A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems

Current methods for determining the influence of xylem cavitationon hydraulic conductance are limited to unbranched stem or rootsegments with hydraulic conductances above c. 2 mmol s^–1MPa^–1. Lower conductances and/or highly branched systemsare encountered in seedlings, arid-land shrubs, herbs, and distalportions of shoot and root systems of trees. In order to quantifythe hydraulic impact of cavitation in such systems, existingtechniques have been modified. Branched shoot or root systemswere prepared for measurement by removal of leaves, or roottips, respectively. The shoot or root system was enclosed ina vacuum chamber with the proximal end protruding and suppliedwith perfusing solution. Flow through the xylem was inducedby chamber vacuum. Hydraulic conductance was determined fromthe slope of the flow rate versus pressure relationship. Xylemembolism was quantified from the increase in hydraulic conductancefollowing high pressure (100 kPa) perfusion of solution throughthe plant. Examples are provided of the application of the methodto cavitation studies in the cold desert shrub Artemisia tridentata. Key words: Hydraulic conductance, xylem cavitation, embolism, whole root/shoot system     

Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architecture

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.     

Relationships between xylem vessel characteristics, calculated axial hydraulic conductance and size-controlling capacity of peach rootstocks

Background and Aims

Previous studies indicate that the size-controlling capacity of peach rootstocks is associated with reductions of scion water potential during mid-day that are caused by the reduced hydraulic conductance of the rootstock. Thus, shoot growth appears to be reduced by decreases in stem water potential. The aim of this study was to investigate the mechanism of reduced hydraulic conductance in size-controlling peach rootstocks.

Methods

Anatomical measurements (diameter and frequency) of xylem vessels were determined in shoots, trunks and roots of three contrasting peach rootstocks grown as trees, each with different size-controlling characteristics: ‘Nemaguard’ (vigorous), ‘P30-135’ (intermediate vigour) and ‘K146-43’ (substantially dwarfing). Based on anatomical measurements, the theoretical axial xylem conductance of each tissue type and rootstock genotype was calculated via the Poiseuille–Hagen law.

Key Results

Larger vessel dimensions were found in the vigorous rootstock (‘Nemaguard’) than in the most dwarfing one (‘K146-43’) whereas vessels of ‘P30-135’ had intermediate dimensions. The density of vessels per xylem area in ‘Nemaguard’ was also less than in ‘P30-135’and ‘K146-43’. These characteristics resulted in different estimated hydraulic conductance among rootstocks: ‘Nemaguard’ had higher theoretical values followed by ‘P30-135’ and ‘K146-43’.

Conclusions

These data indicate that phenotypic differences in xylem anatomical characteristics of rootstock genotypes appear to influence hydraulic conductance capacity directly, and therefore may be the main determinant of dwarfing in these peach rootstocks.Key words: Prunus, rootstock, vessel diameter, hydraulic conductance, dwarfing, xylem anatomy, Poiseuille–Hagen     

Fluid ionic composition influences hydraulic conductance of xylem conduits

The direct effect of fluid composition on xylem hydraulic conductance is investigated in excised stem segments of chrysanthemum (Dendranthema x grandiflorum Tzvelev cv. Cassa) plants. Dynamic changes in hydraulic conductance are accurately measured at 30 s intervals before and after modifications of the composition of the standard fluid (deionized water). It is investigated whether osmotic properties of the flowing solution influence overall hydraulic conductance by affecting the hydraulic conductance of vessel-to-vessel pit membranes, as has previously been suggested. Various iso-osmotic salt solutions (20 mOsm kg-1) of different composition raised the hydraulic conductance of 20 cm long stem segments approximately 5-8% compared to deionized water. In contrast, carbohydrate solutions with similar osmotic strength and pH did not cause any change in hydraulic conductance. KCl solutions that greatly differed in osmotic strength all increased hydraulic conductance, but the response was not correlated with the osmotic strength of the solution. Increasing the number of vessels that were open from one cut end to the other by shortening the stem segments greatly increased the hydraulic conductance response. Changing from deionized water to a salt solution caused an immediate increase in hydraulic conductance, while a shift back to deionized water resulted in a slow decline. This decline lasted longer when the salt solution contained divalent cations compared to monovalent cations. It is concluded that the presence of cations and not the osmotic strength in the flowing solution influenced the hydraulic conductance. The phenomenon is not caused by the vessel-to-vessel pit membranes, which in fact suppressed the effect, due to their large contribution to the overall resistance to water flow.     

