Leaf ABA Research Articles

Comparative responses to water stress in stay-green, rapid- and slow senescing genotypes of the biomass crop, Miscanthus.

• Although hybrids of the perennial rhizomatous grass Miscanthus give high yields under irrigation, their use as a biomass crop in many locations is limited by inadequate water availability. Here, the effects of reduced water availability on growth are reported for three Miscanthus genotypes with differing responses to drought. • In a controlled environment experiment during a drought treatment M. sinensis hybrid (Sin-H6) had no detectable photosynthesis and negligible leaf conductance, whereas M. × giganteus (Gig-3) continued to photosynthesize and transpire but it shed leaf area through senescence. A M. sacchariflorus line (Sac-5) was generally intermediate in its response. Measurements on 3-y-old stands of Gig-3 and Sin-H6 in the field during a short summer drought in August 1999 confirmed the controlled environment observations. • Stay-green characteristics in Sin-H6 appeared to rely on tight control of transpiration through lowered leaf conductance, although the highest leaf ABA concentrations were not found in this genotype. • Stay-green genotypes, such as Sin-H6, are likely to be important for further breeding of Miscanthus because they appear to make the most effective use of available water.

Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.).

Early signals potentially regulating leaf growth and stomatal aperture in field-grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.2 units one day after withholding irrigation (DAWI) and conductivity decreased by about 0.25 mS cm(-1) 1-3 DAWI. Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50% at 1-5 DAWI and potassium concentration decreased by about 50% at 7-8 DAWI. Xylem ABA concentration consistently increased by 45-70 pmol ml(-1) at 7 DAWI. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Leaf nitrogen significantly decreased 8-16 DAWI in droughted plants. Midday leaf water potential and photosynthesis were significantly decreased in droughted plants late in the drying period. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g(-1) dry weight at 7 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.

Changes in phytohormone levels following inoculation of two soybean lines differing in nodulation.

Changes in the concentration of free and conjugated ABA, zeatin riboside (ZR), and IAA in response to Bradyrhizobium inoculation and subsequent nodulation were monitored in xylem sap, phloem sap, and leaves of soybean [Glycine max (L.) Merr. cv. Williams 82] and its hypernodulating mutant, NOD1-3. In this study, pre-inoculation concentrations of phloem and xylem sap ABA and ZR were lower in NOD1-3 than in Williams 82, a difference that was accentuated in phloem after inoculation. The concentration of xylem ABA increased within 6h of inoculation, while the concentration of phloem and leaf ABA did not change until 48-96 h after inoculation. Leaf uptake of [3H]ABA and distribution to phloem sap was greater in Williams 82 than in NOD1-3 during 48-72h after inoculation. Inoculation resulted in similar increases in phloem and leaf IAA concentrations in both cultivars. While inoculation increased xylem sap ZR in both lines, the concentration of ZR increased much earlier in NOD1-3. Of particular interest is that ratios between hormones were altered during nodulation. Leaf and phloem ABA/IAA ratios were higher in Williams 82 than in the hypernod mutant, while the phloem IAA/ZR was greater from inoculation until nodulation in the NOD1-3 hypernod mutant. The xylem ABA/ZR ratio, as well as phloem ABA/ZR ratio, decreased in Williams 82 following inoculation, and leaf ABA concentration was elevated. The most noteworthy results of this study, therefore, came from an examination of the ratios between hormones in xylem and phloem sap, and the demonstration that hormone transport may play an important role in autoregulation of root nodulation.

Physiological responses of Catharanthus roseus (periwinkle) to ash yellows phytoplasmal infection

