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PubMed2001

Hydraulic conductance and mercury-sensitive water transport for roots of Opuntia acanthocarpa in relation to soil drying and rewetting.

Drought-induced changes in root hydraulic conductance (LP) and mercury-sensitive water transport were examined for distal (immature) and mid-root (mature) regions of Opuntia acanthocarpa. During 45 d of soil drying, LP decreased by about 67% for distal and mid-root regions. After 8 d in rewetted soil, LP recovered to 60% of its initial value for both regions. Axial xylem hydraulic conductivity was only a minor limiter of LP. Under wet conditions, HgCl2 (50 microM), which is known to block membrane water-transport channels (aquaporins), decreased LP and the radial hydraulic conductance for the stele (L(R, S)) of the distal root region by 32% and 41%, respectively; both LP and L(R, S) recovered fully after transfer to 2-mercaptoethanol (10 mM). In contrast, HgCl2 did not inhibit LP of the mid-root region under wet conditions, although it reduced L(R, S) by 41%. Under dry conditions, neither LP nor L(R, S) of the two root regions was inhibited by HgCl2. After 8 d of rewetting, HgCl2 decreased LP and L(R, S) of the distal region by 23% and 32%, respectively, but LP and L(R, S) of the mid-root region were unaltered. Changes in putative aquaporin activity accounted for about 38% of the reduction in LP in drying soil and for 61% of its recovery for the distal region 8 d after rewetting. In the stele, changes in aquaporin activity accounted for about 74% of the variable L(R, S) during drought and after rewetting. Thus, aquaporins are important for regulating water movement for roots of O. acanthocarpa.

Aquaporins↗
PubMed2001

Ethylene induces antifreeze activity in winter rye leaves.

Antifreeze activity is induced by cold temperatures in winter rye (Secale cereale) leaves. The activity arises from six antifreeze proteins that accumulate in the apoplast of winter rye leaves during cold acclimation. The individual antifreeze proteins are similar to pathogenesis-related proteins, including glucanases, chitinases, and thaumatin-like proteins. The objective of this study was to study the regulation of antifreeze activity in response to ethylene and salicyclic acid, which are known regulators of pathogenesis-related proteins induced by pathogens. Nonacclimated plants treated with salicylic acid accumulated apoplastic proteins with no antifreeze activity. In contrast, when nonacclimated plants were exposed to ethylene, both antifreeze activity and the concentration of apoplastic protein increased in rye leaves. Immunoblotting revealed that six of the seven accumulated apoplastic proteins consisted of two glucanases, two chitinases, and two thaumatin-like proteins. The ethylene-releasing agent ethephon and the ethylene precursor 1-aminocyclopropane-1-carboxylate also induced high levels of antifreeze activity at 20 degrees C, and this effect could be blocked by the ethylene inhibitor AgNO(3). When intact rye plants were exposed to 5 degrees C, endogenous ethylene production and antifreeze activity were detected within 12 and 48 h of exposure to cold, respectively. Rye plants exposed to drought produced both ethylene and antifreeze activity within 24 h. We conclude that ethylene is involved in regulating antifreeze activity in winter rye in response to cold and drought.

Antifreeze Proteins↗
PubMed1971

Detrimental effect of rust infection on the water relations of bean.

Bean plants (Phaseolus vulgaris L.) infected with the rust Uromyces phaseoli became unusually susceptible to drought as sporulation occurred. Under the conditions used (1,300 ft-c, 27 C, and 55% relative humidity) such plants wilted at soil water potentials greater than -1 bar, whereas healthy plants did not wilt until the soil water potential fell below -3.4 bars. Determinations of leaf water and osmotic potentials showed that an alteration in leaf osmotic potential was not responsible for the wilting of diseased plants. When diffusive resistance was measured as a function of decreasing leaf water content, the resistance of healthy leaves increased to 50 sec cm(-1) by the time relative water content decreased to 70%, whereas the resistance of diseased leaves remained less than 8 sec cm(-1) down to 50% relative water content. Apparently, water vapor loss through cuticle damaged by the sporulation process, together with the reduction in root to shoot ratio which occurs in diseased plants, upset the water economy of the diseased plant under mild drought conditions.

