Stress induction of abscisic acid in maize roots

Moderate water stresses in the range 0 to −0.6 MPa applied with PEG 6000 to excised roots of Zea mays L. var. LG 11 induced increases of up to four-fold in the amount of abscisic acid (ABA) determined in the tissue after a 12 h period of xylem exudation. The ABA concentration of xylem exudate collected after a 2 h water stress also increased by up to four-fold. Salt stresses, induced with NaCl solutions, resulted in similar increases in the ABA concentrations. ABA concentrations in both root tissue and xylem exudate were highest 4 h after removal of the stress and then declined over a subsequent 8 h period. These results are interpreted in support of the concept that root-produced ABA may have a role in the fine control of the plant's water balance.     

Accumulation rate of ABA in detached maize roots correlates with root water potential regardless of age and branching order

How much ABA can be supplied by the roots is a key issue for modelling the ABA-mediated influence of drought on shoot physiology. We quantified accumulation rates of ABA ( S _ABA) in maize roots that were detached from well-watered plants and dehydrated to various extents by air-drying. S _ABA was estimated from changes in ABA content in root segments incubated at constant relative water content (RWC). Categories of root segments, differing in age and branching order, were compared (root branches, and nodal roots subdivided into root tips, subapical unbranched sections, and mature sections). All categories of roots accumulated ABA, including turgid and mature tissues containing no apex. S _ABA measured in turgid roots changed with root age and among root categories. This variability was largely accounted for by differences in water content among different categories of turgid roots. The response of S _ABA to changes in root water potential ( Ψ _root) induced by dehydration was common to root tips, nodal roots and branches of several ages, while this was not the case if root dehydration was expressed in terms of RWC. Differences among root categories in the response of S _ABA to RWC were due to different RWC values among categories at a given Ψ _root, and not to differences in the response of S _ABA to Ψ _root.     

Effects of norflurazon, an inhibitor of carotenogenesis, on abscisic acid and xanthoxin in the caps of gravistimulated maize roots

Maize seeds were germinated in the dark in the presence of the carotenoid synthesis inhibitor norflurazon and the teveis of abscisic acid, xanthoxin and total carotenoids were measured in the root cap and in the adjacent 1.5 mm segment. In norflurazon-treated roots abscisic acid levels were markedly reduced, but an increase occurred in the levels of xanthoxin, a compound structurally and physiologically similar to abscisic acid. In the cultivar of maize ( Zea mays L. cv. Merit) used for this work, brief illumination of the root is required for gravitropic curving. Following illumination both control and norflurazon-treated roots showed normal gravitropic curvature, however, the rate of curvature was delayed in norflurazon-treated roots. Our data from norflurazon-treated roots are consistent with a role for xanthoxin in maize root gravitropism. The increase in xanthoxin in the presence of an inhibitor of carotenoid synthesis suggests that xanthoxin and abscisic acid originate, at least in part, via different metabolic pathways.     

Xylem pressure response in maize roots subjected to osmotic stress: determination of radial reflection coefficients by use of the xylem pressure probe

The absolute pressure in conducting xylem vessels of roots of 2-week-old, slowly transpiring intact maize plants (bathed in nutrition medium) was determined to be +0·024 ± 0·044 MPa using the xylem pressure probe. When the roots were subjected to osmotic stress (NaCI, KCI or sucrose), the xylem pressure decreased immediately and became more negative. However, the response of xylem pressure to osmotic stress was considerably attenuated, indicating that the radial reflection coefficients, σ₁₃ of the maize root for these solutes were rather low (between 0·2 and 0·4 depending on the concentration of the osmoticum). The low values of a, may be caused (partly) by unstirred layer effects. In repeated osmoticum/nutrition regimes a complex pattern of changes in xylem pressure was observed which was apparently linked to the interplay between transpiration and (passive and/or active) solute loading of the xylem. These processes were not observed when the roots were subjected to osmotic stress after excision. In this case, a biphasic response was observed comparable to that found for excised roots using the root pressure probe.     

