Showing papers in "Plant Physiology in 1983"

Nitrogen and Photosynthesis in the Flag Leaf of Wheat (Triticum aestivum L.)

Abstract: Wheat (Triticum aestivum L. cv Yecora 70) plants were grown with various concentrations of nitrate nitrogen available to the roots. Sampling of flag leaves began after they had reached full expansion and continued throughout senescence. Rates of gas exchange, ribulose-1,5-bisphosphate (RuP(2)) carboxylase activity, and the amounts of chlorophyll, soluble protein, nitrogen, and phosphorus were determined for each flag leaf. Rate of CO(2) assimilation was uniquely related to total leaf nitrogen irrespective of nutrient treatment, season, and leaf age. Assimilation rate increased with leaf nitrogen, but the slope of the relationship declined markedly when leaf nitrogen exceeded 125 millimoles nitrogen per square meter. Chlorophyll content and RuP(2) carboxylase activity were approximately proportional to leaf nitrogen content. As leaves aged, RuP(2) carboxylase activity and calculated Hill activity declined in parallel. With normal ambient partial pressure of CO(2), the intercellular partial pressure of CO(2) was always such that rate of assimilation appeared colimited by RuP(2) carboxylation and RuP(2) regeneration capacity.The initial slope of rate of CO(2) assimilation against intercellular partial pressure of CO(2) varied nonlinearly with carboxylase activity. It is suggested that this was due to a finite conductance to CO(2) diffusion in the wall and liquid phase which causes a drop in CO(2) partial pressure between the intercellular spaces and the site of carboxylation. A double reciprocal plot was used to obtain an estimate of the transfer conductance.

Evaluation of the water potentials of solutions of polyethylene glycol 8000 both in the absence and presence of other solutes.

Abstract: Published and additional data for polyethylene glycol 8000 (PEG), formerly PEG 6000, solution water potentials (Ψ) are compared. Actual bars Ψ over the concentration range of 0 to 0.8 gram PEG per gram H2O and temperature (T) range of 5 to 40°C are best predicted (probably within ± 5%) by this equation: Ψ = 1.29[PEG]2T − 140[PEG]2 − 4.0[PEG]. Although transformable through division by [PEG] to virial equation form, results indicate that the coefficients are not virial. Mannitol (MAN) interacts with PEG to produce Ψ significantly lower than additive. Vapor pressure osmometer (VPO) data for MAN-PEG synergism compared favorably with those from thermocouple hygrometry; and VPO data showing the interactions between PEG and four salts are presented. The synergism of MAN-PEG and of NaCl-PEG are related linearly to the concentration of solute added with PEG.

Host-Pathogen Interactions : XXIII. The Mechanism of the Antibacterial Action of Glycinol, a Pterocarpan Phytoalexin Synthesized by Soybeans.

Abstract: The biochemical basis for the ability of the pterocarpan phytoalexin glycinol (3,6a,9-trihydroxypterocarpan) to inhibit the growth of bacteria was examined. Glycinol at bacteriostatic concentrations ( e.g. 50 micrograms per milliliter) inhibits the ability of Erwinia carotovora to incorporate [ 3 H]leucine, [ 3 H]thymidine, or [ 3 H]uridine into biopolymers. Exposure of Escherichia coli membrane vesicles to glycinol at 20 micrograms per milliliter results in inhibition of respiration-linked transport of [ 14 C]lactose and [ 14 C]glycine into the vesicles when either d-lactate or succinate is supplied as the energy source. The ability of E. coli membrane vesicles to transport [ 14 C]α-methyl glucoside, a vectorial phosphorylation-mediated process, is also inhibited by glycinol at 20 micrograms per milliliter. Furthermore, exposure of membrane vesicles to glycinol (50 micrograms per milliliter) at 20°C results in the leakage of accumulated [ 14 C]α-methyl glucoside-6-phosphate. The effects of the phytoalexins glyceollin, capsidiol, and coumestrol, and daidzein, a compound structurally related to glycinol but without antibiotic activity, upon the E. coli membrane vesicle respiration-linked transport of [ 14 C]glycine and of [ 14 C]α-methyl glucoside was also examined. Glyceollin and coumestrol (50 micrograms per milliliter), but not daidzein, inhibit both membrane-associated transport processes. These data imply that the antimicrobial activity of glycinol, glyceollin, and coumestrol are due to a general interaction with the bacterial membrane. Capsidiol (50 micrograms per milliliter) inhibits d-lactate-dependent transport of [ 14 C]glycine but not vectorial phosphorylation-mediated transport of [ 14 C]α-methyl glucoside. Thus, capsidiol9s mechanism of antimicrobial action seems to differ from that of the other phytoalexins examined.

