Abscisic acid accumulation in the roots of nutrient-limited plants: its impact on the differential growth of roots and shoots

We describe the involvement of abscisic acid (ABA) in the control of differential growth of roots and shoots of nutrient limited durum wheat plants. A ten-fold dilution of the optimal concentration of nutrient solution inhibited shoot growth, while root growth remained unchanged, resulting in a decreased shoot/root ratio. Addition of fluridone (inhibitor of ABA synthesis) prevented growth allocation in favour of the roots. This suggests the involvement of ABA in the redirecting of growth in favour of roots under limited nutrient supply. The ABA content was greater in shoots and growing apical root parts of starved plants than in nutrient sufficient plants. Accumulation of ABA in shoots of nutrient deficient plants was linked to a decrease in leaf turgor. Increased flow of ABA in the phloem apparently contributed to the accumulation of ABA in the apical part of the roots. Thus, partitioning of growth between roots and shoots of wheat plants limited in mineral nutrients appears to be modulated by accumulation of ABA in roots. This ABA may originate in the shoots, where its synthesis is stimulated by the loss of leaf turgor.     

Carotenoid composition and down regulation of photosystem II in three conifer species during the winter

Seedlings and coppice shoots of Betula pubescens Ehrh. were grown under controlled conditions designed to simulate the annual growth cycle, and a water stress was introduced during the short day (SD). Alleviation of hud dormancy after increasing periods at chilling temperatures was tested under long day (LD) conditions. Abscisic acid (ABA) was analysed in leaf and bud samples by gas chromatography-mass spectrometry using [²H₄]ABA as the internal standard. Elongation growth of coppice shoots was faster than that of seedlings under both LD and SD conditions, while the final growth cessation occurred in a similar manner and was not affected by water stress, which significantly reduced growth rate in both plant types. Bud dormancy gradually decreased with increasing length of chilling, starting from the basal parts of the plant axis. Water stress did not retard hudhurst. but rather improved it in the chilled coppice shoots and in the non-chilled and partially chilled seedlings. Water content of buds was higher in coppice shoots than in seedlings, but after exposure to SD. it gradually decreased to 45% in both plant types and was not affected by water stress or chilling. The ABA level in both leaves and buds increased during SD treatment and was" enhanced by water stress. No clear differences in bud ABA level were found between the seedlings and coppice shoots under SD conditions, although coppice shoots had less ABA during the preceding LD conditions. There was, in general, no clear effect of chilling on bud ABA level. Budbursl in chilled, single-node cuttings was inhibited by external ABA treatment, which raised the internal ABA levels 10 to 150 times above normal. The observed correlation between ABA level and water content in buds during induction of dormancy under SD and water stress conditions indicates a possible role for ABA in the regulation of dormancy.     

Control of plant growth resides in the shoot,and not in the root,in reciprocal grafts of flacca and wild-type tomato (Lysopersicon esculentum), in the presence and absence of salinity stress

Grafted plants of flacca, an ABA-deficient mutant of tomato (Lycopersicon esculentum), and the wild-type variety Rheinlands Ruhm were grown with and without salinity stress to test the roles of roots and shoots in the regulation of plant growth. Fourteen days after exposure to 200 mM NaCl, shoot and root fresh weight, endogenous ABA concentrations, nitrate concentration, activities of selected enzymes related to nitrogen assimilation, and cation accumulation were determined. Rootstock genotype had little influence on the growth of the grafted plants, whereas grafted plants having wild-type shoots (Ws) produced more biomass than those having flacca shoots (Fs), irrespective of the salinity level. Growth of flacca shoots grafted onto wild-type rootstock (Fs/Wr) was superior to that of flacca shoots grafted onto flacca rootstock (Fs/Fr). The improved growth correlated with enhanced levels of ABA in the flaccashoots of Fs/Wr. In all the graft combinations, ABA content was higher in wild-type shoots than in flacca shoots, with or without salinity. There were no significant differences in root ABA concentrations among the different grafted types. Enhanced growth correlated with higher nitrate levels and higher nitrate reductase activity in the roots and shoots of plants with wild-type shoots and with higher shoot concentrations of ABA in plants with wild-type shoots. There were no significant differences in glutamine synthetase and phosphoenol pyruvate carboxylase activities in the shoots and roots of all the grafted plants, regardless of the salinity level. While shoot genotype determined the accumulation of K⁺ and Na⁺ in grafted plants regardless of salinity, it had no influence on Ca²⁺ concentrations. Regardless of the salinity, the total concentration of cations was the same in all the plants, while salinity decreased Mg²⁺ concentration in roots and shoots of all grafts, with the exception of flacca grafted shoots. The scion genotype – and its ABA level – thus played the major role in the growth of grafted plants, regardless of the rootstock genotype and the salinity of the growth medium.     