Changes in water relation parameters under osmotic and salt stresses in maize and sorghum

A relatively drought tolerant cultivar of maize ( Zea mays L. cv. Pioneer 3950) and a drought tolerant line of sorghum ( Sorghum bicolor [L.] Moench cv. ICSV 112) were grown hydroponically for 11 days. Treatments for non-ionic osmotic and salt stresses were started at the 8th day by addition of polyethylene glycol 6000 and NaCl, respectively, at 200 mOsm equivalent concentrations in the presence or absence of 0. 1 μ M abscisic acid. Relative growth rate was depressed by both stress factors, more severely for maize than sorghum. Abscisic acid increased the growth rate and reverted the negative effect of NaCl in maize, while sorghum was only slightly affected. In general, sorghum had higher levels of K⁺ and lower levels of Na⁺ and the K⁺/Na⁺ ratio was further increased by abscisic acid treatment. From the pressure-volume curves, osmotic potential, the water potential at turgor loss point, bulk elastic modulus and the water saturation deficit at initial turgor loss were estimated. Most significantly, sorghum had a higher elastic modulus than maize and it decreased under osmotic treatment, while in maize it increased under NaCl stress. The results suggest that bulk tissue turgor was not limiting growth under these conditions and underscores the possible implications of changes in the elastic condition of the cell walls in stress responses.     

Effects of nitrogen nutrition and root medium water potential on growth, nitrogen uptake and osmotic adjustment of rice

The effects of nitrogen (N) nutrition on growth, N uptake and leaf osmotic potential of rice plants (Oryza sativa L. ev. IR 36) during simulated water stress were determined. Twenty-one-day-old seedlings in high (28.6 × 10 ^?4M) and low (7.14 × 10 ⁴M) N levels were exposed to decreased nutrient solution water potentials by addition of polyethylene glycol 6000. The roots were separated from the solution by a semi-permeable membrane. Nutrient solution water potential was ?0.6 × 10⁵ Pa and was lowered stepwise to ?1 × 10⁵, ?2 × 10⁵, ?4 × 10⁵ and ?6 × 10⁵ Pa at 2-day intervals. Plant height, leaf area and shoot dry weight of high and low nitrogen plants were reduced by lower osmotic potentials of the root medium. Osmotic stress caused greater shoot growth reduction in high N than in low N plants. Stressed and unstressed plants in 7.14 × 10⁴M N had more root dry matter than the corresponding plants in 28.6 × 10⁴M N. Dawn leaf water potential of stressed plants was 1 × 10⁵ to 5.5 × 10⁵ Pa lower than nutrient solution water potential. Nitrogen-deficient water-stressed plants, however, maintained higher dawn leaf water potential than high nitrogen water-stressed plants. It is suggested that this was due to higher root-to-shoot ratios of N deficient plants. The osmotic potentials of leaves at full turgor for control plants were about 1.3 × 10⁵ Pa higher in 7.14 × 10^?4M than in 28.6 × 10^?4M N and osmotic adjustment of 2.6 × 10⁵ and 4.3 × 10⁵ Pa was obtained in low and high N plants, respectively. The nitrogen status of plants, therefore, affected the ability of the rice plant to adjust osmotically during water stress. Plant water stress decreased transpiration and total N content in shoots of both N treatments. Reduced shoot growth as a result of water stress caused the decrease in amount of water transpired. Transpiration and N uptake were significantly correlated. Our results show that nitrogen content is reduced in water-stressed plants by the integrated effects of plant water stress per se on accumulation of dry matter and transpiring leaf area as well as the often cited changes in soil physical properties of a drying root medium.     

Comparison of exportation and metabolism of xylem-delivered ABA in maize leaves at different water status and xylem sap pH

³H-ABA was introduced into the xylem stream of maize ( Zea mays}) leaves on intact plants by incubation of a semi-attached flap of the sheath in solutions. The relative contribution of exportation and metabolism to the fate of xylem-delivered ABA was assessed in leaves which were either kept at different water potentials through soil drying treatments or subjected to different xylem pHs (pH 7.4 vs. pH 5.5) through a phosphate buffer in the feeding solutions. Xylem-delivered ABA was rapidly metabolised in well-watered leaves with a half-life of 2.19 h in the relatively mature leaves used in this study. Re-exportation of xylem-delivered ABA from leaves was much slower than metabolism. It took 24 h for half of the fed radioactivity to disappear from the well-watered leaves, and very possibly this radioactivity was in the form of metabolites of fed ³H-ABA. Although soil drying usually increases the output of ABA through phloem as reported in previous studies, it greatly reduced the re-exportation of xylem-fed ABA and/or its metabolites. Metabolism was also significantly reduced by the treatment of soil drying (half-life extended from 2.19 to 3.63 h), although the magnitude of change was much less than that of exportation. Manipulation of the pH in the feeding solution also had its effect on the re-exportation. A shift of pH from 5.5 to 7.4 reduced the rate of disappearance of the total radioactivity fed into the attached leaves, but showed no significant effect on the rate of ABA metabolism. It was concluded that it was the ABA metabolism, rather than a re-exportation from leaves, which was mainly responsible for the disposal of the ABA signal from the xylem and therefore preventing an accumulation in leaves. Water stress and pH increase of xylem sap would increase the time of such ABA's presence in the leaves. Since xylem-imported ABA is unlikely to be re-exported from leaves in its intact form, we believe a recycling of ABA from xylem to phloem through leaves plays only a minor role.     

