Photosynthesis of Ivy Leaves (Hedera helix) after Heat Stress I. CO2-Gas Exchange and Diffusion Resistances

For an analysis of the inhibition of the photosynthetic CO₂-uptake after heat stress attached leaves of Hedera helix L. were heat-stressed for 30 min at various temperatures. Subsequently their photosynthetic CO₂-uptake, transpiration, respiration in light and darkness, and CO₂-compensation concentration were measured under optimal conditions. After heat stress the stomatal resistance increased only corresponding to the raised CO₂-concentration inside the leaves (due to the reduced CO₂-uptake). The physical resistance between the mesophyll cell walls and the chloroplasts remained unchanged after heat stress. A non-stomatal inhibition of the CO₂-uptake is indicated by a strong increase of the CO₂-compensation concentration after heat stress. This is hardly due to a stimulation of the respiration in light, as the CO₂-evolution into CO₂-free air in light was even reduced. Therefore, it must be concluded that the photosynthetic process itself is impaired after heat stress.     

Effects of water stress on gas exchange and water relations of a succulent epiphyte,Kalanchoë uniflora

Summary Measurements of gas exchange, xylem tension and nocturnal malate synthesis were conducted with well-watered and droughted plants of Kalanchoë uniflora. Corresponding results were obtained with plants grown in 9 h and 12 h photoperiods. In well-watered plants, 50 to 90% of total CO₂-uptake occurred during the light period. Nocturnal CO₂-uptake and malate synthesis were higher and respiration rate was lower in old leaves (leaf pairs 6 to 10) compared to young leaves (leaf pairs 1 to 5). Within four days of drought distinct physiological changes occurred. Gas exchange during the light period decreased and CO₂-uptake during the dark period increased. Nocturnal malate synthesis significantly increased in young leaves.Respiration rate decreased during periods of drought, this decrease being more pronounced in young leaves compared to old leaves. Restriction of gas exchange during the light period resulted in a decrease of transpiration ratio from more than 100 to about 20. The difference between osmotic pressure and xylem tension decreased in young leaves, indicating a reduction in bulk leaf turgor-pressure.We conclude that both the CAM-enhancement in young leaves and the decrease of respiration rate are responsible for the increase of nocturnal CO₂-uptake during water stress. During short drought periods, which frequently occur in humid habitats, the observed physiological changes result in a marked reduction of water loss while net CO₂-uptake is maintained. This might be relevant for plant growth in the natural habitat.Abbreviations LP light period - DP dark period - CAM crassulacean acid metabolism     

CO(2) and O(2) Exchanges in the CAM Plant Ananas comosus (L.) Merr: Determination of Total and Malate-Decorboxylation-Dependent CO(2)-Assimilation Rates; Study of Light O(2)-Uptake

Photosynthesis and light O₂-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO₂ and O₂ gas exchange measurements. The amount of CO₂ which was fixed during a complete day-night cycle was equal to the amount of total net O₂ evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O₂-evolution and of net light atmospheric CO₂-uptake give the rates of malate-decarboxylation-dependent CO₂ assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O₂ evolved/net CO₂ fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO₂ for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO₂ assimilation deduced from net O₂-evolution was 3 times higher than the maximum rate of atmospheric CO₂-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO₂-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO₂-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO₂ fixed during the last hour of the light period was stored as malate. Light O₂-uptake determined with ¹⁸O₂ showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O₂-uptake increased progressively; during the following part of the light period, the rate of O₂-uptake was 3.5 times higher than the maximum rate of CO₂-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO₂-free atmosphere or in continuous illumination, the rate of O₂-uptake was higher than in the control. This supports the hypothesis that the low rate of O₂-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO₂ concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O₂ and CO₂ gas exchange suggest that at the end of the light period the intracellular CO₂ concentration was very low. We propose that the high ratio of O₂-uptake/CO₂-fixation is principally caused by the stimulation of photorespiration during this period.     

