Foliar water and solute absorption: an update

The absorption of water and solutes by plant leaves has been recognised since more than two centuries. Given the polar nature of water and solutes, the mechanisms of foliar uptake have been proposed to be similar for water and electrolytes, including nutrient solutions. Research efforts since the 19th century focussed on characterising the properties of cuticles and applying foliar sprays to crop plants as a tool for improving crop nutrition. This was accompanied by the development of hundreds of studies aimed at characterising the chemical and structural nature of plant cuticles from different species and the mechanisms of cuticular and, to a lower extent, stomatal penetration of water and solutes. The processes involved are complex and will be affected by multiple environmental, physico-chemical and physiological factors which are only partially clear to date. During the last decades, the body of evidence that water transport across leaf surfaces of native species may contribute to water balances (absorption and loss) at an ecosystem level has grown. Given the potential importance of foliar water absorption for many plant species and ecosystems as shown in recent studies, the aim of this review is to first integrate current knowledge on plant surface composition, structure, wettability and physico-chemical interactions with surface-deposited matter. The different mechanisms of foliar absorption of water and electrolytes and experimental procedures for tracing the uptake process are discussed before posing several outstanding questions which should be tackled in future studies.     

ECTODESMATA AND FOLIAR ABSORPTION

Franke , Wolfgang . (U. Bonn, Germany.) Ectodesmata and foliar absorption. Amer. Jour. Bot. 48(8): 683–691. Illus. 1961.—Plasmodesmata, called ectodesmata, in the outer walls of epidermal cells, have been investigated. Their occurrence and distribution in the epidermis of leaves of Plantago major and Helxine soleirolii have been examined in connection with foliar absorption. Leaf structures such as guard-cells, conical hairs, anticlinal walls and the epidermal cells adjacent to the leaf veins have been shown consistently to contain large numbers of ectodesmata, while in neighboring cells ectodesmata may be low in number or lacking. The same areas also are known to be especially pervious to water and dissolved dyes applied to the surface of the leaf. From special investigations, it appears that certain solutions that form visible crystals and precipitates in the outer wall enter the epidermal cell wall in localized pathways. The localization of these bodies coincides with that of ectodesmata. Therefore, it is concluded that the ectodesmata may be the pathways for transport of substances from the outside to the interior of tissues and vice versa. Nutrients applied to the surface of leaves are thought to enter by the same pathways, i.e., the ectodesmata, as those in which the penetration can be visibly detected. Some phenomena of foliar absorption which confirm this theory are explained in connection with the presence of ectodesmata.     

The periodic wetting of leaves enhances water relations and growth of the long‐lived conifer Araucaria angustifolia

The importance of foliar absorption of water and atmospheric solutes in conifers was recognised in the 1970s, and the importance of fog as a water source in forest environments has been recently demonstrated. Araucaria angustifolia (Araucariaceae) is an emergent tree species that grows in montane forests of southern Brazil, where rainfall and fog are frequent events, leading to frequent wetting of the leaves. Despite anatomical evidence in favour of leaf water absorption, there is no information on the existence and physiological significance of a such process. In this study, we test the hypothesis that the use of atmospheric water by leaves takes place and is physiologically relevant for the species, by comparing growth, water relations and nutritional status between plants grown under two conditions of soil water (well‐watered and water‐stressed plants) and three types of leaf spraying (none, water and nutrient solution spray). Leaf spraying had a greater effect in improving plant water relations when plants were under water stress. Plant growth was more responsive to water available to the leaves than to the roots, and was equally increased by both types of leaf spraying, with no interaction with soil water status. Spraying leaves with nutrient solution increased shoot ramification and raised the concentrations of N, P, K, Zn, Cu and Fe in the roots. Our results provide strong indications that water and nutrients are indeed absorbed by leaves of A. angustifolia, and that this process might be as important as water uptake by its roots.     

The role of marginal hydathodes in foliar water absorption

Summary

Marginal epithem hydathodes may, in some conditions, absorb water. A toothed leaf margin can be regarded as a water-holding structure. It is suggested that the lack of hydathodes in north-temperate evergreens and in the leaves of rain forest trees is related to the need for such species to reduce foliar water absorption. The high proportion of deciduous species with hydathodes in woodland vegetation is thought to be related to the summer potential soil water deficit in Britain, leaf-absorbed water supplementing water from the soil. The term ‘potomorphic’ is suggested for leaves with water absorbing structures.     

