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.     

Effect of Changes of Temperature Around Roots in Relation to Water Uptake by Roots and Leaf Transpiration

Effects of changes in temperature around roots on water uptake by roots and leaf transpiration were studied in Leucaena leucocephala (Lam.) de Wit., a subtropical woody plant species, and in Zea mays L. When the temperature around roots was rapidly lowered from 25 ℃ to 15 ℃, the water uptake by the roots and leaf transpiration were stimulated significantly within a short period ( 14 min). However, this effect did not occur when the cooling time was prolonged neither did if occur when the temperature around the roots was resumed from 15 ℃ to 25 ℃. Both the hydraulic conductivity of roots and leaf transpiration were increased substantially at first (within 20 min)and then decreased steadily to a level lower than those of the control in which the roots were continuous exposed to a low temperature ( 15 ℃ ). Low temperature also promoted the biosynthesis of ABA in roots and enhanced the xylem ABA concentration, but such stimulation did not occur untill about 30 min after cooling treatment, leaf transpiration was reduced markedly, but the hydraulic conductivity of roots increased when the root system was treated with exogenous ABA. It was suggested that some mechanisms other than ABA may be involved in the short-time cryostimulation of water uptake by roots and leaf transpiration.     

Effect of increased irradiance on the hormone content,water relations,and leaf elongation in wheat seedlings

In wheat (Triticum durum Desf., cv. Bezenchukskaya 139) seedlings, an increase in irradiance from 20 to 400 μmol/(m² s) PAR enhanced transpiration and increased stomatal conductance by three times on the background of reduced relative water content (RWC). After this treatment, leaves quickly ceased to grow and became even shrunk later. In 40 or 50 min, leaf growth was resumed. At this period, we observed an increase in hydraulic conductivity and RWC and also in leaf extensibility. As soon as 10 min after treatment, some changes in hormone content were noted. In the zones of leaf growth and its mature part, zeatin and zeatin riboside were accumulated, whereas ABA accumulation was observed in the zone of leaf growth and in the roots. The results obtained indicate that leaf expansion at increased irradiance was related to changes in cell-wall extensibility and hydraulic conductivity. The first effect could be due to cytokinin accumulation, whereas the second one, to ABA accumulation.     

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.     

Effect of soil compaction on barley (Hordeum vulgare L.) growth: I. Possible role for ABA as a root-sourced chemical signal

Wild-type (Steptoe) and abscisic acid (ABA)-deficient mutant(Az34) genotypes of barley were grown in compacted soil to examinethe potential role of ABA as a root-to-shoot signal. Root andshoot growth and leaf conductance were all reduced when plantswere grown in compacted soil with a bulk density of 1.7g cm^–3,relative to uncompacted control plants (1.1 g cm^–3. Theseeffects occurred in the absence of detectable changes in leafwater status or foliar abscisic acid (ABA) content. Analysisof Steptoe and Az34 xylem sap showed that the ABA concentrationwas greatly increased at 6 d after emergence (6 DAE) when seedlingswere grown in compacted soil (1.7 g cm^–3); however, ABAconcentrations were never as high in the mutant as in the wildtype. The increase in xylem sap ABA concentration observed athigh bulk density was closely correlated with reductions inleaf conductance, but not leaf area. These increases were transitory,and xylem sap ABA concentrations subsequently decreased towardsthe control level by 18 DAE in both genotypes. The ABA-deficient mutant, Az34, produced a much lower leaf areathan Steptoe at a bulk density of 1.6 g cm^–3. Examinationof epidermal characteristics indicates that this effect resultedmainly from reductions in cell expansion rather than cell division,suggesting that the higher ABA concentrations detected in xylemsap from the wild-type Steptoe may have exerted a positive rolein maintaining leaf expansion in this treatment. The possibleinvolvement of ABA as a root-to-shoot signal mediating the effectsof compaction stress is discussed. Key words: Soil compaction, bulk density, ABA, ABA-deficient mutant, leaf growth     

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.     