Summary Photosynthetic responses of periwinkle (Catharanthus roseus) were compared during disease development after infection by two strains of ash yellows (AshY) phytoplasmas that differed in aggressiveness. Healthy plants were inoculated by grafting with diseased scions. Mature leaves formed before grafting of diseased scion, and leaves formed during disease development were monitored for physiological changes. Leaves formed before inoculation remained visually and physiologically asymptomatic until normal senescence. However, leaves formed after inoculation had reduced photosynthesis, due to reductions in both the maximum carboxylation rate (Vcmax) and noncyclic electron transport (Jmax). In these leaves, photoinhibition resulted from photoprotective processes rather than photooxidative damage to PSII reaction centres. Stomatal conductance (Gs) was reduced, but this did not increase the stomatal limitation to photosynthesis. Changes in Gs were not correlated with changes in leaf ABA concentration. Known differences in phytoplasmal aggressiveness were correlated with degrees of inhibition of photosynthesis and occurence of photoinhibition. AshY phytoplasmas appear to cause metabolic perturbations associated with localized accumulation or multiplication of the pathogen in symptomatic leaves. AbbreviationsA, net photosynthesis rate; Aa, net photosynthesis rate at ambient [CO2]; Amax, net photosynthesis rate at saturated [CO2]; AshY, ash yellows; Ci, intercellular [CO2]; Fo, Fm, Fv, minimal, maximum and variable fluorescence, respectively; F′m, Fs, maximal and steady state fluorescence in light‐adapted state; Fv/Fm, intrinsic efficiency of PSII; Gs, stomatal conductance; Jmax, maximum noncyclic electron transport; Ls, stomatal limitation; ΦPSII, quantum yield of PSII; qNP, nonphotochemical quenching coefficient; RuBP, ribulose‐1,5‐biophosphate; Vcmax, maximum rate of carboxylation of Rubisco; vpd, leaf‐to‐air vapor pressure deficit; WAG, week after grafting.

Phosphorus concentrations in the leaves of defoliated white clover affect abscisic acid formation and transpiration in drying soil

Increased leaf phosphorus (P) concentration improved the water-use efficiency (WUE) and drought tolerance of regularly defoliated white clover plants by decreasing the rate of daily transpiration per unit leaf area in dry soil. Night transpiration was around 17% of the total daily transpiration. The improved control of transpiration in the high-P plants was associated with an increased individual leaf area and WUE that apparently resulted from net photosynthetic assimilation rate being reduced less than the reductions in the transpiration (27% vs 58%). On the other hand, greater transpiration from low-P plants was associated with poor stomatal control of transpirational loss of water, less ABA in the leaves when exposed to dry soil, and thicker and smaller leaf size compared with high-P leaves. The leaf P concentration was positively related with leaf ABA, and negatively with transpiration rates, under dry conditions (P < 0.001). However, leaf ABA was not closely related to the transpiration rate, suggesting that leaf P concentration has a greater influence than ABA on the transpiration rates.

Functional conservation between yeast and plant endosomal Na +/H + antiporters

Vacuolar compartmentation of Na + is an essential mechanism for salinity tolerance since it lowers cytosolic Na + levels while contributing to osmotic adjustment for cell turgor and expansion. The AtNHX1 protein of Arabidopsis thaliana substituted functionally for ScNHX1, the endosomal Na +/H + antiporter of yeast. Ion tolerance conferred by AtNHX1 and ScNHX1 correlated with ion uptake into an intracellular pool that was energetically dependent on the vacuolar (H +)ATPase. AtNHX1 localized to vacuolar membrane fractions of yeast. Hence, both transporters share an evolutionarily conserved function in Na + compartmentation. AtNHX1 mRNA levels were upregulated by ABA and NaCl treatment in leaf but not in root tissue.

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around −2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.

Drought adaptation of field grown wheat in relation to soil physical conditions

In order to investigate the effects of soil texture on possible non-hydraulic signals under field conditions, spring wheat plants (Triticum aestivum L. cv. Cadensa) grown in sand and loam soils and with a well developed root system were exposed to slow soil drying in the late vegetative stage of growth. Soil water potential and content were measured daily at different depths and plant responses were measured in flag leaves. When the average soil water potential in the top soil layers (0–25 cm depth in sand and 0–45 cm depth in loam) dropped to –60 or –70 kPa and the lower soil layers were still at field capacity, morning xylem [ABA] (0.03–0.04 vs. 0.06–0.08 mmol m-3) and midday leaf ABA concentration increased (250–300 vs. 400–450 ng/g DW) and leaf conductance decreased relatively to well-watered (control) plants (0.75–0.88 vs. 0.64–0.70 mol m-2 s-1). These responses took place before any decrease in leaf water potential occurred as compared with control plants, indicating that they were triggered by root-borne signals due to reduced root water status in the top soil layers. At this stage the soil water content was as low as 6% by volume, the fraction of roots in ‘wet’ soil was 0.12 and relative available soil water was 45% in sand and still high 20%, 0.48 and 70%, respectively, in loam of the whole soil profile indicating that roots were responding to soil water availability and not soil water content at a certain evaporative demand. In addition, similar responses occurred at high and low evaporative demands (3.4–5.2 vs. 0.6–4.0 mm/day of potential evapotranspiration).

