Wide vessels sustain marginal transpiration flux and do not optimize inefficient gas exchange activity under impaired hydraulic control and salinity

Plants optimize water use and carbon assimilation via transient regulation of stomata resistance and by limiting hydraulic conductivity in a long-term response of xylem anatomy. We postulated that without effective hydraulic regulation plants would permanently restrain water loss and photosynthetic productivity under salt stress conditions. We compared wild-type tomatoes to a transgenic type (TT) with impaired stomatal control. Gas exchange activity, biomass, starch content, leaf area and root traits, mineral composition and main stems xylem anatomy and hydraulic conductivity were analyzed in plants exposed to salinities of 1 and 4 dS m⁻¹ over 60 days. As the xylem cannot easily readjust to different environmental conditions, shifts in its anatomy and the permanent effect on plant hydraulic conductivity kept transpiration at lower levels under unstressed conditions and maintained it under salt-stress, while sustaining higher but inefficient assimilation rates, leading to starch accumulation and decreased plant biomass, leaf and root area and root length. Narrow conduits in unstressed TT plants were related to permanent restrain of hydraulic conductivity and plant transpiration. Under salinity, TT plants followed the atmospheric water demand, sustained similar transpiration rate from unstressed to salt-stressed conditions and possibly maintained hydraulic integrity, due to likely impaired hydraulic regulation, wider conduits and higher hydraulic conductivity. The accumulation of salts and starch in the TT plants was a strong evidence of salinity tolerance via osmotic regulation, also thought to help to maintain the assimilation rates and transpiration flux under salinity, although it was not translated into higher growth.     

Mycorrhizal and non-mycorrhizal Lactuca sativa plants exhibit contrasting responses to exogenous ABA during drought stress and recovery

The arbuscular mycorrhizal (AM) symbiosis enhances plant tolerance to water deficit through the alteration of plant physiology and the expression of plant genes. These changes have been postulated to be caused (among others) by different contents of abscisic acid (ABA) between AM and non-AM plants. However, there are no studies dealing with the effects of exogenous ABA on the expression of stress-related genes and on the physiology of AM plants. The aim of the present study was to evaluate the influence of AM symbiosis and exogenous ABA application on plant development, physiology, and expression of several stress-related genes after both drought and a recovery period. Results show that the application of exogenous ABA had contrasting effects on AM and non-AM plants. Only AM plants fed with exogenous ABA maintained shoot biomass production unaltered by drought stress. The addition of exogenous ABA enhanced considerably the ABA content in shoots of non-AM plants, concomitantly with the expression of the stress marker genes Lsp5cs and Lslea and the gene Lsnced. By contrast, the addition of exogenous ABA decreased the content of ABA in shoots of AM plants and did not produce any further enhancement of the expression of these three genes. AM plants always exhibited higher values of root hydraulic conductivity and reduced transpiration rate under drought stress. From plants subjected to drought, only the AM plants recovered their root hydraulic conductivity completely after the 3 d recovery period. As a whole, the results indicate that AM plants regulate their ABA levels better and faster than non-AM plants, allowing a more adequate balance between leaf transpiration and root water movement during drought and recovery.     

Metabolic profile and antioxidant responses during drought stress recovery in sugarcane treated with humic acids and endophytic diazotrophic bacteria