Journal Article↗
PubMed1975

Plant Desiccation and Protein Synthesis: II. On the Relationship between Endogenous Adenosine Triphosphate Levels and Protein-synthesizing Capacity.

Rehydration of Tortula ruralis in 2,4-dinitrophenol inhibits protein synthesis, polysome formation, and ATP production. Polysomes are conserved intact and are active in vitro in hydrated Tortula placed in this chemical, although in vivo protein synthesis is inhibited. Hydrated moss placed under nitrogen in the dark shows a reduced capacity for ATP and protein synthesis, but polysomes are conserved. During anaerobiosis in light, ATP and protein synthesis are unaffected. Rehydration of slow-dried Tortula in nitrogen in the dark results in reduced in vivo protein synthesis, but not polysome formation; this reduction is much less in the light. Slow-dried moss, but not fast-dried, has a greatly reduced ATP content in the dry state, but this rapidly returns to normal levels on rehydration. The prolonged burst in respiration observed previously on rehydration of Tortula is not paralleled by ATP accumulation. Changes in energy charge in all treatments tested follow the changes in ATP. The aquatic moss, Hygrohypnum luridum, which is intolerant to drought, loses ATP during fast drying and this is not replenished on subsequent rehydration.We consider that the relationship between levels of ATP and protein synthesis is more meaningful during rehydration of mosses (the time when repair to desiccation-induced cellular damage can occur) than during desiccation, and that drought-induced cessation of protein synthesis may not be mediated directly through ATP availability.

Journal Article↗
PubMed1977

Stomatal and nonstomatal regulation of water use in cotton, corn, and sorghum.

Stomata of corn (Zea mays L.) and sorghum (Sorghum bicolor L.) responded to changes in leaf water potential during the vegetative growth phase. During reproductive growth, leaf resistances were minimal and stomata were no longer sensitive to bulk leaf water status even when leaf water potentials approached -27 bars. Stomata of corn, cotton (Gossypium hirsutum L.), and sorghum appear to respond to changes in the humidity deficit between the leaf and air and in this manner, regulated transpirational flux to some degree. Distinct differences in water transport efficiency were observed in the three species. Under nonlimiting soil water conditions, sorghum exhibited the greatest efficiency of water transport while under limiting soil moisture conditions, cotton appeared most efficient. Corn was the least efficient with respect to nonstomatal regulation of water use. Differences in drought tolerance among the three species are partially dependent on stomatal regulation of water loss, but efficiency of the water transport system may be more related to drought adaptation. This is particularly important since stomata of all three species did not respond to bulk leaf water status during a large portion of the growing season.

Journal Article↗
PubMed1979

Water Relations of Cotton Plants under Nitrogen Deficiency: I. Dependence upon Leaf Structure.

Cotton plants (Gossypium hirsutum L.) grown on deficient levels of N exhibited many of the characteristics associated with drought resistance. In N-deficient plants, leaf areas and leaf epidermal cells were smaller than at the same nodes in high-N plants. N-deficient leaves lost only about half as much water per unit change in water potential as did high-N leaves. In addition, they maintained a greater relative water content than high-N leaves at any given potential. Osmotic potentials (determined from pressure-volume curves) were slightly lower in N-deficient leaves. This difference in solute concentration was not from organic acids, which were almost unchanged. Sugar concentrations could account for only about 25% of the difference.Leaves of N-deficient plants contained considerably more dry matter per unit moisture. Most of this difference in dry weight was in the crude cell wall fraction. The pressure-volume curves and other indirect evidence strongly suggested that cell walls of N-deficient leaves were substantially more rigid than cell walls of high-N leaves. The effects of N deficiency on cell wall properties mimic the changes which occur during drought adaptation.

