Xylem Sap Abscisic Acid Concentration Research Articles

ABA during reproductive development in non-irrigated grapevines (Vitis vinifera L. cv. Tempranillo)

In grapevines, stomatal aperture decreases after a mid-morning peak during summer days. Afternoon stomatal closure increases in non-irrigated plants as water limitation progresses, which suggests the involvement of abscisic acid (ABA) in the control of stomatal aperture. The objective of this work was to study the seasonal and diurnal time-courses of CO2 assimilation rate, leaf conductance, leaf water potential (Y), and ABA concentration in xylem sap, leaves, flowers and berries in non-irrigated field-grown Tempranillo grapevines throughout reproductive development. Leaf Y decreased throughout fruit development because water availability decreased towards the end of the reproductive cycle. CO2 assimilation rate, leaf conductance and xylem ABA concentration also decreased during the course of the growing season. Combining all measurements xylem ABA was either not correlated, or only slightly correlated, with leaf water status (Y) and daily leaf conductance, respectively. This lack of relationship indicates that xylem ABA during fruit ripening had functions other than provision of a non-hydraulic signal. On a seasonal basis, xylem ABA concentration measured in non-irrigated grapevines was well related to berry ABA concentration, especially at the end of fruit development (veraison and harvest).

Can ABA mediate responses of salinity stressed tomato

We tested the hypothesis that responses to root-sourced salt-stress, in the root and shoot, are coordinated by elevated abscisic acid (ABA) concentrations. To do this we examined the responses of wild-type (Ailsa Craig) and ABA-deficient mutant ( notabilis) genotypes of tomato to a range of salinity (0 (control), 30, 60, 90, 120 mM NaCl). Leaf area was reduced and root:shoot ratio increased in response to increasing salinity in the absence of changes in leaf turgor for both genotypes. Stomatal conductance was however reduced suggesting that a non-hydraulic factor may be coordinating responses to the salinity in the rooting medium. ABA concentrations were increased in response to salinity and correlative analysis showed that stomatal conductance exhibited a negative relationship with increasing xylem ABA for both genotypes ( P<0.001). The possibility that ABA controlled leaf area was less apparent as reduced leaf areas were found in both wild-type and notabilis under high salinity (120 mM; P<0.001), but xylem sap and tissue ABA concentrations were much lower in notabilis. More specific evidence for the role of enhanced ABA concentrations on leaf and root growth was found under moderate levels of salinity. At 90 mM NaCl there was a large decrease in leaf area for notabilis compared with the near control levels of leaf area exhibited by the wild-type ( P<0.001). We hypothesised that the poorer growth in the ABA-deficient mutant was due to its inability to produce ABA at wild-type levels. This was confirmed, by feeding synthetic ABA to the rooting medium, and that 1 and 10 μM±ABA appeared necessary for the maintenance of leaf and root growth at wild-type levels, respectively. These data suggested that there was differential sensitivity to ABA in the roots and shoots but that enhanced concentrations at wild-type levels acted primarily to maintain root and shoot growth under moderate salt stress. How ABA was synthesised to affect concentrations within the leaf and root tissue under salinity stress was investigated by the analysis of mRNAs for two enzymes involved in ABA biosynthesis (9- cis-epoxycarotenoid dioxygenase (NCED) and zeaxanthin epoxidase (ZEP). Evidence for increased synthesis in the roots was detected but there was little change in the leaves in response to salinity and alternative mechanisms that may regulate ABA content in the leaf are explored. The possibility that ABA can interact with other hormones to coordinate whole plant responses to high salinity stress is discussed.

Leaf growth and stomatal sensitivity to ABA in droughted pepper plants

The role of xylem sap abscisic acid (ABA) in regulating leaf growth and stomatal conductance was investigated on droughted pepper cultivars. Two pepper cultivars Capsicum annuum var. grossum, Bellboy and Capsicum annuum var. annuum ‘Cili Padi’ were established in growth chamber in soil compost mixture-filled polyvinyl-chloride tubes, 200 mm long and of an internal diameter of 105 mm. Soil was fertilized and wet to field capacity during plant establishment. One group of plants was given one last watering and left unwatered for 9 days and another group of plants was watered daily. Under gradually decreasing soil moisture content, C. annuum var. grossum ‘Bellboy’ and C. annuum var. annuum ‘Cili Padi’ differed in their leaf water status, stomatal conductance, leaf growth and xylem sap ABA concentrations. The reduction in stomata and leaf growth was independent to the changes in the internal water relations as soil drying progressed. Stomatal conductance of ‘Bellboy’ began to decline within 2 days of cessation of watering, while ‘Cili Padi’ showed a reduction in this variable approximately 2 days later. Leaf growth of well watered ‘Bellboy’ was more rapid than that of ‘Cili Padi’. When watering was stopped, ‘Bellboy’ showed an earlier inhibition of leaf growth. There were differences in the sensitivity of stomata and leaf growth to leaf and soil water status. Leaf growth was apparently responsive to variation in soil water availability in a range where leaf water status did not change. Both cultivars showed the same responses in stomatal conductance and leaf growth when expressed as a function of ABA concentration in the xylem. The study showed that under gradual water stress condition, any production of ABA from roots to be ineffective in triggering early stomatal closure and cessation of leaf growth.