Water deficit is the major yield‐limiting factor for sugarcane crop production that can be enhanced by inoculating with plant growth promoting bacteria (PGPB) combined with humic substances. The aim of this work was to examine changes to the metabolic profile and antioxidant enzyme activity of sugarcane treated with PGPB and humic acid (HA) after drought and then rehydration. The drought was imposed by withholding irrigation for 21 days thereby measuring enzyme activity, metabolic profile and photosynthetic rate 1 week after rehydratation. Growth of plants treated with HA, PGPB and with both treatments combined (PGPB + HA) was higher than control plants. The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities remained higher after rehydration only in plants treated with HA. Plants treated with HA and PGPB + HA exhibited increased transpiration, stomatal conductance and net photosynthesis than plants treated with PGPB. The PGPB‐treated plants exhibited drought resistance that resembled ‘delayed stress onset’, which is a new term for preserving water in the plants tissues. Water preservation in plants treated with PGPB was corroborated by higher relative water content (RWC) than control plants at the end of the drought period. Plants treated with HA + PGPB exhibited the highest water potential after rehydration and high RWC. Osmotic adjustment in the other treatments (control, HA and PGPB) was indicated by a new pattern of metabolic response after rehydration, including generally enhanced carbohydrates and proteins and specific changes induced by HA‐enhancing aromatic compounds, whereas PGPB exhibited enhanced fatty acids and other aliphatic H species. Humic acids assist with drought stress recovery by inducing antioxidant enzyme activity whereas PGPB induced preservation of leaf water potential and RWC by closing stomata efficiently, resulting in plant water preservation.     

Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporative demand

We studied the possible involvement of ABA in the control of water relations under conditions of increased evaporative demand. Warming the air by 3°C increased stomatal conductance and raised transpiration rates of hydroponically grown Triticum durum plants while bringing about a temporary loss of relative water content (RWC) and immediate cessation of leaf extension. However, both RWC and extension growth recovered within 30 min although transpiration remained high. The restoration of leaf hydration and growth were enabled by increased root hydraulic conductivity after increasing the air temperature. The use of mercuric chloride (an inhibitor of water channels) to interfere with the rise on root hydraulic conductivity hindered the restoration of extension growth. Air warming increased ABA content in roots and decreased it in shoots. We propose this redistribution of ABA in favour of the roots which increased the root hydraulic conductivity sufficiently to permit rapid recovery of shoot hydration and leaf elongation rates without the involvement of stomatal closure. This proposal is based on known ability of ABA to increase hydraulic conductivity confirmed in these experiments by measuring the effect of exogenous ABA on osmotically driven flow of xylem sap from the roots. Accumulation of root ABA was mainly the outcome of increased export from the shoots. When phloem transport in air-warmed plants was inhibited by cooling the shoot base this prevented ABA enrichment of the roots and favoured an accumulation of ABA in the shoot. As a consequence, stomata closed.     

Plasma membrane aquaporins play a significant role during recovery from water deficit

The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5- to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants. From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.     

Hydraulic regulation strategies for whole-plant water balance of two maize inbred lines differing in drought resistance under short-term osmotic stress

The conservation of water in agriculture requires an understanding of the mechanisms of plant–water relations. This study aimed to reveal hydraulic regulation strategies of maize (Zea mays L.) for maintaining the plant water balance during drought. The water relations of two maize inbred lines (Tian4 and 478) that differ in their resistance to drought in the field were investigated under well-watered conditions and osmotic stress induced with 10 % PEG 6000. The leaf transpiration rate and leaf water potential of 478 varied diurnally, but remained constant in Tian4, which is more drought resistant. Tian4 plants showed morphological, anatomical and physiological advantages that protected them from foliar water loss. The strategies of leaf hydraulics to regulate leaf water balance during the day and during short-term osmotic stress also differed between Tian4 and 478. The leaf hydraulic conductivity of Tian4 and 478 increased temporarily, but their root hydraulic conductivities were reduced under osmotic stress. However, the root hydraulic conductivity of Tian4 subsequently recovered. Lower and rapidly reduced leaf transpiration and the ability of root hydraulics to recover from short-term osmotic stress can help explain the strategies for plant water balance of drought-tolerant maize.     

