Simulating ozone detoxification in the leaf apoplast through the direct reaction with ascorbate (original) (raw)
Related papers
Planta, 2002
To evaluate reactive absorption of ozone (O 3) in the leaf apoplast, amphistomatous leaves of Phaseolus vulgaris L. were allowed to take up O 3 through the stomata on the lower leaf surface at high rates for 3±5 min. Up to 5% of the O 3 taken up diused through the leaf and emerged from the stomata on the upper surface, suggesting above-zero O 3 concentrations in the leaf intercellular air space, [O 3 ] i. Moreover, measurements revealed that [O 3 ] i increased during exposure to the pollutant. Time patterns of O 3¯u xes through the gas phase and into the aqueous apoplast indicated that the increase in [O 3 ] i was the result of a decrease in the diffusion-reaction conductance of the aqueous apoplast, g aq. Under an intense O 3 pulse, g aq approached the value of the pure diusional conductance within 2.5 min of exposure, suggesting the exhaustion of protective resources in the leaf apoplast. Toward the end of the exposure g aq tended to increase, suggesting either a recovery in the protective resources in the leaf apoplast and/or the induction of new defences. The possibility of estimating the degree of protection aorded by apoplast constituents and the rate of recovery of these protective systems in intact leaves using brief O 3 pulses is discussed. Keywords Apoplast á Detoxi®cation (O 3) á Ozone á Phaseolus (O 3 detoxi®cation) á Reactive absorption Abbreviations A: O 3-exposed mesophyll area relative to leaf projective area á D: aqueous-phase diusivity for O 3 á g: conductance for O 3 á H: solubility of O 3 in water á J: O 3¯u x density á K: ®rst-order coecient of reactive O 3 absorption á L: cell wall thickness á [O 3 ]: concentration of O 3 á q: composite argument in Eq. 5 á Subscripts: a: air boundary layer á aq: aqueous
Estimation of plasmalemma conductivity to ascorbic acid in intact leaves exposed to ozone
2000
To establish the capacity of the leaf mesophyll plasmalemma to 450 nmol ozone mol − 1 during re-illumination, [TAA] cw reached zero at 2.7-fold higher [TAA] l than without ozone, of Phaseolus 7ulgaris L. to supply ascorbate (ASC) into the cell wall by simple diffusion, a method for calculating plas-and the slope of the relationship increased twofold. Plasmalemma diffusional conductivities to AA of 2.9 ×10 − 6 and malemma diffusional conductivity to ascorbic acid (AA) in 1.8 ×10 − 6 m s − 1 , needed to maintain [TAA] cw at the ob-intact leaves was evaluated. The core of the approach is that served level, were calculated from the increase in [TAA] l at in the presence of a sink for ascorbate in the cell wall, cell wall total ascorbic acid concentration [TAA] cw ( =[ASC] cw + [TAA] cw = 0 and from the two different estimates of the sink [AA] cw ) reaches zero at some positive whole-leaf total ascorbic for ASC. A value of 1.3 × 10 − 6 m s − 1 was calculated on the basis of the oil-water distribution coefficient for TAA. It is acid concentration [TAA] l . It is shown that [TAA] l at [TAA] cw =0 is proportional to the sink for ASC in the cell concluded that the demand for ASC in the mesophyll cell wall wall and the reciprocal of plasmalemma conductivity. The of the investigated leaves could be met by simple diffusion of AA through the plasmalemma. From the measured increase in predicted proportional relationship between [TAA] cw and [TAA] l was confirmed by decreasing TAA levels in leaves the slope of the relationship [TAA] cw versus [TAA] l , an increase in the cell wall pH of 0.3 units was estimated under the through predarkening. Furthermore, increasing the sink intensity for ASC in the cell wall by the acute exposure of leaves influence of ozone.
