Hydraulic Root Research Articles

Effects of Root Excision and Water Stress on Root Hydraulics and TaPIPs Expression in Wheat Seedlings

Root excision can break water balance between water uptake by roots and transpriation in shoots and induce hydraulic responses of roots,which is helpful to explain how hydrostatic gradient regulates root hydraulic traits.To explore the regulation mechanisms of root hydraulics to the broken whole plant water balance by root excision and water stress,we investigated the changes of individual root(Lproot) and cortex cell(Lpcell) hydraulic conductivities of wheat(Triticum aestivum,cv.Changwu 134) after root excision and water stress with root and cell pressure probes.The transcription levels of TaPIP1;2 and TaPIP2;5 were also measured with quantitative real-time PCR.Root excision or water stress treatment significantly reduced the leaf transpiration rate and stomatal conductance of wheat seedlings,but the Lproot or Lpcell of the remained roots had no significant changes.The transpiration rate,stomatal conductance,leaf water potential,Lproot,and Lpcell of root-excised plants were significantly higher than those of plants under water stress.However,these parameters of plants with both root excision and water stress were significantly lower than those with a single treatment or the control.Irrespectively of root excision or water stress,Lpcell had a significantly positive correlation with Lproot(R2= 0.97,P 0.05),which suggested the accordance in root water uptake ability at cell level and individual root level.Root excision significantly up-regulated the relative mRNA contents of TaPIP1;2 and TaPIP2;5 in wheat roots,but water stress had an effect of down-regulation.The relative mRNA contents of TaPIP1;2 and TaPIP2;5 of root excised and water stressed seedlings were the lowest in the four treatments.These results may suggest that root excision could reduce the tolerance of wheat seedling to the following water stress;cortex cell hydraulic conductivity could be accordant with individual root hydraulic conductivity during wheat root hydraulics regulation;and TaPIP1;2 and TaPIP2;5 may be involved in regulating root hydraulics of wheat.

The relationship between root hydraulics and scion vigour across Vitis rootstocks: what role do root aquaporins play?

Vitis vinifera scions are commonly grafted onto rootstocks of other grape species to influence scion vigour and provide resistance to soil-borne pests and abiotic stress; however, the mechanisms by which rootstocks affect scion physiology remain unknown. This study characterized the hydraulic physiology of Vitis rootstocks that vary in vigour classification by investigating aquaporin (VvPIP) gene expression, fine-root hydraulic conductivity (Lp r), % aquaporin contribution to Lp r, scion transpiration, and the size of root systems. Expression of several VvPIP genes was consistently greater in higher-vigour rootstocks under favourable growing conditions in a variety of media and in root tips compared to mature fine roots. Similar to VvPIP expression patterns, fine-root Lp r and % aquaporin contribution to Lp r determined under both osmotic (Lp r Osm) and hydrostatic (Lp r Hyd) pressure gradients were consistently greater in high-vigour rootstocks. Interestingly, the % aquaporin contribution was nearly identical for Lp r Osm and Lp r Hyd even though a hydrostatic gradient would induce a predominant flow across the apoplastic pathway. In common scion greenhouse experiments, leaf area-specific transpiration (E) and total leaf area increased with rootstock vigour and were positively correlated with fine-root Lp r. These results suggest that increased canopy water demands for scion grafted onto high-vigour rootstocks are matched by adjustments in root-system hydraulic conductivity through the combination of fine-root Lp r and increased root surface area.

The grapevine root-specific aquaporin VvPIP2;4N controls root hydraulic conductance and leaf gas exchange under well-watered conditions but not under water stress.

