Biomedical subjects

T D Sharkey

Publications and source records attributed to T D Sharkey.

47 records · Page 3Linked to original sources

PubMed1986

Limitation of Photosynthesis by Carbon Metabolism : II. O(2)-Insensitive CO(2) Uptake Results from Limitation Of Triose Phosphate Utilization.

The occurrence of O(2)-insensitive photosynthesis at high quantum flux and moderate temperature in Spinacia oleracea was characterized by analytical gas exchange measurements on intact leaves. In addition photosynthetic metabolite pools were measured in leaves which had been rapidly frozen under defined gas conditions. Upon switching to low O(2) in O(2)-insensitive conditions the ATP/ADP ratio fell dramatically within one minute. The P-glycerate pool increased over the same time. Ribulose bisphosphate initially declined, then increased and exceeded the pool size measured in air. The pools of hexose monophosphates and UDPglucose were higher at a partial pressure of O(2) of 21 millibars than at 210 millibars. These results are consistent with the hypothesis that the rate of sucrose synthesis limited the overall rate of assimilation under O(2)-insensitive conditions.

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PubMed1986

Salinity and Nitrogen Effects on Photosynthesis, Ribulose-1,5-Bisphosphate Carboxylase and Metabolite Pool Sizes in Phaseolus vulgaris L.

Salinity (100 millimolar NaCl) was found to reduce photosynthetic capacity independent of stomatal closure in Phaseolus vulgaris. This reduction was shown to be a consequence of a reduction in the efficiency of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) rather than a reduction in the leaf content of photosynthetic machinery. In control plants, photosynthesis became RuBP-limited at approximately 1.75 moles RuBP per mole 2-carboxyarabinitol bisphosphate binding sites. Salinization caused the RuBP pool size to reach this limiting value for CO(2) fixation at much lower values of intercellular CO(2). Plants grown at low nitrogen and +/- NaCl became RuBP limited at similar RuBP pool sizes as the high nitrogen-grown plants. At limiting RuBP pool sizes and equal values of intercellular CO(2) photosynthetic capacity of salt-stressed plants was less than control plants. This effect of salinity on RuBPCase activity could not be explained by deactivation of the enzyme or inhibitor synthesis. Thus, salinity reduced photosynthetic capacity by reducing both the RuBP pool size by an effect on RuBP regeneration capacity and RuBPCase activity by an unknown mechanism when RuBP was limiting.

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PubMed1986

Contribution of Metabolites of Photosynthesis to Postillumination CO(2) Assimilation in Response to Lightflects.

In the shade plant Alocasia macrorrhiza grown in low light, photosynthetic CO(2) assimilation during a 5 second lightfleck plus postillumination CO(2) assimilation can allow up to 60% more photosynthesis than that which occurs during 5 seconds of steady state light of the same intensity (RL Chazdon, RW Pearcy 1986 Oecologia. 69: 524-531). Metabolites of photosynthesis were measured to determine if the pool of ribulose 1,5-bisphosphate (RuBP) could account for all of the postillumination CO(2) assimilation following a lightfleck in Alocasia. It was found that the pool of triose-P was much larger than that of RuBP and could account for five times more postillumination CO(2) assimilation than could RuBP. The same trend was seen in the sun plant Phaseolus vulgaris when it was grown in the shade. In contrast, sun-grown Alocasia and Phasiolus did not have a large pool of triose-P relative to RuBP following a lightfleck. In sun plants, carbon may rapidly be converted to RuBP in the light whereas in shade plants there may be a restriction in the path between the triose-P and RuBP pools. It is hypothesized that in shade plants the buildup of triose-P rather than RuBP during the lightfleck prevents inhibition of electron transport which may otherwise occur because of competition for ATP between the two kinases of the photosynthetic carbon reduction cycle. Utilization of the triose-P for postillumination CO(2) fixation would require the capacity for significant postillumination ATP synthesis. The extensive grana stacking and large intrathylakoid space which accompanies the high level of chlorophyll in low-light-grown Alocasia could be an important contributing factor to postillumination ATP formation.

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PubMed1985

Starch and Sucrose Synthesis in Phaseolus vulgaris as Affected by Light, CO(2), and Abscisic Acid.