Ozone effects on trees, where uptake and detoxification meet
2002
Ozone is the most important air pollutant and its concentration in ambient air is still rising. Ozone concentrations measured at reference height (50 m is EMEP ozone modelling height), do not reflect the real concentration at the top of the vegetative canopy and do not provide sufficient information about the ozone flux entering the leaves. Modelling stomatal conductance is leading to estimations of cumulative ozone uptake and enables much better to evaluate the impact of ozone on trees. The negative impact of ozone exposure has a measurable effect on physiological processes such as stomatal conductance, photosynthesis and respiration. Disturbance of the basic physiological processes is leading to growth and wood production losses. There have been several attempts to establish critical levels (CL) for ozone effects on forest trees. Average concentrations and cumulative exposure indices are satisfactory to some extent, but do not fully describe the potential impact of ozone exposure. Much more promising is an evaluation based on the effective ozone flux, which is a function of the absorbed ozone flux and the defensive response. Ozone uptake takes place primarily through the stomata and reactions of ozone with hydrocarbons released by the plant cells and transformations of dissolved ozone in the apoplastic fluid create many reactive oxygen species of which free radicals are able to initiate membrane lipid peroxidation and destruction of cell membranes. The defence of a plant against absorbed ozone starts in the apoplastic fluid. Ascorbate is believed to be a very important radical scavenger avoiding detrimental effects of reactive oxygen species to the membranes. Other important antioxidants are phenolics. The defensive response can be linked to the abundance of ascorbate or the ability of the plants to regenerate (reduce) ascorbate from monodehydroascorbate and dehydroascorbate. The reduction of dehydroascorbate takes place in the symplast where ascorbate can be transported back through the plasma membrane into the apoplast. Ozone exposure also causes oxidative stress of the plant cell interior by the formation of reactive oxygen species. Plants can cope with those toxic substances in the symplast by using antioxidants such as ascorbate,-tocopherol, glutathione and carotenoids and enzymes such as superoxide dismutases, catalases and several peroxidases. The complexity of the apoplastic and symplastic antioxidative capacity with different turnover rates and transport of antioxidants makes it difficult to determine the total antioxidative power.
Ascorbate in the leaf apoplast is a factor mediating ozone resistance in Plantago major
Plant Physiology and Biochemistry, 2000
The role of ascorbate in mediating ozone resistance was examined in Plantago major L. Seedlings of eleven populations which exhibited differential resistance to ozone were fumigated in controlled environment chambers with charcoal/Purafilt-filtered air (CFA) or CFA plus 15 nmol·mol -1 ozone overnight rising to a maximum between 12:00-16:00 hours of 75 nmol·mol -1 for 14 d. Measurements of ascorbate content were made on apoplastic and symplastic extracts. Populations differed in their constitutive level of ascorbate in youngest fully expanded leaves, and regression analysis revealed a significant correlation between ascorbate content in ozone-treated leaves and the ozone resistance of the populations. The relationship was stronger using apoplastic ascorbate levels than with corresponding symplastic measurements. The ascorbate content of the youngest fully expanded leaf of an ozone sensitive population was increased by foliar application of ascorbate. No significant difference in stomatal conductance was found between control and ascorbate-treated plants. Following spraying, plants were fumigated with 400 nmol·mol -1 ozone for 7 h. In control plants, ozone exposure resulted in extensive visible leaf damage (20-70 % at the end of the fumigation period) and decreased rates of CO 2 assimilation (-57 %). However, ascorbate treatment prevented the appearance of visible injury, and ameliorated the decline in photosynthesis induced by ozone (-26 %). Modelled data estimating the extent of protection afforded by apoplastic ascorbate against ozone supported the experimental observations. The results suggested that although apoplastic ascorbate plays an important role, other factors must also contribute to the mediation of ozone resistance in P. major. © 2000 Éditions scientifiques et médicales Elsevier SAS Apoplast / ascorbate / detoxification / ozone / Plantago major / resistance A, rate of CO 2 assimilation / ASC, ascorbate / AWF, apoplastic washing fluid / CFA, charcoal / Purafil t -filtered air / DHA, dehydroascorbate / g H2O , stomatal conductance to water vapour / O 3 , ozone / RLE, residual leaf extract / total ASC, ascorbate+dehydroascorbate Plant Physiol. Biochem., 2000, 38 (5), 403−411 /
