Hydathode anatomy in Potentilla palustris (Rosaceae)

Guttating leaf teeth of Potentilla palustris plants from Wisconsin, USA, were cleared or processed for plastic sectioning or scanning electron microscopy. Anatomical features include: 1) long slender hydathode area occupying most of the tooth, 2) adaxial pad of small, flat epidermal cells with 50 or more sunken water pores about the size of ordinary abaxial stomates, 3) three converged bundles that extend distally, where their tracheary files are separated by intervening files of xylem parenchyma cells with sinuous walls, 4) adaxial mass of small, loosely arranged epithem cells above the xylem, 5) one slender phloem strand that extends only about a third of the way into the hydathode, and 6) bundle sheath extending distally only abaxially and along the flanks of the hydathode. Potentilla hydathodes differ significantly from non-guttating ones described earlier in Physocarpus (Rosaceae).     

Marginal and laminar hydathode-like structures in the leaves of the desiccation-tolerant angiosperm Myrothamnus flabellifolius Welw.

The fan-shaped leaves of the resurrection plant Myrothamnus flabellifolius Welw. fold during episodes of drought and consequent desiccation of the tissue. The leaf teeth of M. flabellifolius have several features characteristic of hydathodes. Tracheary elements from the three vein endings that converge in each leaf tooth subtend and extend into a cluster of cells significantly smaller than those of the adjacent mesophyll. The stomata overlying this putative epithem are larger than the other stomata on the leaf surface. Crystal violet is absorbed via these stomata in non-transpiring leaves, suggesting that they are water pores. Two to four such water pores occur per hydathode and are readily distinguished in desiccated leaves. Laminar hydathodes apparently also occur in the leaves of M. flabellifolius. Branched vein endings that terminate in short, wide tracheary elements subtend the outer edges of the abaxial leaf ridge, which otherwise lack stomata, and coincide with regions of crystal violet uptake. Guttation could not be induced in M. flabellifolius. However, desiccated leaves readily absorb liquid water through the leaf surface. The use of Calcafluor White to trace the pathway of apoplastic water movement suggests a role for both types of hydathode in foliar water uptake during rehydration while the accumulation of Sulphorhodamine G (indicating solute retrieval from the apoplast) in the epithem of transpiring plants suggests the hydathodes may be a pathway of water loss in the desiccating leaf.     

Anatomy of hydathodes and pigment disks in leaves of Ficus diversifolia (Moraceae)

In Ficus diversifolia , "mistletoe fig", hydathodes are scattered over the leaf lamina. They open to the adaxial surface via a circular, slightly depressed epidermal pad studded with 20–30 water pores. The epithem is a cylinder of small cells, irregular in shape, with conspicuous intercellular spaces, bounded by a close-fitting sheath of unpigmented cells. Hydathodes occur above characteristic junctions of three or more minor veins, from which short tracheary strands extend upward into the epithem and end blindly. Phloem does not seem to be involved. Unsuccessful attempts were made to induce guttation. Red pigment spots occur abaxially in the axils of major veins. They are compact internal disks of parenchyma cells containing granules of various sizes. A vascular bundle arches downward, passes over the disk, then curves upward to rejoin other bundles. The central large disk has a palisade epidermis of slender elongate cells separated from each other by cuticle; the other disks have a normal epidermis.     

Dynamics of ultrastructure changes in sheet plate fiber flax with braking transport assimilate by nitrate-anion

Changes in leaf mesophyll cell ultrastructure under nitrate feeding into the apoplast of common flax (Linum usitatissimum L.) in the form of 50 mM KNO3 solution were studied. In 30 min after the beginning of nitrate feeding through the transpiration water stream, swelling of mitochondrial and microbodies, clarification of their matrices, and curling of dictyosome discs into annular structures were observed. These events characterized symplastic domain formed by mesophyll, bundle sheath and phloem parenchyma cells, and were not found in companion cell-sieve element complex. Simultaneously, formation of large central vacuoles in companion cells was noted. Restoration of organelle structures in assimilating cells and phloem parenchyma in 1-2 h after treatment was accompanied by enhancement of morphological changes in phloem elements and companion cells and signs of plasmolysis in the mesophyll cells. It was supposed that the two-phase character of changes in leaf organelle ultrastructure and photosynthesis might reflect duality of leaf cell response to nitrate ion. The rapid alterations of the structure can be coupled with direct influence of the anion on cell metabolism and(or) with signal-regulatory functions of oxidized nitrogen forms, while the slower ones reflect the result of suppression of photoassimilate export from leaves by the anion.     

Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. I. Structural and Cytological Aspects

The structural details of the guttating tips of 7-day-old barleyleaves were studied as a basis for a subsequent report on thephysiology of guttation. The walls of the vessels at the tipsof leaves bear many pits and are rather thin, appearing neithercutinized nor lignified. This could facilitate a direct passageof solutes out of the xylary system through the leaf apoplastand out to the leaf surface via hydathode openings. The latterare formed by modified stomatal guard cells, and there are nospecially differentiated epithem, epithelium or gland hair likestructures that could serve an active elimination of guttation.Xylem parenchyma cells and the peculiar mesophyll cells withdense cytoplasm, numerous mitochondria, an extended ER systemand a considerable formation of small vesicles in the leaf tipcould modify the content of the guttated fluid along the routeof transport. Hordeum vulgare L., barley, hydathodes, guttation     

Untangling the complex issue of dissolved organic carbon uptake: a stable isotope approach

1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole-stream addition of a ¹³C-DOC tracer coupled with laboratory measurements of bioavailability of the tracer and stream water DOC.
2. The tracer, a leachate of ¹³C-labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2-h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer.
3. From the longitudinal ¹³C uptake curve, we resolved labile and semi-labile DOC classes within the ¹³C-DOC tracer comprising 82% and 18% of the tracer respectively.
4. Plug-flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi-labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients.
5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi-labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi-labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans.     

The trans-tissue pathway and chemical fate of 14C photoassimilate in carrot taproot

Axial and radial transport and the accumulation of photoassimilates in carrot taproot were studied using ¹⁴C labelling and autoradiography. Axial transport of the ¹⁴C labelled assimilates inside the taproot was rapid and occurred mainly in the young phloem found in rows radiating from the cambium. The radial transport of the assimilate inward (to cambium, xylem zone and pith) and outward (to phloem zone and periderm) from the conducting phloem was an order of magnitude slower than the longitudinal transport and was probably mainly diffusive. The cambial zone of the taproot presented a partial barrier in the inward path of the assimilate to the xylem zone. We suggest that this is due to the cambium comprising a strong sink for the assimilate on the basis that our previous work has shown that it contains very low concentrations of free sucrose. By contrast, a high accumulation of nonsoluble ¹⁴C was found in the cambium region in good agreement with the active growth of this zone. Autoradiography following the feeding of ¹⁴C labelled sugars to excised sections of taproot indicated that only a ring of cells at and/or just within the cambium take up sugars from the apoplast. This indicates that radial movement in the phloem and pith must be symplastic. An apoplastic step between phloem and xylem is possible. The rapid uptake of sugars from the apoplast at this point might represent a mechanism for keeping photoassimilates away from the transpiration stream and re-location back to the leaves.     

Locating active proton extrusion pumps in leaves

Abstract Stabilized microscopic preparations of an apoplastic fluorescent tracer, sulphorhodamine G (SR), have previously shown it confined to leaf cell walls. SR has a pK of 3.2, is dissociated at normal wall pH, and therefore does not enter cells. In transpiring soybean leaves, the SR showed a major internal water pathway in the walls of the paraveinal mesophyll (PVM), which has been implicated in the temporary storage of protein. Also the SR penetrated the PVM and bundle sheath cells, staining organelles and vacuoles, but not other leaf cells. This implies that sufficient SR is undissociated in these walls to allow penetration, and that the pH of the PVM walls is lower than that of most other cells. It is proposed that proton extrusion pumps are revealed by the low wall pH, and that these pumps are probably involved in collecting ammo acids from the transpiration stream.     

Sodium partitioning within the shoot of soybean

Uptake and partitioning of Na⁺ and Cl⁻ in plants of soybean ( Glycine max L. Merr. cv. Hodgson) exposed to moderate NaCl concentrations were studied over an 8-day period. Plants showed marked retention of Na⁺ in the stems and low transport to laminae of young leaves. The xylem sap ascending the main axis was progressively depleted in Na⁺. The oldest leaf greatly contributed to Na⁺ depletion of the sap flowing to younger leaves. These results in combination with estimates of phloem recirculation indicated that Na⁺ accumulation in the young leaf was prevented both by depletion of Na⁺ from the xylem stream, and by a high recirculation of Na⁺ via the phloem. However, this protection of young leaves was effective only for very mild salinity treatment.     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Short-term responses of phloem transport to mechanical perturbation