Uptake of Hydrophilic Solutes Through Plant Leaves: Current State of Knowledge and Perspectives of Foliar Fertilization

Foliar fertilization is an agricultural practice of increasing importance in practical terms. Since nutrient sprays are mostly applied as water solutions, the focus of the article was placed on the penetration of ionic, polar solutes through the leaf surface, although the mechanisms of cuticular penetration of lipophilics are also taken into consideration. In theory, application of foliar nutrient sprays may indeed be a more target-oriented and environmentally friendly fertilization method since nutrients are directly delivered to the target organism in limited amounts, thereby helping to reduce the environmental impact associated with soil fertilization. However, response to foliar sprays is often variable and not reproducible due to the existing lack of knowledge of many factors related to the penetration of the leaf-applied solution.

To overcome the current “spray and pray” situation, there is a need to critically analyze the major factors involved and the existing experimental approaches to safely assess the penetration mechanisms, which is the final aim of this review. Beginning with the significance of foliar fertilization in agriculture, a historical overview regarding foliar uptake related studies is given, with especial emphasis on the penetration of solutes through the cuticle and stomata. The existing models of cuticular and stomatal uptake are analyzed separately considering among other factors the physico-chemistry of the solution including the role of adjuvants and the effect of the environment. Methods employed to estimate the process of cuticular and stomatal penetration of solutes are critically assessed. Finally, conclusions related to multidisciplinary research perspectives for improving the efficiency of foliar sprays are drawn.     

Inferring foliar water uptake using stable isotopes of water

A growing number of studies have described the direct absorption of water into leaves, a phenomenon known as foliar water uptake. The resultant increase in the amount of water in the leaf can be important for plant function. Exposing leaves to isotopically enriched or depleted water sources has become a common method for establishing whether or not a plant is capable of carrying out foliar water uptake. However, a careful inspection of our understanding of the fluxes of water isotopes between leaves and the atmosphere under high humidity conditions shows that there can clearly be isotopic exchange between the two pools even in the absence of a change in the mass of water in the leaf. We provide experimental evidence that while leaf water isotope ratios may change following exposure to a fog event using water with a depleted oxygen isotope ratio, leaf mass only changes when leaves are experiencing a water deficit that creates a driving gradient for the uptake of water by the leaf. Studies that rely on stable isotopes of water as a means of studying plant water use, particularly with respect to foliar water uptake, must consider the effects of these isotopic exchange processes.     

Mucilages and polysaccharides in Ziziphus species (Rhamnaceae): localization, composition and physiological roles during drought-stress.

The drought-tolerant tree species Ziziphus mauritiana Lamk. and Z. rotundifolia Lamk. were shown to have similar high mucilage concentrations (7-10% dry weight) in their leaves, with large numbers of mucilage-containing cells in the upper epidermis and extracellular mucilage-containing cavities in the leaf veins and stem cortex. The main sugar constituents of the water-soluble mucilage extract were rhamnose, glucose and galactose. During drought-stress in two independent studies, foliar mucilage content was unaffected in both species, but glucose and starch contents declined significantly in crude mucilage extracts from droughted leaves. Enzymatic hydrolysis of the mucilage extract using alpha-amylase and amyloglucosidase released glucose, indicating that a mucilage-associated water-soluble glucan, with alpha-1,4- and alpha-1,6-linkages, may exist which was extracted together with the mucilage. From the current data, it is not possible to localize the glucan to determine whether or not it is associated with mucilage-containing cells. Data from pressure-volume analyses of drought-stressed and control leaves showed that, in line with their similar mucilage contents, the relative leaf capacitance isotherm (change in relative water content per unit change in water potential) was similar in both species. During drought-stress, reduced relative capacitance resulted from osmotic adjustment and decreased wall elasticity. Data suggest that in Ziziphus leaves, intracellular mucilages play no part in buffering leaf water status during progressive drought. In Ziziphus species, growing in environments with erratic rainfall, the primary role of foliar mucilage and glucans, rather than as hydraulic capacitors, may be as sources for the remobilization of solutes for osmotic adjustment, thus enabling more effective water uptake and assimilate redistribution into roots and stems prior to defoliation as the drought-stress intensified.     