Abscisic acid inhibits shoot elongation of Scirpus mucronatus

The relationships between free ABA levels and shoot elongation were investigated in shoots of Scirpus mucronatus L. Under submergence, shoot elongation increased but free ABA levels decreased. The extent of the increase in length and the decrease in free ABA in submerged shoots increased with the increase of water depth. When the shoots were transferred to air after 12 days of submergence, they ceased to elongate and the free ABA levels recovered to the values of air-grown shoots. ABA, at concentrations from 1 μ M to 1 m M , inhibited the submergence-induced shoot elongation. In ambient air, fluridone, an inhibitor of ABA biosynthesis, at 7 μ M decreased the free ABA levels in shoots but increased shoot elongation. The effects could be reversed by 10 μ M ABA. These results indicate that ABA is an internal inhibitor of shoot growth in Scirpus .     

Effect of chilling on the opening and abscisic acid content of dormant lateral buds of willow

A reduction in abscisic acid (ABA) content was not a pre-requisite for the breaking of dormancy of vegetative lateral buds of both field-grown trees and shoots of willow (Salix viminalis L.) maintained in controlled conditions. Similar variations in bud ABA levels were observed whether the shoots were stored in a warm (22 ± 1 °C) or cold (6 ± 0.5 °C) environment. Following transfer to a growth room the ABA content of chilled buds declined more rapidly than did that of non-chilled buds.     

Leaf growth responses to ABA are temperature dependent

The robustness of a leaf elongation bioassay was evaluated byconducting trials with detached shoots of wheat at several differenttemperatures. Leaf elongation rate (LER) was monitored for shootsfed either an artificial xylem solution or xylem solution plus10^–3mol m^–3 abscisic acid (ABA). Consistent resultswere obtained when periodic ruler measurements of many shootswere made and compared with simultaneous measurements on a singleshoot made with a linearly variable displacement transducer(LVDT). ABA treatment consistently inhibited leaf growth; however,the magnitude of the inhibition was dependent on the temperatureat which the assay was conducted. Interpretation of resultsfrom such bioas-says in terms of ABA concentration suppliedto the detached shoots is complicated by this observation sincethere is no unique relationship between leaf growth inhibitionand ABA concentration. The results are discussed in terms ofchemical signalling affecting the growth rate of plants in dryingsoil. Key words: ABA, leaf growth, temperature, leaf elongation bioassay     

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

Detached 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.     

Seasonal changes in growth and metabolic activity of underground shoots of yarrow

Growth and metabolic activity of underground shoots of a long-rhizome perennial herbaceous species yarrow (Achillea millefolium L.) were studied. The active growth of rhizomes and the formation of new meristematic zones were observed during the second half of the growing season after termination of aboveground shoot growth. In this period, underground shoots had a rather high rate of respiration (1.3 mg CO₂/(g dry wt h)), a considerable content of nonstructural carbohydrates (15% of dry weight), and the elevated activities of IAA, cytokinins, and ABA. In autumn, the rate of respiration of underground shoots decreased to 1.0 mg CO₂/(g dry wt h), soluble sugars accumulated, the ratio between unsaturated and saturated fatty acids rose as well as the ratios GA/ABA and cytokinins/ABA. Temperature optimum for the rhizome growth lay in the range of low and moderate above-zero temperatures (5–20°C), and the freezing point of water in the apices of under-ground shoots was about ?10°C. It is concluded that rhizome quiescence predominantly depends on low temperatures and is not associated with the accumulation of growth inhibitors. In the course of plant preparation to winter, morphogenetic transformations in underground shoots depend on changes in the hormonal balance directed in favor of growth hormones and relatively high respiratory activity in the apical zones of the rhizomes.     