Role of sodium in the ABA‐mediated long‐term growth response of bean to salt stress

Long‐term salt effects on plant growth have often been related to direct ion toxicity due to the accumulation of high ion concentrations in plant tissue. This work examines the relative importance of endogenous ABA, as well as Na+ and Cl− toxicity, in the inhibition of leaf growth and photosynthesis, in bean plants grown at 1, 25, 50 and 75 mM NaCl until the fruit‐bearing stage. All salt‐treated plants showed very high leaf Cl− concentrations, with little difference between plants exposed to 50 or 75 mM NaCl. The 25 and 50 mM salt‐treated plants were able to successfully exclude Na+ from their leaves, and only suffered an initial decline in the rate of leaf growth. Plants exposed to 75 mM NaCl showed an increase in Na+ leaf concentrations with an accompanying decrease in growth and photosynthesis as salt exposure progressed. A high correlation was found between leaf Na+ and leaf growth. Leaf ABA significantly increased with salt supply, and was highly correlated with both leaf Na+ and leaf growth. Our results suggest that in bean plants under long‐term salt stress, leaf ABA may participate in the regulation of leaf growth, and leaf Na+ would be at least partly responsible for increased ABA levels.

Relative importance of osmotic-stress and ion-specific effects on ABA-mediated inhibition of leaf expansion growth in Phaseolus vulgaris

Abbreviations: The osmotic and ion-specific components of salt-induced inhibition of leaf expansion growth were investigated in beans grown from 12 h to several days in either NaCl-containing solution cultures, an isosmotic concentrated macronutrient solution, or a vermiculite–compost mixture with low Na+ but high Cl– availability. Inhibition of leaf expansion and leaf ABA increase was more intense in the NaCl than in the isosmotic macronutrient treatment. Root Na+ was highly correlated to inhibition of leaf expansion and leaf or xylem sap ABA. When Na+ was sequestered in soil, salinized plants showed no reduction in leaf expansion or ABA increase, regardless of the presence of high leaf Cl– concentrations. Stomatal conductance exhibited an exponential relationship with the reciprocal value of xylem sap ABA. Our results indicate that an ion-specific effect caused by Na+ in roots may account for an ABA-mediated reponse of both stomatal closure and leaf expansion inhibition.

Effect of foliar applications of glycinebetaine on stomatal conductance, abscisic acid and solute concentrations in leaves of salt- or drought-stressed tomato

Tomato (Lycopersicon esculentum Mill.) was used as a model system for investigating the effects of glycinebetaine applications on plants, as this species is unable to synthesise glycinebetaine, but is sensitive to exogenous supply. Glycinebetaine application significantly increased stomatal conductance of tomato plants grown in well-watered, water-deficient or saline conditions; however, glycinebetaine did not affect the leaf ABA concentration, relative water content, sap osmotic potential, or shoot water potential. Glycinebetaine applied to the foliage was readily absorbed, but the concentrations in the leaves made a negligible contribution to the total leaf sap osmotic potential. Our results indicate that applied glycinebetaine was involved in regulation of stomatal conductance but not via ABA metabolism or water relations. Glycinebetaine may have been accumulated in specific cells or cellular compartments, with consequences for stomatal functions. Further work is required to elucidate the cellular and subcellular localisation of applied glycinebetaine.