The effect of abscisic acid on cell growth, cell division and DNA synthesis in the maize root meristem

Cis -abscisic acid (ABA), when applied to maize ( Zea mays L. cv. LG 11) roots, decreases the rates of cell growth and cell division in the meristem. It also decreases the rate at which nuclei become labelled with [³H]-thymidine and enter mitosis. Removing the root cap accelerates the entry of nuclei into the DNA synthetic phase of the mitotic cycle and enhances the rate of cell proliferation in the quescent centre. ABA diminishes these effects, but does not suppress them. Thus, ABA cannot wholely substitute for the presence of a cap. One of the primary effects of applied ABA is to retard cell enlargement which may in turn affect the rate of cell division; natural endogenous ABA may act similarly. ABA might in this way assist in maintaining the quiescent centre in intact roots, but cannot be the sole agent involved.     

Effect of zeatin on the growth and indolyl-3-acetic acid and abscisic acid levels in maize roots

Elongation, indolyl-3-acetic acid (IAA) and abscisic acid (ABA) levels, – gas chromatography-mass spectrometry quantification –, in the elongating zone were analysed for maize ( Zea mays L., Cv. LG11) roots immersed in buffer solution with or without zeatin (Z). The effect of Z depends on the initial extension rate of roots. The slower growing roots are more strongly inhibited by Z (10⁻⁷−10₋₅ M ) and they show a greater increase in IAA and ABA content. When compared to the rapidly growing roots, the larger reactivity of the 'slow'ones cannot be attributed to a higher Z uptake as shown when using [¹⁴C]-Z. It is suggested that Z could regulate root elongation by acting on the IAA and/or ABA level. The comparative action of these two hormones is discussed.     

长期水分胁迫对小麦生育中后期根叶渗透调节能力、渗透调节物质的影响

以 2个抗旱性强的和 2个抗旱性弱的小麦品种为材料 ,研究了中度及严重水分胁迫对根系及叶片渗透调节能力的影响。结果表明 :随着水分胁迫的加剧 ,叶片的渗透调节能力增强 ,但在籽粒迅速扩大的灌浆期 ,叶片的渗透调节能力下降。去穗处理明显地提高叶片的渗透调节能力。说明叶片渗透调节能力的高低与同化物的供应及分配有关。不同品种根系渗透调节能力与叶片基本一致 ,但根系的渗透调节能力低于叶片。开花、灌浆期根系的渗透调节能力大大降低 ,严重水分胁迫下根系的渗透调节能力低于中度水分胁迫。这一方面与同化物的供应有关 ,另一方面严重水分胁迫还会对根细胞造成损伤 ,对根系的渗透调节能力产生影响。渗透调节物质的变化趋势与渗透调节能力基本一致。叶片中 K+对渗透调节的贡献最大 ;其次是可溶性糖 ,6种渗透调节物质排列顺序为 K+>可溶性糖 >游离氨基酸 >Ca2 +>Mg2 +>Pro。根系中仍以 K+占绝大部分 ,但根系中 Ca2 +也是不可忽视的成分之一。     

Water stress‐induced abscisic acid accumulation in relation to reducing agents and sulfhydryl modifiers in maize plant

Signalling process of water stress‐induced abscisic acid (ABA) accumulation was investigated in maize (Zea mays L.) leaf and root tissues. Potent free‐radical scavengers and reducing agents, N‐acetyl cysteine (NAC) and cystein (Cys), significantly inhibited or nearly completely blocked dehydration‐induced ABA accumulation. Dithiothreitol (DTT), a reducing agent but not a free‐radical scavenger, also significantly inhibited such accumulation whereas solely free‐radical scavengers, dimethyl sulphoxide (DMSO) and melatonin (Mela), had no effects. Moreover, water stress‐induced ABA accumulation was not affected either by free radicals, such as superoxide anion and hydrogen peroxide, or by oxidants such as KIO_4. These observations suggest that the blocking of water stress‐induced ABA accumulation resulted from the reducing effect, rather than from anything associated with free radicals. The disulphide bond might be crucial to the reactivity of some signal element(s) in the signalling process of water stress‐induced ABA accumulation. To test the hypothesis, we used a sulfhydryl modifier, iodoacetamide (IOA), and found that it nearly totally blocked the water stress‐induced ABA accumulation. Furthermore, an impermeable sulfhydryl modifier, p‐chloromercuriphenylsulphonic acid (PCMBS), could also inhibit the water stress‐induced ABA accumulation in the leaf tissues. These results indicate that water stress‐perception protein(s) or receptor(s) may be located on the plasmalemma and a sulfhydryl group in the extracellular domain is critical to the reactivity of the speculated water stress receptors. Cys, DTT and IOA did not lead to a decrease of the baseline ABA level, i.e. in non‐stressed roots. Result indicates that their blocking of water stress‐induced ABA accumulation occurred upstream of the ABA biosynthesis pathway, i.e. in the signalling process that initiates such accumulation.     

