Photosynthetic and biochemical characterization of in vitro-derived African violet (Saintpaulia ionantha H. Wendl) plants to ex vitro conditions

African violet (Saintpaulia ionantha H. Wendl) is one of the most easily and commonly tissue-cultured ornamental plants. Despite this, there are limited reports on photosynthetic capacity and its impact on the plant quality during acclimatization. Various growth, photosynthetic and biochemical parameters and activities of antioxidant enzymes and dehydrins of micropropagated plants were assessed under three light intensities (35, 70, and 100 µmol m^?2 s^?1 photosynthetic photon flux density – PPFD). Fresh and dry plant biomass, plant height, and leaf area were optimal with high irradiance (70–100 µmol m^?2 s^?1 PPFD). Chlorophyll and carotenoid contents and net photosynthesis were optimal in plants grown under 70 µmol m^?2 s^?1 PPFD. Stomatal resistance, malondialdehyde content, and F_v/F_m values were highest at low light irradiance (35 µmol m^?2 s^?1 PPFD). The activities of three antioxidant enzymes, superoxide dismutase, catalase, and glutathione peroxidase, increased as light irradiance increased, signaling that high light irradiance was an abiotic stress. The accumulation of 55, 33, and 25 kDa dehydrins was observed with all light treatments although the expression levels were highest at 35 µmol m^?2 s^?1 PPFD. Irradiance at 70 µmol m^?2 s^?1 PPFD was suitable for the acclimatization of African violet plants. Both low and high irradiance levels (35 and 100 µmol m^?2 s^?1 PPFD) induced the accumulation of antioxidants and dehydrins in plants which reveals enhanced stress levels and measures to counter it.     

In vitro growth and single-leaf photosynthetic response of Cymbidium plantlets to super-elevated CO2 under cold cathode fluorescent lamps

To examine the effectiveness of super-elevated (10,000 μmol mol⁻¹) CO₂ enrichment under cold cathode fluorescent lamps (CCFL) for the clonal propagation of Cymbidium, plantlets were cultured on modified Vacin and Went (VW) medium under 0, 3,000 and 10,000 μmol mol⁻¹ CO₂ enrichment and two levels of photosynthetic photon flux density (PPFD, 45 and 75 μmol m⁻² s⁻¹). Under high PPFD, 10,000 μmol mol⁻¹ CO₂ increased root dry weight and promoted shoot growth. In addition, a decrease in photosynthetic capacity and chlorosis at leaf tips were observed. Rubisco activity and stomatal conductance of these plantlets were lower than those of plantlets at 3,000 μmol mol⁻¹ CO₂ under high PPFD, which had a higher photosynthetic capacity. On the other hand, plantlets on Kyoto medium grown in 10,000 μmol mol⁻¹ CO₂ under high PPFD had a higher photosynthetic rate than those on modified VW medium; no chlorosis was observed. Furthermore, growth of plantlets, in particular the roots, was remarkably enhanced. This result indicates that a negative response to super-elevated CO₂ under high PPFD could be improved by altering medium components. Super-elevated CO₂ enrichment of in vitro-cultured Cymbidium could positively affect the efficiency and quality of commercial production of clonal orchid plantlets.     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.     

Gas exchange and growth responses to elevated CO2 and light levels in the CAM species Opuntia ficus-indica

Gas exchange and dry-weight production in Opuntia ficus-indica, a CAM species cultivated worldwide for its fruit and cladodes, were studied in 370 and 750 μmol mol⁻¹ CO₂ at three photosynthetic photon flux densities (PPFD: 5, 13 and 20 mol m⁻² d⁻¹). Elevated CO₂ and PPFD enhanced the growth of basal cladodes and roots during the 12-week study. A rise in the PPFD increased the growth of daughter cladodes; elevated CO₂ enhanced the growth of first-daughter cladodes but decreased the growth of the second-daughter cladodes produced on them. CO₂ enrichment enhanced daily net CO₂ uptake during the initial 8 weeks after planting for both basal and first-daughter cladodes. Water vapour conductance was 9 to 15% lower in 750 than in 370 μmol mol⁻¹ CO₂. Cladode chlorophyll content was lower in elevated CO₂ and at higher PPFD. Soluble sugar and starch contents increased with time and were higher in elevated CO₂ and at higher PPFD. The total plant nitrogen content was lower in elevated CO₂. The effect of elevated CO₂ on net CO₂ uptake disappeared at 12 weeks after planting, possibly due to acclimation or feedback inhibition, which in turn could reflect decreases in the sink strength of roots. Despite this decreased effect on net CO₂ uptake, the total plant dry weight at 12 weeks averaged 32% higher in 750 than in 370 μmol mol⁻¹ CO₂. Averaged for the two CO₂ treatments, the total plant dry weight increased by 66% from low to medium PPFD and by 37% from medium to high PPFD.     