Variation in Photorespiration in Lolium

The rate of photorespiration in several grass species was shownto be highly variable and dependent on the species, genotype,and conditions under which the plants were grown. Photorespiration,measured as oxygen uptake, was negligible in Cenchrus ciliarisand Paspalum dilatatum but significant in Lolium spp. and Festucaarundinacea. There were significant differences in the rateof photorespiration among plants within a Lolium populationof diverse origin and these differences were independent ofthe conditions under which the plants were grown. Among thetemperate grasses there was a significant correlation betweenphotorespiration and the CO₂-compensation concentration andboth parameters were very low in P. dilatatum. Plants grownin day/night temperatures of 15/10 °C compared with 25/20°C had faster rates of dark respiration but slower ratesof light respiration when measured at the same temperature.Photorespiration was faster than dark respiration although differencesin respiration among plants in the light were not shown in thedark.     

Light-enhanced carbon dioxide fixation in isolated chloroplasts

Preillumination of leaves of spinach, soybean and maize in theabsence of CO₂ greatly enhanced the capacity for fixing CO₂in an immediately following dark period. Lightenhanced darkCO₂-fixation was further observed in isolated chloroplasts ofspinach and soybean. When isolated chloroplasts were illuminated,CO₂-fixing capacity in the subsequent dark period increasedrapidly at first and later more slowly attaining a stationaryvalue in about 20 min. When the light was turned off at thisstage, the capacity decreased very rapidly becoming zero inabout 10 min. The magnitude of the enhanced dark fixation andits decay in the dark were not influenced by the presence orabsence of atmospheric oxygen. In both leaves and isolated chloroplasts,no significant change in oxygen (21%) occurred in distributionpatterns of radioactivity in products fixed by photosynthetic,or light-enhanced, dark, ¹⁴CO₂-fixation. In preilluminated leaves¹⁴C was incorporated into sucrose in the subsequent dark period,indicating that the photosynthetic carbon reduction cycle isoperating in light-enhanced dark fixation in higher plants. ¹Present address: Noda Institute for Scientific Research, Noda,Chiba Prefecture (Received August 10, 1970; )     

Photosynthesis and transpiration of tomato and CO2 fluxes in a greenhouse under changing environmental conditions in winter

Responses of tomato leaves in a greenhouse to light and CO₂ were examined at the transient stage at the end of winter, when both photoperiod and irradiance gradually increase. Additionally, CO₂ fluxes were calculated for a greenhouse without supplementary lighting and without CO₂ enrichment based on CO₂ sinks (plant photosynthesis) and CO₂ sources (plant and substrate respiration). In January, tomato leaves in the greenhouse showed low photosynthesis with a maximum assimilation of 6–8 μmol CO₂ m⁻² s⁻¹, a quantum yield of 0.06 μmol CO₂ μmol⁻¹ photosynthetic active radiation (PAR) and a low light compensation point of 26 μmol PAR m⁻² s⁻¹, a combination which classifies them as shade leaves. In February, tomato leaves increased their light compensation point to 39 μmol PAR m⁻² s⁻¹ and quantum yield to 0.08, the former indicating the adaptation to increased irradiance and photoperiod. These tomato leaves increased their transpiration from 0.4 to 0.9 in January to ∼2 mmol H₂O m⁻² s⁻¹ in February. Both photosynthesis and transpiration were primarily limited by light but neither by stomatal conductivity nor by CO₂. In January, light response of photosynthesis, dark respiration and transpiration were negligibly affected by increasing CO₂ concentrations from 600 to 900 ppm CO₂ under low light conditions, indicating no benefit of CO₂ enrichment unless light intensity increased. In February, tomato leaves were photoinhibited at inherent greenhouse CO₂ concentrations on the first sunny day; this photoinhibition was further enhanced by an increased CO₂ concentration of 1000 ppm. CO₂ fluxes in the greenhouse appeared strongly dependent on solar radiation. After exceeding the light compensation point in the morning, greenhouse CO₂ concentrations decreased by 58 or by 110 ppm CO₂ h⁻¹ on a sunny day in January or February and by 23 ppm on overcast days in both months. Calculated per overall tomato canopy, plant photosynthesis contributed 42–50% to the morning CO₂ depletion in the greenhouse. Dark respiration of tomato leaves was ∼2 μmol CO₂ m⁻² s⁻¹ in January and ∼3 μmol CO₂ m⁻² s⁻¹ in February. This dark respiration resulted in rises of 15 and 17 ppm CO₂ h⁻¹ at night in the greenhouse compartment and was identified as primary source of CO₂. Respiration of the substrate used to grow the plants, which produced 7.3 ppm CO₂ h⁻¹, was identified as secondary source of CO₂. The combined plant and substrate respiration resulted in peaks of up to 900 ppm CO₂ in the greenhouse before dawn.     