We functionally characterized the grape (Vitis vinifera) VvPIP2;4N (for Plasma membrane Intrinsic Protein) aquaporin gene. Expression of VvPIP2;4N in Xenopus laevis oocytes increased their swelling rate 54-fold. Northern blot and quantitative reverse transcription-polymerase chain reaction analyses showed that VvPIP2;4N is the most expressed PIP2 gene in root. In situ hybridization confirmed root localization in the cortical parenchyma and close to the endodermis. We then constitutively overexpressed VvPIP2;4N in grape 'Brachetto', and in the resulting transgenic plants we analyzed (1) the expression of endogenous and transgenic VvPIP2;4N and of four other aquaporins, (2) whole-plant, root, and leaf ecophysiological parameters, and (3) leaf abscisic acid content. Expression of transgenic VvPIP2;4N inhibited neither the expression of the endogenous gene nor that of other PIP aquaporins in both root and leaf. Under well-watered conditions, transgenic plants showed higher stomatal conductance, gas exchange, and shoot growth. The expression level of VvPIP2;4N (endogenous + transgene) was inversely correlated to root hydraulic resistance. The leaf component of total plant hydraulic resistance was low and unaffected by overexpression of VvPIP2;4N. Upon water stress, the overexpression of VvPIP2;4N induced a surge in leaf abscisic acid content and a decrease in stomatal conductance and leaf gas exchange. Our results show that aquaporin-mediated modifications of root hydraulics play a substantial role in the regulation of water flow in well-watered grapevine plants, while they have a minor role upon drought, probably because other signals, such as abscisic acid, take over the control of water flow.

Influence of evaporative demand on aquaporin expression and root hydraulics of hybrid poplar

When light levels and evaporative demand increase, dynamic physiological changes in roots may be required to restore the water balance at the whole plant level. We hypothesized that a dynamic increase in root hydraulic conductance (L(P)) and aquaporin (AQP) expression could moderate the transpiration-induced drop in water potential (Ψ), allowing continued gas exchange in hybrid poplar (Populus trichocarpa × deltoides) saplings. Fifty-six AQPs have been identified in poplar, but little information about their expression patterns in roots is available, especially from a whole-plant water relations perspective. We measured AQP expression and L(P) in plants subjected to different levels of light and evaporative demand. Shaded plants had only one-tenth the root area of plants growing at higher light levels. Shade-grown saplings experiencing a sudden increase in light exhibited a threefold higher L(P) than plants remaining in shade. This dynamic increase in L(P) corresponded with increased transcript abundance of 15 AQPs out of a total of 33 genes simultaneously assessed by quantitative RT-PCR. The tissue-level localization of transcripts of four AQPs was studied with in situ hybridization. Comprehensive expression profiling in conjunction with physiological and morphological measurements is a valuable reference for future studies on AQP function in poplar.

Influence of salinity on root hydraulic properties of three olive varieties

Three varieties of olive, Barnea, Arbequina and Proline, varying in salt tolerance, were examined to check the sensitivity of their root system hydraulic properties to salinity. Up to three levels of saline water (EC = 1.2, 4.2 and 7.5 dS m−1) were used for long-term irrigation of mature trees. Specific conductivities and embolism rates of roots and branches were estimated by low-pressure conductivity measurement; variability and plasticity of root and branch axial conductivities were calculated. Cross-sections of roots were analysed with respect to xylem anatomy. Barnea, and to a minor degree Arbequina, were found to be more salt-resistant than Proline. Axial root hydraulics under salt stress reacted in a more plastic fashion than branch conductivities. Increased specific conductivities of roots, different plasticities of root hydraulics and modifications in mean conduit diameters can be dismissed as foremost reasons of the observed differences in salt resistance. Instead, a high within-population variability in root conductivity, as found in the salt-tolerant Barnea and Arbequina varieties, coming to full effect in high conductivity roots of Barnea trees, and an increased bimodal distribution of conduit sizes may represent favourable traits to enhance water uptake in soils with heterogeneous salinity.