Responses of C4 grasses to atmospheric CO2 enrichment

Summary The growth and photosynethetic responses to atmospheric CO₂ enrichment of 4 species of C₄ grasses grown at two levels of irradiance were studied. We sought to determine whether CO₂ enrichment would yield proportionally greater growth enhancement in the C₄ grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1^-1 CO₂ and 1,000 or 150 mol m^-2 s^-1 photosynthetic photon flux density (PPFD). An increase in CO₂ concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO₂. Plants grown in CO₂-enriched atmosphere had lower photosynthetic capacity relative to the low CO₂ grown plants when exposed to lower CO₂ concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO₂ compensation point for photosynthesis.     

Light induction of nonphotochemical quenching,CO2 fixation,and photoinhibition in woody and fern species adapted to different light regimes

We aimed to find out relations among nonphotochemical quenching (NPQ), gross photosynthetic rate (P _G), and photoinhibition during photosynthetic light induction in three woody species (one pioneer tree and two understory shrubs) and four ferns adapted to different light regimes. Pot-grown plants received 100% and/or 10% sunlight according to their light-adaptation capabilities. After at least four months of light acclimation, CO₂ exchange and chlorophyll fluorescence were measured simultaneously in the laboratory. We found that during light induction the formation and relaxation of the transient NPQ was closely related to light intensity, light-adaption capability of species, and P _G. NPQ with all treatments increased rapidly within the first 1–2 min of the light induction. Thereafter, only species with high P _G and electron transport rate (ETR), i.e., one pioneer tree and one mild shade-adapted fern, showed NPQ relaxing rapidly to a low steady-state level within 6–8 min under PPFD of 100 μmol(photon) m^?2 s^?1 and ambient CO₂ concentration. Leaves with low P _Gand ETR, regardless of species characteristics or inhibition by low CO₂ concentration, showed slow or none NPQ relaxation up to 20 min after the start of low light induction. In contrast, NPQ increased slowly to a steady state (one pioneer tree) or it did not reach the steady state (the others) from 2 to 30 min under PPFD of 2,000 μmol m^?2 s^?1. Under high excess of light energy, species adapted to or plants acclimated to high light exhibited high NPQ at the initial 1 or 2 min, and showed low photoinhibition after 30 min of light induction. The value of fastest-developing NPQ can be quickly and easily obtained and might be useful for physiological studies.     