Variation in Quantum Yield for CO2 Uptake among C3 and C4 Plants

Abstract: The quantum yield for CO2 uptake was measured on a number of C3 and C4 monocot and dicot species. Under normal atmospheric conditions (330 microliters per liter CO2, 21% O2) and a leaf temperature of 30°C, the average quantum yields (moles CO2 per einstein) were as follows: 0.052 for C3 dicots, 0.053 for C3 grasses, 0.053 for NAD-malic enzyme type C4 dicots, 0.060 for NAD-malic enzyme type C4 grasses, 0.064 for phosphoenolpyruvate carboxykinase type C4 grasses, 0.061 for NADP-malic enzyme C4 dicots, and 0.065 for NADP-malic enzyme type C4 grasses. The quantum yield under normal atmospheric conditions was temperature dependent in C3 species, but apparently not in C4 species. Light and temperature conditions during growth appeared not to influence quantum yield. The significance of variation in the quantum yields of C4 plants was discussed in terms of CO2 leakage from the bundle sheath cells and suberization of apoplastic regions of the bundle sheath cells.

Phloem Loading of Sucrose

Abstract: Autoradiographic, plasmolysis, and 14 C-metabolite distribution studies indicate that the majority of exogenously supplied 14 C-sucrose enters the phloem directly from the apoplast in source leaf discs of Beta vulgaris . Phloem loading of sucrose is pH-dependent, being markedly inhibited at an apoplast pH of 8 compared to pH 5. Kinetic analyses indicate that the apparent K m of the loading process increases at the alkaline pH while the maximum velocity, V max , is pH-independent. The pH dependence of sucrose loading into source leaf discs translates to phloem loading in and translocation of sucrose from intact source leaves. Studies using asymmetrically labeled sucrose 14 C-fructosyl-sucrose, show that sucrose is accumulated intact from the apoplast and not hydrolyzed to its hexose moieties by invertase prior to uptake. The results are discussed in terms of sucrose loading being coupled to the co-transport of protons (and membrane potential) in a manner consistent with the chemiosmotic hypothesis of nonelectrolyte transport.

Mechanism of Iron Uptake by Peanut Plants: I. FeIII Reduction, Chelate Splitting, and Release of Phenolics

Abstract: Iron deficiency in peanuts (Arachis hypogeae L.) caused an increase in release of caffeic acid, a higher rate of Fe(III) reduction, and increased rates of both Fe(III) chelate splitting and iron uptake.Experiments on Fe(III) reduction by phenolics (in vitro experiments) and by roots of Fe-deficient peanuts exclude the direct involvement of released phenolics in Fe(III) reduction by roots: Fe(III) reduction by phenolics had a pH optimum higher than 8.0 and was strongly dependent on the concentration and the stability of the supplied Fe(III) chelates. In contrast, Fe(III) reduction by roots of Fe-deficient peanuts had a pH optimum of about 5.0 and was less dependent on the stability of the supplied Fe(III) chelates. Furthermore, the observed release of phenolics into nutrient solution would have to be at least 200 times higher to attain the reduction rates of roots of Fe-deficient peanuts. The results of these experiments support the idea of an enzymic reduction of Fe(III) on the plasmalemma of cortical cells of roots.

Inhibition of photosynthesis by carbohydrates in wheat leaves

Abstract: The rate of net CO2 assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% O2, 340 microbars CO2) declined with time of illumination at temperatures lower than 25°C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO2 concentration (700-825 microbars). In this latter case, the decline in the rate of net CO2 assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to CO2, but this was not the primary cause of the decrease in CO2 assimilation because internal partial pressure of CO2 remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO2 assimilation of leaves in atmospheric and high CO2 concentrations. The decreased net CO2 assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO2 assimilation in high carbohydrate leaves as a function of intercellular CO2 partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO2 assimilation by 2% O2 (Warburg effect), and the apparent quantum yield, decreased after several hours of light.

On the Factors Which Determine Massive β-Carotene Accumulation in the Halotolerant Alga Dunaliella bardawil

Abstract: Dunaliella bardawil, a beta-carotene-accumulating halotolerant alga, has been analyzed for the effect of various growth conditions on its pigment content, and compared with Dunaliella salina, a beta-carotene nonaccumulating species In D bardawil, increasing light intensity and light period or inhibiting growth by various stress conditions such as nutrient deficiency or high salt concentration caused a decrease in the content of chlorophyll per cell and an increase in the amount of beta-carotene per cell As a result, the beta-carotene-to-chlorophyll ratio increased from about 04 to 13 grams per gram and the alga changed its visual appearance from green to deep orange D salina grown similarly decreased in content of both chlorophyll and beta-carotene per cell and the culture turned from green to yellowish Low chlorophyll-containing cells of D bardawil or D salina exhibit very high photosynthetic rates when expressed on a chlorophyll basis ( approximately 600 micromoles O(2) evolved per milligram chlorophyll per hour)Variation of pigment content in D bardawil by a large variety of environmental agents has been correlated with the integral irradiance received by the algal culture during a division cycle The higher the integral irradiance per division cycle, the lower the chlorophyll content per cell; the higher the beta-carotene content per cell, and therefore the higher the beta-carotene-to-chlorophyll ratio The results are interpreted as indicating a protecting effect of beta-carotene against injury by high irradiance under conditions of impairment in chlorophyll content per cell