The relationship of hydraulic conductance to root system characteristics of peach (Prunus persica) rootstocks

Specific rootstocks can differentially influence the vegetative growth and development of fruit trees. However, the physiological mechanism involved in this phenomenon has been elusive. Recent research comparing different peach ( Prunus persica L. Batsch) rootstocks suggests that the rootstock effect on vegetative growth in peach trees is associated to water relations and more specifically to differences in rootstock hydraulic conductance. This study was intended to confirm differences in hydraulic characteristics of similar size peach trees grafted on different rootstocks and to examine root system characteristics that could be associated with rootstock hydraulic limitations. Trees on rootstocks that were known to have a size-controlling effect when grown under field conditions had lower rootstock conductance than trees on the vigorous (control) rootstock when rootstock hydraulic conductance was measured with both the high-pressure and evaporative flow methods. Rootstocks with the lowest hydraulic conductance had less fine root surface area and length per unit root dry weight than the more vigorous (control) rootstock. However, contrary to previous field studies, in this study there were no significant differences in dry matter production and distribution among trees on the different rootstocks suggesting that whatever the normal growth control mechanism was, it did not differentially influence growth under the specific conditions of this study. This research confirmed that peach rootstocks exhibiting size-controlling behavior under field conditions differed in their hydraulic and morphological characteristics under controlled growth conditions even when those growth conditions negated the expression of the size-controlling behavior.     

Influence of saline stress on root hydraulic conductance and PIP expression in Arabidopsis

Measurements of the root hydraulic conductance (L0) of roots of Arabidopsis thaliana were carried out and the results were compared with the expression of aquaporins present in the plasma membrane of A. thaliana. L0 of plants treated with different NaCl concentrations was progressively reduced as NaCl concentration was increased compared to control plants. Also, L0 of plants treated with 60 mmol/L NaCl for different lengths of time was measured. Variations during the light period were seen, but only for the controls. A good correlation between mRNA expression and L0 was observed in both experiments. Control plants and plants treated with 60 mmol/L NaCl were incubated with Hg and then with DTT. For these plants, L0 and cell-to-cell pathway contributions to root water transport were determined. These results revealed that in control plants most water movement occurs via the cell-to-cell pathway, thus implying aquaporin involvement. But, in NaCl-stressed plants, the Hg-sensitive cell-to-cell pathway could be inhibited already by the effect of NaCl on water channels. Therefore, short periods of NaCl application to Arabidopsis plants are characterised by decreases in the L0 of roots, and are related to down-regulation of the expression of the PIP aquaporins. This finding indicates that the well known effect of salinity on L0 could involve regulation of aquaporin expression.     

Phosphorus nutritional effects on root hydraulic conductance,xylem water flow and flux of magnesium and calcium in squash plants