Are Roots a Source of Abscisic Acid for the Shoots of Flooded Pea Plants?

Flooding the soil for 2–5 d decreased stomatal conductancesof pea plants (Pisum sativum L., cv. Sprite) with six or sevenleaves. This coincided with slower transpiration, increasedleaf water potentials and increased concentrations of abscisicacid (ABA) in the leaves. No increase in ABA was found in theterminal 20 mm of roots of flooded plants over the same timeperiod. Small stomatal conductances associated with increases in foliarABA were also found in plants grown in nutrient solution whenaeration was halted, causing the equilibrium partial pressuresof dissolved oxygen to fall below 05 It Pa. No increase in ABAconcentration in young secondary roots of the non-aerated plantswas detected after 24, 48 or 72 h, even when the shoot, thepresumed site of deposition for any ABA from the roots, wasremoved 5–6 h before analysis. Similarly, ABA concentrations in roots were not increased whenthe nutrient solution was de-oxygenated by continuous purgingwith nitrogen gas. The abscisic acid concentration in leaf epidermis,the tissue most likely to be the recipient of any ABA movingin the transpiration stream from oxygen-deficient roots, waslower than in the remaining parts of the leaf when examinedin the mutant Argenteum which possesses easily removable epidermallayers. It is concluded that the leaves of plants subjectedto flooding of the soil or oxygen shortage in the root environmentare not enriched substantially with ABA from the roots. A moreprobable source of this growth regulator is the leaf itself. Key words: Pisum sativum, flooding, roots, hormones, aeration stress, abscisic acid, Argenteum mutant     

Water Relations and Growth of the flacca Tomato Mutant in Relation to Abscisic Acid

The flacca mutant in tomato (Lycopersicon esculentum Mill. cv Rheinlands Ruhm) was employed to examine the effects of a relatively constant diurnal water stress on leaf growth and water relations. As the mutant is deficient in abscisic acid (ABA) and can be phenotypically reverted to the wild type by applications of the growth substance, inferences can be made concerning the involvement of ABA in responses to water stress. Water potential and turgor were lower in leaves of flacca than of Rheinlands Ruhm, and were increased by ABA treatment. ABA decreased transpiration rates by causing stomatal closure and also increased the hydraulic conductance of the sprayed plants. Osmotic adjustment did not occur in flacca plants despite the daily leaf water deficits. Stem elongation was inhibited by ABA, but leaf growth was promoted. It is concluded that, in some cases, ABA may promote leaf growth via its effect on leaf water balance.     

Growth rate, plant development and water relations of the ABA-deficient tomato mutant sitiens

Given the close relationship between a plant's growth rate and its pattern of biomass allocation and the effects of abscisic acid (ABA) on biomass allocation, we studied the influence of ABA on biomass allocation and growth rate of wildtype tomato ( Lycopersicon esculentum Mill. cv. Moneymaker) plants and their strongly ABA-deficient mutant sitiens. The relative growth rate of sitiens was 22% lower than that of the wildtype, as the result of a decreased specific leaf area. The net assimilation rate and the leaf weight ratio were not affected. The mutant showed a much higher transpiration rate and lower hydraulic conductance of the roots. These two factors resulted in sitiens having a significantly lower leaf water potential and turgor. resulting in reduced leaf expansion and, consequently, a lower specific leaf area relative to the wildtype. Addition of ABA to the sitiens roots resulted in phenotypic reversion to the wildtype. We conclude that the influence of ABA-deficiency on biomass allocation and relative growth rate is the result of altered water relations in the plants, rather than of a direct effect on sink strength of different plant organs.     