Photosynthetic induction in C4 leaves

Simultaneous measurements of CO₂ uptake, transpiration rate, and chlorophyll a fluorescence in leaf strips of C₄ plants during the induction phase of photosynthesis are described. The timecourse of CO₂ fixation is biphasic with the initial phase occurring within the first 1 to 5 min and the secondary phase consisting of a slow rise to the steady-state rate of photosynthesis. Transpiration rate follows the CO₂-fixation timecourse closely but the intercellular CO₂ concentration never falls below saturation for C₄ plants. Chlorophyll a fluorescence quenching occurs exclusively during the initial fast phase of the CO₂-fixation timecourse. The effect of duration of dark pretreatment of leaves on these parameters and the effects of light intensity and CO₂ concentration are examined. These results are discussed with respect to the C₄ cycle and photochemical and non-photochemical chlorophyll fluorescence quenching.Abbreviations IRGA infra-red gas analyser - NADP-ME, NAD-ME and PEP-CK the three groups of C₄ plants utilising the enzymes NADP-malic enzyme, NAD-malic enzyme and phosphoenolpyruvate carboxykinase, respectively, for C₄-acid decarboxylation - PEP phosphoenolpyruvate - 3-PGA 3-phosphoglyceric acid     

Influence of light intensity at different temperatures on rate of respiration of douglas-fir seedlings

The rate of photorespiration of Douglas-fir seedlings was measured under different light intensities by: (1) extrapolating the curve for CO₂ uptake in relation to atmospheric CO₂ content to zero CO₂ content, and (2) measuring CO₂ evolution of the plants into a CO₂-free airstream. Different results, obtained from these techniques, were believed to be caused by a severe restriction of the photosynthetic activity when the latter was used. With the first method, CO₂ evolution was lower than the dark respiration rate at low light intensity. For all temperatures studied (6°, 20°, 28°) a further increase in light intensity raised the CO₂ evolution above dark respiration before it leveled off. The rate of CO₂ evolution was stimulated by increase in temperature at all light intensities. With the CO₂-free air method, CO₂ evolution in the light was less than dark respiration at all light intensities.     

Light control of the respiration of exogenous glycerol in the red macroalga Grateloupia doryphora

Heterotrophic activity in macroalgae has been little studied, but the red macroalga Grateloupia doryphora is known to grow in light at a higher rate in a glycerol-containing medium than in seawater. The effects of 0·1 M exogenous glycerol in seawater (SW90-gly) on the respiration rate of G. doryphora and the role played by light were investigated. The algae pretreated for 2 h in the light and in SW90-gly evolved oxygen and fixed carbon dioxide (H¹⁴CO₃ ^?), but also evolved radioactive ¹⁴CO₂ from [¹⁴C]glycerol. The rate of oxygen evolution was lower than that of samples in seawater, due to a high respiration rate and/or a partial inhibition of photosynthesis induced by glycerol. In contrast, the rate of inorganic carbon fixation was higher in SW90-gly than in control samples in seawater, suggesting that non-photosynthetic patterns were operating. In darkness, after pretreatment in the light in SW90-gly, samples showed a high oxygen uptake rate just after the light was turned off. Twenty minutes of darkness were enough to decrease this high respiration rate to that of samples in seawater. The oxygen uptake observed in all experiments with glycerol was mitochondrial as it was inhibited by potassium cyanide and salicylhydroxamic acid (SHAM). Pretreatment of samples in the light in SW90-gly with the photosynthetic inhibitor DCMU did not inhibit ensuing dark respiration, thus providing evidence for a non-photosynthetic effect of the light. The highest dark respiration rate was observed after the samples were pretreated in monochromatic blue light in glycerol-containing media.     