Previous studies have found that P nutrition of plants is an important factor in the uptake and translocation of Mg and Ca, and increasing root osmotic hydraulic conductance (L_o) and osmotically driven xylem exudate flow (J_v). Experiments were designed to determine if the observed changes in Mg and Ca uptake and translocation, J_v, and L_o from altered P nutrition are related or are separate functions. When six-week old squash (Cucurbita pepo L.) plants grown in perlite were treated with P levels ranging from 50 to 400 μM P for seven days, J_v and L_o increased as P treatment level increased. Xylem exudate concentrations of Mg and Ca were maintained as J_v increased, resulting in an increase in total flux of these mineral elements. The increase in Mg and Ca flux in the xylem exudate correlated with increased shoot Mg and Ca levels as P nutritional level was raised. Further studies with greenhouse grown plants indicated that the increases in J_v, L_o, and Mg and Ca flux were more responsive to changes in P nutritional level than to similar changes in levels of other anions. In hydroponically grown squash plants, xylem exudate was collected for a 20 min period after 0, 2 and 4 h in treatments of 50 and 500 μM P or after P treatment was increased from 50 to 500 μM. Immediately after nutrient solution P was increased (time 0), there was a 33% increase in J_v and a 22% increase in L_o when compared to the 50 μM P treatment. The J_v and L_o of the 50–500 μM P treatment did not equal levels of the continuous 500 μM control at time 0, but were similar after 2 and 4 h. Flux of Mg and Ca did not increase as rapidly as J_v in the 50–500 treatment indicating that regulation of Mg and Ca uptake and xylem loading by P may lag behind that of water movement. This revised version was published online in June 2006 with corrections to the Cover Date.     

A technique for measuring xylem hydraulic conductance under high negative pressures

A technique for measuring hydraulic conductances of excised xylem segments exposed to high negative pressures is described. A centrifugal force is used to generate negative pressures (P) in the sample and to create a positive hydrostatic pressure difference (ΔP) between its two ends. ΔP forces water through the sample at a flow rate (F) determined optically during centrifugation. The sample hydraulic conductance k is derived from F and ΔP. The sample vulnerability curve is given by the dependence of k on P. Results for Cedrus atlantica Manetti and Laurus nobilis L. shoots are given. The technique is appropriate for the analysis of xylem refilling under negative pressure.     

Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis

The extent to which stomatal conductance (g_s) was capable of responding to reduced hydraulic conductance (k)and preventing cavitation-inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), g_s, and xylem pressure (Ψ_p-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and g_s. The k measurement was expressed per unit leaf area (k_l, leaf-specific conductance). Leaves measured within 2 d of shoot injection showed reduced g_s and E relative to non-injected controls, and both parameters were strongly correlated with k_l At this time, there was no difference in leaf Ψ_p-xylem between injected shoots and controls, and leaf Ψ_p-xylem was not significantly different from the highest cavitation-inducing pressure (Ψ_p-cav) in the branch xylem (-1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of g_s and E values to the control range. This was associated with a decrease in leaf Ψ_p-xylem below Ψ_p-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.     

Sensitivity of stem and petiole hydraulic conductance of deciduous trees to xylem sap ion concentration

Hydraulic conductance of stem and petioles increased in response to an increase in xylem sap ion concentration, and decreased in response to a decrease in the ion concentration in six temperate deciduous tree species. The ion sensitivity of hydraulic conductance of stem and petioles was higher than the ion sensitivity of the stem alone. The ion sensitivity was lowest in the earliest developmental stages of the xylem, and had a seasonal maximum in the second half of summer. The ion sensitivity was highest in slow-growing species and lowest in fast-growing species. The ion sensitivity correlated negatively with mean radius of xylem conduits, hydraulic conductance of stem and petioles, hydraulic conductance of leaf laminae, and stomatal conductance, and positively with response of the hydraulic conductance of leaf laminae to HgCl₂, and stomatal response to a decrease in leaf water potential or abscisic acid. It was concluded that the high ion sensitivity of xylem hydraulic conductance is a relevant characteristic of slow growth and a conservative water use strategy.     