Host-Pathogen Interactions : XXII. A Galacturonic Acid Oligosaccharide from Plant Cell Walls Elicits Phytoalexins.

Abstract: Elicitors of phytoalexin accumulation in soybean (Glycine max L. Merr., cv Wayne) cotyledons were released from soybean cell walls and from citrus pectin by partial acid hydrolysis. These two hydrolysates yielded nearly identical distributions of elicitor activity when fractionated on anion-exchange columns. Chromatography of the pectin elicitor on gel filtration and high-pressure anion-exchange columns did not further purify the elicitor. Elicitor activity of the preparation was lost by treatment with either endo-alpha-1,4-polygalacturonase or pectate lyase. Glycosyl residue compositions of the purified elicitors from cell walls and pectin were both found to be approximately 98% galacturonosyl residues. Linkage analysis of the pectin elicitor showed that most, if not all, of the galacturonosyl residues were alpha-1,4-linked. The high-mass molecular ions detected by fast atom bombardment-mass spectrometry of the most active elicitor fractions from cell walls and pectin both corresponded precisely to a molecule composed of 12 galacturonosyl residues. These results suggest that dodeca-alpha-1,4-d-galacturonide is the active elicitor, but the possibility remains that the active component could be a slightly modified oligogalacturonide present, but not detected, in the purified fractions.

Stomatal Sensitivity to Carbon Dioxide and Humidity A Comparison of Two C3 and Two C4 Grass Species

Abstract: The sensitivity of stomatal conductance to changes of CO2 concentration and leaf-air vapor pressure difference (VPD) was compared between two C3 and two C4 grass species. There was no evidence that stomata of the C4 species were more sensitive to CO2 than stomata of the C3 species. The sensitivity of stomatal conductance to CO2 change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO2 concentration, Ci/Ca, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, Ci/Ca was 0.7 and 0.5 (approximately) in C3 and C4 species, respectively. Transpiration efficiency (net CO2 assimilation rate/transpiration rate) was larger in the C4 species than in the C3 species at current atmospheric CO2 concentrations, but the relative increase due to high CO2 was larger in the C3 than in the C4 species.

Involvement of Abscisic Acid in Potato Cold Acclimation

Abstract: Upon exposure to 2 degrees C day/night (D/N), leaves of Solanum commersonii (Sc) began acclimating on the 4th day from a -5 degrees C (killing temperature) hardy level to -12 degrees C by the 15th day. Leaves of S. tuberosum L. (St) cv ;Red Pontiac' typically failed to acclimate and were always killed at -3 degrees C. Leaves of control (20/15 degrees C, D/N) and treated plants (2 degrees C, D/N) of St showed similar levels of free abscisic acid (ABA) during a 15-day sampling period. In treated Sc plants, however, free ABA contents increased 3-fold on the 4th day and then declined to their initial level thereafter. The increase was not observed in leaves of Sc control plants.Treated St plants showed a slightly higher content of leaf soluble protein than controls. In Sc, leaves of controls maintained relatively constant soluble proteins, but leaves of treated plants showed a distinct increase. This significant increase was initiated on the 4th day, peaked on the 5th day, and remained at a high level throughout the 15-day sampling period.Exogenously applied ABA induced frost hardiness in leaves of Sc plants whether plants were grown under a 20 degrees C or 2 degrees C temperature regime. When cycloheximide was added to the medium of stem-cultured plants at the beginning of 2 degrees C acclimation, or at the beginning of the ABA treatment in the 20 degrees C regime, it completely inhibited the development of frost hardiness. However, when cycloheximide was added to plants on the 5th day during 2 degrees C acclimation, the induction of frost hardiness was not inhibited. The role of ABA in triggering protein synthesis needed to induce frost hardiness is discussed.

Relationship between Photosynthesis and Respiration: The Effect of Carbohydrate Status on the Rate of CO2 Production by Respiration in Darkened and Illuminated Wheat Leaves

Abstract: The rate of dark CO(2) efflux from mature wheat (Triticum aestivum cv Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO(2) efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO(2) efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O(2) and 330 microbar CO(2) before darkening. After 30 minutes of darkness, a relatively steady rate of CO(2) efflux was obtained. The temperature dependence of steady-state dark CO(2) efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO(2) efflux following photosynthesis is correlated with accumulated net CO(2) assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO(2) compensation point in 21% O(2), and an increase in the light compensation point. The interactions between CO(2) efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO(2) efflux by respiration is comparable in darkened and illuminated wheat leaves.