Crop tolerance to saline sprinkling water

Summary Crops sprinkled with saline irrigation water are subject to foliar salt absorption and injury as well as to injury from soil salinity. Yield reductions caused by soil salinity alone are well documented, and data are presented here for 71 agricultural crops. Factors affecting these data and their applicability to sprinkler-irrigated crops are discussed. Although foliar injury has been observed with many sprinkled crops, particularly tree crops, essentially no information is available to predict yield losses as a function of the salt concentration of the irrigation water. Salinity thresholds for sprinkling-induced foliar injury are estimated for some crops; however, climatic conditions greatly affect the onset and degree of injury. Managment strategies that minimize sprinkling injury are mentioned.     

甘南高寒草甸植物元素含量与土壤因子对坡向梯度的响应

通过测定甘南高寒草甸不同坡向条件下25科86种植物叶片氮(N)、磷(P)、钾(K)含量、有机碳(C)含量、叶片含水量和相对叶绿素(SPAD)值,以及不同坡向的土壤含水量、有机碳、全氮、全磷含量等土壤指标,分析了不同坡向植物叶片元素含量与土壤环境因子之间的关系。研究结果表明,在南坡-北坡梯度上,随着土壤含水量的增加,植物叶片P含量、叶K含量和叶片含水量显著增加,而相对叶绿素显著降低。土壤养分含量与植物叶片P、叶K含量和叶含水量显著正相关,与叶片相对叶绿素显著负相关。说明不同坡向条件下叶片养分含量受土壤因子的影响显著,土壤的水分及养分状况对植物叶片元素含量的贡献不同。土壤含水量是坡向梯度上影响植物叶片特征的最主要因子。坡向梯度上土壤含水量对植物叶片各种元素含量的影响和植物叶片含水量对不同土壤因子的响应模式支持了生长在南坡的植物能以提高水分和养分利用效率而适应南坡较为干旱和贫瘠的生境。     

Effects of kelp (Macrocystis integrifolia andEcklonia maxima) foliar applications on bean crop growth and nitrogen nutrition under varying soil moisture regimes

A greenhouse experiment was designed to test the effects of two kelp(Macrocystis integrifolia andEcklonia maxima) concentrates, when prepared as foliar sprays, upon bean (Phaseolus vulgaris) growth and N nutrition under three soil moisture regimes. Plant growth and developmental responses in this greenhouse experiment have demonstrated the effectiveness of the two kelp foliar sprays as plant growth regulating substances. Bean growth and developmental responses to the kelp foliar spray treatments were dependent upon the soil moisture regime to which they have been subjected. Although the two kelp foliar sprays had varying and sometimes contrasting effects on bean growth and N nutrition, which were dependent on the soil moisture treatment, their developmental effects upon the number of nodes, shoot/root ratio, leaf area ratio and specific leaf area were quite similar. A soybean callus bioassay demonstrated the presence of cytokinin-like substances and a callus growth antagonist in the kelp concentrate. Increasing dilution of the kelp concentrate disproportionately reduced the callus growth antagonist relative to the growth promoting or cytokinin-like activity.     

Foliar water uptake: a common water acquisition strategy for plants of the redwood forest

Evaluations of plant water use in ecosystems around the world reveal a shared capacity by many different species to absorb rain, dew, or fog water directly into their leaves or plant crowns. This mode of water uptake provides an important water subsidy that relieves foliar water stress. Our study provides the first comparative evaluation of foliar uptake capacity among the dominant plant taxa from the coast redwood ecosystem of California where crown-wetting events by summertime fog frequently occur during an otherwise drought-prone season. Previous research demonstrated that the dominant overstory tree species, Sequoia sempervirens, takes up fog water by both its roots (via drip from the crown to the soil) and directly through its leaf surfaces. The present study adds to these early findings and shows that 80% of the dominant species from the redwood forest exhibit this foliar uptake water acquisition strategy. The plants studied include canopy trees, understory ferns, and shrubs. Our results also show that foliar uptake provides direct hydration to leaves, increasing leaf water content by 2–11%. In addition, 60% of redwood forest species investigated demonstrate nocturnal stomatal conductance to water vapor. Such findings indicate that even species unable to absorb water directly into their foliage may still receive indirect benefits from nocturnal leaf wetting through suppressed transpiration. For these species, leaf-wetting events enhance the efficacy of nighttime re-equilibration with available soil water and therefore also increase pre-dawn leaf water potentials.     