Leaf hydraulic conductance and mesophyll conductance are not closely related within a single species

Stomata represent one resistor in a series of resistances for carbon and water exchange between the leaf and the atmosphere; the remaining resistors occurring within the leaf, commonly represented as mesophyll conductance to CO₂, g_m, and leaf hydraulic conductance, k_Leaf. Recent studies have proposed that g_m and k_Leaf may be coordinated across species because of shared pathways. We assessed the correlation between g_m and k_Leaf within cotton, under growth CO₂ partial pressure and irradiance treatments and also with short‐term variation in irradiance and humidity. g_m was estimated using two isotopic techniques that allowed partitioning of total g_m (Δ¹³C‐g_m) into cell wall plus plasma membrane conductance (Δ¹⁸O‐g_m) and chloroplast membrane conductance (g_cm). A weak correlation was found between Δ¹³C‐g_m and k_Leaf only when measured under growth conditions. However, Δ¹⁸O‐g_m was related to k_Leaf under both short‐term environmental variation and growth conditions. Partitioning g_m showed that g_cm was not affected by short‐term changes in irradiance or correlated with k_Leaf, but was strongly reduced at high growth CO₂ partial pressure. Thus, simultaneous measurements of g_m, k_Leaf and g_cm suggest independent regulation of carbon and water transport across the chloroplast membrane with limited coordinated regulation across the cell wall and plasma membrane.     

Evidence for xylem embolism as a primary factor in dehydration-induced declines in leaf hydraulic conductance

Hydraulic conductance of leaves (K(leaf)) typically decreases with increasing water stress and recent studies have proposed different mechanisms responsible for decreasing K(leaf) . We measured K(leaf) concurrently with ultrasonic acoustic emissions (UAEs) in dehydrating leaves of several species to determine whether declining K(leaf) was associated with xylem embolism. In addition, we performed experiments in which the surface tension of water in the leaf xylem was reduced by using a surfactant solution. Finally, we compared the hydraulic vulnerability of entire leaves with the leaf lamina in three species. Leaf hydraulic vulnerability based on rehydration kinetics and UAE was very similar, except in Quercus garryana. However, water potentials corresponding to the initial decline in K(leaf) and the onset of UAE in Q. garryana were similar. In all species tested, reducing the surface tension of water caused K(leaf) to decline at less negative water potentials compared with leaves supplied with water. Microscopy revealed that as the fraction of embolized xylem increased, K(leaf) declined sharply in Q. garryana. Measurements on leaf discs revealed that reductions in lamina hydraulic conductance with dehydration were not as great as those observed in intact leaves, suggesting that embolism was the primary mechanism for reductions in K(leaf) during dehydration.     

Estimation of hydraulic conductance within field-grown apricot using sap flow measurements

Using the heat pulse and other techniques, the hydraulic architecture of apricot trees was mapped out. The flows (overall flow, flow across the four main branches) and forces (water potential differences between xylem and leaves) measured allowed us to quantify hydraulic conductance of branches and of the root/soil resistance. The experiment was carried out in a commercial orchard of 11-year-old apricot trees (Prunus armeniaca L., cv. Búlida, on Real Fino apricot rootstock) during 1 week (October 27–November 3, 1998). Three representative trees with a cylindrical trunk divided into four main branches of different sizes, orientation and local microclimate were chosen for the experiment. Sap flow was measured throughout the experimental period. Twelve sets of heat-pulse probes were used, one for each main branch. The diurnal course of the environmental conditions, the fraction of the area irradiated and leaf water relations were also considered in each main branch. The relationships between leaf water potential, xylem water potential and transpiration were established for different branches and also for the total plant. Using the slopes of these regressions, total plant conductance, the hydraulic conductance of the stem and root pathway, the hydraulic conductance of the canopy and the hydraulic conductance of each branch were estimated. Our findings show that the root conductance and the canopy hydraulic conductance are similar in magnitude. Leaf hydraulic conductance per leaf area unit was similar for each of the four branch orientations, indicating that, while the light microclimate has a dominant influence on transpiration, in this case it had little effect on the hydraulic properties of the canopy.     

