Effect of inhibition of abscisic Acid accumulation on the spatial distribution of elongation in the primary root and mesocotyl of maize at low water potentials

Previous work showed that accumulation of endogenous abscisic acid (ABA) acts both to maintain primary root growth and inhibit shoot growth in maize seedlings at low water potentials (ψ_w) (IN Saab, RE Sharp, J Pritchard, GS Voetberg [1990] Plant Physiol 93: 1329-1336). In this study, we have characterized the growth responses of the primary root and mesocotyl of maize (Zea mays L. cv FR27 × FRMo 17) to manipulation of ABA levels at low ψ_w with a high degree of spatial resolution to provide the basis for studies of the mechanism(s) of ABA action. In seedlings growing at low ψ_w and treated with fluridone to inhibit carotenoid (and ABA) biosynthesis, ABA levels were decreased in all locations of the root and mesocotyl growing zones compared with untreated seedlings growing at the same ψ_w. In the root, low ψ_w (−1.6 megapascals) caused a shortening of the growing zone, as reported previously. The fluridone treatment was associated with severe inhibition of root elongation rate, which resulted from further shortening of the growing zone. In the mesocotyl, low ψ_w (−0.3 megapascal) also resulted in a shortened growing zone. In contrast with the primary root, however, fluridone treatment prevented most of the inhibition of elongation and the shortening of the growing zone. Final cell length measurements indicated that the responses of both root and mesocotyl elongation to ABA manipulation at low ψ_w involve large effects on cell expansion. Measurements of the relative changes in root and shoot water contents and dry weights after transplanting to a ψ_w of −0.3 megapascal showed that the maintenance of shoot elongation in fluridone-treated seedlings was not attributable to increased water or seed-reserve availability resulting from inhibition of root growth. The results suggest a developmental gradient in tissue responsiveness to endogenous ABA in both the root and mesocotyl growing zones. In the root, the capacity for ABA to protect cell expansion at low ψ_w appears to decrease with increasing distance from the apex. In the mesocotyl, in contrast, the accumulation of ABA at low ψ_w appears to become increasingly inhibitory to expansion as cells are displaced away from the meristematic region.     

Water Deficit and Abscisic Acid Cause Differential Inhibition of Shoot versus Root Growth in Soybean Seedlings : Analysis of Growth, Sugar Accumulation, and Gene Expression

Roots often continue to elongate while shoot growth is inhibited in plants subjected to low-water potentials. The cause of this differential response to water deficit was investigated. We examined hypocotyl and root growth, polysome status and mRNA populations, and abscisic acid (ABA) content in etiolated soybean (Glycine max [L.] Merr. cv Williams) seedlings whose growth was inhibited by transfer to low-water potential vermiculite or exogenous ABA. Both treatments affected growth and dry weight in a similar fashion. Maximum inhibition of hypocotyl growth occurred when internal ABA levels (modulated by ABA application) reached the endogenous level found in the elongating zone of seedlings grown in water-deficient vermiculite. Conversely, root growth was affected to only a slight extent in low-water potential seedlings and by most ABA treatments (in some, growth was promoted). In every seedling section examined, transfer of seedlings into low-water potential vermiculite caused ABA levels to increase approximately 5- to 10-fold over that found in well-watered seedlings. Changes in soluble sugar content, polysome status, and polysome mRNA translation products seen in low-water potential seedlings did not occur with ABA treatments sufficient to cause significant inhibition of hypocotyl elongation. These data suggest that both variation in endogenous ABA levels, and differing sensitivity to ABA in hypocotyls and roots can modulate root/shoot growth ratios. However, exogenous ABA did not induce changes in sugar accumulation, polysome status, and mRNA populations seen after transfer into low-water potential vermiculite.     

Drought-induced increases in abscisic Acid levels in the root apex of sunflower

Abscisic acid (ABA) levels in 3-mm apical root segments of slowly droughted sunflower plants (Helianthus annuus L. cv Russian Giant) were analyzed as the methyl ester by selected ion monitoring gas chromatography-mass spectrometry using characteristic ions. An internal standard, hexadeuterated ABA (d6ABA) was used for quantitative analysis. Sunflower seedlings, grown in aeroponic chambers, were slowly droughted over a 7-day period. Drought stress increased ABA levels in the root tips at 24, 72, and 168 hour sample times. Control plants had 57 to 106 nanograms per gram ABA dry weight in the root tips (leaf water potential, −0.35 to −0.42 megapascals). The greatest increase in ABA, about 20-fold, was found after 72 hours of drought (leaf water potential, −1.34 to −1.47 megapascals). Levels of ABA also increased (about 7− to 54-fold) in 3-mm apical root segments which were excised and then allowed to dessicate for 1 hour at room temperature.     