Canopy stomatal conductance and xylem sap abscisic acid (ABA) in mature Scots pine during a gradually imposed drought.

We investigated the effect of drought on canopy stomatal conductance (g(c)), and examined the hypothesis that g(c) is controlled by the chemical messenger abscisic acid (ABA) produced in roots. Beginning in November 1994, we subjected a mature stand of Scots pine (Pinus sylvestris L.) to an imposed 11-month drought. Control plots were maintained at average-season soil water content. Xylem sap was extracted from shoots at regular intervals from April to November 1995. Soil water, sap flow and leaf water potentials (predawn to dusk) were recorded at the same time. Canopy stomatal conductance was calculated from sap flow data and xylem sap ABA concentrations ([ABA(xyl)]) were measured by radioimmunoassay. Mean [ABA(xyl)] in control trees was 250 micromol m(-3). No diurnal variation in [ABA(xyl)] was detected. With soil drying, [ABA(xyl)] increased to a maximum in summer (600 micromol m(-3)), but decreased again toward autumn; however, no significant increase in ABA flux to the leaves occurred. A decline in g(c) was detected when volumetric soil water content declined below 0.12. The decline in g(c) could not have been mediated by increasing [ABA(xyl)] because stomatal closure appeared to precede any increase in [ABA(xyl)]. Peak sap flow velocity data were used to estimate delivery times for root-to-shoot signals in 15-m tall trees. Under normal field conditions, a signal would take 12 days to travel from the site of production (roots) to the presumed site of action (shoots). However, under drought conditions it may take a chemical signal in excess of 6 weeks. We conclude that a feedforward model of short-term stomatal response to soil drying, based solely on the action of a chemical messenger from the roots, is not applicable in mature conifer trees because signal transmission is too slow.

How the roots contribute to the ability of Phaseolus vulgaris L. to cope with chilling-induced water stress.

Intact plants and stem-girdled plants of Phaseolus vulgaris grown hydroponically were exposed to 5 degrees C for up to 4 d; stem girdling was used to inhibit the phloem transport from the leaves to the roots. After initial water stress, stomatal closure and an amelioration of root water transport properties allowed the plants to rehydrate and regain turgor. Chilling augmented the concentration of abscisic acid (ABA) content in leaves, roots and xylem sap. In intact plants stomatal closure and leaf ABA accumulation were preceded by a slight alkalinization of xylem sap, but they occurred earlier than any increase in xylem ABA concentration could be detected. Stem girdling did not affect the influence of chilling on plant water relations and leaf ABA content, but it reduced slightly the alkalinization of xylem sap and, principally, prevented the massive ABA accumulation in root tissues and the associated transport in the xylem that was observed in non-girdled plants. When the plants were defoliated just prior to chilling or after 10 h at 5 degrees C, root and xylem sap ABA concentration remained unchanged throughout the whole stress period. When the plants were chilled under conditions preventing the occurrence of leaf water deficit (i.e. at 100% relative humidity), there were no significant variations in endogenous ABA levels. The increase in root hydraulic conductance in chilled plants was a response neither to root ABA accretion, nor to some leaf-borne chemical signal transported downwards in the phloem, nor to low temperature per se, as indicated by the results of the experiments with defoliated or girdled plants and with plants chilled at 100% relative humidity. It was concluded that the root system contributed substantially to the bean's ability to cope with chilling-induced water stress, but not in an ABA-dependent manner.

Stomatal and leaf growth responses to partial drying of root tips in willow.