黄花倒水莲生长与内源激素水平动态变化关系研究北大核心CSCD

为探索黄花倒水莲春梢生理生化特性的差异以及不同内源激素的变化规律,该文对黄花倒水莲春梢的生长动态进行监测,采用间接酶联免疫吸附法(ELISA)测定脱落酸(ABA)、生长素(IAA)、赤霉素(GA)、乙烯(ETH)和玉米素核苷(ZR)五种内源激素含量的动态变化,并对两者间的相关性进行分析。结果表明:(1)黄花倒水莲春梢生长发育过程可分为快速增长期(0~12 d)、生长转折期(16~20 d)和缓慢增长期(24~32 d)三个阶段。(2)内源激素ABA、GA、ETH和ZR含量在缓慢增长期显著高于快速增长期和生长转折期,IAA含量各时期差异较小。(3)春梢长、底部叶长和叶宽在快速增长期与ABA、GA、ETH和ZR含量呈负相关,且与ZR含量具有一定显著性,与IAA含量呈正相关;生长转折期,各指标与GA、ETH和ZR含量呈正相关,与GA含量具有一定显著性,与ABA含量呈负相关;缓慢增长期,各指标与五种内源激素含量均呈正相关,与IAA和ZR含量具有一定显著性。该研究结果为生产上利用外源激素调控黄花倒水莲春梢抽出以及生长提供了理论基础。     

Effect of Competition and Treatment with Inhibitor of Ethylene Perception on Growth and Hormone Content of Lettuce Plants

We elucidated the effect of increased planting density (single and grouped competing plants) on concentrations of auxin, abscisic acid, and cytokinins in normal lettuce plants and in those with ethylene perception inhibited by 1-methylcyclopropene (1-MCP). An attempt was made to relate the changes in hormone concentration induced by competition and inhibition of ethylene sensitivity to growth responses of lettuce planting. The results showed changes in concentrations of auxins, cytokinins, and ABA in the response of lettuce to crowding. Accumulation of ABA in shoots was likely to contribute to inhibition of transpiration of the plants grown in the presence of neighbors. This assumption was supported by the results of application of an inhibitor of ABA synthesis (fluridone and carotenoid biosynthesis herbicide) resulting in increased transpiration of grouped, but not single plants. Increased planting density led to the decline in root auxins paralleled by inhibition of root growth. This effect was likely to be due to decreased auxin transport to the roots from the shoots suggested by accumulation of auxins in the shoots and inhibition of root growth by application of the auxin transport inhibitor [N-(1-naphtyl)phtalamic acid (NPA)]. Importance of the changes in hormone concentrations was confirmed by data showing that disturbance of auxin and cytokinin distribution detected in MCP-treated plants was accompanied by corresponding modification of the growth response.

     

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.     

Persistence of plant hormone levels in rice shoots grown under microgravity conditions in space: its relationship to maintenance of shoot growth

We investigated the effects of microgravity environment on growth and plant hormone levels in dark‐grown rice shoots cultivated in artificial 1 g and microgravity conditions on the International Space Station (ISS). Growth of microgravity‐grown shoots was comparable to that of 1 g‐grown shoots. Endogenous levels of indole‐3‐acetic acid (IAA) in shoots remained constant, while those of abscisic acid (ABA), jasmonic acid (JA), cytokinins (CKs) and gibberellins (GAs) decreased during the cultivation period under both conditions. The levels of auxin, ABA, JA, CKs and GAs in rice shoots grown under microgravity conditions were comparable to those under 1 g conditions. These results suggest microgravity environment in space had minimal impact on levels of these plant hormones in rice shoots, which may be the cause of the persistence of normal growth of shoots under microgravity conditions. Concerning ethylene, the expression level of a gene for 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthase, the key enzyme in ethylene biosynthesis, was reduced under microgravity conditions, suggesting that microgravity may affect the ethylene production. Therefore, ethylene production may be responsive to alterations of the gravitational force.     