Growth of the maize primary root at low water potentials : I. Spatial distribution of expansive growth

Seedlings of maize (Zea mays L. cv WF9 × Mo 17) were grown in vermiculite at various water potentials. The primary root continued slow rates of elongation at water potentials which completely inhibited shoot growth. To gain an increased understanding of the root growth response, we examined the spatial distribution of growth at various water potentials. Time lapse photography of the growth of marked roots revealed that inhibition of root elongation at low water potentials was not explained by a proportional decrease in growth along the length of the growing zone. Instead, longitudinal growth was insensitive to water potentials as low as − 1.6 megapascal close to the root apex, but was inhibited increasingly in more basal locations such that the length of the growing zone decreased progressively as the water potential decreased. Cessation of longitudinal growth occurred in tissue of approximately the same age regardless of spatial location or water status, however. Roots growing at low water potentials were also thinner, and analysis revealed that radial growth rates were decreased throughout the elongation zone, resulting in greatly decreased rates of volume expansion.     

Translatable RNA Populations Associated with Maintenance of Primary Root Elongation and Inhibition of Mesocotyl Elongation by Abscisic Acid in Maize Seedlings at Low Water Potentials

Previous work indicated that accumulation of abscisic acid (ABA) acts differentially to maintain elongation of the primary root and inhibit elongation of the mesocotyl of maize (Zea mays L.) seedlings at low water potentials ([psi]w). Subsequent results indicated specific locations in the elongation zones where elongation is maintained, inhibited, or unaffected by endogenous ABA at low [psi]w. This information was utilized in this study to identify in vitro translation products of RNA associated with the maintenance or inhibition of elongation in the primary root and mesocotyl, respectively, by endogenous ABA at low [psi]w. The results distinguished products associated specifically with the elongation responses from those nonspecifically associated with ABA accumulation or low [psi]w, as well as normal cell development and maturation. In the primary root, the maintenance of elongation at low [psi]w by ABA was associated with the maintenance of expression of three products that were also expressed during elongation at high [psi]w, the expression of a novel product, and the suppression of two products. In the mesocotyl, the inhibition of elongation by ABA after transplanting to low [psi]w was associated with the induction of a novel translation product. However, the induction of this product, as well as accumulation of ABA and inhibition of elongation, occurred without a decline in tissue water content. The results demonstrate the necessity of examining the association of gene expression with elongation responses to low [psi]w with a high degree of spatial resolution.     

Abscisic acid accumulation maintains maize primary root elongation at low water potentials by restricting ethylene production

Previous work showed that primary root elongation in maize (Zea mays L.) seedlings at low water potentials (psi(w)) requires the accumulation of abscisic acid (ABA) (R.E. Sharp, Y. Wu, G.S. Voetberg, I.N. Saab, M.E. LeNoble [1994] J Exp Bot 45: 1743-1751). The objective of the present study was to determine whether the inhibition of elongation in ABA-deficient roots is attributable to ethylene. At a psi(w) of -1.6 MPa, inhibition of root elongation in dark-grown seedlings treated with fluridone to impose ABA deficiency was largely prevented with two inhibitors of ethylene synthesis (aminooxyacetic acid and aminoethoxyvinylglycine) and one inhibitor of ethylene action (silver thiosulfate). The fluridone treatment caused an increase in the rate of ethylene evolution from intact seedlings. This effect was completely prevented with aminooxyacetic acid and also when ABA was supplied at a concentration that restored the ABA content of the root elongation zone and the root elongation rate. Consistent results were obtained when ABA deficiency was imposed using the vp5 mutant. Both fluridone-treated and vp5 roots exhibited additional morphological symptoms of excess ethylene. The results demonstrate that an important role of ABA accumulation in the maintenance of root elongation at low psi(w) is to restrict ethylene production.     