Monocots

Green nectaries have been frequently mentioned in the literature, leading to the assumption that photosynthesis of nectaries can supply the carbohydrates secreted in the nectar, especially when storage of starch is seen in the plastids in nectaries and this starch disappears during secretion. Photosynthesis in nectaries can also provide reduction equivalents for the nectar–redox cycle and energy for secretion. However, quantitative data on the photosynthetic capacity of nectaries are largely missing. Therefore, in the present study, the photosynthetic capacity of green nectaries from a range of plants was screened; 20 floral nectaries (including six septal nectaries) and six extrafloral nectaries were studied. For the screening, chlorophyll fluorescence parameters were measured as depending on photosynthetic photon flux density (PPFD). Parameters measured were basic ground fluorescence (F) and quantum yield (Y₀) of the dark adapted sample at 0 PPFD. From the light saturation curves saturating PPFD (PPFD_sat), quantum yield at saturation (Y_sat) and maximum apparent photosynthetic electron transport rates (ETR_max) were obtained. For comparison, leaves of the plants were also measured. In most cases, the performance of the nectaries was lower than that of the leaves. F was lower in 14 floral and four extrafloral nectaries (69% of total), ETR_max was lower in 18 floral and four extrafloral nectaries (85%), Y_sat was lower in 15 floral and three extrafloral nectaries (69%). In 18 floral and two extrafloral nectaries (77%) Y₀ was well below 0.8, indicating photoinhibition. In contrast, the range of ETR_max for green nectaries was 25–140 μmol m^?2 s^?1 and overlaps well with that of green tissues in general. The lower end of the range of rates of photosynthetic carbon dioxide (CO₂) uptake of sun leaves in the literature is 10 μmol CO₂ m^?2 s^?1. Taking this value for sun‐adapted green nectaries, i.e. having a PPFD_sat > 1000 μmol m^?2 s^?1, with an area of nectar tissue measured as 3–50 mm² per flower, sugar secretion related to photosynthetic CO₂ fixation in the green nectaries is estimated at approximately 0.2–3.0 μmol hexose units flower^?1 day^?1. This is compares well in order of magnitude with the range of secretion given in the literature and clearly suggests that photosynthetic activity of green nectaries can explain a significant part, if not all, of the sugar secreted. In some nectaries ETR did not saturate with PPFD. This could be attributable to spillover from photosystem II to photosystem I and cyclic photosynthetic electron transport. It is in agreement with observations in the literature and my preliminary findings that nectary plastids often lack grana thylakoids where photosytem II is located. Cyclic photophosphorylation could provide adenosine triphosphate (ATP) energy for the nectaries. This needs further investigation. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 1–11.     

High-light-like photosynthetic responses of Cucumis sativus leaves acclimated to fluorescent illumination with a high red:far-red ratio: interaction between light quality and quantity

This study evaluated the photosynthetic responses of Cucumis sativus leaves acclimated to illumination from three-band white fluorescent lamps with a high red:far-red (R:FR) ratio (R:FR = 10.5) and the photosynthetic responses of leaves acclimated to metal-halide lamps that provided a spectrum similar to that of natural light (R:FR = 1.2) at acclimation photosynthetic photon flux density (PPFD) of 100 to 700 μmol m^?2 s^?1. The maximum gross photosynthetic rate (P _G) of the fluorescent-acclimated leaves was approximately 1.4 times that of the metal-halide-acclimated leaves at all acclimation PPFDs. The ratio of quantum efficiency of photosystem II (Φ_PSII) of the fluorescent-acclimated leaves to that of the metal-halide-acclimated leaves tended to increase with increasing acclimation PPFD, whereas the corresponding ratios for the leaf mass per unit area tended to decrease with increasing acclimation PPFD. These results suggest that the greater maximum P _G of the fluorescent-acclimated leaves resulted from an interaction between the acclimation light quality and quantity, which was mainly caused by the greater leaf biomass for photosynthesis per area at low acclimation PPFDs and by the higher Φ_PSII as a result of changes in characteristics and distribution of chloroplasts, or a combination of these factors at high acclimation PPFDs.     

Photosynthesis of individual field-grown cotton leaves during ontogeny

Photosynthetic characteristics of field-grown cotton (Gossypium hirsutum L.) leaves were determined at several insertion levels within the canopy during the growing season. Single-leaf measurements of net photosynthesis (Pn), stomatal conductance to CO₂ (g_s·CO₂), substomatal CO₂, leaf area expansion, leaf nitrogen, and light intensity (PPFD) were recorded for undisturbed leaves within the crop canopy at 3–4 day intervals during the development of all leaves at main-stem nodes 8, 10, and 12. Patterns of Pn during leaf ontogeny exhibited three distinct phases; a rapid increase to maximum at 16–20 days after leaf unfolding, a relatively short plateau, and a period of linear decline to negligible Pn at 60–65 days. Analysis of the parameters which contributed to the rise and fall pattern of Pn with leaf age indicated the primary involvement of leaf area expansion, leaf nitrogen, PPFD, and g_s·CO₂ in this process. The response of Pn and g_s·CO₂ to incident PPFD conditions during canopy development was highly age dependent. For leaves less than 16 days old, the patterns of Pn and g_s·CO₂ were largely controlled by non-PPFD factors, while for older leaves Pn and g_s·CO₂ were more closely coupled to PPFD-mediated processes. Maximum values of Pn were not significantly different for any of the leaves monitored in this study, however, those leaves at main-stem node 8 did possess a significantly diminished photosynthetic capacity with age compared to upper canopy leaves. This accelerated decline in Pn could not be explained by age-related variations in g_s·CO₂ since all leaves showed similar changes in g_s·CO₂ with leaf age.Abbreviations g_s·CO₂ stomatal conductance to CO₂ - Pn net photosynthesis - PPFD photosynthetic photon flux density     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Effect of CO2 concentration on the PIC/POC ratio in the coccolithophore Emiliania huxleyi grown under light-limiting conditions and different daylengths