Early signals in field grown wheat in response to shallow soil drying

The aim of the present work was to test under field condition earlier climate room findings that root-borne signals may control stomatal closure and leaf extension rate during mild soil drying. Stomatal conductance (g H2O) of flag leaves, leaf and stem extension rates, leaf water relations, leaf ABA content and predawn xylem [ABA] were measured daily in wheat grown in two soil types (sand and loam) in lysimeters in the field during a period of soil drying with high and low evaporative demands. At 3 days after withholding irrigation (DAI) on both soil types, when soil water potential (Ψsoil) in the upper soil profile of the droughted treatment had dropped to –70 kPa and with the lower layers still at field capacity, predawn xylem [ABA] increased. At 4 DAI the leaf extension rate decreased and midday leaf ABA content increased. Stem extension rates decreased at 5 DAI in loam and 7 DAI in sand. g H2O started to decrease 6 DAI in loam and 9 DAI in sand. These responses were observed before any detectable decrease in the midday leaf water status of the droughted plants had occurred relative to well- watered plants. The responses were closely related to Ψsoil and independent of evaporative demand. We conclude that, in wheat, root-borne signals probably control stomatal conductance and leaf extension rate during mild soil drying in the field.

Endogenous abscisic acid levels are linked to decreased growth of bush bean plants treated with NaCl

This paper studies the relative importance of endogenous ABA and ion toxicity in the leaf growth inhibition caused by NaCl in salt‐adapted and unadapted bush beans. Adaptation to salt‐stress was achieved by germination of seeds in 75 mM NaCl, while unadapted plants were germinated in tap water. The adaptation process caused a transitory increase in leaf ABA, Na+ and Cl− concentrations, while leaf expansion was inhibited. However, when grown for 8 or 13 days in 75 mM NaCl‐containing nutrient solution, primary and first trifoliolate leaves of salt‐adapted plants had greater areas than those of unadapted plants. Concentrations of ABA, Na+ and Cl− in these leaves were lower in adapted plants, and a strong negative correlation between leaf expansion growth and either leaf Na+, Cl− or ABA concentrations could be established. However, in the second trifoliolate leaves only the ABA, but not the Na+ or Cl−, concentrations were significantly correlated with leaf expansion. Our results suggest that salt‐induced inhibition of leaf expansion growth in bush beans is mediated by ABA rather than Na+ or Cl− toxicity. Moreover, the increase of ABA, induced by the salt‐pretreatment, seems to play an important role in limiting the accumulation of Na+ and Cl− in the leaves, leading to adaptation of bush beans to salt‐stress.

Re-export and metabolism of xylem-delivered ABA in attached maize leaves under different transpirational fluxes and xylem ABA concentrations

By feeding radioactive 3 H-ABA into attached maize leaves, the re-export and metabolism of xylem-delivered ABA and their relationships with xylem ABA transpirational fluxes and concentrations were investigated. ABA entering leaves in the transpirational stream was re-exported out of leaves slowly. Within 24h the proportion of fed radioactivity that was re-exported was less than 45%. When different concentrations of 3 H-ABA (100 nM versus 500 nM) was fed, no difference between the two concentrations was found in their rates of re-export of the fed radioactivity during the first 5 h. After 5 h, very little fed radioactivity was re-exported in leaves that were fed with 100 nM 3 H-ABA, while leaves that were fed with 500 nM 3 H-ABA continued to re-export such that the final proportion remaining in leaves after 24 h was less as a result, suggesting a concentration-stimulated re-export. When 3 H-ABA was fed at two different transpiration rates which were induced by different air humidity, a 4-fold difference in transpirational fluxes did not produce any difference in terms of re-exportation of fed radioactivity. The rate of catabolism of xylem-fed 3 H-ABA in the attached leaves was much faster than that of re-export. On average fed 3 H-ABA had a half-life of 2.2 h and only 8% remained unmodified after 24 h of incubation, suggesting that re-exported radioactivity might not be the intact form of ABA at all. Using the parameters obtained from the feeding experiment, we calculated that in a real soil-drying situation the possible maximum amount of xylem-delivered ABA that could accumulate in leaves during a day. It was found that the proportion of daily accumulated ABA was only 5% of the leaf ABA in well-watered plants. In soil-dried plants the maximum amount of daily accumulation by xylem ABA could reach 20% of the leaf ABA at the beginning of soil drying, but it soon declined to about 5% again. The declined contribution was mainly due to a reduced transpiration and an increased total leaf ABA as a result of aggravated leaf water deficit. A tight relationship between leaf conductance and the accumulation of xylem-delivered ABA was not found.