Journal of Experimental Botany, 2000
reaction of O 3 with ASC apo in the leaves of Vicia faba is potentially sufficient to intercept a substantial pro-Broad bean (Vicia faba L.) plants were exposed, in portion (30-40%) of the O 3 entering the plant under duplicate controlled environment chambers, to charenvironmentally-relevant conditions. The potential role coal/PurafilB-filtered air (CFA-grown plants) or to of apoplastic ASC in mediating the tolerance of leaves 75 nmol mol−1 ozone (O 3) for 7 h d−1 (O 3-grown plants) to O 3 is discussed. for 28 d, and then exposed to 150 nmol mol−1 O 3 for 8 h. The concentration of ascorbate (ASC) was deter-Key words: Apoplast, cell wall, ozone, detoxification, mined in leaf extracellular washing fluid (apoplast) and antioxidants, ascorbate. in the residual leaf tissue (symplast) after 0, 4 and 8 h acute fumigation, and after a 16 h 'recovery' period in CFA. Changes in stomatal conductance were meas
Ozone Concentration in Leaf Intercellular Air Spaces Is Close to Zero
PLANT PHYSIOLOGY, 1989
Transpiration and ozone uptake rates were measured simultaneously in sunflower leaves at different stomatal openings and various ozone concentrations. Ozone uptake rates were proportional to the ozone concentration up to 1500 nanoliters per liter. The leaf gas phase diffusion resistance (stomatal plus boundary layer) to water vapor was calculated and converted to the resistance to ozone multiplying it by the theoretical ratio of diffusion coefficients for water vapor and ozone in air (1.67). The ozone concentration in intercellular air spaces calculated from the ozone uptake rate and diffusion resistance to ozone scattered around zero. The ozone concentration in intercellular air spaces was measured directly by supplying ozone to the leaf from one side and measuring the equilibrium concentration above the other side, and it was found to be zero. The total leaf resistance to ozone was proportional to the gas phase resistance to water vapor with a coefficient of 1.68. It is concluded that ozone enters the leaf by diffusion through the stomata, and is rapidly decomposed in cell walls and plasmalemma. diffusion resistance of the whole gaseous pathway from cell surfaces to ambient air:
Dark-induced ascorbate deficiency in leaf cell walls increases plasmalemma injury under ozone
Planta, 1998
To assess protection of the mesophyll cell plasmalemma against O 3 by apoplasmic reduced ascorbate (AA), its concentration in the leaf cell wall of common bean (Phaseolus vulgaris L.) was lowered from 0.6 mM to 0.1 mM by pre-exposing plants to continuous darkness for up to 48 h. Subsequent ozonization of ascorbate-de®cient leaves with 350±450 nmol O 3 mol A1 resulted in a rapid rise of apoplasmic AA within the second hour of the treatment, the concomitant appearance of cytoplasmic marker enzymes in cell wall solute extracts and the development of waterlogged spots on leaves. Prior to these events, stomatal conductances had just reached values close to those observed in AA-nonde®cient leaves, whereas AA concentration in the cell wall was still 2±4 times lower than in leaves pre-exposed to the normal 10-h dark period. In AA-nonde®cient leaves the inital apoplasmic AA level of 0.6 mM was maintained under O 3 for 2.5 h; thereafter, it increased moderately. There appeared to be no signs of injury even 2 d after the whole 4.5-h treatment. During the period of equal stomatal conductances, the O 3 decay rate in direct reaction with AA in AA-de®cient cell walls was estimated to be 50±70% of that occurring in AA-nonde®cient leaves. It is suggested that under AA de®ciency some threshold for the stability of the plasmalemma was surpassed owing to the more``O 3-permeable'' cell wall. The mesophyll conductance was found to be stable throughout O 3 exposure, indicating that the cytoplasmic O 3 defense barrier was not exceeded. Possible changes in oxyradical reactions and in cell wall phenolics are discussed. It is suggested that after prolonged darkness the¯ow rate of reactive oxygen intermediates to the plasmalemma may also be higher because they are less trapped in direct and peroxidase-catalyzed reactions.