Über die Beziehungen der Ektodesmen zur Stoffaufnahme durch Blätter

Summary When droplets of radioactive solutions are applied to the cuticle, micro-autoradiograms show that the guard cells and the anticlinal walls of the leaf epidermis ofSpinacia oleracea and ofViola tricolor are the favoured sites of absorption. Scattered black spots in the stripping film could also be observed above the periclinal walls of the epidermis.A comparison of such micro-autoradiograms with the distribution of the ectodesmata in the leaf epidermis of the same two species demonstrates that the areas of favoured foliar absorption correspond to the areas of the densest accumulation of the ectodesmata, while the ectodesmata in the less absorbing periclinal walls are more loosely dispersed. Thus, though ectodesmata cannot be localized exactly by the occurrence of silver grains in the stripping film, identity exists between the areas of foliar absorption and the distribution of ectodesmata in the leaves which proves that ectodesmata are functioning as pathways of transport in foliar absorption.The experiments reported cofirm earlier statements that guard cells participate essentially in foliar absorption. The micro-autoradiograms also clearly show that absorption does not take place through the pores of the guard cells.

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不同土壤含水量下侧柏幼树叶片水分利用效率

对植物水分利用效率的研究,可以揭示植物的内在耗水机制,为区域森林生态系统经营与维护提供依据.本研究以侧柏幼树为研究对象,通过室内控制试验设置不同的土壤水分梯度,分别用气体交换法和稳定同位素法对其不同土壤含水量条件下的瞬时水分利用效率(WUE_gs)和短期水分利用效率(WUE_cp)进行研究.结果表明:受气孔导度(g_s)的影响,叶片净光合速率(P_n)和蒸腾速率(T_r)随土壤含水量的增加呈现相同的变化趋势,均在土壤含水量为70%～80%田间持水量(FC)时达到最大值;叶片WUE_gs则在土壤含水量最低(35%～45% FC)时达到最大值(7.26 mmol·m^-2·s^-1).叶片可溶性糖、枝条韧皮部渗出液的δ¹³C都在土壤含水量最低(35%～45% FC)条件下达到最大值,且叶片可溶性糖的δ¹³C明显高于枝条韧皮部渗出液的δ¹³C,未产生明显分馏;而叶片WUE_cp也在土壤含水量最低(35%～45% FC)时达到最大值(7.26 mmol·m^-2·s^-1).相同条件下,叶片WUE_gs和WUE_cp存在一定差异(平均相差0.52 mmol·m^-2·s^-1),WUE_gs时空变异性较大,而WUE_cp更具有代表性.侧柏幼树通过降低生理生态活动和提高水分利用效率来适应干旱的土壤条件.     

The water relations of the root–soil interface

The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway. Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly. A resistance at the root–soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy. We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non-destructive measurement of the hydrostatic pressure in the leaf xylem. When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance. However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil. We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.     

Complete turgor maintenance at low water potentials in the elongating region of maize leaves

Leaf elongation rate, water potential, and osmotic potential were measured in the fifth leaf of maize (Zea mays L.) plants growing in soil from which water was withheld for varying times. Elongation occurred in the basal region, which was enclosed by other leaf sheaths. When water was withheld from the soil, leaf elongation decreased and eventually ceased even though enough solutes accumulated in the elongating region to maintain turgor virtually constant. In the exposed blade, however, turgor was lost and wilt symptoms developed. If the night was prolonged, the elongating region lost much of its ability to accumulate solute, which suggests that the accumulating solutes were of recent photosynthetic origin. Under these conditions, leaf elongation was restricted to higher water potentials than under the usual photoperiodic regime.     

Divergent patterns of foliar δ13C and δ15N in Quercus aquifolioides with an altitudinal transect on the Tibetan Plateau: an integrated study based on multiple key leaf functional traits

