Environmental Responses of the Post-lower Illumination CO(2) Burst as Related to Leaf Photorespiration

As leaf irradiance is decreased in increments, a single transient CO₂ burst is exhibited by C₃ plant leaves. This post-lower illumination CO₂ burst (PLIB) is sensitive to changes in irradiance, to changes in the concentrations of O₂ and CO₂, and to temperature. Increasing O₂ concentrations above ambient produces a progressively larger PLIB while increasing CO₂ concentrations above ambient produces a progressively smaller PLIB. The PLIB, which exhibits many responses to environment common with other methods for measuring photorespiration and photosynthesis, is proposed as a measure of photorespiration in illuminated leaves of C₃ plants. Although the PLIB cannot be used as a quantitative measurement of photorespiration, we propose that the PLIB is a rapid, easy, relatively inexpensive, nondestructive method for evaluating photorespiration in intact illuminated C₃ leaves in air.     

Effect of Light Intensity during Growth on Photoinhibition of Intact Attached Bean Leaflets

In the study reported here, two different photoinhibitory phenomena were compared within a single plant species. Bean plants were grown in three different light intensities to simulate sun and shade environments. The effects of photoinhibitory treatments on in vivo CO₂ assimilation rates and in vitro chloroplast electron transport reactions were investigated and the extent to which carbon metabolism served to prevent photoinhibition was characterized. It was shown that the photoinhibition which follows exposure of intact leaflets of low light-grown bean plants to high light intensity in normal air is essentially similar to that which occurs when leaflets of plants grown in full sunlight are illuminated in the absence of CO₂ at low O₂ partial pressures.     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

Relationship between Photosynthesis and Respiration: The Effect of Carbohydrate Status on the Rate of CO(2) Production by Respiration in Darkened and Illuminated Wheat Leaves

The rate of dark CO₂ 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 CO₂ efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO₂ efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O₂ and 330 microbar CO₂ before darkening. After 30 minutes of darkness, a relatively steady rate of CO₂ efflux was obtained. The temperature dependence of steady-state dark CO₂ efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO₂ efflux following photosynthesis is correlated with accumulated net CO₂ assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO₂ compensation point in 21% O₂, and an increase in the light compensation point. The interactions between CO₂ efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO₂ efflux by respiration is comparable in darkened and illuminated wheat leaves.     

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

The occurrence of O₂-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 O₂ in O₂-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 O₂ 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 O₂-insensitive conditions.     

Glyphosate effects on carbon assimilation and gas exchange in sugar beet leaves

The mechanism responsible for the inhibition of net carbon exchange (NCE) which was reported previously (DR Geiger et al. 1986 Plant Physiol 82: 468-472) was investigated by applying glyphosate [N-(phosphonomethyl)glycine] to exporting leaves of sugar beet (Beta vulgaris L.). Leaf internal CO₂ concentration (C_i) remained constant despite decreases in stomatal conductance and NCE following glyphosate treatment, indicating that the cause of the inhibition was a slowing of carbon assimilation rather than decreased conductance of CO₂. Throughout a range of CO₂ concentrations, NCE rate at a given C_i declined gradually, with the time-series of response curves remaining parallel. Gas exchange measurements revealed that disruption of chloroplast carbon metabolism was an early and important factor in mediating these glyphosate effects, perhaps by slowing the rate of ribulose bisphosphate regeneration. An increase in the CO₂ compensation point accompanied the decrease in NCE and this increase was hastened by stepwise lowering of the ambient CO₂ concentration. Eventually the CO₂ compensation point approached the CO₂ level of air and the difference between internal and external CO₂ concentrations decreased. In control and in glyphosate-treated plants, both carbon assimilation and photorespiration at atmospheric CO₂ level were inhibited to a similar extent of air level of O₂. Maintaining leaves in low O₂ concentration did not prevent the decline in NCE rate.     