Natural Variation of Root Hydraulics in Arabidopsis Grown in Normal and Salt-Stressed Conditions

To gain insights into the natural variation of root hydraulics and its molecular components, genotypic differences related to root water transport and plasma membrane intrinsic protein (PIP) aquaporin expression were investigated in 13 natural accessions of Arabidopsis (Arabidopsis thaliana). The hydraulic conductivity of excised root systems (Lpr) showed a 2-fold variation among accessions. The contribution of aquaporins to water uptake was characterized using as inhibitors mercury, propionic acid, and azide. The aquaporin-dependent and -independent paths of water transport made variable contributions to the total hydraulic conductivity in the different accessions. The distinct suberization patterns observed among accessions were not correlated with their root hydraulic properties. Real-time reverse transcription-polymerase chain reaction revealed, by contrast, a positive overall correlation between Lpr and certain highly expressed PIP transcripts. Root hydraulic responses to salt stress were characterized in a subset of five accessions (Bulhary-1, Catania-1, Columbia-0, Dijon-M, and Monte-Tosso-0 [Mr-0]). Lpr was down-regulated in all accessions except Mr-0. In Mr-0 and Catania-1, cortical cell hydraulic conductivity was unresponsive to salt, whereas it was down-regulated in the three other accessions. By contrast, the five accessions showed qualitatively similar aquaporin transcriptional profiles in response to salt. The overall work provides clues on how hydraulic regulation allows plant adaptation to salt stress. It also shows that a wide range of root hydraulic profiles, as previously reported in various species, can be observed in a single model species. This work paves the way for a quantitative genetics analysis of root hydraulics.

Effect of Calcium on Maize Seedling Root Hydraulic Conductivity and Growth under Water Stress and Rehydration Conditions

The effects of extra calcium(additions of 10 mmol L-1 CaCl2 into half-strength Hoagland solution) on maize(Zea mays L.) seedling root hydraulic conductivity(Lpr),morphology and leaf water potential(ψw) were investigated under water stress,which was simulated by 10% PEG-6000 with osmotic potential(ψs) value of -0.2 MPa for seven days,and subsequent two days rehydration.Whole root hydraulic conductivity(Lpr) and leaf water potential(ψw) were determined by pressure chamber.After these treatments,it could be seen that water stress reduced Lpr,which was restored when calcium was added to the solution for growing the water-stressed plants.In addition,the Lpr recovered to the level of the control after re-watering at high Ca2+ level,but the recovery of Lpr was only 59.06% at regular Ca2+ level.HgCl2(50 μmol L-1) treatment caused a sharp decline in Lpr,which was almost restored by treatment with 5 mmol L-1 β-mercaptoethanol.The reduction of Lpr by Hg2+ was 53.20% and 74.55% at regular and high calcium levels,respectively under water stress condition,and 62.1% under normal condition.The results suggest that Ca2+ increased the passage of water through the cell membrane of roots by increasing the activity of Hg-sensitive AQP under water stress.The percentage of reduction by Hg2+ was decreased to the control level two days after re-watering in CaCl2 treated plants.The leaf water potential was declined significantly under water stress,particularly at regular calcium level with respect to the decrease of Lpr.However,ψw recovered rapidly onw day after re-watering.Furthermore,water stress had a detrimental effect on the root growth.The addition of calcium,especially at low water potential,increased the root surface area and primary root diameter,and it promoted primary root enlongation and lateral root development.Compared with the controls,the growth of Ca2+ treated plants was recovered gradually with the prolonging of rehydration.Therefore,these results indicated that the extra calcium,with respect to root growth and water uptake,mitigates the negative effect of water stress and enhances the compensatory effects of rehydration.

Hydraulic resistance partitioning between shoot and root system and plant water status of <I>Haloxyolon ammodendron</I> growing at sites of contrasting soil texture

Hydraulic resistance components and water relations were studied on Haloxyolon ammoden- dron, a small xeric tree, growing at sites significantly differed in soil texture. Soil water content, leaf water potential (ψl), xylem water potential (ψx), root water potential (ψroot), leaf transpiration rate (TR) and stomatal conductance (gs) were measured at the two sites during the growing season of 2005 and 2006. Leaf spe- cific hydraulic resistance (Rplant) during the whole growing season, hydraulic resistance of plants (Rp), shoots (Rshoot) and roots (Rroot) in the August of both years were calculated and expressed on leaf area basis. The results showed the proportion of the hydraulic resistance of the aerial part (Rshoot) to the Rp was the same to the proportion of the hydraulic resistance of the soil part (Rroot) to the Rp, indicating that both parts were equivalent important to plant water hydraulic system from soil to leaf. Positive significant corre- lations were found between Rp and Rroot, suggesting that root hydraulics resistance was a major determi- nant of plant hydraulic resistance (Rp) and transpiration rate. The integrated effect of stomatal control, hy- draulic regulation and morphology adjustment enabled plants at heavy soil site surviving the extreme water deficit period.