Phaseolus vulgaris L. leaves were subjected to various light, CO(2), and O(2) levels and abscisic acid, then given a 10 minute pulse of (14)CO(2) followed by a 5 minute chase with unlabeled CO(2). After the chase period, very little label remained in the ionic fractions (presumed to be mostly carbon reduction and carbon oxidation cycle intermediates and amino acids) except at low CO(2) partial pressure. Most label was found in the neutral, alcohol soluble fraction (presumed sucrose) or in the insoluble fraction digestable by amyloglucosidase. Sucrose formation was linearly related to assimilation rate (slope = 0.35). Starch formation increased linearly with assimilation rate (slope = 0.56) but did not occur if the assimilation rate was below 4 micromoles per square meter per second. Neither abscisic acid, nor high CO(2) in combination with low O(2) (thought to disrupt control of carbon metabolism) caused significant perturbations of the sucrose/starch formation ratio. These studies indicate that the pathways for starch and sucrose synthesis both are controlled by the rate of net CO(2) assimilation, with sucrose the preferred product at very low assimilation rates.

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PubMed1985

O(2)-insensitive photosynthesis in c(3) plants : its occurrence and a possible explanation.

Leaves of C(3) plants which exhibit a normal O(2) inhibition of CO(2) fixation at less than saturating light intensity were found to exhibit O(2)-insensitive photosynthesis at high light. This behavior was observed in Phaseolus vulgaris L., Xanthium strumarium L., and Scrophularia desertorum (Shaw.) Munz. O(2)-insensitive photosynthesis has been reported in nine other C(3) species and usually occurred when the intercellular CO(2) pressure was about double the normal pressure. A lack of O(2) inhibition of photosynthesis was always accompanied by a failure of increased CO(2) pressure to stimulate photosynthesis to the expected degree. O(2)-insensitive photosynthesis also occurred after plants had been water stressed. Under such conditions, however, photosynthesis became O(2) and CO(2) insensitive at physiological CO(2) pressures. Postillumination CO(2) exchange kinetics showed that O(2) and CO(2) insensitivity was not the result of elimination of photorespiration.It is proposed that O(2) and CO(2) insensitivity occurs when the concentration of phosphate in the chloroplast stroma cannot be both high enough to allow photophosphorylation and low enough to allow starch and sucrose synthesis at the rates required by the rest of the photosynthetic component processes. Under these conditions, the energy diverted to photorespiration does not adversely affect the potential for CO(2) assimilation.

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PubMed1983

Gas Exchange, Stomatal Behavior, and deltaC Values of the flacca Tomato Mutant in Relation to Abscisic Acid.

The relationship between stomatal conductance and capacity for assimilation was investigated in flacca, a mutant of tomato (Lycopersicon esculentum Mill.) that has abnormal stomatal behavior and low abscisic acid (ABA) content. The assimilation capacity, determined by measuring assimilation rate as a function of intercellular CO(2) pressure, did not differ in leaves of flacca and its parent variety, Rheinlands Ruhm (RR). On the other hand, stomatal conductance of flacca leaves was greater than that of RR, and could be phenotypically reverted by spraying with 30 micromolar ABA. Stomatal conductance of flacca leaves was also reduced by increasing CO(2) pressure, increasing leaf to air vapor pressure difference, and decreasing quantum flux, irrespective of ABA treatment.The high conductance of flacca leaves resulted in a high intercellular CO(2) pressure. This allowed greater discrimination against (13)CO(2), as evidenced by more negative delta (13)C values for flacca as compared to RR. The delta (13)C values of both flacca and RR plants as influenced by ABA treatment were consistent with predictions based on gas exchange measurements, using a recent model of discrimination.

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PubMed1982

A Direct Confirmation of the Standard Method of Estimating Intercellular Partial Pressure of CO(2).

The partial pressure of CO(2) inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber allowed normal CO(2) and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO(2) in the closed system rapidly reached a steady pressure which it is believed was identical to the CO(2) pressure inside the leaf, because there was no flux of CO(2) across the epidermis. This measured partial pressure was in close agreement with that estimated from a consideration of the fluxes of CO(2) and vapor at the other surface.

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PubMed1982

Effects of G, a Growth Regulator from Eucalyptus grandis, on Photosynthesis.

A growth regulator (G; 4-ethyl-1-hydroxy-4,8,8,10,10 pentamethyl-7,9-dioxo-2,3 dioxyabicyclo (4.4.0) decene-5) from Eucalyptus grandis (Maiden) reduced stomatal conductance and also photosynthetic capacity when fed through the transpiration stream of detached leaves. The concentration of G required for this effect was high (10(-4) molar), but the amount of G taken up (dose) was below the level which has previously been found in E. grandis leaves. Similar effects were observed in detached leaves of Xanthium strumarium L. though almost 10 times more G was required. G reduced CO(2)-dependent O(2) evolution from isolated cells of X. strumarium. In spinach (Spinacia oleracea L.) chloroplasts, electron transport through photosystem II was reduced by G. It is proposed that G affects stomatal conductance and photosynthesis by reducing photosystem II activity in both the guard cell chloroplasts and mesophyll cell chloroplasts.