Abscisic Acid accumulates at positive turgor potential in excised soybean seedling growing zones

Abscisic acid (ABA) accumulated in soybean (Glycine max [L.] Merr. cv Williams) hypocotyl elongating regions when seedlings were transferred to low water potential vermiculite (Ψ = −0.3 megapascals) even though positive turgor is retained in this tissue. Accumulation of ABA in growing zones could occur from de novo biosynthesis within this tissue or transport from adjacent nongrowing zones. Both growing and nongrowing hypocotyl and root tissues accumulated significant levels of ABA when excised and dehydrated to reduce turgor. Surprisingly, excised growing zones (which experienced no water loss) also accumulated ABA when incubated in darkness for 4 hours at 100% relative humidity and 29°C. Induction of ABA accumulation in the excised elongating region of the hypocotyl was not caused by disruption of root pressure or wounding. While excision of hypocotyl elongating regions induced ABA accumulation, no change in either extensin or p33 mRNA levels was observed. Accumulation of extensin or p33 mRNA required more severe wounding. This suggests that ABA is not involved in the response of these genes in wounded tissue and that wound signals are not causing ABA accumulation in excised tissue. Accumulation of ABA in excised elongating regions was correlated with growth inhibition and a decline in turgor to the yield threshold (Ψ;_p = 0.37 megapascals; R Matyssek, S Maruyama, JS Boyer [1988] Plant Physiol 86: 1163-1167). Inhibiting hypocotyl growth by transferring seedlings to lower temperatures or light did not cause ABA accumulation. We conclude that induction of ABA accumulation in growing zones is more sensitive to changes in turgor than the induction which occurs in mature tissues.     

Spatial distribution of turgor and root growth at low water potentials

Spatial distributions of turgor and longitudinal growth were compared in primary roots of maize (Zea mays L. cv FR27 × FRMo 17) growing in vermiculite at high (−0.02 megapascals) or low (−1.6 megapascals) water potential. Turgor was measured directly using a pressure probe in cells of the cortex and stele. At low water potential, turgor was greatly decreased in both tissues throughout the elongation zone. Despite this, longitudinal growth in the apical 2 millimeters was the same in the two treatments, as reported previously. These results indicate that the low water potential treatment caused large changes in cell wall yielding properties that contributed to the maintenance of root elongation. Further from the apex, longitudinal growth was inhibited at low water potential despite only slightly lower turgor than in the apical region. Therefore, the ability to adjust cell wall properties in response to low water potential may decrease with cell development.     

Water Deficit Rapidly Stimulates the Activity of a Protein Kinase in the Elongation Zone of the Maize Primary Root

The mechanisms by which plants detect water deficit and transduce that signal into adaptive responses is unknown. In maize (Zea mays L.) seedlings, primary roots adapt to low water potentials such that substantial rates of elongation continue when shoot growth is completely inhibited. In this study, in-gel protein kinase assays were used to determine whether protein kinases in the elongation zone of the primary root undergo activation or inactivation in response to water deficit. Multiple differences were detected in the phosphoprotein content of root tips of water-stressed compared with well-watered seedlings. Protein kinase assays identified water-deficit-activated protein kinases, including a 45-kD, Ca2+-independent serine/threonine protein kinase. Water-deficit activation of this kinase occurred within 30 min after transplanting seedlings to conditions of low water potential and was localized to the elongation zone, was independent of ABA accumulation, and was unaffected by cycloheximide-mediated inhibition of protein translation. These results provide evidence that the 45-kD protein kinase acts at an early step in the response of maize primary roots to water deficit and is possibly involved in regulating the adaptation of root growth to low water potential.     

Stomatal response to abscisic Acid is a function of current plant water status

We investigated, under laboratory and field conditions, the possibility that increasing abscisic acid (ABA) concentrations and decreasing water potentials can interact in their effects on stomata. One experiment was carried out with epidermal pieces of Commelina communis incubated in media with a variety of ABA and polyethylene glycol concentrations. In the media without ABA, incubation in solutions with water potentials between −0.3 and −1.5 megapascals had no significant effect on stomatal aperture. Conversely, the sensitivity of stomatal aperture to ABA was trebled in solutions at −1.5 megapascals compared with sensitivity at −0.3 megapascals. The effect of the change in sensitivity was more important than the absolute effect of ABA at the highest water potential. In a field experiment, sensitivity of maize stomatal conductance to the concentration of ABA in the xylem sap varied strongly with the time of the day. We consider that the most likely explanation for this is the influence of a change in leaf or epidermal water potential that accompanies an increase in irradiance and saturation deficit as the day progresses. These observations suggest that epidermal water relations may act as a modulator of the responses of stomata to ABA. We argue that such changes must be taken into account in studies or modeling of plant responses to drought stress.     

Reduced de-etiolation of hypocotyl growth in a tomato mutant is associated with hypersensitivity to, and high endogenous levels of, abscisic acid.