Xylem tension affects growth-induced water potential and daily elongation of maize leaves

Diurnal rates of leaf elongation vary in maize (Zea mays L.) and are characterized by a decline each afternoon. The cause of the afternoon decline was investigated. When the atmospheric environment was held constant in a controlled environment, and water and nutrients were adequately supplied to the soil or the roots in solution, the decline persisted and indicated that the cause was internal. Inside the plants, xylem fluxes of water and solutes were essentially constant during the day. However, the forces moving these components changed. Tensions rose in the xylem, and gradients of growth-induced water potentials decreased in the surrounding growing tissues of the leaf. These potentials, measured with isopiestic thermocouple psychrometry, changed because the roots became less conductive to water as the day progressed. The increased tensions were reversed by applying pressure to the soil/root system, which rehydrated the leaf. Afternoon elongation immediately recovered to rapid morning rates. The rapid morning rates did not respond to soil/root pressurization. It was concluded that increased xylem tension in the afternoon diminished the gradients in growth-induced water potential and thus inhibited elongation. Because increased tensions cause a similar but larger inhibition of elongation if maize dehydrates, these hydraulics are crucial for shaping the growth-induced water potential and thus the rates of leaf elongation in maize over the entire spectrum of water availability.     

Genotype-specific differences in chilling tolerance of maize in relation to chilling-induced changes in water status and abscisic acid accumulation

Four inbred maize lines differing in chilling tolerance were used to study changes in water status and abscisic acid (ABA) levels before, during and after a chilling period. Seedlings were raised in fertilized soil at 24/22°C (day/night), 70% relative humidity. and a 12-h photoperiod with 200 μmol m⁻² s⁻¹ from fluorescent tubes. At an age of 2 weeks the plants were conditioned at 14/12°C for 4 days and then chilled for 5 days at 5/3°C. The other conditions (relative humidity, quantum flux, photoperiod) were unchanged. After the chilling period the plants were transferred to the original conditions for recovery. The third leaves were used to study changes in leaf necrosis, ion efflux, transpiration, water status and ABA accumulation. Pronounced differences in chilling tolerance between the 4 lines as estimated by necrotic leaf areas, ion efflux and whole plant survival were observed. Conditioning significantly increased tolerance against chilling at 5/3°C in all genotypes. The genotypes with low chilling tolerance had lower water and osmotic potentials than the more tolerant genotypes during a chilling period at 5/3°C. These differences were related to higher transpiration rates and lower diffusive resistance values of the more susceptible lines. During chilling stress at 5/3°C ABA levels were quadrupled. Only a small rise was measurable during conditioning at 14/12°C. However, conditioning enhanced the rise of ABA during subsequent chilling. ABA accumulation in the two lines with a higher chilling tolerance was triggered at a higher leaf water potential and reached higher levels than in the less tolerant lines. We conclude that chilling tolerance in maize is related to the ability for fast and pronounced formation of ABA as a protective agent against chilling injury.     

The expansion of maize root-cap mucilage during hydration. 3. Changes in water potential and water content

Root-cap mucilage from aerial nodal roots of maize has been found to have water potential values of −11 MPa or lower when air dried. The value approaches 0 MPa within 2 min of hydration in distilled water. In this time the expanding gel absorbs only about 0.3% of the water content of fully expanded mucilage. It is concluded that the root-cap mucilage per se has almost no capacity to retain water in the rhizosphere. Any function that it may play in the slowing of root desiccation would be indirect. For example, mucilage might decrease pore size between and within soil aggregates by pulling the particles together in a cycle of nocturnal efflux of water from the root surface, and diumal dyring during transpiration.     