Cadmium Uptake Kinetics in Intact Soybean Plants

Abstract: The absorption characteristics of Cd 2+ by 10- to 12-day-old soybean plants ( Glycine max cv Williams) were investigated with respect to influence of Cd concentration on adsorption to root surfaces, root absorption, transport kinetics and interaction with the nutrient cations Cu 2+ , Fe 2+ , Mn 2+ , and Zn 2+ . The fraction of nonexchangeable Cd bound to roots remained relatively constant at 20 to 25% of the absorbed fraction at solution concentration of 0.0025 to 0.5 micromolar, and increased to 45% at solution concentration in excess of 0.5 micromolar. The exchangeable fraction represented 1.4 to 32% of the absorbed fraction, and was concentration dependent. Using dinitrophenol as a metabolic inhibitor, the `metabolically absorbed9 fraction was shown to represent 75 to 80% of the absorbed fraction at concentration less than 0.5 micromolar, and decreased to 55% at 5 micromolar. At comparatively low Cd concentrations, 0.0025 to micromolar 0.3, root absorption exhibited two isotherms with K 2 values of 0.08 and 1.2 micromolar. Root absorption and transfer from root to shoot of Cd 2+ was inhibited by Cu 2+ , Fe 2+ , Mn 2+ , and Zn 2+ . Analyses of kinetic interaction of these nutrient cations with Cd 2+ indicated that Cu 2+ , Fe 2+ , Zn 2+ , and possibly Mn 2+ inhibited Cd absorption competitively suggesting an involvement of a common transport site or process.

Nitrogen Redistribution during Grain Growth in Wheat (Triticum aestivum L.) : IV. Development of a Quantitative Model of the Translocation of Nitrogen to the Grain

Abstract: Translocation of nitrogen was measured in wheat (Triticum aestivium L. cv SUN 9E) plants grown without an exogenous supply of nitrogen from the time that the flagleaf began to emerge, and a model of nitrogen translocation was constructed to describe translocation on one day during the linear period of grain growth. Nitrogen for grain development was derived entirely by the redistribution of nitrogen from vegetative organs. Leaves contributed 40%, glumes 23%, stem 23%, and roots 16% of the nitrogen incorporated by the grains on the fifteenth day after anthesis. Less than 50% of the nitrogen exported from leaves was translocated directly to the grain via the phloem, the rest was translocated to the roots and was cycled in the roots and exported to the shoot in the transpiration stream. Nitrogen imported by leaves and glumes via the xylem was not accumulated in these organs but was transferred to the phloem for reexport from the organs. A large proportion (60%) of the nitrogen in the transpiration stream was cycled in the glumes. The glumes were also a major source of nitrogen for grain development. It was considered likely that this organ always plays an important role in nitrogen metabolism in wheat.

Abscisic Acid-induced freezing resistance in cultured plant cells.

Abstract: The effect of abscisic acid (ABA) on the cold hardiness of cell suspension was investigated. Cell suspension cultures of winter wheat (Triticum aestivum L. cv Norstar), winter rye (Secale cereale L. cv Cougar), and bromegrass Bromo inermis Leyss treated with 7.5 × 10−5 molar ABA for 4 days at 20°C could tolerate −30°C, whereas the control cultures tolerated only −7 to −8°C. The optimum concentration for increasing the cold hardiness of the cultures was 7.5 × 10−5 molar. The degree of cold hardiness and the rate of hardening obtained by ABA treatment was significantly higher than that induced by low temperature alone. Of ten species tested, ABA was only effective on those cultures which were capable of cold hardening upon exposure to low temperatures. The results suggest that ABA bypasses the cold requirement for hardening and also suggests that ABA triggers the genetic system(s) responsible for inducing the hardening process.

Glutamine Synthetases of Higher Plants Evidence for a Specific Isoform Content Related to Their Possible Physiological Role and Their Compartmentation within the Leaf

Abstract: The chromatographic properties of glutamine synthetase isoforms have been investigated in a wide range of higher plant leaves and shoots using ion exchange chromatography. Different patterns of glutamine synthetase isoform content were observed. Among higher plants, four patterns or groups could be recognized. The first group is characterized by having only cytosolic glutamine synthetase, whereas the second group is distinguished by having only chloroplastic glutamine synthetase. The third group is characterized by cytosolic glutamine synthetase being a minor component of the total leaf glutamine synthetase activity. The fourth group is distinct from the other groups in having high cytosolic and chloroplast glutamine synthetase activity. Immunological studies have been undertaken on a few species from each group to identify unambiguously both cytosolic and chloroplastic glutamine synthetases.