Journal Article↗
PubMed1981

Growth, water content, and solute accumulation of two tobacco cell lines cultured on sodium chloride, dextran, and polyethylene glycol.

Simulated drought tolerance was compared for an NaCl-adapted and a nonadapted cell line of tobacco (Nicotiana tabacum var. Samsum) to determine the relationship of salt and drought tolerances. The osmotic adjustment and growth of these two lines was followed when cultured on solid media which contained isosmotic concentrations of NaCl, KCl, polyethylene glycol (PEG) or dextran. One line was adapted to growth on 130 millimolar NaCl, but the other was not.The growth of NaCl-adapted and nonadapted cell lines was equally inhibited (61 per cent of control) by 130 millimolar NaCl. Growth inhibition was greater on PEG or dextran than on NaCl. Growth ceased on the second passage of dextran for the nonadapted cells, while the NaCl-adapted cells grew slowly for four passages on dextran. Water contents for both cell lines were 95 per cent on NaCl or KCl and 70 to 88 per cent on PEG 1540 or 4000 or dextran after the second passage on these media.On dextran or PEG 4000, 46 to 89 per cent of the cellular osmotic potential was produced by the solutes initially present in the medium. Similarly, on NaCl, almost 100 per cent was attributable to solutes in the medium. It was concluded that cells grown on the nonpenetrating solutes had a more negative osmotic potential than those grown in the absence of added solute due to partial dehydration, greater uptake of external ions, and possibly the production of unidentified osmotica. Adjustment to growth on penetrating solutes may have enabled the adapted line to overcome the osmotic stress produced by nonpenetrating dextran.

Journal Article↗
PubMed1982

Nodule and Leaf Nitrate Reductases and Nitrogen Fixation in Medicago sativa L. under Water Stress.

The effect of water stress on patterns of nitrate reductase activity in the leaves and nodules and on nitrogen fixation were investigated in Medicago sativa L. plants watered 1 week before drought with or without NO(3) (-). Nitrogen fixation was decreased by water stress and also inhibited strongly by the presence of NO(3) (-). During drought, leaf nitrate reductase activity (NRA) decreased significantly particularly in plants watered with NO(3) (-), while with rewatering, leaf NRA recovery was quite important especially in the NO(3) (-)-watered plants. As water stress progressed, the nodular NRA increased both in plants watered with NO(3) (-) and in those without NO(3) (-) contrary to the behavior of the leaves. Beyond -15.10(5) pascal, nodular NRA began to decrease in plants watered with NO(3) (-). This phenomenon was not observed in nodules of plants given water only.Upon rewatering, it was observed that in plants watered with NO(3) (-) the nodular NRA increased again, while in plants watered but not given NO(3) (-), such activity began to decrease. Nitrogen fixation increased only in plants without NO(3) (-).

Journal Article↗
PubMed1983

C(3) Photosynthesis and Crassulacean Acid Metabolism in a Kansas Rock Outcrop Succulent, Talinum calycinum Engelm. (Portulacaceae).

The potential for Crassulacean acid metabolism (CAM) was investigated in the sandstone outcrop succulent Talinum calycinum in central Kansas. Field studies revealed CAM-like diurnal acid fluctuations in these plants. These fluctuations persisted under all moisture and temperature regimes in the laboratory. Despite this CAM-like acid metabolism, simultaneous gravimetric determinations of day- and nighttime transpiration rates indicated the presence of a C(3) gas exchange pattern. Subsequent analyses of diurnal CO(2) and H(2)O exchange patterns under well-watered conditions and after 3, 5, and 7 days of drought confirmed these findings, though low rates of nocturnal CO(2) uptake were observed on the fifth night after continuous drought. Finally, the delta(13)C/(12)C value of this succulent, -27.8 per thousand, emphasizes the insignificance of any nocturnal CO(2) uptake in the lifelong accumulation of carbon in this species. Thus, it is proposed that T. calycinum is a C(3) plant with some CAM characteristics, including the ability to re-fix respiratory CO(2) at night under all moisture regimes, potentially resulting in a conservation of carbon, and occasionally to fix atmospheric CO(2) at night. These findings may prove to be common among rock outcrop succulents.