Root tips of intact willow (Salix dasyclados Wimm., Clone 81-090) plants were partially dried by exposure to ambient greenhouse air and then kept in water-vapor-saturated air for up to 3 days. The drying treatment increased abscisic acid (ABA) concentrations in both the root tips subjected to drying and in the xylem sap, while it reduced leaf stomatal conductance and leaf extension rate. Despite the decrease in stomatal conductance, leaf water potentials were unaffected by the root drying treatment, indicating that the treatment reduced hydraulic conductivity between roots and foliage. After roots subjected to drying were returned to a nutrient solution or excised, ABA concentrations in the remaining roots and in the xylem sap, stomatal conductance of mature leaves and extension rate of unfolding leaves all returned to values observed in control plants. The 4-fold increase in xylem sap ABA concentration following the root drying treatment was not solely the result of reduced sap flow, and thus may be considered a potential cause, not merely a consequence, of the observed reduction in stomatal conductance.

Does engineering abscisic acid biosynthesis in Nicotiana plumbaginifolia modify stomatal response to drought?

ABSTRACTThe consequences of manipulating abscisic acid (ABA) biosynthesis rates on stomatal response to drought were analysed in wild‐type, a full‐deficient mutant and four under‐producing transgenic lines of N. plumbaginifolia. The roles of ABA, xylem sap pH and leaf water potential were investigated under four experimental conditions: feeding detached leaves with varying ABA concentration; injecting exogenous ABA into well‐watered plants; and withholding irrigation on pot‐grown plants, either intact or grafted onto tobacco. Changes in ABA synthesis abilities among lines did not affect stomatal sensitivity to ABA concentration in the leaf xylem sap ([ABA]xyl), as evidenced with exogenous ABA supplies and natural increases of [ABA]xyl in grafted plants subjected to drought. The ABA‐deficient mutant, which is uncultivable under normal evaporative demand, was grafted onto tobacco stock and then presented the same stomatal response to [ABA]xyl as wild‐type and other lines. This reinforces the dominant role of ABA in controlling stomatal response to drought in N. plumbaginifolia whereas roles of leaf water potential and xylem sap pH were excluded under all studied conditions. However, when plants were submitted to soil drying onto their own roots, stomatal response to [ABA]xyl slightly differed among lines. It is suggested, consistently with all the results, that an additional root signal of soil drying modulates stomatal response to [ABA]xyl.

Salt, nutrient uptake and transport, and ABA of Populus euphratica; a hybrid in response to increasing soil NaCl

The uptake and transport of salt ions (Na+, Cl–), macronutrients (K+, Ca2+, Mg2+) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom × (P. euphratica + Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206 mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354 mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Cl–. Salt treatment altered nutrient balance of the hybrid, leaf K+, Ca2+ and Mg2+ concentrations significantly declined and the same trends were observed in roots with the exception of K+. Although K+ levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca2+ and Mg2+ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca2+ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables P. euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.

Does an antagonistic relationship between ABA and ethylene mediate shoot growth when tomato (Lycopersicon esculentum Mill.) plants encounter compacted soil?

ABSTRACTThis study tested the hypothesis that antagonistic interactions between abscisic acid (ABA) and ethylene mediate the effects of soil compaction on shoot growth. Isogenic wild‐type (Ailsa Craig), ABA‐deficient (notabilis) and a transgenic (ACO1AS) tomato genotype with a reduced capacity to synthesize ethylene were examined. Exogenous ABA was also applied. Leaf area was comparable when Ailsa Craig and ACO1AS were grown in uncompacted (1·1 g cm−3) or compacted (1·5 g cm−3) soil, but was lower in notabilis. However, a 1·1/1·5 g cm−3 split‐pot treatment invoked marked genotypic differences, whereby leaf area was comparable to 1·1 g cm−3 control plants in ACO1AS but was intermediate between the 1·1 and 1·5 g cm−3 treatments in Ailsa Craig and notabilis. ABA may be discounted as the root‐sourced signal responsible for reducing leaf area when the roots encountered compacted soil as Ailsa Craig and ACO1AS showed differing responses despite similar increases in xylem sap ABA concentration; leaf area was invariably lower in notabilis. These genotypic differences were correlated with ethylene evolution; thus the greater leaf area in ACO1AS was associated with its reduced ability to synthesize ethylene, whereas the reductions in leaf expansion observed when Ailsa Craig and notabilis encountered compacted soil were accompanied by increased ethylene production. Application of ABA had little effect on ACO1AS, but promoted a recovery of leaf expansion in notabilis, and more surprisingly in Ailsa Craig. These results suggest that antagonistic interactions between ABA and ethylene may regulate leaf expansion when the root system simultaneously encounters uncompacted and compacted soil.