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PubMed1981

Separation and measurement of direct and indirect effects of light on stomata.

Conductance for water vapor, assimilation of CO(2), and intercellular CO(2) concentration of leaves of five species were determined at various irradiances and ambient CO(2) concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO(2) concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO(2) concentration was constant, and when CO(2) concentration changed and irradiance was constant. On leaves of Xanthium strumarium L., Gossypium hirsutum L., Phaseolus vulgaris L., and Perilla frutescens (L.), Britt., the stomatal response to light was determined to be mainly a direct response to light and to a small extent only a response to changes in intercellular CO(2) concentration. This was also true for stomata of Zea mays L., except at irradiances < 150 watts per square meter, when stomata responded primarily to the depletion of the intercellular spaces of CO(2) which in turn was caused by changes in the assimilation of CO(2).Stomata responded to light even in leaves whose net exchange of CO(2) was reduced to zero through application of the inhibitor of photosynthetic electron transport, cyanazine (2-chloro-4[1-cyano-1-methylethylamino]-6-ethylamino-S-triazine). When leaves were inverted and irradiated on the abaxial surface, conductance decreased in the shaded and increased in the illuminated epidermis, indicating that the photoreceptor pigment(s) involved are located in the epidermis (presumably in the guard cells). In leaves of X. strumarium, the direct effect of light on conductance is primarily a response to blue light.Stomatal responses to CO(2) and to light opposed each other. In X. strumarium, stomatal opening in response to light was strongest in CO(2) free air and saturated at lower irradiances than in CO(2) containing air. Conversely, stomatal closure in response to CO(2) was strongest in darkness and it decreased as irradiance increased. In X. strumarium, P. vulgaris, and P. frutescens, an irradiance of 300 watts per square meter was sufficient to eliminate the stomatal response to CO(2) altogether. Application of abscisic acid, or an increase in vapor pressure deficit, or a decrease in leaf temperature reduced the stomatal conductance at light saturation, but when the data were normalized with respect to the conductance at the highest irradiance, the various curves were congruent.

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PubMed1981

Effect of Light Quality on Stomatal Opening in Leaves of Xanthium strumarium L.

Flux response curves were determined at 16 wavelengths of light for the conductance for water vapor of the lower epidermis of detached leaves of Xanthium strumarium L. An action spectrum of stomatal opening resulted in which blue light (wavelengths between 430 and 460 nanometers) was nearly ten times more effective than red light (wavelengths between 630 and 680 nanometers) in producing a conductance of 15 centimoles per square meter per second. Stomata responded only slightly to green light. An action spectrum of stomatal responses to red light corresponded to that of CO(2) assimilation; the inhibitors of photosynthetic electron transport, cyanazine (2-chloro-4[1-cyano-1-methylethylamino]-6-ethylamino-s-triazine) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, eliminated the response to red light. This indicates that light absorption by chlorophyll is the cause of stomatal sensitivity to red light. Determination of flux response curves on leaves in the normal position (upper epidermis facing the light) or in the inverted position (lower epidermis facing the light) led to the conclusion that the photoreceptors for blue as well as for red light are located on or near the surfaces of the leaves; presumably they are in the guard cells themselves.

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PubMed1980

Effects of phaseic Acid and dihydrophaseic Acid on stomata and the photosynthetic apparatus.

Plant extracts containing phaseic acid (PA), as well as solutions of purified PA and dihydrophaseic acid (DPA) were applied to leaves, isolated mesophyll cells, and isolated epidermal strips. In Commelina communis, stomatal closure began 4 minutes after the addition of either 20 micromolar (+/-)-abscisic acid or 10 micromolar PA. Stomata closed less rapidly after treatment with 10 micromolar PA than after treatment with 10 micromolar (+/-)-abscisic acid in Amaranthus powelli, Hordeum vulgare, Xanthium strumarium, and Zea mays and did not respond at all to PA in Vicia faba. DPA (10 micromolar) did not cause stomatal closure in any species.Plant extracts containing PA reduced photosynthesis, as reported by Kriedemann et al. (Aust J Plant Physiol 2: 553-567, 1975). Subsequent experiments with PA purified by crystallization and with residues of solvents employed in the extraction of PA proved that it was not PA that impaired photosynthetic O(2) evolution or CO(2) uptake but unidentified contaminants of the allegedly pure solvents.

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