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.

     

Carbon exchange rates of shoots required to utilize available acetylene reduction capacity in soybean and alfalfa root nodules

The CO₂-exchange rate required to make full use of available N₂-fixation capacity, measured as acetylene reduction, was determined in soybean and alfalfa. Carbohydrates of root systems were depleted during a 40-hour dark treatment; then plants were exposed to a 24-hour light period during which different CO₂-exchange rates were maintained with various CO₂ concentrations. In three- and four-week-old soybeans and four-week-old alfalfa plants, acetylene-reduction capacity was used fully with CO₂-exchange rates as low as 10 milligrams CO₂ per plant per hour. In six-week-old alfalfa plants, however, acetylene reduction rates increased linearly, and apparent N₂-fixation capacity was not used fully when CO₂-exchange rates were higher than 40 milligrams CO₂ per plant per hour. Under the conditions established, the energy cost of N₂ fixation, measured as Δ(respiration of roots + nodules)/Δacetylene reduction over dark-treatment values, was 0.453 milligrams CO₂ per micromole C₂H₄ for all rates of acetylene reduction and for both ages of soybean and alfalfa plants. Thus, root-plus-nodule respiration was not promoted by higher rates of apparent photosynthesis after C₂H₂-reduction capacity became saturated, and all available capacity for apparent N₂ fixation had the same energy requirement.     

Effect of changes in shoot carbon-exchange rate on soybean root nodule activity

The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean (Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N₂ fixation. Shoot CER, root + nodule respiration, and apparent N₂ fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N₂ fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N₂ fixation was measured within 4 hours, and decreasing CO₂ concentration from 320 to 90 microliters CO₂ per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO₂ concentration around the shoot from 320 to 1,000 microliters CO₂ per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO₂-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO₂ enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N₂ fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N₂-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known.     

Photosynthetic characteristics of the shade-adapted C4 grass Muhlenbergia sobolifera (Muhl.) Trin.: control of development of photorespiration by growth temperature

Muhlenbergia sobolifera (Muhl.) Trin., a C₄ grass, occurs in understory habitats in the northeastern United States. Plants of M. sobolifera were grown at 23 and 30°C at 150 and 700 μmol photons m⁻² s⁻¹. The photosynthetic CO₂ compensation point, maximum CO₂ assimilation, dark respiration and the absorbed quantum use efficiency (QUE) were measured at 23 and 30°C at 2 and 20% O₂. Photosynthetic CO₂ compensation points ranged from 4 to 14mm³ dm⁻³ CO₂ and showed limited O₂ sensitivity. The mean photosynthetic CO₂ compensation point of plants grown at 30°C (4·5 mm³ dm⁻³) was 57% lower and 80% less inhibited by O₂ than that of plants grown at 23°C. Photosynthesis was similarly affected by growth temperature, with 70% more O₂ inhibition in plants grown at 23°C; suppression over all treatments ranging from 2 to 11%. Unlike typical C₄ species, plants of M. sobolifera from both temperature regimes exhibited higher CO₂ assimilation rates when grown at low light. Growth temperature and light also affected QUE; plants grown at low light and 23°C had the highest value (0·068 mol CO₂/mol quanta). Measurement temperature and growth light regime significantly affected dark respiration; however, O² did not affect QUE or dark respiration under any growth or measurement conditions. The results indicate that M. sobolifera is adapted to low PPFD, and that complete suppression of photorespiration is dependent upon high growth temperature.     