Journal Article↗
PubMed1986

Relationship of Xylem Embolism to Xylem Pressure Potential, Stomatal Closure, and Shoot Morphology in the Palm Rhapis excelsa.

Xylem failure via gas embolism (cavitation) induced by water stress was investigated in the palm Rhapis excelsa (Thumb.) Henry. Xylem embolism in excised stems and petioles was detected using measurements of xylem flow resistance: a decrease in resistance after the removal of flow-impeding embolisms by a pressure treatment indicated their previous presence in the axis. Results supported the validity of the method because increased resistance in an axis corresponded with: (a) induction of embolism by dehydration, (b) increased numbers of cavitations as detected by acoustic means, (c) presence of bubbles in xylem vessels. The method was used to determine how Rhapis accommodates embolism; results suggested four ways. (a) Embolism was relatively rare because pressure potentials reach the embolism-inducing value of about -2.90 megapascals only during prolonged drought. (b) When embolism did occur in nature, it was confined to the relatively expendable leaf xylem; the stem xylem, which is critical for shoot survival, remained fully functional. (c) Even during prolonged drought, the extent of embolism is limited by complete stomatal closure, which occurred at the xylem pressure potential of -3.20 +/- 0.18 megapascals. (d) Embolism is potentially reversible during prolonged rains, since embolisms dissolved within 5 h at a pressure potential of 0.00 megapascals (atmospheric), and xylem sap can approach this pressure during rain.

Journal Article↗
PubMed1986

Physiological Changes in Cultured Sorghum Cells in Response to Induced Water Stress : II. Soluble Carbohydrates and Organic Acids.

Eight cultivars Sorghum bicolor (L.) Moench were grown as callus cultures under induced, prolonged water stress (8 weeks), with polyethylene glycol in the medium. Concentrations of soluble carbohydrates and organic acids in callus were measured at the end of the growth period to determine differences in response to prolonged water stress. Sucrose, glucose, fructose, and malate were the predominant solutes detected in all callus at all water potentials. All cultivars had high levels of solutes in the absence of water stress and low levels in the presence of prolonged water stress. However, at low water potentials, low levels of solutes were observed in drought-tolerant cultivar callus and high solute levels were observed in drought-susceptible cultivar callus. Estimated sucrose concentrations were significantly higher in water-stressed, susceptible cultivar callus. Large solute concentrations in susceptible cultivar callus were attributed to osmotic adjustment and/or reduced growth during water stress.

Journal Article↗
PubMed1987

Alteration of Cell-Wall Water Content and Elasticity in Douglas-Fir during Periods of Water Deficit.

Three populations of seedlings of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were subjected to recurrent drought cycles. Pressure-volume curves were developed to derive the osmotic and pressure potentials as functions of the symplastic water over a wide range of relative water content. The drought cycles induced large reversible changes in the relative water content of the apoplasm and in the bulk modulus of elasticity. These responses have not been reported previously in any species of higher plant. Our data suggest that turgor regulation in Douglas-fir is mediated by structural and biochemical factors associated with the cell wall, rather than by osmotic adjustment.

Journal Article↗
PubMed1987

Plant Morphological and Biochemical Responses to Field Water Deficits : II. Responses of Leaf Glycerolipid Composition in Cotton.