Similarities and differences in lipid metabolism of chloroplasts isolated from 18:3 and 16:3 plants.

Abstract: Photosynthetically active chloroplasts retaining high rates of fatty acid synthesis from [1- 14 C]acetate were purified from leaves of both 16:3 ( Solanum nodiflorum, Chenopodium album ) and 18:3 plants ( Amaranthus lividus, Pisum sativum ). A comparison of lipids into which newly synthesized fatty acids were incorporated revealed that, in 18:3 chloroplasts, enzymic activities catalyzing the conversion of phosphatidate to diacylglycerol and of diacylglycerol to monogalactosyl diacylglycerol (MGD) were significantly less active than in 16:3 chloroplasts. In contrast, labeling rates of MGD from UDP-[ 14 C]gal were similar for both types of chloroplasts. The composition and positional distribution of labeled fatty acids within the glycerides synthesized by isolated 16:3 and 18:3 chloroplasts were similar and in each case only a C18/C16 diacylglycerol backbone was synthesized. In nodiflorum chloroplasts, C18:1/C16:0 MGD assembled de novo was completely desaturated to the C18:3/C16:3 stage. Whereas newly synthesized C18/C18 MGD could not be detected in any of these chloroplasts if incubated with [ 14 C]acetate after isolation, chloroplasts isolated from acetate-labeled leaves contained MGD with labeled C18 fatty acids at both sn -1 and sn -2 positions. Taken together, these results provide further evidence on an organellar level for the operation of pro- and eucaryotic pathways in the biosynthesis of MGD in different groups of plants.

β-1,3-Endoglucanase from Soybean Releases Elicitor-Active Carbohydrates from Fungus Cell Walls

Abstract: Two enzymes from soybean (Glycine max L. Merr. cv Harosoy 63) cotyledons released elicitor-active carbohydrates from cell walls of the phytopathogenic fungus Phytophthora megasperma f.sp. glycinea. They were identified as isoenzymes of β-1,3-endoglucanase (EC 3.2.1.39) with isoelectric points of pH 8.7 and 10.5. The pI 10.5 enzyme was extracted in the greatest amount and was isolated as a homogeneous protein of about 33,000 daltons as determined by gel filtration and sodium dodecyl sulfategel electrophoresis. The purified enzymes hydrolyzed several β-1,3-glucans in a strictly random manner, but degraded neither β-1,6- nor β-1,4-glucans.

Photosynthesis and ion content of leaves and isolated chloroplasts of salt-stressed spinach.

Abstract: Spinach (Spinacia oleracea) plants were subjected to salt stress by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar. Plants were harvested 3 weeks after starting NaCl treatment. Fresh and dry weight of both shoots and roots was decreased more than 50% compared to control plants but the salt-stressed plants appeared healthy and were still actively growing. The salt-stressed plants had much thicker leaves. The salt-treated plants osmotically adjusted to maintain leaf turgor. Leaf K(+) was decreased but Na(+) and Cl(-) were greatly increased.The potential photosynthetic capacity of the leaves was measured at saturating CO(2) to overcome any stomatal limitation. Photosynthesis of salt-stressed plants varied only by about 10% from the controls when expressed on a leaf area or chlorophyll basis. The yield of variable chlorophyll a fluorescence from leaves was not affected by salt stress. Stomatal conductance decreased 70% in response to salt treatment.Uncoupled rates of electron transport by isolated intact chloroplasts and by thylakoids were only 10 to 20% below those for control plants. CO(2)-dependent O(2) evolution was decreased by 20% in chloroplasts isolated from salt-stressed plants. The concentration of K(+) in the chloroplast decreased by 50% in the salt-stressed plants, Na(+) increased by 70%, and Cl(-) increased by less than 20% despite large increases in leaf Na(+) and Cl(-).It is concluded that, for spinach, salt stress does not result in any major decrease in the photosynthetic potential of the leaf. Actual photosynthesis by the plant may be reduced by other factors such as decreased stomatal conductance and decreased leaf area. Effective compartmentation of ions within the cell may prevent the accumulation of inhibitory levels of Na(+) and Cl(-) in the chloroplast.