Aims With a close association with plant water availability, foliar δ 13 C had been investigated extensively in alpine regions; however, foliar δ 15 N has rarely been concurrently used as an indicator of plant nitrogen availability. Due to the positive correlations between leaf nitrogen content and foliar δ 13 C and δ 15 N found in previous studies, we expected that they should show consistent patterns along an altitudinal gradient.Methods To test our hypothesis, we measured foliar δ 13 C and δ 15 N in conjunction with multiple key leaf functional traits of Quercus aquifolioides, a dominant species of alpine forest on the eastern slopes of the Sygera Mountains, southeastern Tibetan Plateau from 2500 to 3800 m.Important findings (i) Contrary to our hypothesis, foliar δ 13 C exhibited a significant positive linear relationship with altitude; in contrast, foliar δ 15 N initially increased and subsequently decreased with altitude, the change in trend occurring around 3300 m. (ii) Our analyses indicated that leaf internal resistance and stomatal conductance, rather than photosynthetic capacity indicated by leaf N concentration, apparently explained the altitudinal variation in foliar δ 13 C, while differences in foliar δ 15 N were likely the result of soil N availability. (iii) Principal component analysis revealed a clear association between δ 13 C and a tradeoff between water loss and carbon gain, indicated by traits related to gas exchange such as leaf thickness, density, stomatal properties. In contrast, the second axis was associated with δ 15 N and nitrogen acquisition strategy in Q. aquifolioides across its altitudinal distribution, represented by traits related to nitrogen concentration and stomata per gram of leaf nitrogen.     

Research progress on water uptake through foliage

Leaf is one of the most sensitive organs that response to atmospheric environment changes. In arid region, especially under water stress conditions, water from foliar uptake is one of the main water subsidy sources, which plays an important role in plant survival, growth and ecosystem sustainable development. Foliar water uptake attracts more attention in physiological and ecological research all over the world. This essay summarized the main measurement methods of foliar water uptake at home and abroad in detail: vessel control method, stable isotope technology and heat ratio method. And analyzed the significance of foliar water uptake from physiological and ecological aspects; and discussed the influence factors of foliar water uptake from water condition perspective, water absorption and leaf structure aspects. The following aspects should be paid more attention in future researches: (1) measure the amount of foliar water uptake by using stable isotope technology and heat ratio method; (2) the relationship among foliar uptake of fog, dew, rain and water vapor in arid region; (3) the foliar uptake amount and rate should be simulated by models; (4) the mechanisms of foliar water uptake should be researched; (5) study foliar water uptake of different plants at ecosystem scale.     

Research progress on water uptake through foliage

Leaf is one of the most sensitive organs that response to atmospheric environment changes. In arid region, especially under water stress conditions, water from foliar uptake is one of the main water subsidy sources, which plays an important role in plant survival, growth and ecosystem sustainable development. Foliar water uptake attracts more attention in physiological and ecological research all over the world. This essay summarized the main measurement methods of foliar water uptake at home and abroad in detail: vessel control method, stable isotope technology and heat ratio method. And analyzed the significance of foliar water uptake from physiological and ecological aspects; and discussed the influence factors of foliar water uptake from water condition perspective, water absorption and leaf structure aspects. The following aspects should be paid more attention in future researches: (1) measure the amount of foliar water uptake by using stable isotope technology and heat ratio method; (2) the relationship among foliar uptake of fog, dew, rain and water vapor in arid region; (3) the foliar uptake amount and rate should be simulated by models; (4) the mechanisms of foliar water uptake should be researched; (5) study foliar water uptake of different plants at ecosystem scale.     

沿土壤水分梯度黄囊苔草碳同位素组成及其适应策略的变化

 选取了内蒙古锡林河流域6个水分条件不同的典型植物群落,测定了各群落中黄囊苔草 (Carex korshinskyi) 叶片δ13C值、叶片含水量（LWC）及其种群特征的变化。结果表明：1）不同生境下,黄囊苔草叶片的碳同位素组成发生明显变化（变幅为1.8‰）。沿土壤水分梯度,随着土壤含水量的降低,黄囊苔草叶片δ13C值显著增大,水分利用方式更加保守。2）虽然不同生境下,黄囊苔草叶片含水量变化不大,但其叶片δ13C值与LWC表现出显著的负相关关系(p=0.051)。这表明黄囊苔草水分利用效率对其叶片水分状况变化的反应非常敏感。3）在不同生境下,黄囊苔草种群的植株高度、密度、地上生物量及其在群落中的出现频度明显不同。具有较高δ13C值的黄囊苔草种群在群落中出现的频度和地上生物量所占比例都显著增加。以上结果表明,生长在不同生境下的黄囊苔草种群能够通过改变其水分利用效率适应不同的土壤水分状况,使其在植物群落中表现出更强的竞争能力和生态适应性。     

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h⁻¹) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.