Effects of elevated CO2 on growth,photosynthesis and respiration of sweet chestnut (Castanea sativa Mill.)

Two year old sweet chestnut seedlings (Castanea sativa Mill) were grown in pots at ambient (350 µmol·mol^–1) and double (700 µmol·mol^–1) atmospheric CO₂ concentration in constantly ventilated greenhouses during entire growing seasons. CO₂ enrichment caused either no significant change or a decrease in shoot response, depending on yearly weather conditions. Similarly, leaf area was either reduced or unchanged under elevated CO₂. However, when grown under controlled conditions in a growth chamber, leaf area was enlarged with elevated CO₂.The CO₂ exchanges of whole plants were measured during the growing season. In elevated CO₂, net photosynthetic rate was maximum in May and then decreased, reaching the level of the control at the end of the season. End of night dark respiration of enriched plants was significantly lower than that of control plants; this difference decreased with time and became negligible in the fall. The original CO₂ level acted instantaneously on the respiration rate: a double concentration in CO₂ decreased the respiration of control plants and a reduced concentration enhanced the respiration of enriched plants. The carbon balance of a chestnut seedling may then be modified in elevated CO₂ by increased carbon inputs and decreased carbon outputs.     

CO2-Fixation and ATP synthesis in continuous cultures of Chlorella fusca

In continuous cultures of Chlorella fusca under steady state conditions, the CO₂-fixation rate, the ATP-level, the apparent rate of photophosphorylation as calculated from the changes in the ATP-level during light to dark or dark to light transients and the energy charge were measured at various environmental conditions. During growth the energy charge was around 0.64. CO₂-assimilation and the apparent ATP-synthesis were strongly dependant on light intensity, however the ATP-level was independant on it. Since the rates of apparent ATP-synthesis and of the CO₂-fixation do not seem to be strictly correlated in a logic way when environmental factors are changed and furthermore the stoichiometry of 3 ATP necessary per CO₂ fixed was never achieved, the described method frequently used for procaryotes to determine the in vivo rate of phosphorylation does not give valid results in highly compartimented eukaryotic cells.     

A Model of the Photosynthesizing Leaf

A model is proposed for the relationship between net photosynthetic rate (N) and light Intensity at a given concentration of CO₂ in the air ([CO₂]_a). The model provides a prediction of the sum of the diffusion resistances (Σr), the capacity (K) of the leaf to fix CO₂, the concentration of CO₂ at the point of photosynthesis ([CO₂]_g), and the respiration rate (R). The model fits the available data well and provides a frame work by which future research may be guided. The calculated values of [CO₂]_g decreased from [CO₂]_g at the compensation point to a nearly constant value at high tight intensities. [CO₂]_g high light infensitit-s range from 32 to 144 μ/l (volume) depending on the species. When these values of [CO₂]_g, are used in the diffusion equation, the resulting levels of the mesophyll resistance (r_m) are lower than those calculated by using the assumptions that [CO₂]_g equals zero. The plants which had (he higher photosynthetic rates at a given light intensity and [CO₂]_a had grealer values of [CO₂]_g than those with lower photo-synthetic rates. The calculated rates of respiration of wheat leaves were twice as high as those measured in the dark. This suggests that the light respiration rate may be twice as great as the dark respiration rate at the same temperature. The calculated values of K demonstrate variability within and between species. The maximum N was independent of K. A relationship between K and the maximum quantum efficiency, at constant levels of [CO₂]_g, was demonstrated in several species. The Σr was inversely related to the maximum rate of photosynthesis for the species investigated. The values of r_m calculated for cotton were inversely related to [CO₂]_a suggesting that the transfer of [CO₂] in the cell may involve a concentration dependent chemical reaction in addition to or rather than a physical diffusion process.     

Prechilling of Xanthium strumarium L. Reduces Net Photosynthesis and, Independently, Stomatal Conductance, While Sensitizing the Stomata to CO(2)

Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO₂ exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO₂ concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO₂ concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO₂ concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO₂ concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO₂; they responded to changes in CO₂ concentration in the range from 100 to 1000 microliters per liter.     