We compared the effect of CO₂ concentration ([CO₂], ranging from ∼5 to ∼34 μmol l⁻¹) at four different photon flux densities (PFD=15, 30, 80 and 150 μmol m⁻² s⁻¹) and two light/dark (L/D) cycles (16/8 and 24/0 h) on the coccolithophore Emiliania huxleyi. With increasing [CO₂], a decrease in the particulate inorganic carbon to particulate organic carbon (PIC/POC) ratio was observed at all light intensities and L/D cycles tested. The individual response in cellular PIC and POC to [CO₂] depended strongly on the PFD. POC production increased with rising [CO₂], irrespective of the light intensity, and PIC production decreased with increasing [CO₂] at a PFD of 150 μmol m⁻² s⁻¹, whereas below this light level it was unaffected by [CO₂]. Cell growth rate decreased with decreasing PFD, but was largely independent of ambient [CO₂]. The diurnal variation in PIC and POC content, monitored over a 38-h period (16/8 h L/D, PFD=150 μmol m⁻² s⁻¹), exceeded the difference in carbon content between cells grown at high (∼29 μmol l⁻¹) and low (∼4 μmol l⁻¹) [CO₂]. However, consistent with the results described above, cellular POC content was higher and PIC content lower at high [CO₂], compared to the values at low [CO₂], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO₂] may be of importance in regulating species-specific primary production and species composition.     

The effect of light quality on the induction of efficient photosynthesis under low CO2 conditions in Chlamydomonas reinhardtii and Chlorella pyrenoidosa

The effect of blue and red light on the adaptation to low CO₂ conditions was studied in high-CO₂ grown cultures of Chlorella Pyrenoidosa (82T) and Chlamydomonas reinhardtii(137⁺) by measuring O₂ exchange under various inorganic carbon (Cⁱ) concentrations. At equal photosynthetic photon flux density (PPFD), blue light was more favourable for adaptation in both species, compared to red light. The difference in photosynthetic oxygen evolution between cells adapted to low C_iunder blue and red light was more pronounced when oxygen evolution was measured under low C_i compared to high C_i conditions. The effect of light quality on adaptation remained for several hours. The different effects caused by blue and red light was observed in C. pyrenoidosa over a wide range of PPFD with increasing differences at increasing PPFD. The maximal difference was obtained at a PPFD above 1 500 μmol m^?2s^?1. We found no difference in the extracellular carbonic anhydrase activity between blue- and red light adapted cells. The light quality effect recorded under C_i-limiting conditions in C. reinhardtii cells adapted to air, was only 37% less when instead of pure blue light red light containing 12.5% of blue light (similar PPFD as blue light) was used during adaptation to low carbon. This indicates that in addition to affecting photosynthesis, blue light affected a sensory system involved in algal adaptation to low C_i conditions. Since the affinity for C_i of C. Pyrenoidosa and C. reinhardtii cells adapted to air under blue light was higher than that of cells adapted under red light, we suggest that induction of some component(s) of the C_i accumulating mechanism is regulated by the light quality.     

Growth potential of Lemna gibba: Effect of CO2 enrichment at high photon flux rate

Lemna gibba L. was cultivated in continuous light (800–1200 μmol quanta m^?2s^?1, 320–400 W m^?2) and normal or CO₂-enriched air (1500 μl CO₂ l^?1), with a continuous nutrient supply. Increased CO₂ concentration increased the unit leaf rate (ULR) or net assimilation rate and decreased the leaf area ratio (LAR) (photosynthetic area per unit dry weight), but the relative growth rate was unchanged (0.43 g g^?1 day^?1). The changes in ULR and LAR indicate that organic matter production can be increased with CO₂ enrichment at high photon flux rate (PFR).     