An abscisic acid-related reduced transpiration promotes gradual embolism repair when grapevines are rehydrated after drought

* Proposed mechanisms of embolism recovery are controversial for plants that are transpiring while undergoing cycles of dehydration and rehydration. * Here, water stress was imposed on grapevines (Vitis vinifera), and the course of embolism recovery, leaf water potential (Psi(leaf)), transpiration (E) and abscisic acid (ABA) concentration followed during the rehydration process. * As expected, Psi(leaf) and E decreased upon water stress, whereas xylem embolism and leaf ABA concentration increased. Upon rehydration, Psi(leaf) recovered in 5 h, whereas E fully recovered only after an additional 48 h. The ABA content of recovering leaves was higher than in droughted controls, both on the day of rewatering and the day after, suggesting that ABA accumulated in roots during drought was delivered to the rehydrated leaves. In recovering plants, xylem embolism in petioles, shoots, and roots decreased during the 24 h following rehydration. * A model is proposed to describe plant recovery after rehydration based on three main points: embolism repair occurs progressively in shoots and further in roots and in petioles, following an almost full recovery of Psi(leaf); hydraulic conductance recovers during diurnal transpiring hours, when formation and repair of embolisms occurs in all plant organs; an ABA residual signal in rehydrated leaves hinders stomatal opening even when water relations have recovered, suggesting that an ABA-induced transpiration control promotes gradual embolism repair in rehydrated grapevines.     

Early stomatal closure in waterlogged pea plants is mediated by abscisic acid in the absence of foliar water deficits

Abstract Soil waterlogging decreased leaf conductance (interpreted as stomatal closure) of vegetative pea plants (Pisuin sativum L. cv. ‘Sprite’) approximately 24 h after the start of flooding, i.e. from the beginning of the second 16 h-long photo-period. Both adaxial and abaxial surfaces of leaves of various ages and the stipules were affected. Stomatal closure was sustained for at least 3 d with no decrease in foliar hydration measured as water content per unit area, leaf water potential or leaf water saturation deficit. Instead, leaves became increasingly hydrated in association with slower transpiration. These changes in the waterlogged plants over 3 d were accompanied by up to 10-fold increases in the concentration of endogenous abscisic acid (ABA). Waterlogging also increased foliar hydration and ABA concentrations in the dark. Leaves detached from non-waterlogged plants and maintained in vials of water for up to 3 d behaved in a similar way to leaves on flooded plants, i.e. stomata closed in the absence of a water deficit but in association with increased ABA content. Applying ABA through the transpiration stream to freshly detached leaflets partially closed stomata within 15 min. The extractable concentrations of ABA associated with this closure were similar to those found in flooded plants. When an ABA-deficient ‘wilty’ mutant of pea was waterlogged, the extent of stomatal closure was less pronounced than that in ordinary non-mutant plants, and the associated increase in foliar ABA was correspondingly smaller. Similarly, waterlogging closed stomata of tomato plants within 24 h, but no such closure was seen in ‘flacca’, a corresponding ABA-deficient mutant. The results provide an example of stomatal closure brought about by stress in the root environment in the absence of water deficiency. The correlative factor operating between the roots and shoots appeared to be an inhibition of ABA transport out of the shoots of flooded plants, causing the hormone to accumulate in the leaves.     

Drought, abscisic acid and transpiration rate effects on the regulation of PIP aquaporin gene expression and abundance in Phaseolus vulgaris plants

BACKGROUND AND AIMS: Drought causes a decline of root hydraulic conductance, which aside from embolisms, is governed ultimately by aquaporins. Multiple factors probably regulate aquaporin expression, abundance and activity in leaf and root tissues during drought; among these are the leaf transpiration rate, leaf water status, abscisic acid (ABA) and soil water content. Here a study is made of how these factors could influence the response of aquaporin to drought. METHODS: Three plasma membrane intrinsic proteins (PIPs) or aquaporins were cloned from Phaseolus vulgaris plants and their expression was analysed after 4 d of water deprivation and also 1 d after re-watering. The effects of ABA and of methotrexate (MTX), an inhibitor of stomatal opening, on gene expression and protein abundance were also analysed. Protein abundance was examined using antibodies against PIP1 and PIP2 aquaporins. At the same time, root hydraulic conductance (L), transpiration rate, leaf water status and ABA tissue concentration were measured. KEY RESULTS: None of the treatments (drought, ABA or MTX) changed the leaf water status or tissue ABA concentration. The three treatments caused a decline in the transpiration rate and raised PVPIP2;1 gene expression and PIP1 protein abundance in the leaves. In the roots, only the drought treatment raised the expression of the three PIP genes examined, while at the same time diminishing PIP2 protein abundance and L. On the other hand, ABA raised both root PIP1 protein abundance and L. CONCLUSIONS: The rise of PvPIP2;1 gene expression and PIP1 protein abundance in the leaves of P. vulgaris plants subjected to drought was correlated with a decline in the transpiration rate. At the same time, the increase in the expression of the three PIP genes examined caused by drought and the decline of PIP2 protein abundance in the root tissues were not correlated with any of the parameters measured.     