Hormonal changes induced by partial rootzone drying of irrigated grapevine

Partial rootzone drying (PRD) is a new irrigation technique which improves the water use efficiency (by up to 50%) of wine grape production without significant crop reduction. The technique was developed on the basis of knowledge of the mechanisms controlling transpiration and requires that approximately half of the root system is always maintained in a dry or drying state while the remainder of the root system is irrigated. The wetted and dried sides of the root system are alternated on a 10-14 d cycle. Abscisic acid (ABA) concentration in the drying roots increases 10-fold, but ABA concentration in leaves of grapevines under PRD only increased by 60% compared with a fully irrigated control. Stomatal conductance of vines under PRD irrigation was significantly reduced when compared with vines receiving water to the entire root system. Grapevines from which water was withheld from the entire root system, on the other hand, show a similar reduction in stomatal conductance, but leaf ABA increased 5-fold compared with the fully irrigated control. PRD results in increased xylem sap ABA concentration and increased xylem sap pH, both of which are likely to result in a reduction in stomatal conductance. In addition, there was a reduction in zeatin and zeatin-riboside concentrations in roots, shoot tips and buds of 60, 50 and 70%, respectively, and this may contribute to the reduction in shoot growth and intensified apical dominance of vines under PRD irrigation. There is a nocturnal net flux of water from wetter roots to the roots in dry soil and this may assist in the distribution of chemical signals necessary to sustain the PRD effect. It was concluded that a major effect of PRD is the production of chemical signals in drying roots that are transported to the leaves where they bring about a reduction in stomatal conductance.

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.)

ABSTRACTNovel 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.

Does root‐sourced ABA have a role in mediating growth and stomatal responses to soil compaction in tomato (Lycopersicon esculentum)?

Isogenic wild‐type (Ailsa Craig) and abscisic acid (ABA)‐deficient mutant (flacca) genotypes of tomato were used to examine the role of root‐sourced ABA in mediating growth and stomatal responses to compaction. Plants were grown in uniform soil columns providing low to moderate bulk densities (1.1–1.5 g cm−3), or in a split‐pot system, which allowed the roots to divide between soils of the same or differing bulk density (1.1/1.5 g cm−3). Root and shoot growth and leaf expansion were reduced when plants were grown in compacted soil (1.5 g cm−3) but leaf water status was not altered. However, stomatal conductance was affected, suggesting that non‐hydraulic signal(s) transported in the transpiration stream were responsible for the observed effects. Xylem sap and foliar ABA concentrations increased with bulk density for 10 and 15 days after emergence (DAE), respectively, but were thereafter poorly correlated with the observed growth responses. Growth was reduced to a similar extent in both genotypes in compacted soil (1.5 g cm−3), suggesting that ABA is not centrally involved in mediating growth in this severely limiting ‘critical’ compaction stress treatment. Growth performance in the 1.1/1.5 g cm−3 split‐pot treatment of Ailsa Craig was intermediate between the uniform 1.1 and 1.5 g cm−3 treatments, whereas stomatal conductance was comparable to the compacted 1.5 g cm−3 treatment. In contrast, shoot dry weight and leaf area in the split‐pot treatment of flacca were similar to the 1.5 g cm−3 treatment, but stomatal conductance was comparable to uncompacted control plants. These results suggest a role for root‐sourced ABA in regulating growth and stomatal conductance during ‘sub‐critical’ compaction stress, when genotypic differences in response are apparent. The observed genotypic differences are comparable to those previously reported for barley, but occurred at a much lower bulk density, reflecting the greater sensitivity of tomato to compaction. By alleviating the severe growth reductions induced when the entire root system encounters compacted soil, the split‐pot approach has important applications for studies of the role of root‐sourced signals in compaction‐sensitive species such as tomato.

Drought tolerance, xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of canopy trees of three temperate deciduous angiosperms.

Patterns of water relations, xylem sap abscisic acid concentration ([ABA]) and stomatal aperture were characterized and compared in drought-sensitive black walnut (Juglans nigra L.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.) trees co-occurring in a second-growth forest in Missouri, USA. There were strong correlations among reduction in predawn leaf water potential, increased xylem sap [ABA] and stomatal closure in all species. Stomatal conductance was more closely correlated with xylem sap ABA concentration than with ABA flux or xylem sap pH and cation concentrations. In isohydric black walnut, increased concentrations of ABA in the xylem sap appeared to be primarily of root origin, causing stomatal closure in response to soil drying. In anisohydric sugar maple and white oak, however, there were reductions in midday leaf water potential associated with stomatal closure, making it uncertain whether drought-induced xylem sap ABA was of leaf or root origin. The role of root-originated xylem sap ABA in these species as a signal to the shoot of the water status of the roots is, therefore, less certain.

