Leaf xylem embolism, detected acoustically and by cryo-SEM, corresponds to decreases in leaf hydraulic conductance in four evergreen species

Hydraulic conductance of leaves ( K _leaf) typically decreases with increasing water stress. However, the extent to which the decrease in K _leaf is due to xylem cavitation, conduit deformation or changes in the extra-xylary pathway is unclear. We measured K _leaf concurrently with ultrasonic acoustic emission (UAE) in dehydrating leaves of two vessel-bearing and two tracheid-bearing species to determine whether declining K _leaf was associated with an accumulation of cavitation events. In addition, images of leaf internal structure were captured using cryo-scanning electron microscopy, which allowed detection of empty versus full and also deformed conduits. Overall, K _leaf decreased as leaf water potentials ( Ψ _L) became more negative. Values of K _leaf corresponding to bulk leaf turgor loss points ranged from 13 to 45% of their maximum. Additionally, Ψ _L corresponding to a 50% loss in conductivity and 50% accumulated UAE ranged from −1.5 to −2.4 MPa and from −1.1 to −2.8 MPa, respectively, across species. Decreases in K _leaf were closely associated with accumulated UAE and the percentage of empty conduits. The mean amplitude of UAEs was tightly correlated with mean conduit diameter ( R ² = 0.94, P  = 0.018). These results suggest that water stress-induced decreases in K _leaf in these species are directly related to xylem embolism.     

Xylem hydraulic characteristics of subtropical trees from contrasting habitats grown under identical environmental conditions

Five evergreen subtropical tree species growing under identical environmental conditions were investigated to establish which hydraulic properties are genotypically rigid and which show phenotypic plasticity. Maximum xylem-specific conductivity ( k _s) correlated well with the anatomical characteristics (conduit diameter and density) for the four angiosperms Tecomaria capensis , Trichilia dregeana , Cinnamomum camphora and Barringtonia racemosa ; the anatomy of the gymnosperm Podocarpus latifolius was not assessed. Huber values (functional xylem cross-sectional area : leaf area) varied inversely with k _s among species. Maximum leaf-specific conductivity was similar in the five unrelated species. Vulnerability of xylem to cavitation differed between species, as did the relationship between transpiration and water potential. Models of these parameters and isolated midday readings confirm that these trees operate at similar maximum leaf-specific conductivity ( k _l) values. The data are consistent with the hypothesis that conductivity characteristics ( k _l, k _s) are influenced by environment, whereas vulnerability to cavitation is genetically determined.     

Correlations of stomatal conductance with hydraulic and chemical factors in several deciduous tree species in a natural habitat

Recent research in whole-plant stomatal physiology, conducted largely with potted plants in controlled environments, suggests that stomatal conductance ( g _s) might be more closely linked to plant chemical variables than to hydraulic variables. To test this in a field situation, seasonal g _s was examined in relation to a number of plant and environmental variables in 11 temperate, deciduous forest tree species. Stomatal conductance was generally better correlated with environmental variables (air temperature, vapor pressure deficit, PPFD) than with plant variables, and slightly better correlated with plant hydraulic variables (shoot water and osmotic potentials) than with plant chemical variables (xylem sap ABA concentration, xylem sap pH). We examined a model, developed previously for maize, which describes regulation of g _s by xylem sap ABA concentration with leaf water status acting to modify stomatal sensitivity to the ABA signal. This model explained slightly more variation in seasonal g _s in the forest trees than did single plant variables but not more variation than most single environmental variables. Response surface models, especially those incorporating environmental variables, were more consistently successful at explaining g _s across species.     

The importance of xylem constraints in the distribution of conifer species

Vulnerability of stem xylem to cavitation was measured in 10 species of conifers using high pressure air to induce xylem embolism. Mean values of air pressure required to induce a 50% loss in hydraulic conductivity (φ₅₀) varied enormously between species, ranging from a maximum of 14.2±0.6 MPa (corresponding to a xylem water potential of −14.2 MPa) in the semi-arid species Actinostrobus acuminatus to a minimum of 2.3±0.2 MPa in the rainforest species Dacrycarpus dacrydioides . Mean φ₅₀ was significantly correlated with the mean rainfall of the driest quarter within the distribution of each species. The value of φ₅₀ was also compared with leaf drought tolerance data for these species in order to determine whether xylem dysfunction during drought dictated drought response at the leaf level. Previous data describing the maximum depletion of internal CO₂ concentration (c_i) in the leaves of these species during artificial drought was strongly correlated with φ₅₀ suggesting a primary role of xylem in effecting leaf drought response. The possibility of a trade-off between xylem conductivity and xylem vulnerability was tested in a sub-sample of four species, but no evidence of an inverse relationship between φ₅₀ and either stem-area specific (K_a) or leaf-area specific conductivity (K₁) was found.     

An external heat pulse method for measurement of sap flow through fruit pedicels, leaf petioles and other small-diameter stems

The external heat ratio method is described for measurement of low rates of sap flow in both directions through stems and other plant organs, including fruit pedicels, with diameters up to 5 mm and flows less than 2 g h⁻¹. Calibration was empirical, with heat pulse velocity ( v _h) compared to gravimetric measurements of sap flow. In the four stem types tested ( Actinidia sp. fruit pedicels, Schefflera arboricola petioles, Pittosporum crassifolium stems and Fagus sylvatica stems), v _h was linearly correlated with sap velocity ( v _s) up to a v _s of approximately 0.007 cm s⁻¹, equivalent to a flow of 1.8 g h⁻¹ through a 3-mm-diameter stem. Minimum detectable v _s was approximately 0.0001 cm s⁻¹, equivalent to 0.025 g h⁻¹ through a 3-mm-diameter stem. Sensitivity increased with bark removal. Girdling had no effect on short-term measurements of in vivo sap flow, suggesting that phloem flows were too low to be separated from xylem flows. Fluctuating ambient temperatures increased variability in outdoor sap flow measurements. However, a consistent diurnal time-course of fruit pedicel sap flow was obtained, with flows towards 75-day-old kiwifruit lagging behind evaporative demand and peaking at 0.3 g h⁻¹ in the late afternoon.     