Effect of Photosynthesis and Carbohydrate Status on Respiratory Rates and the Involvement of the Alternative Pathway in Leaf Respiration

Abstract: In spinach ( Spinacia oleracea Hybrid 102 [New World seeds]) and wheat ( Triticum aestivum L cv Gabo) leaves, O 2 uptake rates in the dark were faster after the plants had been allowed to photosynthesize for a period of several hours Alternative path activity also increased following a period of photosynthesis in these leaves No such effects were observed with isolated mitochondria In spinach and wheat leaves, the level of fructose plus glucose decreased during a period of darkness In pea ( Pisum sativum cv Alaska) leaves, the level of these sugars did not vary significantly during the day, and respiratory rates were also constant In slices cut from wheat leaves harvested at the end of the night, addition of sugars increased the rate of respiration and engaged the previously latent alternative oxidase In pea leaves, O 2 uptake in the first few minutes following illumination was faster than that observed before illumination, but declined during the next 15 to 20 minutes Adding the alternative oxidase inhibitor salicylhydroxamic acid, or imposing high bicarbonate concentrations during the period of photosynthesis, prevented the rise in O 2 uptake rate during the immediate post illumination period We conclude that the level of respiratory substrate in leaves determines their rate of O 2 uptake, and the degree to which the alternative path contributes to that O 2 uptake

The Role of Ethylene in the Growth Response of Submerged Deep Water Rice

Abstract: We investigated the effect of partial submergence on internode elongation in a Bangladesh variety of floating or deep water rice (Oryza sativa L., cv. Habiganj Aman II). In plants which were at least 21 days old, 7 days of submergence led to a 3- to 5-fold increase in internodal length. During submergence, the ethylene concentration in the internodes increased from about 0.02 to 1 microliters per liter. Treatment of nonsubmerged plants with ethylene also stimulated internode elongation. When ethylene synthesis in partially submerged plants was blocked with aminooxyacetic acid and aminoethoxyvinylglycine, internode elongation was inhibited. This growth inhibition was reversed when ethylene biosynthesis was restored with 1-aminocyclopropane-1-carboxylic acid (ACC). Radio-labeling studies showed that ethylene in floating rice was synthesized from methionine via ACC. Internodal tissue from submerged plants had a much higher capacity to form ethylene than did internodal tissue from nonsubmerged plants. This increase in ethylene synthesis appeared to be due to enhanced ACC formation rather than to increased conversion of ACC to ethylene. Our results indicate that ethylene produced during submergence is required for the stimulation of growth in submerged floating rice plants.

β-Galactosidases in Ripening Tomatoes

Abstract: Tomatoes (Lycopersicon esculentum L.) contained a high level of beta-galactosidase activity which was due to three forms of the enzyme. During tomato ripening, the sum of their activities remained relatively constant, but the levels of the individual forms of beta-galactosidase changed markedly. The three enzymes were separated by a combination of chromatography of DEAE-Sephadex A-50 and Sephadex G-100. During ripening of tomatoes, beta-galactosidases I and III levels decreased but the beta-galactosidase II level increased more than 3-fold. The three enzymes were optimally active near pH 4, and all were inhibited by galactose and galactonolactone. However, the enzymes differed in molecular weight, K(m) value with p-nitrophenyl-beta-galactoside, and stability with respect to pH and temperature. beta-Galactosidase II was the only enzyme capable of hydrolyzing a polysaccharide that was isolated from tomatoes and that consisted primarily of beta-1, 4-linked galactose. The ability of beta-galactosidase II to degrade the galactan and the increase in its activity during tomato ripening suggest a possible role for this enzyme in tomato softening.

Effects of Low Concentrations of O3 on Net Photosynthesis, Dark Respiration, and Chlorophyll Contents in Aging Hybrid Poplar Leaves

Abstract: Chronic exposure of hybrid poplar (Populus deltoides × trichocarpa) plants to low concentrations of ozone had negative impact upon net photosynthetic capacity, dark respiration, and leaf chlorophyll contents. Exposure to as much as 0.20 microliters per liter O3 had no immediate effect on net photosynthesis (Pn), but chronic exposure to 0.125 or 0.085 microliters per liter had a number of gradual effects on CO2 exchange. These included increased dark respiration and consequently increased light compensation points in very young leaves (4-6 days old); and decreased Pn, leaf chlorophyll a and b contents, light saturation points, and apparent quantum yields in leaves 10 to 70 days old. Decreased Pn was partially due to accelerated aging in leaves exposed to O3, and lightsaturated Pn was linearly related to chlorophyll a + b contents. Differences in light-saturated Pn between control and O3-treated leaves of the same age were mostly due to photosaturation in O3-treated leaves and to a much lesser extent to lowered apparent quantum yields. Also, since Pn and dark respiration were most affected by O3 at different leaf ages, distinct modes of action are suggested. The effects of leaf aging on CO2 exchange were considerable, but typical of other species. However, careful monitoring of the interacting effects of leaf age and pollutant exposure was needed in order to characterize the impact of chronic O3 exposure upon CO2 exchange.