Activity regulation and physiological impacts of maize C4-specific phosphoenolpyruvate carboxylase overproduced in transgenic rice plants

Phosphoenolpyruvate carboxylase (PEPC) was overproduced in the leaves of rice plants by introducing the intact maize C₄-specific PEPC gene. Maize PEPC in transgenic rice leaves underwent activity regulation through protein phosphorylation in a manner similar to endogenous rice PEPC but contrary to that occurring in maize leaves, being downregulated in the light and upregulated in the dark. Compared with untransformed rice, the level of the substrate for PEPC (phosphoenolpyruvate) was slightly lower and the product (oxaloacetate) was slightly higher in transgenic rice, suggesting that maize PEPC was functioning even though it remained dephosphorylated and less active in the light. ¹⁴CO₂ labeling experiments indicated that maize PEPC did not contribute significantly to the photosynthetic CO₂ fixation of transgenic rice plants. Rather, it slightly lowered the CO₂ assimilation rate. This effect was ascribable to the stimulation of respiration in the light, which was more marked at lower O₂ concentrations. It was concluded that overproduction of PEPC does not directly affect photosynthesis significantly but it suppresses photosynthesis indirectly by stimulating respiration in the light. We also found that while the steady-state stomatal aperture remained unaffected over a wide range of humidity, the stomatal opening under non-steady-state conditions was destabilized in transgenic rice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photorespiration and Glycolate Metabolism: A Re-examination and Correlation of Some Previous Studies

Some previous studies of photorespiration and glycolate oxidation were re-examined and correlated by infra-red CO₂ analysis. Data about rate of photosynthesis and oxygen sensitivity indicated that complete inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1 dimethyl urea (DCMU) allowed dark respiration to continue in the light. Photorespiration was also inhibited. The oxygen sensitivity of glycolate-stimulated CO₂ production was found to be compatible with the proposal that glycolate is a substrate of photorespiration. Both `in vivo' and `in vitro' studies of the alga Nitella flexilis have revealed a pathway of glycolate oxidation similar to that of higher plants. DCMU inhibition of photosynthesis by Nitella gave results similar to those for the monocotyledons tested. Under very low light intensity, carbon dioxide compensation in corn was measurable but was not sensitive to high oxygen concentration. It appears that the lack of photorespiration in this plant is not the end result of efficient internal recycling of CO₂ to photosynthesis.     

The Regulation of Carbon Transport and the Carbon Balance of Mature Tomato Leaves

Rates of carbon transport from a single mature tomato leaf inthe light period (day transport) and the dark period (nighttransport) were estimated from the rate of carbon fixation inthe light period, the rate of respiration in the dark periodand the changes in carbon contents over these two periods. Plantswere grown initially at 40 W m^–2 light intensity witheither 350 vpm (nonenriched plants) or 1000 vpm CO₂ (enrichedplants). Various light flux densities or CO₂ concentrationswere then applied to the experimental leaves in the light periodduring the experiment When leaves were temporarily exposed to contrasting light fluxdensities both day transport and night transport were linearlyrelated to the rate of carbon fixation. If leaves were shadedbelow the light compensation point for up to five days, or transferredto contrasting CO₂ concentrations for up to ten days, the linearrelationship between carbon fixation and carbon transport nolonger held. During acclimatization, therate of wbon fixationwas simply related to thecurrent light flux density and CO₂concentration, but the rate of carbon transport changed withtime. Day and night transports responded differently to changesin environment: night transport was more related to the contentof reserve, particularly starch, than to the rate of concurrentwbon fixation. It is concluded that the rate of carbon transport of a maturetomato leaf in a single photoperiod is regulated not merelyby the rate of concurrent carbon fixation but by the contentof reserve in the leaf. The latter results from previous cumulativewbon fixation and carbon transport. As a result of changingthe rate of carbon transport, a balance of carbon input andoutput was achieved within 10 days of acclimatization in a maturetomato leaf.