A recessive single gene mutant, 7B-1, in tomato was originally selected for its photoperiod-dependent male sterility. The 7B-1 mutant also has some pleiotropic effects including reduced light-induced inhibition, i.e. de-etiolation, of the hypocotyl in long days (LD), increased seed size and weight, and reduced transpiration rate. These traits led us to investigate the sensitivity of 7B-1 to exogenous hormones and the interaction of these responses with daylength. In LD, but not in short days (SD), 7B-1 was more sensitive than wild-type (WT) to exogenous abscisic acid (ABA) for inhibition of seed germination, root elongation and transpiration rate. 7B-1 mutant also exhibited reduced responses to exogenous gibberellin (GA(3)) for hypocotyl elongation, and to inhibitors of GA biosynthesis for seed germination and root and hypocotyl elongation. 7B-1 hypocotyls contained a higher level of endogenous ABA than WT in both photoperiods, although ABA levels were higher in LD than in SD. In contrast, growth-active GAs, i.e. GA(1), GA(3) and GA(4), and IAA were low in the mutant hypocotyls. The 7B-1 mutant appears to be an ABA-overproducer, and the photoperiod-regulated ABA levels may be responsible for the hypersensitivity of the mutant to exogenous ABA.     

Influence of soil water deficits on root growth of cotton seedlings

Summary Cotton (Gossypium hirsutum L. cv. H14) seedlings were raised in soil of differing soil water content in specially designed pots in which the roots had access to freely available water and nutrients located 2.5 cm below the base of the soil core. The time for root emergence from the soil core and the rate of root growth were measured daily from sowing to harvest. The root and shoot dry weight and leaf water potential were measured at the final harvest 16 days after sowing. As soil water content decreased, the root emerged from the soil earlier and the initial rate of root elongation was faster. In spite of the availability of freely available water, the plants in the soil at low water contents had significantly lower leaf water potentials than those in soil at high water contents. The root: shoot ratio increased as the soil water content decreased. This arose from an absolute increase in root weight, with shoot weight not being significantly affected.     

Water relations of the tos1 tomato mutant at contrasting evaporative demand

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

An assessment of the role of ethylene in mediating lettuce (Lactuca sativa) root growth at high temperatures

Growth of temperate lettuce (Lactuca sativa) plants aeroponically in tropical greenhouses under ambient root-zone temperatures (A-RZTs) exposes roots to temperatures of up to 40 degrees C during the middle of the day, and severely limits root and shoot growth. The role of ethylene in inhibiting growth was investigated with just-germinated (24-h-old) seedlings in vitro, and 10-d-old plants grown aeroponically. Compared with seedlings maintained at 20 degrees C, root elongation in vitro was inhibited by 39% and root diameter increased by 25% under a temperature regime (38 degrees C/24 degrees C for 7 h/17 h) that simulated A-RZT in the greenhouse. The effects on root elongation were partially alleviated by supplying the ethylene biosynthesis inhibitors aminooxyacetic acid (100-500 microM) or aminoisobutyric acid (5-100 microM) to the seedlings. Application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid to seedlings grown at 20 degrees C mimicked the high temperature effects on root elongation (1 microM) and root diameter (1 mM). Compared with plants grown at a constant 20 degrees C root-zone temperature, A-RZT plants showed decreased stomatal conductance, leaf relative water content, photosynthetic CO(2) assimilation, shoot and root biomass, total root length, the number of root tips, and root surface area, but increased average root diameter. Addition of 10 microM ACC to the nutrient solution of plants grown at a constant 20 degrees C root-zone temperature mimicked the effects of A-RZT on these parameters but did not influence relative water content. Addition of 30 microM aminoisobutyric acid or 100 microM aminooxyacetic acid to the nutrient solution of A-RZT plants increased stomatal conductance and relative water content and decreased average root diameter, but had no effect on other root parameters or root and shoot biomass or photosynthetic CO(2) assimilation. Although ethylene is important in regulating root morphology and elongation at A-RZT, the failure of ethylene biosynthesis inhibitors to influence shoot carbon gain limits their use in ameliorating the growth inhibition induced by A-RZT.     