Chilling of drought-hardened and non-hardened plants of different chilling-sensitive maize lines Changes in water relations and ABA contents

The effect of chilling acclimation by drought was investigated in two maize ( Zea mays L.) inbred lines differing in chilling sensitivity. Two-leaf-stage seedlings of the two lines were drought-hardened or not and then chilled. Subsequent recovery was evaluated. Plant growth parameters, water status, transpiration, root water uptake and ABA levels were measured. Chilling produced a greater decrease in relative growth rates (RGR) and net assimilation rates (NAR) in Penjalinan than in Z7. This was also observed in non-drought-hardened plants versus drought-hardened ones. Drought-hardening decreased chilling-induced water deficit. At the end of chilling, relative water content and leaf water potential were higher in drought-hardened plants than in non-drought-hardened ones. The beneficial effect of drought-hardening was more evident in the chilling-sensitive line. Penjalinan. Osmotic potential at full turgor showed the absence of osmotic adjustment. Root water uptake decreased similarly in drought-hardened as in non-drought-hardened plants. However, transpiration of drought-hardened plants during chilling was lower. During chilling, root and leaf ABA content was higher in drought-hardened plants, while no differences were observed between lines. At the end of the recovery, nearly all measured parameters reached control values. The beneficial effect of drought-hardening on plant growth and water status during chilling is discussed on the basis of ABA content and its effect on transpiration.

The relationship between leaf growth and ABA accumulation in the grass leaf elongation zone

ABSTRACTDetached barley (Hordeum vulgare L.) shoots, maintained at different air temperatures and VPDs, were fed ABA via the sub‐crown internode in a leaf elongation assay. Analysis of variance of leaf elongation rate (LER) showed significant effects of temperature (T), fed [ABA] and the interaction T × [ABA]. However, the interaction became non‐significant when LER was modelled against the [ABA] of the elongation zone, [EZ‐ABA]When detached barley shoots were fed sap from droughted maize (Zea mays L.) plants, sap [ABA] could not explain the growth inhibitory activity. Measurement of [EZ‐ABA] accounted for this ‘unexplained’ growth inhibition.The detached shoot experiments indicated that [EZ‐ABA], and not xylem sap [ABA], was an appropriate explanatory variable to measure in droughted plants. However, ABA accumulation in the elongation zone could not explain a 35% growth reduction in intact droughted plants; thus we considered an interaction of water status and ABA.Using a coleoptile growth assay, we applied mild osmotic stresses (ψ=0 to −0.06 MPa) and 10−4 mol m−3 ABA. Individually, these treatments did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants.

Effects of low temperature, high salinity and exogenous ABA on the synthesis of two chloroplastic drought‐induced proteins in Solanum tuberosum

Two chloroplastic proteins of 32 and 34 kDa were previously shown to be substantially synthesized in response to a progressive water deficit in whole Solanum tuberosum plants (G. Pruvot, S. Cuiné, N. Gault, G. Peltier and P. Rey, unpublished data; G. Pruvot, S. Cuiné, G. Peltier and P. Rey. 1996. Planta 198: 471–479). These chloroplastic drought‐induced stress proteins, named CDSP 32 and CDSP 34, accumulated in the stroma and in the thylakoids, respectively. In this study, we investigated the effects of low temperature and high salinity on the synthesis of the CDSP proteins. Whereas the CDSP 32 synthesis was not modified in response to a cold treatment, an increased synthesis of CDSP 32 was observed in salt‐stressed plants, resulting in accumulation of the protein. The thylakoid CDSP 34 protein exhibited enhanced synthesis and substantial accumulation in response to cold and high salinity. A significant increase in the leaf abscisic acid content (at least 2.5‐fold) was measured in plants subjected to water deficit, high salinity or low temperature. The contribution of ABA to the synthesis of the two proteins was investigated by spraying well‐watered plants with a 100 μM/ ABA solution for 15 days. This treatment resulted in a 15‐fold increase in the leaf ABA content. Whereas synthesis of the CDSP 32 protein was not affected by exogenous ABA, synthesis of the CDSP 34 protein was substantially enhanced. Based on these results, we conclude that ABA likely mediates the increased synthesis of CDSP 34 upon drought, low temperature and high salinity and suggest that another signal, likely related to high osmolarity, is involved in the induction of CDSP 32 synthesis.