The significance of roots as hydraulic rheostats

Roots are the primary sites of water uptake by plants. Roots also sense most of the physico-chemical parameters of the soil, perceive signals from the shoots, and adjust their growth and water transport properties accordingly. The present opinion paper discusses the significance of the variable water transport capacity (hydraulic conductance) of roots during development and in response to environmental stimuli. It is shown that root hydraulics determines water uptake intensities but also water potential gradients within the plant. It is indicated how the dynamics of root hydraulics contributes to many integrated plant nutritional and growth functions. For instance, the heterogeneity of soil water and nutrient availability and the heterogeneity of root hydraulic properties feed each other and play critical roles in root transport functions. Another important aspect is the integration of root hydraulics within the mutual interactions of roots and shoots, for co-ordinated growth and water-saving responses to drought.

Effects of water storage in the stele on measurements of the hydraulics of young roots of corn and barley

In standard techniques (root pressure probe or high-pressure flowmeter), the hydraulic conductivity of roots is calculated from transients of root pressure using responses following step changes in volume or pressure, which may be affected by a storage of water in the stele. Storage effects were examined using both experimental data of root pressure relaxations and clamps and a physical capacity model. Young roots of corn and barley were treated as a three-compartment system, comprising a serial arrangement of xylem/probe, stele and outside medium/cortex. The hydraulic conductivities of the endodermis and of xylem vessels were derived from experimental data. The lower limit of the storage capacity of stelar tissue was caused by the compressibility of water. This was subsequently increased to account for realistic storage capacities of the stele. When root water storage was varied over up to five orders of magnitude, the results of simulations showed that storage effects could not explain the experimental data, suggesting a major contribution of effects other than water storage. It is concluded that initial water flows may be used to measure root hydraulic conductivity provided that the volumes of water used are much larger than the volumes stored.

Roles of Morphology, Anatomy, and Aquaporins in Determining Contrasting Hydraulic Behavior of Roots

The contrasting hydraulic properties of wheat (Triticum aestivum), narrow-leafed lupin (Lupinus angustifolius), and yellow lupin (Lupinus luteus) roots were identified by integrating measurements of water flow across different structural levels of organization with anatomy and modeling. Anatomy played a major role in root hydraulics, influencing axial conductance (L(ax)) and the distribution of water uptake along the root, with a more localized role for aquaporins (AQPs). Lupin roots had greater L(ax) than wheat roots, due to greater xylem development. L(ax) and root hydraulic conductance (L(r)) were related to each other, such that both variables increased with distance from the root tip in lupin roots. L(ax) and L(r) were constant with distance from the tip in wheat roots. Despite these contrasting behaviors, the hydraulic conductivity of root cells (Lp(c)) was similar for all species and increased from the root surface toward the endodermis. Lp(c) was largely controlled by AQPs, as demonstrated by dramatic reductions in Lp(c) by the AQP blocker mercury. Modeling the root as a series of concentric, cylindrical membranes, and the inhibition of AQP activity at the root level, indicated that water flow in lupin roots occurred primarily through the apoplast, without crossing membranes and without the involvement of AQPs. In contrast, water flow across wheat roots crossed mercury-sensitive AQPs in the endodermis, which significantly influenced L(r). This study demonstrates the importance of examining root morphology and anatomy in assessing the role of AQPs in root hydraulics.