Jaeger, C. H., Goeschl, J. D., Magnuson, C. E., Fares, Y. and Strain, B. R. 1988. Short-term responses of phloem transport to mechanical perturbation. - Physiol. Plant. 72: 588–594.
Phloem transport was monitored using a continuous stream of ¹¹CO₂-labelled air administered to one leaf while gamma detectors measured ¹¹C activity at intervals along the stem. The effect of gentle, non-injurious mechanical perturbation on phloem transport was tested in cotton ( Gossypium hirsutum L. cv. Stoneville 213). Mechanical stimuli such as shaking, localized vibration and gentle massage were applied while the plants were at isotope equilibrium. Localized phloem blockages were observed within 1–2 min of the stimuli. The blockages lasted from 6–55 min and full recovery of transport required 20–175 min. The effect of preconditioning to mechanical perturbation on phloem transport was tested in bush beans ( Phaseolus vulgaris L. cv. Cherokee Bush). Preconditioning of a bean seedling to gentle stem massage resulted in a shorter blockage response and quicker transport recovery period when the seedling was massaged during a ¹¹C tracer experiment compared to a control seedling. These results indicate that measurements of phloem transport on recently disturbed plants will probably show depressed phloem transport velocities. Measurements should be made after at least a 24-h disturbance-free recovery period.     

Long distance transport of potassium in cereals during grain filling in detached ears

Ears of wheat plants ( Triticum aestivum L. cv. Kolibri), which were given different and uniform K⁺-nutrition in two experiments, were cut at 2, 4 and 6 weeks after anthesis at 15 cm below the ear. These detached ears were fed 30 m M (experiment 1) or 15, 30, 60 or 90 m M ⁸⁶Rb-K₂ malate (experiment 2) and 146 m M [¹⁴C]-sucrose. After a pulse period of 6 and 4 h, respectively, the ears were transferred to identical non-labeled solutions for additional 0, 4, 8 or 20 h.
About 50% of the K⁺ and sucrose supplied was absorbed by detached ears. This rate declined with plant age and decreasing transpiration. Within the 6 and 4 h uptake period less than 7% of the absorbed K⁺, but 20% of the sucrose taken up were incorporated into the grain. During the chase period labeled K⁺ in the grain increased to 15% and ¹⁴C even to 50% of total tracer uptake. Incorporation of labeled K⁺ into the grain was not affected by the previous K⁺ nutrition of the plant and was proportional to the K⁺ concentration in the uptake solution. Transition of K⁺ from xylem into phloem during its acropetal transport is assumed. No evidence was found that the grain itself could control its uptake of K⁺.     

Effect of high external NaCl concentrations on ion transport within the shoot of Lupinus albus. I. Ions in xylem sap

Abstract. Xylem sap was collected from individual leaves of intact transpiring lupin plants exposed to increasing concentrations of NaCl by applying pneumatic pressure to the roots. Concentrations of Na⁺ and Cl⁻ in the xylem sap increased linearly with increases in the external NaCl concentration, averaging about 10% of the external concentration. Concentrations of K⁺ and NO₃⁻, the other major inorganic ions in the sap, were constant at about 2.5 and 1.5 mol m⁻³, respectively. There was no preferential direction of Na ⁺ or Cl⁻ to either young or old leaves: leaves of all ages received xylem sap having similar concentrations of Na⁺ and Cl⁻, and transpiration rates (per unit leaf area) were also similar for all leaves. Plants exposed to 120–160 mol m⁻³ NaCl rapidly developed injury of oldest leaves; when this occurred, the Na⁺ concentration in the leaflet midrib sap had increased to about 40 mol m⁻³ and the total solute concentration to 130 osmol m⁻³. This suggests that uptake of salts from the transpiration stream had fallen behind the rate of delivery to the leaf and that salts were building up in the apoplast.     

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.     

Reduction of nitrate in the perennial tissues of aerial parts of Alnus glutinosa

In vivo nitrate reductase (NR, EC 1.6.6.1.) activity was measured in leaves, branches and trunk of field-grown Alnus glutinosa (L.) Gaertn. All of the assayed tissues enzymatically reduced nitrate with a decreasing activity [μmol NO₂⁻ (g dry weight)⁻¹ h⁻¹] in the order: leaves > branch bark > inner branch tissues > trunk xylem. The NR activity of the various tissues of excised branches was inhibited by tungstate added to the transpiration stream. Part of the nitrate added to the feeding solution (0.2, 0.5 or 1 m M KNO₃) of excised branches disappeared during its transport via the transpiration stream in the perennial tissues. This disappearance was enzymatic since it was decreased by tungstate.
No evidence was obtained for the presence of nitrate in natural xylem sap nor for a significant correlation between nitrate content of soil and leaf NR activity. These results indicate that in the field-grown black alder, the nitrate not reduced in the roots could be reduced in the perennial tissues of aerial parts. Since the leaf NR activity does not reflect the actual in situ nitrate reduction, the existence of a constitutive NR activity in Alnus leaves is suggested.     