Antioxidant response and Lea genes expression under exogenous ABA and water deficit stress in wheat cultivars contrasting in drought tolerance

Two wheat (Triticum aestivum) cultivars, C306 (drought tolerant) and PBW343 (drought susceptible) were compared for their response to exogenous ABA, water stress (WS) and combined (ABA plus WS) during their seedlings growth. Their responses were studied in the form of seedlings growth, antioxidant potential of roots and shoots and expression levels of LEA genes in shoots. ABA treatment led to increase in levels of ascorbate, ascorbate to dehydroascorbate ratio, antioxidant enzymes and decreases in levels of dehydroascorbate, malondialdehyde (MDA). Decrease in biomass, ascorbate contents, ascorbate to dehydroascorbate ratios and antioxidant enzymes was more in PBW343 than in C306 under WS. Dehydroascorbate and MDA levels were higher in PBW343 than in C306 under WS. ABA plus WS improved some of these features from their levels under WS in PBW343. Proline contents were not increased significantly under ABA in both cultivars. Out of ten LEA genes studied, six LEA genes were induced more under WS than under ABA in C306 but equally induced in PBW343. Four LEA genes were induced earlier in PBW343 but later in C306. Wdhn13 was induced more under ABA than under WS in C306 while it was non-responsive to both stresses in PBW343.     

Nitrate signalling to stomata and growing leaves: interactions with soil drying, ABA, and xylem sap pH in maize

Increasing the nitrate (N) concentration in the rooting substrate above deficiency decreased stomatal conductance and leaf growth rate compared with sufficient N in maize seedlings (Zea mays L.) growing in drying substrate. Novel effects were detected when N in the non-deficient range was supplied directly to the xylem of detached shoots: concentrations above 2.0 mol m-3 KNO3 reduced transpiration, and concentrations above 12 mol m-3 KNO3 reduced leaf growth rate. Evidence is provided that the novel effects of N on transpiration and growth were mediated by pH-based ABA redistribution. ABA at 0.05 mol m-3, whilst ineffective alone, sensitized leaf growth to increases in KNO3 concentration (from 3.0 mol m-3), and the capacity of higher concentrations of ABA to reduce growth was enhanced by KNO3. Transpiration was sensitively reduced by KNO3, ABA, or buffers adjusted to pH 6.7-7.0 (compared with buffers adjusted to pH 5.0) alone. Nevertheless, a synergistic effect of KNO3 and either ABA or buffers adjusted to pH 6.7-7.0 was observed. Buffers of pH 5.6 supplied to detached shoots alleviated the depression of transpiration caused by 12 mol m-3 KNO3. Buffers adjusted to pH 6.7 increased the sensitivity of growth to KNO3. Xylem sap extracted from intact seedlings growing in drying soil exhibited an initial increase in N concentration, followed by a decrease at progressively lower soil water potentials. The importance for novel N signalling above deficiency is discussed with reference to the generality of fluctuations in soil and xylem N concentration within this range.     

Foliar sprays with ABA promote growth of Ilex paraguariensis by alleviating diurnal water stress

Stomatal closure, relative water content (RWC) and vegetative growth were monitored in Ilex paraguariensis plants grown under well-watered conditions with a photosynthetic photon flux density (PPFD) varying from 100% to 1.5%, and sprayed weekly with either distilled water (control) or 1.89 mM abscisic acid (ABA). ABA treatments caused stomatal closure, ranging from 62% to 73%. These treatments also increased RWC in the early evening from 82% to 92% and 88% to 94% in mature and immature leaves, respectively. Such alleviation of the water stress was correlated with increases in leaf area, leaf dry weight (DW), shoot length and shoot DW. On day 35 from the beginning of the experiment, the increases in DW of both leaves and shoots were 1.5-fold at the 1.5% PPFD and 3-fold (for leaves) and 4.5-fold (for shoots) under 100% PPFD. In water-sprayed control plants grown under 1.5% PPFD shoot length also increased significantly, although these shoots contained more ABA (assessed by capillary gas chromatography–mass spectrometry) than those of plants grown under 100% PPFD. These results show that ABA sprayed on to leaves promotes growth in I. paraguariensis plants by alleviating diurnal water stress.     