Effects of O2 Concentration on Rice Seedlings

Abstract: The ability of rice, wheat, and oat seedlings to germinate and grow as the O2 concentration was lowered to zero was compared. The germination of rice was completely unaffected by O2 supply, whereas that of oats and wheat was strongly retarded at levels below 5% O2. In contrast to the coleoptiles of oats and wheat and to roots of all three species where growth was progressively diminished as the O2 concentration was lowered, that of the rice coleoptile was progressively increased. However, the dry weight and content of protein, sugars, and cellulose were all depressed in the rice coleoptile in anoxia, and the levels of several respiratory enzymes, particularly those of mitochondria, were also much lower than those of the coleoptiles grown in air. In 1% O2, the growth of the rice coleoptile was similar to that in air. The effect of ethanol concentration on germination and growth of rice was measured. Coleoptile growth was reduced when the ethanol concentration exceeded 40 millimolarity, and root growth was somewhat more sensitive. Coleoptiles of all three species grown in air were transferred to N2, and ethanol accumulation was measured over 24 hours. The rate of ethanol accumulation in oats was close to that in rice, and in all three species the amounts of ethanol lost to the surrounding medium were those expected from simple diffusion from the tissue. The ability of the rice coleoptile to grow in anoxia is apparently not due to a particularly low rate of ethanol formation or to unusual ethanol tolerance. Any explanation of the success of rice in anoxia must encompass the much lower rate of ATP synthesis than that in air and account for the biochemical deficiencies of the coleoptile.

Solutes Contributing to Osmotic Adjustment in Cultured Plant Cells Adapted to Water Stress

Abstract: Osmotic adjustment was studied in cultured cells of tomato (Lycopersicon esculentum Mill cv VFNT-Cherry) adapted to different levels of external water potential ranging from −4 bar to −28 bar. The intracellular concentrations of reducing sugars, total free amino acids, proline, malate, citrate, quaternary ammonium compounds, K+, NO3−, Na+, and Cl− increased with decreasing external water potential. At any given level of adaptation, the maximum contribution to osmotic potential was from reducing sugars followed by potassium ions. The sucrose levels in the cells were 3- to 8-fold lower than reducing sugar levels and did not increase beyond those observed in cells adapted to −16 bar water potential. Concentrations of total free amino acids were 4- to 5-fold higher in adapted cells. Soluble protein levels declined in the adapted cell lines, but the total reduced nitrogen was not significantly different after adaptation. Uptake of nitrogen (as NH4+ or NO3−) from the media was similar for adapted and unadapted cells. Although the level of quaternary ammonium compounds was higher in the nonadapted cells than that of free proline, free proline increased as much as 500-fold compared to only a 2- to 3-fold increase observed for quaternary ammonium compounds. Although osmotic adjustment after adaptation was substantial (up to −36 bar), fresh weight (volume increase) was restricted by as much as 50% in the adapted cells. Altered metabolite partitioning was evidenced by an increase in the soluble sugars and soluble nitrogen in adapted cells which occurred at the expense of incorporation of sugar into cell walls and nitrogen into protein. Data indicate that the relative importance of a given solute to osmotic adjustment may change depending on the level of adaptation.

Effects of Light Quality on Tiller Production in Lolium spp.

Abstract: Variations in phytochrome status in grasses were provoked by end of day red and far-red treatments and above canopy irradiations. Lolium perenne L. and Lolium multiflorum Lam. plants developed more tillers when illuminated with higher red/far-red ratios. These results show that branching of grasses is controlled by phytochrome activity in a way similar to that in dicotyledon plants.

Ultrastructure of Tomato Fruit Ripening and the Role of Polygalacturonase Isoenzymes in Cell Wall Degradation

Abstract: Ultrastructural changes in the pericarp of tomato (Lycopersicon esculentum Mill) fruit were followed during ripening. Ethylene production was monitored by gas chromatography and samples analyzed at successive stages of the ripening process.Changes in the cytoplasmic ultrastructure were not consistent with the suggestion that ripening is a ;senescence' phenomenon. A large degree of ultrastructural organization, especially of the mitochondria, chromoplasts, and rough endoplasmic reticulum, was retained by ripe fruit.Striking changes in the structure of the cell wall were noted, beginning with dissolution of the middle lamella and eventual disruption of the primary cell wall. These changes were correlated with appearance of polygalacturonase (EC 3.2.1.15) isoenzymes. Application of purified tomato polygalacturonase isoenzymes to mature green fruit tissue duplicated the changes in the cell wall noted during normal ripening. Possible roles of the polygalacturonase isoenzymes in cell wall disorganization are discussed.