Novel approaches for examining the effects of differential soil compaction on xylem sap abscisic acid concentration, stomatal conductance and growth in barley (Hordeum vulgare L.)

Novel techniques were devised to explore the mechanisms mediating the adverse effects of compacted soil on plants. These included growing plants in: (i) profiles containing horizons differing in their degree of compaction and; (ii) split-pots in which the roots were divided between compartments containing moderately (1·4 g cm ^{? 3}) and severely compacted (1·7 g cm ^{? 3}) soil. Wild-type and ABA-deficient genotypes of barley were used to examine the role of abscisic acid (ABA) as a root-to-shoot signal. Shoot dry weight and leaf area were reduced and root : shoot ratio was increased relative to 1·4 g cm ^{? 3} control plants whenever plants of both genotypes encountered severely compacted horizons. In bartey cultivar Steptoe, stomatal conductance decreased within 4 d of the first roots encountering 1·7 g cm ^{? 3} soil and increased over a similar period when roots penetrated from 1·7 g cm ^{? 3} into 1·4 g cm ^{? 3} soil. Conductance was again reduced by a second 1·7 g cm ^{? 3} horizon. These responses were inversely correlated with xylem sap ABA concentration. No equivalent stomatal responses occurred in Az34 (ABA deficient genotype), in which the changes in xylem sap ABA were much smaller. When plants were grown in 1·7 : 1·4 g cm ^{? 3} split-pots, shoot growth was unaffected relative to 1·4 g cm ^{? 3} control plants in Steptoe, but was significantly reduced in Az34. Excision of the roots in compacted soil restored growth to the 1·4 g cm ^{? 3} control level in Az34. Stomatal conductance was reduced in the split-pot treatment of Steptoe, but returned to the 1·4 g cm ^{? 3} control level when the roots in compacted soil were excised. Xylem sap ABA concentration was initially higher than in 1·4 g cm ^{? 3} control plants but subsequently returned to the control level; no recovery occurred if the roots in compacted soil were left intact. Xylem sap ABA concentration in the split-pot treatment of Az34 was initially similar to plants grown in uniform 1·7 g cm ^{? 3} soil, but returned to the 1·4 g cm ^{? 3} control level when the roots in the compacted compartment were excised. These results clearly demonstrate the involvement of a root-sourced signal in mediating responses to compacted soil; the role of ABA in providing this signal and future applications of the compaction procedures reported here are discussed.     

Gas exchange is related to the hormone balance in mycorrhizal or nitrogen-fixing alfalfa subjected to drought