Root restriction affects leaf growth and stomatal response: the role of xylem sap ABA

The effects of a restricted root zone (RRZ) on leaf and root growth, water relations, stomatal conductance ( g s) and xylem sap abscisic acid concentration ([ABA]) were studied in pepper ( Capsicum annuum cv Bellboy) plants. Plants were grown in soil columns of two different volumes (0.5 l (restricted root) and 2.25 l (unrestricted root) control) in a controlled environment growth cabinet. Another group of plants were grown without root restriction but a gradual soil drying treatment was imposed. Root restriction for 19 days reduced leaf and root growth almost to the rate shown by plants exposed to soil drying. Leaf water potential of RRZ plants was similar to that of unrestricted plants, but, g s was reduced in plants undergoing root restriction. The slight increase of xylem sap ABA concentration in RRZ plants did not fully account for the large reductions in leaf growth and g s.

Xylem sap abscisic acid concentration and stomatal conductance of mycorrhizal Vigna unguiculata in drying soil

SUMMARYThis study was conducted to determine whether xylem abscisic acid (ABA) concentration is altered by mycorrhizal symbiosis of cowpea plants grown in drying soil, and to determine whether stomatal sensitivity to xylem ABA is altered by the symbiosis. We allowed the entire root zone to dry and found that at high soil water contents (θ), mycorrhizal plants had higher stomatal conductance (gs) and lower xylem [ABA]than did non‐mycorrhizal plants, but the difference disappeared at low tH, probably because of stomatal closure. The altered gs and xylem [ABA] were apparently not related to plant water status since shoot water potential, xylem sap osmotic potential and shoot water content were similar for mycorrhizal and non‐mycorrhizal plants across the range of soil moisture. These differences were also not related to P nutrition or plant size. The relationship of gs to xylem [ABA] was not affected by the symbiosis, indicating that either stomatal sensitivity to xylem ABA was not affected by other xylem constituents, or that more than one xylem constituent was altered by the symbiosis but was offsetting in its effect on gs. We conclude that the symbiosis altered gs non–hydraulically, and that the factor might be xylem ABA.

Growth of Zea mays L. plants with their seminal roots only. Effects on plant development, xylem transport, mineral nutrition and the flow and distribution of abscisic acid (ABA) as a possible shoot to root signal

Maize (Zea mays L.) was grown in quartz sand culture either with a normal root system (controls) or with seminal roots only ('single-rooted'). Development of adventitious roots was prevented by using plants with an etiolated mesocotyl and the stem base was positioned 5-8 cm above the sand. Even though the roots of the single-rooted plants were sufficiently supplied with water and nutrients, the leaves experienced water deficits and showed decreased transpiration as transpirational water flow was restricted by the constant number of xylem vessels present in the mesocotyl. As a consequence of this restriction, transpirational water flow velocities in the metaxylem vessels reached mean values of 270 m h -1 and phloem transport velocities of 5.2 m h -1 . Despite limited xylem transport mineral nutrient concentrations in leaf tissues were not decreased in single-rooted plants, but shoot and particularly stem development was somewhat inhibited. Due to the lack of adventitious roots the shoot:root ratio was strongly increased in the single-rooted plants, but the seminal roots showed compensatory growth compared to those in control plants. Consistent with decreased leaf conductance, ABA concentrations in leaves of single-rooted plants were elevated up to 10-fold, but xylem sap ABA concentrations in these plants were lower than in controls, in good agreement with the well-watered conditions experienced by the seminal roots. Surprisingly, however, ABA concentrations in tissues of the seminal roots of the single-rooted plants were clearly increased compared to the controls, presumably due to increased ABA import via phloem from the water-stressed leaves. The results are discussed in relation to the role of ABA as a shoot to root signal.