Photosynthesis and Ribulose 1,5-Bisphosphate Carboxylase in Rice Leaves: Changes in Photosynthesis and Enzymes Involved in Carbon Assimilation from Leaf Development through Senescence

Abstract: Changes in photosynthesis and the ribulose 1,5-bisphosphate (RuBP) carboxylase level were examined in the 12th leaf blades of rice ( Oryza sativa L.) grown under different N levels. Photosynthesis was determined using an open infrared gas analysis system. The level of RuBP carboxylase was measured by rocket immunoelectrophoresis. These changes were followed with respect to changes in the activities of RuBP carboxylase, ribulose 5-phosphate kinase, NADP-glyceraldehyde 3-phosphate dehydrogenase, and 3-phosphoglyceric acid kinase. RuBP carboxylase activity was highly correlated with the net rate of photosynthesis ( r = 0.968). Although high correlations between the activities of other enzymes and photosynthesis were also found, the activity per leaf of RuBP carboxylase was much lower than those of other enzymes throughout the leaf life. The specific activity of RuBP carboxylase on a milligram of the enzyme protein basis remained fairly constant (1.16 ± 0.07 micromoles of CO 2 per minute per milligram at 25°C) throughout the experimental period. Kinetic parameters related to CO 2 fixation were examined using the purified carboxylase. The K m (CO 2 ) and V max values were 12 micromolar and 1.45 micromoles of CO 2 per minute per milligram, respectively (pH 8.2 and 25°C). The in vitro specific activity calculated at the atomospheric CO 2 level from the parameters was comparable to the in situ true photosynthetic rate per milligram of the carboxylase throughout the leaf life. The results indicated that the level of RuBP carboxylase protein can be a limiting factor in photosynthesis throughout the life span of the leaf.

Potassium Transport in Corn Roots : II. The Significance of the Root Periphery.

Abstract: The relative transport capabilities of the cells of the root periphery and cortex were investigated using a variety of experimental techniques. Brief (30 seconds to 1 minute) exposures with the penetrating sulfhydryl reagent, N-ethyl maleimide (NEM), and the impermeant reagent, p-chloromercuribenzene sulfonic acid (PCMBS), dramatically reduced (86)Rb(+) (0.2 millimolar RbCl) uptake into 2 centimeter corn (Zea mays [A632 x (C3640 x Oh43)]) root segments. Autoradiographic localization studies with [(3)H]NEM and [(203)Hg]PCMBS demonstrated that, during short term exposures with either reagent, sulfhydryl binding occurred almost exclusively in the cells of the root periphery.Corn root cortical protoplasts were isolated, and exhibited significant K(+)((86)Rb(+)) influx. The kinetics for K(+) uptake were studied; the influx isotherms were smooth, nonsaturating curves that approached linearity at higher K(+)(Rb(+)) concentrations (above 1 millimolar K(+)). These kinetics were identical in shape to the complex kinetics previously observed for K(+) uptake in corn roots (Kochian, Lucas 1982 Plant Physiol 70: 1723-1731), and could be resolved into a saturable and a first order kinetic component.The existence of a hypodermal apoplastic barrier was investigated. The apoplastic, cell wall binding dye, Calcofluor White M2R, appeared to be excluded from the cortex by the hypodermis. However, experiments with damaged roots indicated that this result may be an artifact resulting from the binding of dye to the epidermal cell walls. Furthermore, [(203)Hg] PCMBS autoradiography demonstrated that the hypodermis was not a barrier to apoplastic movement of PCMBS.These results suggest that although cortical cells possess the capacity to absorb ions, K(+) influx at low concentrations is limited to the root periphery. Cortical cell uptake appears to be repressed under these conditions. At higher concentrations, cortical cells may function to absorb K(+). Such a model may involve regulation of cortical cell ion transport capacity.

Cavitation Events in Thuja occidentalis L.? : Utrasonic Acoustic Emissions from the Sapwood Can Be Measured.

Abstract: Ultrasonic acoustic emissions (AE) in the frequency range of 0.1 to 1 megahertz appear to originate in the sapwood of Thuja occidentalis L. The AE are vibrations of an impulsive nature. The vibrations can be transduced to a voltage waveform and amplified. The vibrations of each AE event begin at a large amplitude which decays over 20 to 100 microseconds. Strong circumstantial evidence indicates that the ultrasonic AE result from cavitation events because: (a) they occur only when the xylem pressure potential Ψxp is more negative than a threshold level of about —1 megapascal; (b) the rate of AE events increases as Ψxp decreases and when the net rate of water loss increases; (c) the AE can be stopped by raising Ψxp above —1 megapascal. Ultrasonic AE have been measured in whole terminal shoots allowed to dry in the laboratory, in isolated pieces of sapwood as they dried in the laboratory, and in whole terminal shoots in a pressure bomb when Ψxp was decreased by lowering the gas pressure in the pressure bomb.