The beneficial effect of mycorrhization on photosynthetic gas exchange of host plants under drought conditions could be related to factors other than changes in phosphorus nutrition and water uptake. Our objective was to study the influence of drought on phytohormones and gas exchange parameters in Medicago sativa L. cv. Aragón associated with or in the absence of arbuscular mycorrhizal (AM) fungi and/or nitrogen-fixing bacteria. Four treatments were used: (1) plants inoculated with Glomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain (MR); (2) plants inoculated with only Rhizobium (R); (3) plants inoculated with only mycorrhizae (M); and (4) non-inoculated plants (N). When endophytes were well established, treatments received different levels of phosphorus and nitrogen in the nutrient solution in order to obtain plants similar in size. Sixty days after planting, plants were subjected to two cycles of drought and recovery. Midday leaf water potential (Ψ), CO₂ exchange rate (CER), leaf conductance (g_w) and transpiration (T), as well as leaf and root abscisic acid (ABA) and cytokinin concentrations were measured after the second drought period. Gas exchange parameters were determined by infrared gas analysis. Cytokinins and ABA levels in tissues were analysed by ELISA and HPLC, respectively. Nodulated R and MR plants had the lowest ABA concentrations in roots under well-watered conditions. Water stress increased ABA concentrations in leaves of N, R and MR plants, while ABA concentration in M plants did not change. The highest production of ABA under water deficit was in the roots of non-mycorrhizal plants. The ratio of ABA to cytokinin concentration strongly increased in leaves and roots of non-mycorrhizal plants under drought. By contrast, this ratio was lowered in roots of M plants and remained unchanged in leaves and roots of MR plants when stress was imposed. The highest leaf conductances and transpirational fluxes under well-watered conditions were those of nitrogen-fixing R and MR plants, but these results were not impaired with increased CO₂ exchange rates. Photosynthesis, leaf conductance and transpiration rates decreased in all treatments when stress was imposed, with the strongest decrease occurring in non-mycorrhizal plants. The relationships found between these gas exchange parameters and the hormone concentrations in stressed alfalfa tissues suggest that microsymbionts have an important role in the control of gas exchange of the host plant through hormone production in roots and the ABA/cytokinin balance in leaves. The most relevant effect of mycorrhizal fungi was observed under drought conditions.     

Root-to-shoot signalling when soil moisture is heterogeneous: increasing the proportion of root biomass in drying soil inhibits leaf growth and increases leaf abscisic acid concentration

To determine whether root-to-shoot signalling of soil moisture heterogeneity depended on root distribution, wild-type (WT) and abscisic acid (ABA)-deficient (Az34) barley (Hordeum vulgare) plants were grown in split pots into which different numbers of seminal roots were inserted. After establishment, all plants received the same irrigation volumes, with one pot watered (w) and the other allowed to dry the soil (d), imposing three treatments (1 d: 3 w, 2 d: 2 w, 3 d: 1 w) that differed in the number of seminal roots exposed to drying soil. Root distribution did not affect leaf water relations and had no sustained effect on plant evapotranspiration (ET). In both genotypes, leaf elongation was less and leaf ABA concentrations were higher in plants with more roots in drying soil, with leaf ABA concentrations and water potentials 30% and 0.2 MPa higher, respectively, in WT plants. Whole-pot soil drying increased xylem ABA concentrations, but maximum values obtained when leaf growth had virtually ceased (100 nm in Az34, 330 nm in WT) had minimal effects (<40% leaf growth inhibition) when xylem supplied to detached shoots. Although ABA may not regulate leaf growth in vivo, genetic variation in foliar ABA concentration in the field may indicate different root distributions between upper (drier) and lower (wetter) soil layers.     

Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3°C, 0.2-0.4 kPa vapor pressure deficit, 100 μmol·m⁻²·s⁻¹ photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3°C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3°C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3°C compared with 20°C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.     

The effect of root cooling on hormone content, leaf conductance and root hydraulic conductivity of durum wheat seedlings (Triticum durum L.)

Root cooling of 7-day-old wheat seedlings decreased root hydraulic conductivity causing a gradual loss of relative water content during 45 min (RWC). Subsequently (in 60 min), RWC became partially restored due to a decrease in transpiration linked to lower stomatal conductivity. The decrease in stomatal conductivity cannot be attributed to ABA-induced stomatal closure, since no increase in ABA content in the leaves or in the concentration in xylem sap or delivery of ABA from roots was found. However, decreased stomatal conductance was associated with a sharp decline in the content of cytokinins in shoots that was registered shortly after the start of root cooling and linked to increases in the activity of cytokinin-oxidase. This decrease in shoot cytokinin content may have been responsible for closing stomata, since this hormone is known to maintain stomatal opening when applied to plants. In support of this, pre-treatment with synthetic cytokinin benzyladenine was found to increase transpiration of wheat seedlings with cooled roots and bring about visible loss of turgor and wilting.     