Water-water cycle involved in dissipation of excess photon energy in phosphorus deficient rice leaves

The water-water cycle which may be helpful for dissipating the excitation pressure over electron transport chain and minimizing the risk of photoinhibition and photodamage was investigated in rice after 10-d P-deficient treatment. Net photosynthetic rate decreased under P-deficiency, thus the absorption of photon energy exceeded the energy required for CO₂ assimilation. A more sensitive response of effective quantum yield of photosystem 2 (Φ_PS2) to O₂ concentration was observed in plants that suffered P starvation, indicating that more electrons were transported to O₂ in the P-deficient leaves. The electron transport rate through photosystem 2 (PS 2) (J_f) was stable, and the fraction of electron transport rate required to sustain CO₂ assimilation and photorespiration (J_g/J_f) was significantly decreased accompanied by an increase in the alternative electron transport (J_a/J_f), indicating that a considerable electron amount had been transported to O₂ during the water-water cycle in the P-deficient leaves. However, the fraction of electron transport to photorespiration (J_o/J_f) was also increased in the P-deficient leaves and it was less sensitive than that of water-water cycle. Therefore, water-water cycle could serve as an efficient electron sink. The higher non-photochemical fluorescence quenching (q_N) in the P-deficient leaves depended on O₂ concentration, suggesting that the water-water cycle might also contribute to non-radiative energy dissipation. Hence, the enhanced activity of the water-water cycle is important for protecting photosynthetic apparatus under P-deficiency in rice.     

Conversion of photosynthetic products in the light in CO2-free O2 and N2 in leaves of Zea mays L. and Phaseolus vulgaris L.

Summary After 10 min illumination of segments of bean (Phaseolus vulgaris L.) or maize (Zea mays L.) leaves in air with ¹⁴CO₂, the atmosphere was changed to CO₂-free O₂ or N₂ and conversion of photosynthetic products in the light was investigated. The experiments have shown that after the ¹⁴CO₂ assimilation period the bean leaves contain the pool of weakly fixed ¹⁴C (WF-¹⁴C) which is converted into stable products during the subsequent period of illumination in CO₂-free N₂. In O₂ atmosphere the WF-¹⁴C pool is initially the main source of CO₂ evolved. The marked decrease in radioactivity of sucrose and starch during illumination of bean leaves in O₂ atmosphere indicates that these compounds were also the source of CO₂ evolved in the light. The total amount of previously fixed ¹⁴C remained almost on the same level during illumination of maize leaves in N₂ as well as in O₂. However, oxygen changed the distribution of ¹⁴C in photosynthetic products, which is suggested to be the consequence of the photorespiration process in maize.Abbreviation WF-¹⁴C weakly fixed ¹⁴C     

Prior illumination and the respiration of maize leaves in the dark

The course of respiration of attached maize (Zea mays L.) leaves was measured by infrared gas analysis of CO₂ efflux in the dark following illumination in atmospheres of 300 microliters of CO₂ per liter of air, CO₂-free air, and CO₂-free N₂ containing 400 microliters of O₂ per liter. CO₂ efflux from control leaves started 3 to 4 minutes after darkening, increased to a maximum after about 20 minutes, and returned to a steady minimum after 2 to 3 hours. Respiration was quantitatively related to prior illumination, independent of net CO₂ fixation in the light, and depressed by N₂. Light, but not air, was required to produce a substrate for respiration in the subsequent dark period; air was required for oxidation of the substrate to CO₂. The stimulation of respiration by prior illumination in maize leaves differs in its slower onset and greater duration from the postillumination burst of photorespiration.     

Is Transfer of CO2 to C3-Plants Purely by Diffusion?

The dependence of the CO₂ compensation concentration on O₂ partial pressure and the dependence of differential uptake of ¹⁴CO₂ and ¹²CO₂ on CO₂ and O₂ partial pressures are analyzed in illuminated white clover (Trifolium repens L.) leaves. The data show a deviation of the photosynthetic gas exchange from ribulose bisphosphate carboxylase oxygenase kinetics at 10°C but not at 30°C. This deviation is due to an effect of CO₂ partial pressure on the ratio of photosynthesis to photorespiration which can be explained if active inorganic carbon transport is assumed.     