Nitrate induction of root hydraulic conductivity in maize is not correlated with aquaporin expression

Some plant species can increase the mass flow of water from the soil to the root surface in response to the appearance of nitrate in the rhizosphere by increasing root hydraulic conductivity. Such behavior can be seen as a powerful strategy to facilitate the uptake of nitrate in the patchy and dynamically changing soil environment. Despite the significance of such behavior, little is known about the dynamics and mechanism of this phenomenon. Here we examine root hydraulic response of nitrate starved Zea mays (L.) plants after a sudden exposure to 5 mM NO(3)(-) solution. In all cases the treatment resulted in a significant increase in pressure-induced (pressure gradient approximately 0.2 MPa) flow across the root system by approximately 50% within 4 h. Changes in osmotic gradient across the root were approximately 0.016 MPa (or 8.5%) and thus the results could only be explained by a true change in root hydraulic conductance. Anoxia treatment significantly reduced the effect of nitrate on xylem root hydraulic conductivity indicating an important role for aquaporins in this process. Despite a 1 h delay in the hydraulic response to nitrate treatment, we did not detect any change in the expression of six ZmPIP1 and seven ZmPIP2 genes, strongly suggesting that NO(3)(-) ions regulate root hydraulics at the protein level. Treatments with sodium tungstate (nitrate reductase inhibitor) aimed at resolving the information pathway regulating root hydraulic properties resulted in unexpected findings. Although this treatment blocked nitrate reductase activity and eliminated the nitrate-induced hydraulic response, it also produced changes in gene expression and nitrate uptake levels, precluding us from suggesting that nitrate acts on root hydraulic properties via the products of nitrate reductase.

WATER STRESS AND ROOT HYDRAULIC CONDUCTIVITY IN GRAPEVINE GRAFTED ON DIFFERENT ROOTSTOCKS

WATER STRESS AND ROOT HYDRAULIC CONDUCTIVITY IN GRAPEVINE GRAFTED ON DIFFERENT ROOTSTOCKS

During measurements of root hydraulics with pressure probes, the contribution of unstirred layers is minimized in the pressure relaxation mode: comparison with pressure clamp and high-pressure flowmeter

The effects of unstirred layers (USLs) at the endodermis of roots of young maize plants (Zea mays L.) were quantified, when measuring the water permeability of roots using a root pressure probe (RPP) in the pressure relaxation (PR) and pressure clamp (PC) modes. Different from PRs, PCs were performed by applying a constant pressure for certain periods of time. Experimental data were compared with results from simulations based on a convection versus diffusion (C/D) model, with the endodermis being the main barrier for solutes and water. Solute profiles in the stele were calculated as they occurred during rapid water flows across the root. The model quantitatively predicted the experimental finding of two distinct phases during PRs, in terms of a build-up of concentration profiles in the stele between endodermis and xylem vessels. It also predicted that, following a PC, half-times (T1/2) of PRs increased as the time used for clamping (and the build-up of USLs) increased. Following PCs of durations of 15, 30 and 60 s, T1/2 increased by factors of between 2.5 and 7.0, and water permeability of roots (root hydraulic conductivity, Lpr) was reduced by the same factors. When root pressure was immediately taken back to the original equilibrium root pressure following a PC, there was a transient uptake of water into the root stele (transient increase of root pressure), and the size of transients rose with time of clamping, as predicted by the model. The results indicated that the 'real' hydraulic conductivity of roots should be measured during initial water flows, such as during the rapid phase of PRs, when the effect of USLs was minimized. It was discussed that 'pressure-propagation effects' could not explain the finding of two phases during PRs. The results of USL effects threw some doubt on the use of PC and high-pressure flowmeter (HPFM) techniques with roots, where rigorous estimates of USLs were still missing despite the fact that large quantities of water were forced across the root.

Plant morphology and root hydraulics are altered by nutrient deficiency in Pistacia lentiscus (L.)