Over-expression of PIP2;5 aquaporin alleviates gas exchange and growth inhibition in poplars exposed to mild osmotic stress with polyethylene glycol

The effects of mild osmotic stress conditions on aquaporin-mediated water transport are not well understood. In the present study, mild osmotic stress treatments with 20 and 50 g L^?1 polyethylene glycol 6000 (PEG) in Hoagland’s mineral solution were applied for 3 weeks under controlled environmental conditions to transgenic Populus tremula × Populus alba plants constitutively over-expressing a Populus PIP2;5 aquaporin and compared with the wild-type plants. The PEG treatments resulted in growth reductions and triggered changes in net photosynthesis, transpiration, stomatal conductance and root hydraulic conductivity in the wild-type plants. However, height growth, leaf area, gas exchange, and root hydraulic conductivity were less affected by the PEG treatments in PIP2;5-over-expressing poplar lines. These results suggest that water transport across the PIP2;5 aquaporin is an important process contributing to tolerance of mild osmotic stress in poplar. Greater membrane abundance of PIP2;5 was most likely the factor that was responsible for higher root hydraulic conductivity leading to improved plant water flux and, consequently, greater gas exchange and growth rates under mild osmotic stress conditions. The results also provide evidence for the functional significance of PIP2;5 aquaporin in water transport and its strong link to growth processes in poplar.     

The Effect of VA Mycorrhizal Infection and Phosphorus Status on Sunflower Hydraulic and Stomatal Properties

Koide, R. 1985. The effect of VA mycorrhizal infection and phosphorusstatus on sunflower hydraulic and stomatal properties.—J. exp. Bot. 36: 1087–1098. Mycorrhizal (M) and non-mycorrhizal (NM) sunflower plants weregrown in a soil of low phosphorus availability (with and withoutphosphorus amendment) and in a soil of moderate phosphorus availability(without phosphorus amendment). Using the Ohm's law analogyand measured leaf water potentials, stem water potentials, andtranspiration rates, hydraulic resistances were calculated forthe whole plant, leaf, and below leaf components. Mycorrhizalinfection (as high as 89%) was shown to have no effect on theintrinsic hydraulic properties of the soil/plant system overa wide range of transpiration rates in either soil when M andNM plants of equivalent root length were compared. When grownin the soil of moderate phosphorus availability, calculatedhydraulic resistances under given environmental conditions werethe same for M and NM plants, as were stomatal resistances andtranspiration rates. When grown in the soil of low phosphorusavailability, calculated values of hydraulic resistance werelower for M plants than for NM plants under given sets of environmentalconditions. These differences in calculated hydraulic resistancewere not due to a difference in the intrinsic hydraulic propertiesof M and NM plants. The differences were evident because stomatalresistances were lower and transpiration rates higher for Mplants and because hydraulic resistance varied inversely withtranspiration rate. When plants of significantly greater rootlength were compared to plants of lesser root length, the calculatedhydraulic resistances under given environmental conditions weremuch lower for the plants of greater root length. This differencewas largely due to a difference in the intrinsic hydraulic propertiesbetween large and small plants, and not because of differencesin transpiration rate. The elevated transpiration rates exhibitedby M plants were attributed to an enhanced phosphorus status.Short term phosphorus amendments made to phosphorus-deficientNM plants improved transpiration; transpiration rates were similarfor M and NM plants before NM plants became phosphorus-deficient,and phosphorus-amended M and NM plants had similar transpirationrates. The data are discussed in relation to other reports ofmycorrhizal influence on hydraulic and stomatal resistances.Possible mechanisms for the influence of infection on stomatalresistance are also briefly discussed. Key words: Hydraulic resistance, stomatal resistance, mycorrhizas     

Water uptake by seminal and adventitious roots in relation to whole-plant water flow in barley (Hordeum vulgare L.)

Prior to an assessment of the role of aquaporins in root water uptake, the main path of water movement in different types of root and driving forces during day and night need to be known. In the present study on hydroponically grown barley (Hordeum vulgare L.) the two main root types of 14- to 17-d-old plants were analysed for hydraulic conductivity in dependence of the main driving force (hydrostatic, osmotic). Seminal roots contributed 92% and adventitious roots 8% to plant water uptake. The lower contribution of adventitious compared with seminal roots was associated with a smaller surface area and number of roots per plant and a lower axial hydraulic conductance, and occurred despite a less-developed endodermis. The radial hydraulic conductivity of the two types of root was similar and depended little on the prevailing driving force, suggesting that water uptake occurred along a pathway that involved crossing of membrane(s). Exudation experiments showed that osmotic forces were sufficient to support night-time transpiration, yet transpiration experiments and cuticle permeance data questioned the significance of osmotic forces. During the day, 90% of water uptake was driven by a tension of about -0.15 MPa.     