Effect of soil compaction on barley (Hordeum vulgareL.) growth

The abscisic acid (ABA)-deficient mutant of barley, Az34, exhibited a much reduced rate of leaf expansion at a bulk density of 1.6 g cm -3 as compared to the isogenic wild-type variety, Steptoe. Az34 had a consistently lower xylem sap ABA concentration at 7 d and 14 d after emergence (DAE). The xylem sap data suggest that ABA present at Steptoe concentrations may have a direct role in maintaining leaf expansion at the sub-critical bulk density (1.6 g cm -3 ). To test this hypothesis, addition of synthetic ABA either to the rooting environment (100 nM) or directly to the xylem sap (5 pg μl -1 ) to reproduce the xylem sap ABA concentrations found in Steptoe, increased leaf expansion in Az34 to the wild-type level. Furthermore, feeding Steptoe xylem sap to Az34 produced similar effects. These experiments provide direct evidence of a positive role for ABA as a root-to-shoot signal which assists in maintaining leaf growth in plants experiencing sub-critical levels of compaction stress.

Effect of soil compaction on barley (Hordeum vulgareL.) growth

Wild-type (Steptoe) and abscisic acid (ABA)-deficient mutant (Az34) genotypes of barley were grown in compacted soil to examine the potential role of ABA as a root-to-shoot signal. Root and shoot growth and leaf conductance were all reduced when plants were grown in compacted soil with a bulk density of 1.7 g cm -3 , relative to uncompacted control plants (1.1 g cm -3 ). These effects occurred in the absence of detectable changes in leaf water status or foliar abscisic acid (ABA) content. Analysis of Steptoe and Az34 xylem sap showed that the ABA concentration was greatly increased at 6 d after emergence (6 DAE) when seedlings were grown in compacted soil (1.7 g cm -3 ) ; however, ABA concentrations were never as high in the mutant as in the wild type. The increase in xylem sap ABA concentration observed at high bulk density was closely correlated with reductions in leaf conductance, but not leaf area. These increases were transitory, and xylem sap ABA concentrations subsequently decreased towards the control level by 18 DAE in both genotypes. The ABA-deficient mutant, Az34, produced a much lower leaf area than Steptoe at a bulk density of 1.6 g cm -3 . Examination of epidermal characteristics indicates that this effect resulted mainly from reductions in cell expansion rather than cell division, suggesting that the higher ABA concentrations detected in xylem sap from the wild-type Steptoe may have exerted a positive role in maintaining leaf expansion in this treatment. The possible involvement of ABA as a root-to-shoot signal mediating the effects of compaction stress is discussed.

Stomatal conductance in relation to xylem sap abscisic acid concentrations in two tropical trees, Acacia confusa and Litsea glutinosa

ABSTRACTTwo tropical trees, Acacia confusa and Litsea glutinosa, were grown under controlled conditions with their roots subjected to soil drying and soil compaction treatments. In both species, a decline in stomatal conductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resulted in a substantial decrease in stomatal conductance but had little effect on leaf water potential. A rapid and substantial increase in xylem abscisic acid (ABA) concenation ([ABA]), rather than hulk leaf ABA, was closely related to soil drying and soil compaction. A significant relationship between stomatal conductance (gs) and xylem [ABA] was observed in both species. Artificially feeding ABA solutions to excised leaves of both species showed that the relationship bet ween gs and [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between gs and xylem [ABA]. These results suggest that xylem ABA may act as a stress signal in the control of stomatal conductance.

Mycorrhizal influence on hydraulic and hormonal factors implicated in the control of stomatal conductance during drought

During drying, mycorrhizal plants often maintain higher stomatal conductance (g s ) than similarly-sized and -nourished non-mycorrhizal plants, but the mechanism of mycorrhizal influence remains unclear. Several hydraulic and non-hydraulic factors previously implicated in control of stomatal behavior during drought were measured, to learn which are affected when roots of cowpea (Vigna unguiculata [L.] Walp. cv. White Acre) are extensively colonized by Glomus intraradices Schenck and Smith isolate UT143. At low soil water contents (θ), mycorrhizal plants maintained higher g s , transpiration and shoot water potential (ψ) than non-mycorrhizal plants. These higher foliar water status characters were associated with lower xylem-sap abscisic acid concentrations ([ABA]) and lower ABA fluxes to leaves in mycorrhizal plants at low soil θ. Stomatal conductance was most closely correlated with xylem-sap [ABA], ABA flux to leaves and shoot ψ. Stomatal conductance was not correlated with xylem-sap concentrations of calcium or zeatin riboside equivalents, or with xylem-sap pH, nor were these xylem-sap constituents affected by mycorrhizal symbiosis. Stomata of mycorrhizal and non-mycorrhizal leaves showed similar sensitivities to ABA, whether leaves were intact or detached. It is concluded that mycorrhizal fungi probably increased the capability of root systems to scavenge water in drier soil, resulting in less strain to foliage and hence higher g s and shoot ψ at particular soil θ.