The plants in arid and semiarid areas are often limited by water and nutrients. Morpho-functional adjustments to improve nutrient capture may have important implications on plant water balance, and on plant capacity to withstand drought. Several studies have shown that N and P deficiencies may decrease plant hydraulic conductance. Surprisingly, studies on the implications of nutrient limitations on water use in xerophytes are scarce. We have evaluated the effects of strong reductions in nitrogen and phosphorus availability on morphological traits and hydraulic conductance in seedlings of a common Mediterranean shrub, Pistacia lentiscus L.. Nitrogen deficiency resulted in a decrease in aboveground biomass accumulation, but it did not affect belowground biomass accumulation or root morphology. Phosphorus-deficient plants showed a decrease in leaf area, but no changes in aboveground biomass. Root length, root surface area, and specific root length were higher in phosphorus-deficient plants than in control plants. Nitrogen and phosphorus deficiency reduced both root hydraulic conductance and root hydraulic conductance scaled by total root surface area. On the other hand, nutrient limitations did not significantly affect root conductance per unit of foliar surface area. Thus, adaptation to low nutrient availability did not affect seedling capacity for maintaining water supply to leaves. The implications for drought resistance and survival during seedling establishment in semi-arid environments are discussed.

Exposure of roots of cucumber (Cucumis sativus) to low temperature severely reduces root pressure, hydraulic conductivity and active transport of nutrients.

When root temperature dropped below 25 degrees C, there was a sharp drop in the root pressure (P(r)) and hydraulic conductivity of excised roots (Lp(r)) of young cucumber (Cucumis sativus L.) seedlings as measured with the root pressure probe. A detailed analysis of root hydraulics provided evidence for a larger reduction in the osmotic component of Lp(r) (77%) in comparison with the hydrostatic component (34%) in response to the exposure of the root system to 13 degrees C. The activity of the plasma membrane H(+)-ATPase (EC 3.6.1.35) was reduced from 30 to 16 micro mol Pi mg(-1) protein h(-1) upon exposure to 8 degrees C for 1 day. Ultrastructural observations showed no evidence of loosening of the microstructure of endodermal cell walls in low temperature (LT)-treated roots. It is concluded that the rapid drop in the P(r) in response to LT is largely caused by a reduction in the activity of the plasma membrane H(+)-ATPase rather than by loosening of the endodermal wall which would cause substantial solute losses. On the other hand, water permeability of root cell membrane at LT was related to changes in the activity (open/closed state) of water channels.

Drought resistance of Ailanthus altissima: root hydraulics and water relations.

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13-week treatment period, soil water potential was -0.05 MPa for well-watered control seedlings (W) and -0.4, -0.8 and -1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meter. When scaled by leaf surface area, shoot hydraulic conductance did not differ among the treated seedlings, whereas root hydraulic conductance decreased by about 20% in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scaled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.

Periodic Drought Stress Effect on Evapotranspiration, Root Hydraulic Conductance and Fruit Yield Efficiency of Eggplant

Periodic Drought Stress Effect on Evapotranspiration, Root Hydraulic Conductance and Fruit Yield Efficiency of Eggplant

High-Temperature Preconditioning and Thermal Shock Imposition Affects Water Relations, Gas Exchange and Root Hydraulic Conductivity in Tomato

Potted tomato plants (Lycopersicon esculentum Mill. cv. Amalia) were submitted to three different treatments: control (C) plants were maintained at day/night temperature of 25/18 °C; preconditioned plants (PS) were submitted to two consecutive periods of 4 d each, of 30/23 and 35/28 °C before being exposed to a heat stress (40/33 °C lasting 4 d) and non-preconditioned (S) plants were maintained in the same conditions as the C plants and exposed to the heat stress. The inhibition of plant growth was observed only in PS plants. Heat stress decreased chlorophyll content, net photosynthetic rate and stomatal conductance in both PS and S plants. However, PS plants showed good osmotic adjustment, which enabled them to maintain leaf pressure potential higher than in S plants. Furthermore, at the end of the recovery period PS plants had higher pressure potential and stomatal conductance than in S plants.

PHYSIOLOGICAL CHANGES IN GRAPEVINE DURING ADJUSTMENT TO WATER STRESS: ABA, LEAF GAS EXCHANGES AND ROOT HYDRAULIC CONDUCTIVITY

PHYSIOLOGICAL CHANGES IN GRAPEVINE DURING ADJUSTMENT TO WATER STRESS: ABA, LEAF GAS EXCHANGES AND ROOT HYDRAULIC CONDUCTIVITY