The effects of water deficits on leaf glycerolipid composition were analyzed in two photoperiodic strains of field grown cotton (Gossypium kirsutum L.) that differ in sensitivity to drought. Leaves from plants grown under dryland conditions exhibited increased dry weight and specific leaf weight. The average midday leaf water potential in the dryland treatment decreased to -1.9 and -2.4 megapascals, respectively, for the T25 and T185 genotypes. Total leaf lipid content of plants exposed to dryland conditions was 5.9 and 7.5% of leaf dry weight for strain T25 and T185, respectively. The difference in leaf lipid content between these genotypes was caused by water deficits and was attributed to loss of both phospholipids and glycolipids in strain T25. There was no apparent loss of phospholipids due to water deficits in the T185 genotype; however, a significant loss of glycolipids was partially compensated by a 2-fold increase in triacylglycerol. No change in triacylglycerol was found between treatments in T25 leaves. Water deficit caused a significant decline in the relative degree of acylunsaturation in phospholipids and glycolipids from both genotypes; however, the double bond index for triacylglycerol increased in both genotypes. It is believed that the observed responses of leaf lipid composition to dryland conditions may be an additional criterion for characterization and selection of new drought-tolerant cotton genotypes.

Journal Article↗
PubMed1990

Leaf water relations and maintenance of gas exchange in coffee cultivars grown in drying soil.

Plant water status, leaf tissue pressure-volume relationships, and photosynthetic gas exchange were monitored in five coffee (Coffea arabica L.) cultivars growing in drying soil in the field. There were large differences among cultivars in the rates at which leaf water potential (Psi(L)) and gas exchange activity declined when irrigation was discontinued. Pressure-volume curve analysis indicated that increased leaf water deficits in droughted plants led to reductions in bulk leaf elasticity, osmotic potential, and in the Psi(L) at which turgor loss occurred. Adjustments in Psi(L) at zero turgor were not sufficient to prevent loss or near loss of turgor in three of five cultivars at the lowest values of midday Psi(L) attained. Maintenance of protoplasmic volume was more pronounced than maintenance of turgor as soil drying progressed. Changes in assimilation and stomatal conductance were largely independent of changes in bulk leaf turgor, but were associated with changes in relative symplast volume. It is suggested that osmotic and elastic adjustment contributed to maintenance of gas exchange in droughted coffee leaves probably through their effects on symplast volume rather than turgor.

Journal Article↗
PubMed1991

Photophosphorylation in Attached Leaves of Helianthus annuus at Low Water Potentials.

The in situ response of photophosphorylation and coupling factor activity to low leaf water potential (psi(L)) was investigated using kinetic spectroscopy to measure the flash-induced electrochromic absorption change in attached sunflower (Helianthus annuus L. cv IS894) leaves. The electrochromic change is caused by the formation of an electric potential across the thylakoid membrane associated with proton uptake. Since depolarization of the thylakoid membrane following flash excitation is normally dominated by proton efflux through the coupling factor during ATP formation, this measurement can provide direct information about the catalytic activity of the coupling factor. Under low psi(L) conditions in which a clear nonstomatal limitation of net photosynthesis could be demonstrated, we found a strong inhibition of coupling factor activity in dark-adapted leaves which was probably caused by an increase in the energetic threshold for the activation of the enzyme at low psi(L). While this result supported earlier in vitro findings, we further discovered that the light-dependent reduction of coupling factor reversed any observable effect of low psi(L) on the energetics of activation or on photophosphorylation competence. Furthermore, coupling factor was reduced, even in severely droughted sunflower, almost immediately upon illumination. Based on these measurements, we conclude that the nonstomatal limitation of photosynthesis observed by us and others in droughted plants cannot be explained by impaired coupling factor activity.

Journal Article↗
PubMed2003

Arabidopsis AtMYC2 (bHLH) and AtMYB2 (MYB) function as transcriptional activators in abscisic acid signaling.