Endogenous ABA maintains shoot growth in tomato independently of effects on plant water balance: evidence for an interaction with ethylene

To examine whether the reduced shoot growth of abscisic acid (ABA)-deficient mutants of tomato is independent of effects on plant water balance, flacca and notabilis were grown under controlled-humidity conditions so that their leaf water potentials were equal to or higher than those of well-watered wild-type plants throughout development. Most parameters of shoot growth remained markedly impaired and root growth was also greatly reduced. Additional experiments with flacca showed that shoot growth substantially recovered when wild-type levels of ABA were restored by treatment with exogenous ABA, even though improvement in leaf water potential was prevented. The ability of applied ABA to increase growth was greatest for leaf expansion, which was restored by 75%. The ethylene evolution rate of growing leaves was doubled in flacca compared to the wild type and treatment with silver thiosulphate to inhibit ethylene action partially restored shoot growth. The results demonstrate that normal levels of endogenous ABA are required to maintain shoot development, particularly leaf expansion, in well-watered tomato plants, independently of effects on plant water balance. The impairment of shoot growth caused by ABA deficiency is at least partly attributable to ethylene.     

On the role of abscisic Acid and gibberellin in the regulation of growth in rice

Submergence induces rapid elongation of rice coleoptiles (Oryza sativa L.) and of deepwater rice internodes. This adaptive feature helps rice to grow out of the water and to survive flooding. Earlier, we found that the growth response of submerged deepwater rice plants is mediated by ethylene and gibberellin (GA). Ethylene promotes growth, at least in part, by increasing the responsiveness of the internodal tissue to GA. In the present work, we examined the possibility that increased responsiveness to GA was based on a reduction in endogenous abscisic acid (ABA) levels. Submergence and treatment with ethylene led, within 3 hours, to a 75% reduction in the level of ABA in the intercalary meristem and the growing zone of deepwater rice internodes. The level of GA₁ increased fourfold during the same time period. An interaction between GA and ABA could also be shown by application of the hormones. ABA inhibited growth of submerged internodes, and GA counteracted this inhibition. Our results indicate that the growth rate of deepwater rice internodes is determined by the ratio of an endogenous growth promoter (GA) and a growth inhibitor (ABA). We also investigated whether ABA is involved in regulating the growth of rice coleoptiles. Rice seedlings were grown on solutions containing fluridone, an inhibitor of carotenoid and, indirectly, of ABA biosynthesis. Treatment with fluridone reduced the level of ABA in coleoptiles and first leaves by more than 75% and promoted coleoptile growth by more than 60%. Little or no enhancement of growth by fluridone was observed in barley, oat, or wheat. The involvement of ABA in determining the growth rate of rice coleoptiles and deepwater rice internodes may be related to the semiaquatic growth habit of this plant.     

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.) : III. Sensitivity of Germination to Water Potential and Abscisic Acid during Development

Muskmelon (Cucumis melo L.) seeds are germinable 15 to 20 days before fruit maturity and are held at relatively high water content within the fruit, yet little precocious germination is observed. To investigate two possible factors preventing precocious germination, the inhibitory effects of abscisic acid and osmoticum on muskmelon seed germination were determined throughout development. Seeds were harvested at 5-day intervals from 30 to 65 days after anthesis (DAA) and incubated either fresh or after drying on factorial combinations of 0, 1, 3.3, 10, or 33 micromolar abscisic acid (ABA) and 0, −0.2, −0.4, −0.6, or −0.8 megapascals polyethylene glycol 8000 solutions at 30°C. Radicle emergence was scored at 12-hour intervals for 10 days. In the absence of ABA, the water potential (Ψ) required to inhibit fresh seed germination by 50% decreased from −0.3 to −0.8 megapascals between 30 and 60 DAA. The Ψ inside developing fruits was from 0.4 to 1.4 megapascals lower than that required for germination at all stages of development, indicating that the fruit Ψ is sufficiently low to prevent precocious germination. At 0 megapascal, the ABA concentration required to inhibit germination by 50% was approximately 10 micromolar up to 50 DAA and increased to >33 micromolar thereafter. Dehydration improved subsequent germination of immature seeds in ABA or low Ψ. There was a linear additive interaction between ABA and Ψ such that 10 micromolar ABA or −0.5 megapascal osmotic potential resulted in equivalent, and additive, reductions in germination rate and percentage of mature seeds. Abscisic acid had no effect on embryo solute potential or water content, but increased the apparent minimum turgor required for germination. ABA and osmoticum appear to influence germination rates and percentages by reducing the embryo growth potential (turgor in excess of a minimum threshold turgor) but via different mechanisms. Abscisic acid apparently increases the minimum turgor threshold, while low Ψ reduces turgor by reducing seed water content.     