Colloidal suspensions of clay or titanium dioxide nanoparticles can inhibit leaf growth and transpiration via physical effects on root water transport

A laboratory investigation was conducted to determine whether colloidal suspensions of inorganic nanoparticulate materials of natural or industrial origin in the external water supplied to the primary root of maize seedlings ( Zea mays L.) could interfere with water transport and induce associated leaf responses. Water flow through excised roots was reduced, together with root hydraulic conductivity, within minutes of exposure to colloidal suspensions of naturally derived bentonite clay or industrially produced TiO₂ nanoparticles. Similar nanoparticle additions to the hydroponic solution surrounding the primary root of intact seedlings rapidly inhibited leaf growth and transpiration. The reduced water availability caused by external nanoparticles and the associated leaf responses appeared to involve a rapid physical inhibition of apoplastic flow through nanosized root cell wall pores rather than toxic effects. Thus: (1) bentonite and TiO₂ treatments also reduced the hydraulic conductivity of cell wall ghosts of killed roots left after hot alcohol disruption of the cell membranes; and (2) the average particle exclusion diameter of root cell wall pores was reduced from 6.6 to 3.0 nm by prior nanoparticle treatments. Irrigation of soil-grown plants with nanoparticle suspensions had mostly insignificant inhibitory effects on long-term shoot production, and a possible developmental adaptation is suggested.     

荒漠河岸林植物木质部导水与栓塞特征及其对干旱胁迫的响应

 以同处于干旱区的塔里木河下游(铁干里克)和黑河下游(乌兰图格)断面为研究区, 比较了荒漠河岸林主要建群种胡杨(Populus euphratica)、柽柳(Tamarix spp.)、疏叶骆驼刺(Alhagi sparsifolia)和花花柴(Karelinia caspia)在长期遭受不同干旱胁迫下的根、枝条木质部导水力和栓塞化程度的变化特征, 并分析了木质部导水对干旱胁迫的响应及适应策略。结果表明: 1) 黑河下游荒漠河岸林植物的导水能力显著高于塔里木河下游, 其中柽柳、胡杨、疏叶骆驼刺和花花柴根木质部的初始比导率(K_s0)分别高11.97、6.74、7.10和3.73倍, 枝条的K_s0分别高9.48、3.65、2.07和1.88倍, 地下水埋深导致的干旱胁迫程度不同是诱发荒漠植物导水能力差异的根本原因; 2)柽柳耐干旱能力最强, 适应范围较宽, 而花花柴、疏叶骆驼刺的耐旱性相对较弱, 适生范围较窄, 这可能与植物的根系分布有关; 3)干旱胁迫较轻时, 枝条木质部是荒漠河岸林植物水分传输的主要阻力部位, 干旱胁迫严重时, 根木质部是限制植株水流的最大阻碍部位; 4)荒漠河岸林植物主要通过调节枝条木质部的水流阻力来适应干旱胁迫, 且其适应策略与干旱胁迫程度有关, 干旱胁迫轻时, 植物通过限制枝条木质部水流来协调整株植物的均匀生长; 干旱胁迫严重时, 植物通过牺牲劣势枝条、增强优势枝条水流来提高植株整体生存的机会。     

The effects of (2RS, 3RS)-1-(4-chlorophenyl)-4, 4-dimethyl-2-(1H-1,2,4 triazol-1-yl) pentan-3-ol (Paclobutrazol:PP333) and root pruning on the growth,water use and response to drought of Colt Cherry rootstocks

The effect of (2RS, 3RS)-1-(4-Chlorophenyl)-4, 4-dimethyl-2-(1H-1,2,4 triazol-1-yl) pentan-3-ol (PP333) on the growth and transpiration of normal and root pruned colt rootstocks was measured. PP333 reduced plant height, stem diameter increment, leaf number, area and weight and stem weight. Root pruning reduced root, leaf and stem weight, and plant height in control plants. PP333 reduced both total water use and transpiration per unit leaf area and increased stomatal resistance. In control plants root pruning also reduced total water use and increased stomatal resistance. 15 days after the beginning of the experiment half the plants in all treatments were allowed to dry out. The effects of drought, i.e. reduced transpiration, growth and leaf water potentials, were smaller in PP333 treated than in control plants.     