Control of crops leaf growth by chemical and hydraulic influences

Three species of forage grasses (Festuca arundinacea, Eragrostis curvula, Sporobolus stapfianus) commonly grown in the Mediterranean region were subjected to a soil drying treatment. Leaf growth rate in F. arundinacea was highly sensitive to soil drying and low growth rates were associated with high laminar turgors. The production of ABA was stimulated by soil drying and there was a clear relation between increasing ABA accumulation and reduction in leaf growth. Leaf growth of E. cutvula, a C4 warm season grass, was relatively insensitive to soil drying which was not accompanied by a substantial increase in leaf ABA content. S. stapfianus, a resurrection plant, was highly sensitive to decreasing soil water availability. In these two latter species, leaf growth was substantially restricted before ABA accumulation occurred. It is suggested that reductions in laminar turgor of E. curvula and S. stapfianus may be limiting leaf growth as soil dries. The results indicated a different mechanism of sensing and responding to reduction in soil water availability for the three species studied. The relative importance of the chemical and hydraulic control of leaf growth is discussed.

Shoot induction of ABA-requiring genes in response to soil drying

Plant responses to water deficit are dynamic and varied, requiring co-ordination between the shoot and root. Among these responses are alterations in gene expression. The expression of four genes, le4, le16, le20, and le25, which require increased ABA content for expression, was studied in tomato plants in which the root systems were divided between two large pots to impose water deficit gradually and to control signals from the root in response to soil drying without inducing a signal from the shoot. One group of plants had one-half of the roots watered, another group had both halves watered, and another group had neither halves watered. In unwatered plants, the expression of le4 and le25 correlated with ABA content, and that of le16 and le20 occurred before a detectable increase in leaf ABA content. The contrasting patterns of expression indicate a difference in sensitivity of these genes to ABA or an additional signalling mechanism. Ample evidence indicates that shoot processes such as stomatal closure are controlled by signals from the root. This study demonstrates that genes may also be induced in the shoot by signals from the root. Shoots of plants in which only half of the roots were watered showed no decrease in relative water content and no increase in ABA content ; however, three of the four genes, le4, le16, and le20, were induced. Root-to-shoot communication plays a role in changes in gene expression and in alterations in physiological processes.

Characterization of a wilty sunflower (Helianthus annuusL.) mutant

The present study was conducted to evaluate phenotypic interaction in reciprocal grafts between wilty (w-1) sunflower mutant and normal (W-1) plants. The w-1 genotype is a 'leaky' ABA-deficient mutant, characterized by high stomatal conductance, in both light and dark conditions, and high transpiration rate. In well-watered conditions, mutant scions grafted on to normal rootstock (w-1/W-1 showed higher leaf relative water content, leaf water potential and ABA levels than those of control grafts (w-1/w-1). In addition, detached leaves of w-1/W-1 exhibited lower water loss than w-1/w-1 grafts, while mutant rootstock did not affect the transpiration rate of detached W-1 leaves. When drought stress was imposed to potted plants by withholding water, the mutant scions grafted on to normal roots showed a partial phenotypic reversion. A rapid stomatal closure and a rise in ABA levels in response to a small decrease in leaf water potential was observed. By contrast, in w-1/w-1 grafts significant reductions in stomatal conductance and ABA accumulation were detected only in conjunction with a severe water deficit. W-1 scions on mutant stocks (W-1/w-1) maintained the normal phenotype of control wild-type grafts (W1/W-1).