Stomata response to changes in temperature and humidity in wheat cultivars grown under contrasting climatic conditions

Stomatal response to changes in temperature and humidity was studied in wheat (Triticum aestivum L.) cv. Iren’ cultivated under conditions of high water supply and cv. Kazakhstanskaya 10, which is relatively drought tolerant. Experiments were performed under both laboratory and field conditions. It was demonstrated that stomata of cv. Kazakhstanskaya 10 plants closed rapidly with reducing humidity (the response of the first type), whereas, in cv. Iren’, this response was less expressed and, under conditions of a high water content in soil, stomatal conductance could increase in response to reduced humidity (the response of the second type). At an increased stomatal conductance and transpiration, water content in cv. Iren’ plants was maintained due to the increase in hydraulic conductance and water inflow from the roots. A possible role of the first-type response (rapid stomata closure) for growth maintenance under drought and of the second-type response (a parallel increase in the stomatal and hydraulic conductance) for providing of rapid growth and high productivity under sufficient water supply is discussed. A possibility to use the type of stomata behavior for cultivar assessment is considered.     

Diurnal cycles of embolism formation and repair in petioles of grapevine (Vitis vinifera cv. Chasselas)

The impact of water deficit on stomatal conductance (g(s)), petiole hydraulic conductance (K(petiole)), and vulnerability to cavitation (PLC, percentage loss of hydraulic conductivity) in leaf petioles has been observed on field-grown vines (Vitis vinifera L. cv. Chasselas). Petioles were highly vulnerable to cavitation, with a 50% loss of hydraulic conductivity at a stem xylem water potential (Ψ(x)) of -0.95?MPa, and up to 90% loss of conductivity at a Ψ(x) of -1.5?MPa. K(petiole) described a daily cycle, decreasing during the day as water stress and evapotranspiration increased, then rising again in the early evening up to the previous morning's K(petiole) levels. In water-stressed vines, PLC increased sharply during the daytime and reached maximum values (70-90%) in the middle of the afternoon. Embolism repair occurred in petioles from the end of the day through the night. Indeed, PLC decreased in darkness in water-stressed vines. PLC variation in irrigated plants showed the same tendency, but with a smaller amplitude. The Chasselas cultivar appears to develop hydraulic segmentation, in which petiole cavitation plays an important role as a 'hydraulic fuse', thereby limiting leaf transpiration and the propagation of embolism and preserving the integrity of other organs (shoots and roots) during water stress. In the present study, progressive stomatal closure responded to a decrease in K(petiole) and an increase in cavitation events. Almost total closure of stomata (90%) was measured when PLC in petioles reached >90%.     

Water relations of the tos1 tomato mutant at contrasting evaporative demand

The tos1 (tomato osmotically sensitive) mutant, isolated from an in vitro screen of root growth during osmotic stress, was less sensitive to exogenous ABA, but accumulated more ABA under osmotic stress than WT plants. We assessed growth and water relations characteristics of hydroponically grown tos1 seedlings (in the absence of osmotic stress) at low and high evaporative demands. Growth of tos1 was severely inhibited at both high and low evaporative demands. Twenty DAS, WT and tos1 genotypes had a similar leaf water and turgor potential, but mature tos1 plants (45 day old) showed a significant diurnal loss of leaf turgor, with recovery overnight. Increased evaporative demand increased turgor loss of tos1 plants. High evaporative demand at the beginning of the day decreased stomatal conductance of tos1, without diurnal recovery, thus whole plant transpiration was decreased. De-topped tos1 seedlings showed decreased root hydraulic conductance and had a 1.4-fold increase in root ABA concentration. Impaired root function of tos1 plants failed to meet transpirational water demand and resulted in shoot turgor loss, stomatal closure and growth inhibition.