Control of phloem unloading by action potentials in Mimosa

In the sensitive plant, Mimosa pudica , action potentials arise when the leaves are touched and they trigger a sudden decrease in turgor of the pulvinar motor cells, which causes the leaf to close. These potentials may travel through the phloem and they appear to influence pulvinar phloem unloading after stimulation. Mature leaves were exposed to ¹⁴CO₂ and phloem translocation was observed by autoradiography. In unstimulated pulvini, labeled photoassimilates were restricted to the phloem. However, after stimulation, the ¹⁴C-label appeared to be concentrated in the extensor region of the motor cortex. Since stimulation elicits an action potential, it is suggested that it also triggers phloem unloading of sucrose in the pulvini.     

Diurnal water storage in the stems of Picea sitchensis (Bong.) Carr.

Abstract. Two models of the relationship between diurnal variation in shoot water potential and transpiration in 14-year-old Picea sitchensis (Bong.) Carr. were compared. The first model was a physiologically based resistance-capacitance (R-C) analogue with its associated differential equations. The second was a non-physiological discrete-difference (D-D) or stochastic transfer function model. The RC model included only the effect of water storage in the phloem and bark while the D-D model implicity included all storage mechanisms. The R-C and D-D models explained similar fractions (62% and 68% respectively) of the variation in shoot water potential due to diurnal changes in transpiration rate. However, the D-D model had fewer parameters than the R-C model. The results from the D-D model showed that the resistance to flow from soil to shoots along the trunk, (RT), was 5 × 10³ MPa kg^-1s and the capacitance of the phloem and bark treated as a single store, (C_s), was 1.6 kg MPa^-1. It is suggested that the resistance to flow into storage (R_s) is much greater than R_T and can be disregarded. A non-linear version of the D-D model suggested [hat resistance to flow in the trunk increases with increasing transpiration rate.     

Leaf Epidermal Hydathodes and the Ecophysiological Consequences of Foliar Water Uptake in Species of Crassula from the Namib Desert in Southern Africa

Abstract: Epidermal hydathodes were found on leaves of 46 of 48 species of Crassula collected from the Namib Desert in southern Africa. The possibility that these structures might allow the absorption of surface water was investigated in 27 species (including subspecies). The presence of hydathodes on leaf epidermi correlated, in most cases, with increases in leaf thickness and enhanced rates of nocturnal, and sometimes diurnal, CO₂ uptake following wetting of the leaves during the night. The precise nature of these responses varied depending on the species. In addition, wetting only the older leaves on the lower portion of the shoot of C. tetragona ssp. acutifolia not only resulted in increased thickness of these leaves, but also effected an increase in leaf thickness and stimulation of CO₂ uptake rates in the distal, younger portion of the shoot that was not wetted. Overall, foliar hydathodes were implicated in the absorption of surface water in many species of Crassula such that the ecophysiology of these desert succulents was positively affected. Although rainfall in the Namib Desert is infrequent, surface wetting of the leaves is a more common occurrence as a result of nighttime dew or fog deposition. Presumably, species with hydathodes benefit directly from this source of moisture. These findings have important implications in understanding a relatively unexplored adaptation of some xerophytes to an extremely arid environment.     

A xylem sap retrieval pathway in rice leaf blades: evidence of a role for endocytosis?

The structure and transport properties of pit membranes at the interface between the metaxylem and xylem parenchyma cells and the possible role of these pit membranes in solute transfer to the phloem were investigated. Electron microscopy revealed a fibrillar, almost tubular matrix within the pit membrane structure between the xylem vessels and xylem parenchyma of leaf blade bundles in rice (Oryza sativa). These pits are involved primarily with regulating water flux to the surrounding xylem parenchyma cells. Vascular parenchyma cells contain large mitochondrial populations, numerous dictyosomes, endomembrane complexes, and vesicles in close proximity to the pit membrane. Taken collectively, this suggests that endocytosis may occur at this interface. A weak solution of 5,6-carboxyfluorescein diacetate (5,6-CFDA) was applied to cut ends of leaves and, after a minimum of 30 min, the distribution of the fluorescent cleavage product, 5,6-carboxyfluorescein (5,6-CF), was observed using confocal microscopy. Cleavage of 5,6-CFDA occurred within the xylem parenchyma cells, and the non-polar 5,6-CF was then symplasmically transported to other parenchyma elements and ultimately, via numerous pore plasmodesmata, to adjacent thick-walled sieve tubes. Application of Lucifer Yellow, and, separately, Texas Red-labelled dextran (10 kDa) to the transpiration stream, confirmed that these membrane-impermeant probes could only have been offloaded from the xylem via the xylem vessel-xylem parenchyma pit membranes, suggesting endocytotic transmembrane transfer of these membrane-impermeant fluorophores. Accumulation within the thick-walled sieve tubes, but not in thin-walled sieve tubes, confirms the presence of a symplasmic phloem loading pathway, via pore plasmodesmata between xylem parenchyma and thick-walled sieve tubes, but not thin-walled sieve tubes.