crystals might interfere with the FWU. Our findings suggest that a complex three-dimensional network of the cuticular compounds contributes to different strategies of FWU in six plant species from foggy tropical mountaintops.

Photosynthetic heat tolerance in plants with different foliar water -uptake strategies

PREMISE

The distribution and even the survival of plant species are influenced by temperature. In an old climatically buffered infertile landscape (OCBIL) in Brazil, we previously characterized different strategies for foliar water uptake (FWU). It is possible that photosystem II tolerance to heat and excessive light intensity varies among species with different FWU capacities.

METHODS

The relationship between FWU, photoinhibition, and thermotolerance was investigated in seven species from this ecosystem.

RESULTS

The species with slow water absorption and high water absorption are those that presented less photoinhibition. Contrastingly, the species that have fast and low water absorption presented greater thermotolerance when their leaves are totally hydrated. However, when there is greater leaf dehydration, the most thermotolerant species were those with slow but high water absorption.

CONCLUSIONS

Foliar water uptake is an important trait for plants to tolerate excessive light intensity and higher temperatures. Plants in this OCBIL may be differentially affected by future global warming, and the best strategy to deal with this expected climate change is with slow and high absorption of water.

The Neglected Reverse Water Pathway: Atmosphere-Plant-Soil Continuum

The soil-plant-atmosphere continuum (SPAC) describes the continuous water movement from soil via plants to atmosphere. Here, we propose to name the reverse water pathway, driven by foliar water uptake, the atmosphere-plant-soil continuum (APSC). We highlight the different hydraulic resistances this reverse water movement has to overcome.

Foliar Water Uptake in Trees: Negligible or Necessary?

Foliar water uptake (FWU) has been identified as a mechanism commonly used by trees and other plants originating from various biomes. However, many questions regarding the pathways and the implications of FWU remain, including its ability to mitigate climate change-driven drought. Therefore, answering these questions is of primary importance to adequately address and comprehend drought stress responses and associated growth. In this review, we discuss the occurrence, pathways, and consequences of FWU, with a focus predominantly on tree species. Subsequently, we highlight the tight coupling between FWU and foliar fertilizer applications, discuss FWU in a changing climate, and conclude with the importance of including FWU in mechanistic vegetation models.

Oxidative metabolism in plants from Brazilian rupestrian fields and its relation with foliar water uptake in dry and rainy seasons

In an altitudinal Brazilian ecosystem, fog is a frequent event in both the dry and rainy seasons. The drought stress is aggravated in the dry season due to elevated evaporative demand and this constraint can induce oxidative stress in plants. In this ecosystem, there are plants which present different foliar water uptake (FWU) capacities - species that absorb less water more quickly (LQ) and those that absorb more water more slowly (MS). In this study, the relationship between oxidative stress responses and the different FWU strategies was evaluated in dry and rainy seasons. The oxidative stress was assessed by H₂O₂ production and lipid peroxidation as well as by the antioxidant enzymes system as superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX). During the dry season, plants had higher oxidative stress compared to rainy season plants which exhibited moderate oxidative damage. The FWU strategies were closely related to oxidative stress responses, since the LQ species presented the higher H₂O₂ content and oxidative defense system. Contrastingly, it was found that MS species have the lowest values of H₂O₂ and less SOD, CAT and APX activities. However, the lipid peroxidation did not present any relation with FWU strategies. Altogether results revealed that plants, which present MS strategy, are more adapted to cope with the higher H₂O₂ concentrations generated in the dry season and drought stress events than those that present LQ strategy.

Foliar water-uptake strategies are related to leaf water status and gas exchange in plants from a ferruginous rupestrian field

PREMISE

Fog is a frequent event in Brazilian rupestrian field and plays an important role in the physiology of several plant species. Foliar water uptake (FWU) of fog may be fast or slow depending on the species. However, fog water may negatively affect CO₂ assimilation. Thus, the interference in the water and carbon balance as a result of different strategies of FWU was evaluated to verify whether fog may mitigate possible water deficit in leaves.

METHODS

Four plant species with different FWU strategies were studied in a ferruginous rupestrian field with frequent fog. Gas exchange and water potential were measured before dawn and at midday during the dry and rainy seasons, separating foggy from non-foggy days during the dry season.

RESULTS

The FWU speed negatively influences CO₂ assimilation in the dry season, possibly because of its negative relationship with stomatal conductance, since reduced stomatal aperture impairs carbon entrance. Fog presence increased leaf water potential both in early morning and midday during the dry season. However, during the rainy season, the values of leaf water potential were lower at midday, than during the dry season with fog at midday, which favors leaf gas exchanges.

CONCLUSIONS

FWU interferes negatively, but briefly with CO₂ assimilation. Nevertheless, FWU prevents water loss through transpiration and increases the water status of plants in the dry season. That is, FWU results in a compensation between CO₂ assimilation and foliar hydration, which, in fact, is beneficial to the plants of this ecosystem.