Effect of oxygen on photosynthesis, photorespiration and respiration in detached leaves. I. Soybean

The effect of O₂ on the CO₂ exchange of detached soybean leaves was measured with a Clark oxygen electrode and infrared carbon dioxide analysers in both open and closed systems.

The rate of apparent photosynthesis was inhibited by O₂ while the steady rate of respiration after a few minutes in the dark was not affected. Part of the inhibition of apparent photosynthesis was shown to be a result of increased photorespiration. This stimulation of photorespiration by O₂ was manifested by an increase in the CO₂ compensation point.

The differential effects of O₂ on dark respiration (no effect) and photorespiration (stimulation) indicated that these were 2 different processes.

Moreover the extrapolation of the CO₂ compensation point to zero at zero O₂ indicated that dark respiration was suppressed in the light at least at zero O₂ concentration.

     

Photosynthesis at low water potentials in sunflower: lack of photoinhibitory effects

The losses in chloroplast capacity to fix CO₂ when photosynthesis is reduced at low leaf water potential (ψ₁) have been proposed to result from photoinhibition. We investigated this possibility in soil-grown sunflower (Helianthus annuus L. cv IS894) using gas exchange techniques to measure directly the influence of light during dehydration on the in situ chloroplast capacity to fix CO₂. The quantum yield for CO₂ fixation as well as the rate of light- and CO₂-saturated photosynthesis were strongly inhibited at low ψ₁. The extent of inhibition was the same whether the leaves were exposed to high or to low light during dehydration. When intercellular partial pressures of CO₂ were decreased to the compensation point, which was lower than the partial pressures resulting from stomatal closure, the inhibition of the quantum yield was also unaffected. Photoinhibition could be observed only after high light exposures were imposed under nonphysiological low CO₂ and O₂ where both photosynthesis and photorespiration were suppressed. The experiments are the first to test whether gas exchange at low ψ₁ is affected by potentially photoinhibitory conditions and show that the loss in chloroplast capacity to fix CO₂ was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.     

Photoinhibition of photosynthesis: effect of O2 and selective excitation of the photosystems in intact Lemna gibba plants

Intact Lemna gibba plants were photoinhibited under anaerobic conditions on illumination with monochromatic light which selectively excited the photosystems. Photoinhibition was less when PS 1 was excited and greatest when mainly PS 2 was excited, which suggests that PS 2 was most damaged by photoinhibition induced in complete absence of O₂ and CO₂.The illumination of plants with monochromatic light exciting PS 1, at different O₂ concentrations (in CO₂ deficient conditions), showed that PS 1 photoinhibition was increased at the low O₂ concentrations. The damage to PS 1 was more evident at 2% O₂ than at the higher O₂ concentrations.CO₂ as well as O₂ at atmospheric concentration, (air), was necessary for complete protection of the plant from photoinhibition when both photosystems were excited either separately or together.Abbreviations I irradiance, photon fluence rate - PCO photosynthetic carbon oxidation cycle - PCR photosynthetic carbon reduction cycle - PS 1 photosystem 1 - PS 2 photosystem 2     

Leaf Conductance in Relation to Rate of CO(2) Assimilation: III. Influences of Water Stress and Photoinhibition

Rates of CO₂ assimilation and leaf conductances to CO₂ transfer were measured in plants of Zea mays during a period of 14 days in which the plants were not rewatered, and leaf water potential decreased from −0.5 to −8.0 bar. At any given ambient partial pressure of CO₂, water stress reduced rate of assimilation and leaf conductance similarly, so that intercellular partial pressure of CO₂ remained almost constant. At normal ambient partial pressure of CO₂, the intercellular partial pressure of CO₂ was estimated to be 95 microbars. This is the same as had been estimated in plants of Zea mays grown with various levels of nitrogen supply, phosphate supply and irradiance, and in plants of Zea mays examined at different irradiances.

After leaves of Phaseolus vulgaris L. and Eucalyptus pauciflora Sieb. ex Spreng had been exposed to high irradiance in an atmosphere of CO₂-free N₂ with 10 millibars O₂, rates of assimilation and leaf conductances measured in standard conditions had decreased in similar proportions, so that intercellular partial pressure of CO₂ remained almost unchanged. As the conductance of each epidermis that had not been directly irradiated had declined as much as that in the opposite, irradiated surface it was hypothesized that conductance may have been influenced by photoinhibition within the mesophyll tissue.