In Arabidopsis, the induction of a dehydration-responsive gene, rd22, is mediated by abscisic acid (ABA). We reported previously that MYC and MYB recognition sites in the rd22 promoter region function as cis-acting elements in the drought- and ABA-induced gene expression of rd22. bHLH- and MYB-related transcription factors, rd22BP1 (renamed AtMYC2) and AtMYB2, interact specifically with the MYC and MYB recognition sites, respectively, in vitro and activate the transcription of the beta-glucuronidase reporter gene driven by the MYC and MYB recognition sites in Arabidopsis leaf protoplasts. Here, we show that transgenic plants overexpressing AtMYC2 and/or AtMYB2 cDNAs have higher sensitivity to ABA. The ABA-induced gene expression of rd22 and AtADH1 was enhanced in these transgenic plants. Microarray analysis of the transgenic plants overexpressing both AtMYC2 and AtMYB2 cDNAs revealed that several ABA-inducible genes also are upregulated in the transgenic plants. By contrast, a Ds insertion mutant of the AtMYC2 gene was less sensitive to ABA and showed significantly decreased ABA-induced gene expression of rd22 and AtADH1. These results indicate that both AtMYC2 and AtMYB2 proteins function as transcriptional activators in ABA-inducible gene expression under drought stress in plants.

Abscisic Acid↗
PubMed2003

CIPK3, a calcium sensor-associated protein kinase that regulates abscisic acid and cold signal transduction in Arabidopsis.

Plants respond to environmental stress by activating "stress genes." The plant hormone abscisic acid (ABA) plays an important role in stress-responsive gene expression. Although Ca(2+) serves as a common second messenger in signaling stress and ABA, little is known about the molecular basis of Ca(2+) action in these pathways. Here, we show that CIPK3, a Ser/Thr protein kinase that associates with a calcineurin B-like calcium sensor, regulates ABA response during seed germination and ABA- and stress-induced gene expression in Arabidopsis. The expression of the CIPK3 gene itself is responsive to ABA and stress conditions, including cold, high salt, wounding, and drought. Disruption of CIPK3 altered the expression pattern of a number of stress gene markers in response to ABA, cold, and high salt. However, drought-induced gene expression was not altered in the cipk3 mutant plants, suggesting that CIPK3 regulates select pathways in response to abiotic stress and ABA. These results identify CIPK3 as a molecular link between stress- and ABA-induced calcium signal and gene expression in plant cells. Because the cold signaling pathway is largely independent of endogenous ABA production, CIPK3 represents a cross-talk "node" between the ABA-dependent and ABA-independent pathways in stress responses.

Abscisic Acid↗
PubMed2002

Arabidopsis OST1 protein kinase mediates the regulation of stomatal aperture by abscisic acid and acts upstream of reactive oxygen species production.

During drought, the plant hormone abscisic acid (ABA) triggers stomatal closure, thus reducing water loss. Using infrared thermography, we isolated two allelic Arabidopsis mutants (ost1-1 and ost1-2) impaired in the ability to limit their transpiration upon drought. These recessive ost1 mutations disrupted ABA induction of stomatal closure as well as ABA inhibition of light-induced stomatal opening. By contrast, the ost1 mutations did not affect stomatal regulation by light or CO(2), suggesting that OST1 is involved specifically in ABA signaling. The OST1 gene was isolated by positional cloning and was found to be expressed in stomatal guard cells and vascular tissue. In-gel assays indicated that OST1 is an ABA-activated protein kinase related to the Vicia faba ABA-activated protein kinase (AAPK). Reactive oxygen species (ROS) were shown recently to be an essential intermediate in guard cell ABA signaling. ABA-induced ROS production was disrupted in ost1 guard cells, whereas applied H(2)O(2) or calcium elicited the same degree of stomatal closure in ost1 as in the wild type. These results suggest that OST1 acts in the interval between ABA perception and ROS production. The relative positions of ost1 and the other ABA-insensitive mutations in the ABA signaling network (abi1-1, abi2-1, and gca2) are discussed.

Abscisic Acid↗