Hormonal Control of the Shoot-to-Root Ratio Is Not Related to Water Deficiency in Wheat Plants under Mineral Deficiency

We measured the content of hormones, the rate of growth, and some parameters of water regime (water content, transpiration, and stomatal and hydraulic conductivities) one and two days after wheat plant transfer from 10 to 1% Hoagland-Arnon nutrient medium. It was shown that, a day after dilution of nutrient solution, the content of various cytokinin forms decreased in the xylem sap, shoots, and roots. This decrease was most pronounced in the case of zeatin in the xylem sap and zeatin riboside in the mature zone of the first leaf. ABA was found to accumulate in shoots. A day after dilution of nutrient solution, we observed root elongation evidently induced by mineral nutrient deficiency, and this accelerated root growth was maintained later. Two days after dilution of nutrient solution, we observed the slowing of shoot weight accumulation, whereas root weight remained unchanged. Plant growth response could be related to ABA accumulation in shoots and cytokinin depletion in the whole plant. A reduced hydraulic conductivity and water content in the growing leaf zone was detected only two days after dilution of nutrient solution. Thus, changes in the growth rates and hormone contents could not result from disturbances in water regime induced by mineral nutrient deficiency.     

Unusual stomatal behaviour on partial root excision in wheat seedlings

The excision of four out of five primary roots of wheat (Triticum durum Desf.) seedlings often leads to an enhanced rate of transpiration. Surprisingly this enhancement could be maintained for several hours after root excision and was particularly likely to occur at low irradiances or high atmospheric humidity. This long‐term enhancement could not be explained in terms of conventional hydropassive stomatal effects. Elevated rates of transpiration were associated with and possibly caused by increased cytokinin concentrations in shoots of plants with partially excised roots. The single root remaining after excision was able to maintain an adequate water uptake for the continued enhanced transpiration, after only a short transient reduction in leaf water content. The enhanced capacity for water uptake by the remaining root was confirmed by measuring the water flow from detached roots at negative hydrostatic pressure. Even without additional suction, flow from the reduced root system increased about 1.5 h after the start of treatment, suggesting an increase in membrane permeability for water. Although abscisic acid (ABA) concentrations in the roots increased after the root excision treatment, there was no evidence for any enhanced concentration in the xylem sap. The possible role that this accumulation of ABA in roots may have in the apparent increase in hydraulic conductivity after root excision is discussed.     

胡杨木质部水分传导对盐胁迫的响应与适应北大核心CSCD

解析植物木质部导水率对逆境的响应和适应对促进植物抗逆性机理研究和受损植被恢复具有重要意义。该文以荒漠河岸林建群种胡杨(Populus euphratica)为研究对象,系统分析了胡杨幼株根、茎、叶水分传输通道对不同浓度盐胁迫的响应和适应。结果表明:(1)胡杨幼株根系对盐胁迫的敏感性高于茎和叶,盐胁迫下根系生长和根尖数显著受到抑制,根木质部易于发生栓塞,导水率明显降低。(2)胡杨幼株茎木质部导水率对盐胁迫的响应依盐浓度而定,轻度(0.05 mol·L–1 Na Cl)和中度(0.15 mol·L–1 Na Cl)盐胁迫下,胡杨可以通过协调导管输水的有效性和安全性来调节木质部的导水率,维持植物正常生长;重度(0.30 mol·L–1 Na Cl)盐胁迫下,胡杨茎木质部导管输水有效性和安全性均明显降低,木质部导水率显著下降,并伴随叶片气孔导度的显著降低,从而严重抑制了胡杨的光合和生长。     