Measurement of the water potential of stored potato tubers

A method of measuring the water potential of stored potato tubers (Solanum tuberosum L.) was needed to investigate the relationship of bacterial soft rot in tubers to water potential. Pressure chamber measurements, while useful for tubers with functional stolons, cannot be made on stored tubers. Measurements could be made on excised tissue pieces in a hygrometer chamber and with hygrometers implanted into tubers. We report here our evaluation of these hygrometric methods using a comparison with the pressure chamber on tubers harvested with stolons intact.

In tubers of high water potential, measurements on excised tissue were as much as 0.5 megapascals lower than the pressure chamber, probably due to turgor-driven expansion of the sample when freed from constraints imposed by surrounding tissue. Good agreement (±0.05 megapascals) was found between the implanted hygrometer and the pressure chamber at potentials higher than −0.5 megapascals. At lower water potentials, both hygrometer measurements were higher than the pressure chamber. Respirational heating of the tissue contributed to the increase in the excised tissue samples, but not with the implanted hygrometers because of the hygrometer design. The osmotic pressure balanced the pressure chamber measurement of potential at −0.7 megapascals, but was too small to do so at lower potentials. At most, 25% of this discrepancy can be accounted for by dilution by apoplastic water. We believe that the pressure chamber measurement is too low at low water potentials and that the error is associated with air bubbles in the xylem. At low potentials air emerged from xylem vessels along with sap, and fewer xylem emitted sap as potentials decreased.

     

Electrophysiological responses of maize roots to low water potentials: relationship to growth and ABA accumulation

The maintenance of root elongation is an important adaptive response to low water potentials (psi(w)), but little is known about its regulation. An important component may be changes in root cell electrophysiology, which both signal and maintain growth maintenance processes. As a first test of this hypothesis, membrane potentials (E(m)) were measured within the cell elongation zone of maize (Zea mays L.) primary roots. Seedlings were grown in oxygenated solution culture, and low psi(w) was imposed by the gradual addition of polyethylene glycol. Cells hyperpolarized approximately 25 mV in response to low psi(w), and after 48 h resting potentials remained significantly hyperpolarized at psi(w) lower than -0.3 MPa compared with roots at high psi(w). Inhibitor experiments showed that the hyperpolarization was dependent on plasma membrane H(+)-ATPase activity. Previous work showed that accumulation of abscisic acid (ABA) is required for the maintenance of maize primary root elongation at low psi(w). To determine if the mechanism of action of ABA involves changes in root electrophysiology, E(m) measurements were made during long-term exposure to low psi(w). Steady-state resting E(m) were measured in regions in which maintenance of cell elongation was dependent on ABA accumulation (2-3 mm from the apex), or in which elongation was inhibited regardless of ABA status (6-8 mm from the apex). E(m) was substantially more negative in ABA-deficient roots specifically in the 2-3 mm region. The results suggest that set-points for ion homeostasis shifted in association with the maintenance of root cell elongation at low psi(w), and that ABA accumulation plays a role in regulating the ion transport processes involved in this response.     

Endosperm cell division in maize kernels cultured at three levels of water potential

The influence of osmoticum treatments on early kernel development of maize (Zea mays L.) was studied using an in vitro culture method. Kernels with subtending cob sections were placed in culture at 5 days after pollination. Sucrose (0.29, 0.44, or 0.58 molar) and sorbitol (0, 0.15, or 0.29 molar) were used to obtain six media with water potentials of −1.1, −1.6, or −2.0 megapascals. Kernel water potential declined in correspondence with the water potential of the medium; however, fresh weight growth was not significantly inhibited from 5 to 12 days after pollination. In stress treatments with media water potentials of −1.6 or −2.0 megapascals, endosperm tissue accumulated water and solutes from 10 and 12 days after pollination at a rate similar to or greater than that of the control (−1.1 megapascals). In contrast, endosperm cell division was inhibited in all treatments relative to control. At 10 days after pollination, endosperm sucrose concentration was greater in two of the −2.0 megapascal treatments with 0.44 or 0.58 molar media sucrose compared to control kernels cultured in 0.29 molar sucrose at −1.1 megapascals. Significant increases in abscisic acid content per gram of fresh weight were detected in two −2.0 megapascal treatments (0.29 molar sucrose plus 0.29 molar sorbitol and 0.58 molar sucrose) at 10 days after pollination. We conclude that in cultured maize kernels, endosperm cell division was more responsive than fresh weight accumulation to low water potential treatments. Data were consistent with mechanisms involving abscisic acid or lowered tissue water potential, or an interaction of the two factors.