Changes in cellular osmotic potential and mechanical properties of cell walls during light-induced inhibition of cell elongation in maize coleoptiles

The growth rate of maize ( Zea mays L. cv. Cross Bantam T51) coleoptiles in the dark was highest at the basal zone and decreased towards the tip. Growth was strongly inhibited by white fluorescent light (5 W m⁻²), especially in the basal zone of coleoptiles. Light irradiation caused an increase in the values of stress-relaxation parameters, the minimum stress-relaxation time and the relaxation rate and a decrease in the extensibility (strain/stress) of the cell walls at all zones. In addition, during growth, the accumulation of osmotic solutes was strongly inhibited by white light irradiation, resulting in an increased osmotic potential. The influences of white light on the mechanical properties of the cell wall and the osmotic potential of the tissue sap were most prominent in the basal zone. Significant correlations were observed between the increment of coleoptile length and the mechanical properties of the cell walls or the osmotic potential of the tissue sap and osmotic solutes content. Furthermore, light inhibited the outward bending of split coleoptile segments. These facts suggest that white light inhibits elongation of maize coleoptiles by modifying both the mechanical properties of the cell walls and cellular osmotic potential, which control the rate of water uptake.     

Developmental regulation of anoxic stress tolerance in maize

Anoxia associated with flooding stress is detrimental to plant growth and productivity. When maize seedlings 2 to 7 d old were exposed to anoxic stress, 3-d-old seedlings were found to have much lower tolerance than 2-d-old seedlings. Ninety per cent of 2-d-old seedlings survived 72 h of anoxic stress compared with 0% of the 3-d-old seedlings. Since 2-d-old isolated root tips survived anoxic stress better than 3-d-old tips, the anoxic tolerance of 2-d-old seedlings was independent of the translocation of nutrient reserves from the endosperm to the root. The addition of glucose to the medium improved the anoxia tolerance of 2-d-old seedlings by 25% but had no effect on 3-d-old seedlings. Acclimation by pre-cxposure to 4% oxygen and pre-treatment with 100mmol m^?1 abscisic acid (ABA) improved the anoxia tolerance of 3-d-old seedlings by 2- and 4-fold, respectively. However, acclimation and ABA treatment had no effect on 2-d-old seedlings. The results indicate that anoxia tolerance in maize is develop-mentally regulated. The mechanism of anoxia tolerance innate to 2-d-old seedlings was inducible in 3-d-old seedlings by acclimation or treatment with ABA.     

A comparison between 3,5-diiodo-4-hydroxybenzoic acid and 2,3,5-triiodobenzoic acid. I. Effects on growth and gravireaction of maize roots

A comparison between the effects of DIHB and TIBA on growth and gravireaction of 15 mm primary maize ( Zea mays L. cv. LG 11) roots is presented. Intact roots were pretreated in the dark for 1 h with buffered solutions (pH 5.0 or 6.0) containing DIHB (10, 50, 100 μ M ). The plantlets were then maintained either vertically or horizontally in the dark or the light, and growth and gravireaction were recorded using a macrophotographic technique. Pretreatment with DIHB slightly inhibited growth and delayed gravireaction. These effects were most marked with DIHB at 100 μ M and were enhanced when DIHB was applied at pH 5.0. Similar effects were observed in roots pretreated with TIBA, but at a lower concentration (1 μ M ). The similarities between DIHB and TIBA as regards both chemical structure and the inhibition of gravireaction and growth, lead us to suggest that a major mode of action of DIHB, like TIBA, is the inhibition of indol-3yl-acetic acid transport.     

The water status of the roots of soil-grown maize in relation to the maturity of their xylem

The long delayed maturation of the late metaxylem of maize ( Zea mays ) roots imposes a high-resistance barrier between the immature apices and the negative water potential of the leaves. These apices (20+ cm) bear strongly adhering soil sheaths to within 0.5 to 2 cm of the distal end. It was hypothesized that the sheathed immature apices should show less response to transpiration stress than bare regions. Measurements were made of the relative water content (RWC) of the sheathed and bare zones of the axile roots, both at different ages of the plant, and early and late in the day's transpiration. Sheathed roots maintained a steady RWC of about 83% irrespective of age or transpiration. Bare roots had RWCs of about 63% in the morning, but this fell to 55% in the afternoon. The first-order branches on the bare roots in the morning had still lower values of RWC, near 50%. Plots of RWC against water potential were indistinguishable for the three root types. It is concluded that the immature apices are indeed relatively isolated from the fluctuating tensions in the stem xylem, and that these tensions reduce the water content of bare roots and their branches to low values.