Control of Leaf Expansion by Nitrogen Nutrition in Sunflower Plants : ROLE OF HYDRAULIC CONDUCTIVITY AND TURGOR

Nitrogen nutrition strongly affected the growth rate of young sunflower (Helianthus annuus L.) leaves. When plants were grown from seed on either of two levels of N availability, a 33% decrease in tissue N of expanding leaves was associated with a 75% overall inhibition of leaf growth. Almost all of the growth inhibition resulted from a depression of the daytime growth rate. Measurements of pressure-induced water flux through roots showed that N deficiency decreased root hydraulic conductivity by about half. Thus, N deficiency lowered the steady-state water potential of expanding leaves during the daytime when transpiration was occurring. As a result, N-deficient leaves were unable to maintain adequate turgor for growth in the daytime. N deficiency also decreased the hydraulic conductivity for water movement into expanding leaf cells in the absence of transpiration, but growth inhibition at night was much less than in the daytime. N nutrition had no detectable effects on plastic extensibility or the threshold turgor for growth.     

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.     

Growth, Allocation and Water Relations of Shade-grownQuercus rubraL. Saplings Exposed to a Late-season Canopy Gap

For understory saplings to exploit canopy gaps successfully,carbon gain must increase in the gap environment. We predictedthat total biomass of shade-grown red oak saplings would increaseafter exposure to a late-season canopy gap, and that increasedwater and nutrient demand within the canopy gap would drivechanges in the allocation of this carbon. Shade-grown red oaksaplings acclimated to gaps by increasing biomass during theseason of gap formation and increasing the potential for carbongain in the following summer. Within-season carbon gain didnot result from greater production of leaf area, so it mostlikely arose from higher photosynthetic rates of existing shade-developedfoliage, which may be linked to accumulation of leaf nitrogen.During the season of gap formation, shade-gap plants increasedallocation to storage of total non-structural carbohydrates(TNC), and to root growth. The increase in TNC storage suggeststhat shade-developed saplings exposed to gaps were also primedfor fast growth and carbon gain in the following summer. Theincrease in root growth suggests that higher nutrient and waterdemand drove allocation shifts to enhance the capacity for nutrientand water uptake in the gap. Plant hydraulic conductivity (K_a)of shade-grown plants was limited upon exposure to the gap,possibly because of embolism formation resulting from the abruptincrease in water demand. Greater water potential gradientscompensated for limitations to K_a, allowing saplings to maintainhigh transpiration rates, suggesting that actual water uptakeof shade-gap plants was unaffected by gap exposure. Acclimation; canopy gaps; carbon allocation; hydraulic conductivity; nitrogen allocation; non-structural carbohydrates;Quercus rubra L.; red oak; stomatal conductance; transpiration; water-relations     

Plant potassium content modifies the effects of arbuscular mycorrhizal symbiosis on root hydraulic properties in maize plants

It is well known that the arbuscular mycorrhizal (AM) symbiosis helps the host plant to overcome several abiotic stresses including drought. One of the mechanisms for this drought tolerance enhancement is the higher water uptake capacity of the mycorrhizal plants. However, the effects of the AM symbiosis on processes regulating root hydraulic properties of the host plant, such as root hydraulic conductivity and plasma membrane aquaporin gene expression, and protein abundance, are not well defined. Since it is known that K(+) status is modified by AM and that it regulates root hydraulic properties, it has been tested how plant K(+) status could modify the effects of the symbiosis on root hydraulic conductivity and plasma membrane aquaporin gene expression and protein abundance, using maize (Zea mays L.) plants and Glomus intraradices as a model. It was observed that the supply of extra K(+) increased root hydraulic conductivity only in AM plants. Also, the different pattern of plasma membrane aquaporin gene expression and protein abundance between AM and non-AM plants changed with the application of extra K(+). Thus, plant K(+) status could be one of the causes of the different observed effects of the AM symbiosis on root hydraulic properties. The present study also highlights the critical importance of AM fungal aquaporins in regulating root hydraulic properties of the host plant.