Rapid shoot‐to‐root signalling regulates root hydraulic conductance via aquaporins

We investigated how root hydraulic conductance (normalized to root dry weight, L_o) is regulated by the shoot. Shoot topping (about 30% reduction in leaf area) reduced L_o of grapevine (Vitis vinifera L.), soybean (Glycine max L.) and maize (Zea mays L.) by 50 to 60%. More detailed investigations with soybean and grapevine showed that the reduction in L_o was not correlated with the reduction in leaf area, and shading or cutting single leaves had a similar effect. Percentage reduction in L_o was largest when initial L_o was high in soybean. Inhibition of L_o by weak acid (low pH) was smaller after shoot damage or leaf shading. The half time of reduction in L_o was approximately 5 min after total shoot decapitation. These characteristics indicate involvement of aquaporins. We excluded phloem‐borne signals and auxin‐mediated signals. Xylem‐mediated hydraulic signals are possible since turgor rapidly decreased within root cortex cells after shoot topping. There was a significant reduction in the expression of several aquaporins in the plasma membrane intrinsic protein (PIP) family of both grapevine and soybean. In soybean, there was a five‐ to 10‐fold reduction in GmPIP1;6 expression over 0.5–1 h which was sustained over the period of reduced L_o.     

Effect of partial root excision on transpiration, root hydraulic conductance and leaf growth in wheat seedlings.

Removal of four out of five roots did not lower transpiration and stomatal conductivity of wheat (Triticum durum Desf.) seedlings. Water content of mature expanded leaf lamina remained constant at control levels. The results suggest that the only remaining root was capable to supply the shoot with water. This was evidenced by an increase in hydraulic conductivity of the root system following partial root excision measured at low subatmospheric pressures induced by vacuum. In the absence of a hydrostatic gradient, water flow from reduced root system was initially not higher than from an intact system, but increased subsequently. ABA content was increased in roots 1 h after partial root excision, which might contribute to the increase in hydraulic conductivity.     

干旱低磷胁迫对不同品种小麦根系导水率的影响

控制磷素水平,采用控制灌水量(正常供水、中度及重度干旱胁迫)的盆栽试验法,选择抗旱性小麦品种陕合6号(W1)和水分敏感型品种郑引1号(W2)为供试材料。用压力室法测定了三叶期的两品种小麦根系导水率(LPr)的变化规律。结果表明：陕合6号,在有磷正常供水处理( PH)下具有较高的导水率,干旱胁迫时LPr降低较少,且复水后有较强的恢复能力。郑引1号, PH的LPr值相对较小,干旱导致的根系导水率下降非常突出,复水后的恢复能力也较弱。另外,干旱胁迫对小麦苗期根系导水率的影响大于磷胁迫对其导水率的影响,且两品种小麦无磷止常供水处理(-PH)的LPr分别为 PH的31．9%和53．6％,即磷对前者LPr的影响大于后者。     

Different cation stresses affect specifically osmotic root hydraulic conductance, involving aquaporins, ATPase and xylem loading of ions in Capsicum annuum, L. plants

In order to study the effect of nutrient stress on water uptake in pepper plants (Capsicum annuum L.), the excess or deficiency of the main cations involved in plant nutrition (K(+), Mg(2+), Ca(2+)) and two different degrees of salinity were related to the activity of plasma membrane H(+)-ATPase, the pH of the xylem sap, nutrient flux into the xylem (J(s)) and to a number of parameters related to water relations, such as root hydraulic conductance (L(0)), stomatal conductance (g(s)) and aquaporin activity. Excess of K(+), Ca(+) and NaCl produced a toxic effect on L(0) while Mg(2+) starvation produced a positive effect, which was in agreement with aquaporin functionality, but not with ATPase activity. The xylem pH was altered only by Ca treatments. The results obtained with each treatment could suggest that detection of the quality of the nutrient supply being received by roots can be related to aquaporins functionality, but also that each cation stress triggers specific responses that have to be assessed individually.     

Activation of the root xylem proton pump by hydraulic signals from leaves under suppressed transpiration

Journal of Plant Research - Long term field observations have revealed that the inhibition of transpiration by heavy rainfall promotes immediate positive shift in the trans-root electric potential...