Osmotic adjustment to water stress in pearl millet (Pennisetum americanum [L.] Leeke) under field conditions

Abstract. Osmotic adjustment, a mechanism whereby plants maintain positive turgor despite low water potential (ψ), was investigated in pearl millet ( Pennisetum americanum [L.] Leeke) in three types of field experiment at Hyderabad, India:

• (1)

  Osmotic adjustment during the growing season was evaluated by comparing solute potential (ψ_s) of leaves taken at midday from irrigated and droughted plots and allowed to rehydrate in the laboratory. The degree of seasonal adjustment was also estimated by comparing observed values of ψ_s in the field with those expected if ψ_s decreased solely in proportion to water loss. Both types of assessment indicated the maximum seasonal adjustment to be about 0.2 MPa. The cultivars BJ 104 and Serere 39 differed in their capacity to adjust osmotically over the season; Serere 39 was least able to osmoregulate.
• (2)

  Measurements of diurnal variations in ψ and ψ_s in BJ 104 revealed osmotic adjustment during the afternoon hours. At a given value of ψ, turgor (ψ_p) was about 0.1 MPa higher in irrigated, and over 0.2 MPa higher in droughted plants, in the afternoon, than in the morning.
• (3)

  Osmotic adjustment of different leaves within the canopy was investigated. Upper leaves had lower ψ than basal leaves. Differences in ψ were matched by gradients in ψ_s, so that turgor was similar for all leaf layers.

     

Linking physiological processes with mangrove forest structure: phosphorus deficiency limits canopy development, hydraulic conductivity and photosynthetic carbon gain in dwarf Rhizophora mangle

Spatial gradients in mangrove tree height in barrier islands of Belize are associated with nutrient deficiency and sustained flooding in the absence of a salinity gradient. While nutrient deficiency is likely to affect many parameters, here we show that addition of phosphorus (P) to dwarf mangroves stimulated increases in diameters of xylem vessels, area of conductive xylem tissue and leaf area index (LAI) of the canopy. These changes in structure were consistent with related changes in function, as addition of P also increased hydraulic conductivity ( K _s), stomatal conductance and photosynthetic assimilation rates to the same levels measured in taller trees fringing the seaward margin of the mangrove. Increased xylem vessel size and corresponding enhancements in stem hydraulic conductivity in P fertilized dwarf trees came at the cost of enhanced midday loss of hydraulic conductivity and was associated with decreased assimilation rates in the afternoon. Analysis of trait plasticity identifies hydraulic properties of trees as more plastic than those of leaf structural and physiological characteristics, implying that hydraulic properties are key in controlling growth in mangroves. Alleviation of P deficiency, which released trees from hydraulic limitations, reduced the structural and functional distinctions between dwarf and taller fringing tree forms of Rhizophora mangle .     

Short episodes of water stress increase barley root resistance to radial shrinkage in a dehydrating environment

Although plant shoots can be 'hardened' by abiotic stresses, little is known about such changes in roots. In order to investigate possible induction of root-hardening in response to short water-stress episodes, barley seedlings ( Hordeum vulgare L) hydroponically grown under a controlled environment were moderately water-stressed by addition of a non-penetrating osmoticum, polyethylene glycol (PEG) 6000 at −0.4 MPa water potential, to the aerated nutrient solution. Seedlings were then hydrated in dilute nutrient solution without PEG before excision and assay of the seminal roots. Previous water stress treatments for 72 h, 12 h, or even 6 h induced an apparent root-hardening process. Thus, root radial shrinkage during subsequent exposure to strongly dehydrating conditions was remarkably decreased. The root hardening was related to biophysical adjustments: turgor-pressure increased while osmotic potential decreased from −0.45 ± 0.02 MPa to −0.60 ± 0.02 MPa. Moreover, the maximum bulk volumetric modulus of elasticity, ɛ_max determined by pressure–volume analysis, increased from 2.1 ± 0.4 MPa to 3.7 ± 0.4 MPa, i.e. root elasticity was decreased. Root hardening in response to episodes of water stress may have ecological significance for barley plants in regions where intermittent drought episodes are frequent.     

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

Changes in axial hydraulic conductivity along elongating leaf blades in relation to xylem maturation in tall fescue

Xylem maturation in elongating leaf blades of tall fescue ( Festuca arundinacea ) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region in which most of the longitudinal water movements occurred in the LMX. The axial hydraulic conductivity ( K _h) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conductivity ( K _t) computed from the diameter of individual inner vessels. K _t was proportional to K _h throughout the leaf, but K _t was about three times K _h. The changes in K _h and K _t along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, K _h was about 0.30±0.07 mmol s⁻¹ mm MPa⁻¹. Beyond that region, K _h rapidly increased with metaxylem element maturation to a maximum value of 5.0±0.3 mmol s⁻¹ mm MPa⁻¹, 105 mm from the leaf base. It then decreased to 3.5±0.2 mmol s⁻¹ mm MPa⁻¹ near the leaf tip. The basal expanding region was observed to restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.