A comparative study of the effects of abscisic acid and methyl jasmonate on seedling growth of rice

The effects of abscisic acid (ABA) and methyl jasmonate (MJ) on growth of rice seedlings were compared. The lowest tested concentration of ABA and MJ that inhibited seedling growth was found to be 4.5 and 0.9 µM, respectively. Growth inhibition by ABA is reversible, whereas that by MJ is irreversible. GA₃ was found to be more effective in reversing inhibition of shoot growth by ABA than by MJ. KCl partially relieved MJ-inhibited, but not ABA-inhibited, growth of rice seedlings. The beneficial effect of K⁺ on growth of rice seedlings in MJ medium could not be replaced by Li⁺, Na⁺ or Cs⁺. MJ treatment caused a marked release of K⁺ into the medium. In order to understand whether cell wall-bound peroxidase activity was inversely related to rice seedling growth, effects of ABA and MJ on cell wall-bound peroxidase activity were also examined. Results indicated that both ABA and MJ increased cell wall-bound peroxidase activity in roots and shoots of rice seedlings. Although MJ (4.5 µM) was less effective in inhibiting root growth than ABA (9 µM), MJ was found to increase more cell wall-bound peroxidase activity in roots than ABA.     

Root and Shoot Growth of Plants Treated with Abscisic Acid

Young seedlings of Capsicum annum L., Commelina communis L.and maize (Zea mays L.) were subjected to a mild water-stressingtreatment and/or treated with abscisic acid (ABA). Plants rootedin soil received a soil-drying treatment and their leaves weresprayed with a 10–⁴ M solution of ABA. Plants grown insolution culture were stressed by the addition of polyethyleneglycol (PEG) to the rooting medium and ABA was also added tothe rooting medium, either with or without PEG. The effectsof both treatments on the growth of roots and shoots and theultimate root: shoot dry weight ratio were very similar. Shootgrowth was limited both by water stress and by ABA application;while there was some evidence that mild water stress and/orABA application may have resulted in a stimulation of root growth.More severe water stress reduced the growth of roots but theoverall effect of stress was to increase the ratio of rootsto shoots. Capsicum annum L., Commelina communis L., Zea mays L., water stress, abscisic acid     

The Role of Abscisic Acid in Acclimation of Plants Cultivated in vitro to ex vitro Conditions

The content of endogenous free abscisic acid (ABA) in the shoots of in vitro cultivated tobacco (Nicotiana tabacum L. cv. White Burley) and its changes during ex vitro acclimation of these plants to the greenhouse or growth chamber were estimated. The content of free ABA significantly increased at the 1^st and/or 2^nd day after plant transfer from in vitro to ex vitro. The ABA content of plants covered with transparent foil to maintain higher relative humidity (RH), did not significantly differ from ABA content of plants cultivated under ambient RH. Transfer to fresh medium also transiently increased the content of endogenous ABA. The ABA content in plants, which had been acclimated for 1 week to ex vitro conditions, decreased to the content found in the in vitro plants. Acclimation to ex vitro conditions affected the stomata on adaxial and abaxial sides differently: stomata on the adaxial side were less open than those on the abaxial one. The exogenous application of 5 μM ABA increased transiently its endogenous concentration in shoots of in vitro plants more than ten fold, but after 1 week the concentration in the shoots decreased. This revised version was published online in July 2006 with corrections to the Cover Date.     

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

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

Root water flow and leaf stomatal conductance in aspen (Populus tremuloides) seedlings treated with abscisic acid

Exogenous abscisic acid (ABA) applied to the roots and excised shoots of aspen (Populus tremuloides Michx.) inhibited stomatal conductance. However, the effect of ABA on stomatal conductance was more pronounced in the excised shoots compared with the intact seedlings. Approximately 10% of the ABA concentration applied to the roots was found in the xylem exudates of root systems exposed to a hydrostatic pressure of 0.3 MPa. A similar concentration of ABA applied to the excised shoots produced a faster and greater reduction of stomatal conductance. ABA applied to the roots had no effect on root steady-state flow rate over the 5-h experimental period. Moreover, pre-incubating root systems of intact seedlings for 12 h with 5 x 10(-5) M ABA did not significantly reduce volume flow density. Similarly, ABA had no effect on root hydraulic conductivity and the activation energy of root water flow rates.