     

Inhibition of photosynthesis by carbohydrates in wheat leaves

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.     

Investigation of the CO(2) Dependence of Quantum Yield and Respiration in Eucalyptus pauciflora

In leaves of C₃ plants, the rate of nonphotorespiratory respiration appears to be higher in darkness than in the light. This change from a high to a low rate of carbon loss with increasing photon flux density leads to an increase in the apparent quantum yield of photosynthetic CO₂ assimilation at low photon flux densities (Kok effect). The mechanism of this suppression of nonphotorespiratory respiration is not understood, but biochemical evidence and the observation that a Kok effect is often not observed under low O₂, has led to the suggestion that photorespiration might be involved in some way. This hypothesis was tested with snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) using gas exchange methods. The test was based on the assumption that if photorespiration were involved, then it would be expected that the intercellular partial pressure of CO₂ would also have an influence on the Kok effect. Under normal atmospheric levels of CO₂ and O₂, a Kok effect was found. Changing the intercellular partial pressure of CO₂, however, did not affect the estimate of nonphotorespiratory respiraton, and it was concluded that its decrease with increasing photon flux density did not involve photorespiration. Concurrent measurements showed that the quantum yield of net assimilation of CO₂ increased with increasing intercellular partial pressure of CO₂, and this increase agreed closely with predictions based on recent models of photosynthesis.     

Leaf development,gas exchange characteristics,and photorespiratory activity in maize seedlings

Five decades ago, a novel mode of CO₂ assimilation that was later described as C₄-photosynthesis was discovered on mature leaves of maize (Zea mays L.) plants. Here we show that 3- to 5-day-old developing maize leaves recapitulate the evolutionary advance from the ancient, inefficient C₃ mode of photosynthesis to the C₄ pathway, a mechanism for overcoming the wasteful process of photorespiration. Chlorophyll fluorescence measurements documented that photorespiration was high in 3-day-old juvenile primary leaves with non-specialized C₃-like leaf anatomy and low in 5-day-old organs with the typical “Kranz-anatomy” of C₄ leaves. Photosynthetic gas (CO₂)-exchange measurements on 5-day-old leaves revealed the characteristic features of C₄ photosynthesis, with a CO₂ compensation point close to zero and little inhibition of photosynthesis by the normal oxygen concentration in the air. This indicates a very low photorespiratory activity in contrast to control experiments conducted with mature C₃ sunflower (Helianthus annuus L.) leaves, which display a high rate of photorespiration.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

Postillumination respiration of maize in relation to oxygen concentration and glycolic Acid metabolism

Prior illumination in CO₂-free air enhances a respiration from maize (Zea mays L.) leaves different in onset and duration from the postillumination burst of photorespiration. The course of respiration after brief illumination of attached leaves was measured as CO₂ efflux in darkness into CO₂-free atmospheres with four O₂ concentrations. The peak of CO₂ efflux following illumination was suppressed by 2.23% O₂, was completely eliminated by 0.04% O₂, and was not stimulated by 40% O₂ compared with air. Compared with air, steady dark respiration was suppressed by 0.04% O₂ but was not affected by 2.23% nor 40% O₂. Excision and subsequent uptake of distilled water through the vascular system nearly eliminated the enhanced respiration.     

O2-dependent inhibition of photosynthetic capacity in intact isolated chloroplasts and isolated cells from spinach leaves illuminated in the absence of CO2

When isolated intact chloroplasts or cells from spinach (Spinacia oleracea L.) leaves are incubated in the light in the absence of CO₂, their capacity for subsequent CO₂-dependent photosynthetic oxygen evolution is drastically decreased. This inhibition is light and oxygen-dependent and can be prevented by addition of bicarbonate. It is concluded that the normal dissipation of photosynthetic energy by carbon assimilation and in processes related to photorespiration is an essential condition for the physiological stability of illuminated intact chloroplasts and cells.Abbreviation chl chlorophyll