RESPONSE OF TWO OLD FIELD PERENNIALS TO INTERACTIONS OF CO2 ENRICHMENT AND DROUGHT STRESS

Aster pilosus Willd. (aster, C₃) and Andropogon virginicus L. (broomsedge, C₄) were grown in growth chambers at 26/20 C day/night temperatures with a PPFD of 1,000 μmol s^–1 m^–2. Water was withheld for a 2-wk drought period under three CO₂ concentrations (approximately 380, 500, and 650 μl 1^–1). There were significant effects of CO₂ enrichment on aster so that drought stress did not occur in plants grown with CO₂ enrichment. Non-watered plants with CO₂ enrichment had greater leaf water potentials, greater photosynthetic rates, and greater total dry wt than non-watered plants grown at 380 μl 1^–1 CO₂. The response of broomsedge to drought was the same in all CO₂ treatments and there was no significant interaction of CO₂ enrichment and drought. The decreased severity of drought stress and the increased growth of aster with CO₂ enrichment may increase its competitive ability during droughts, allowing it to persist for longer periods during succession in abandoned fields.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

RuBPCO kinetics and the mechanism of CO2 entry in C3 plants

Abstract. The CO₂ partial pressure in the chloroplasts of intact photosynthetic C₃ leaves is thought to be less than the intercellular CO₂ partial pressure. The intercellular CO₂ partial pressure can be calculated from CO₂ and H₂O gas exchange measurements, whereas the CO₂ partial pressure in the chloroplasts is unknown. The conductance of CO₂ from the intercellular space to the chloroplast stroma and the CO₂ partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO₂ partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO₂ compensation concentration; their comparison with differential uptake of ¹⁴CO₂ and ¹²CO₂; and their comparison with O₂ dependence of net CO₂ uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m^?2 s^?1, and an ambient CO₂ partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO₂ partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO₂ across the chloroplast envelope which decreases with increasing temperature. At low CO₂ partial pressure, the diffusion resistance appears to be counterbalanced by active CO₂ (or HCO₃) transport with high affinity and low maximum velocity. At low PPFD, CO₂ partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO₂ flux.     

Green light enhances growth,photosynthetic pigments and CO<Subscript>2</Subscript> assimilation efficiency of lettuce as revealed by ‘knock out’ of the 480–560 nm spectral waveband

Adding green component to growth light had a profound effect on biomass accumulation in lettuce. However, conflicting views on photosynthetic efficiency of green light, which have been reported, might occur due to nonuniform light sources used in previous studies. In an attempt to reveal plausible mechanisms underlying the differential photosynthetic and developmental responses to green light, we established a new way of light treatment modeled according to the principle of gene “knock out”. Lettuce (Lactuca sativa L. var. youmaicai) was grown under two different light spectra, including a wide spectrum of light-emitting diode (LED) light (CK) and a wide spectrum LED light lacking green (480–560 nm) (LG). Total PPFD was approximately 100 µmol(photon) m^?2 s^?1 for each light source. As compared to lettuce grown under CK, shoot dry mass, photosynthetic pigment contents, total chlorophyll to carotenoids ratio, absorptance of PPFD, and CO₂ assimilation showed a remarkable decrease under LG, although specific leaf area did not show significant difference. Furthermore, plants grown under LG showed significantly lower stomatal conductance, intercellular CO₂ concentration, and transpiration compared with CK. The plants under CK exhibited significantly higher intrinsic quantum efficiency, respiration rate, saturation irradiance, and obviously lower compensation irradiance. Finally, we showed that the maximum ribulose-1,5-bisphosphate-saturated rate of carboxylation, the maximum rate of electron transport, and rate of triosephosphate utilization were significantly reduced by LG. These results highlighted the influence of green light on photosynthetic responses under the conditions used in this study. Adding green component (480–560 nm) to growth light affected biomass accumulation of lettuce in controllable environments, such as plant factory and Bioregenerative Life Support System.     

High rates of net ecosystem carbon assimilation by Brachiara pasture in the Brazilian Cerrado

To investigate the consequences of land use on carbon and energy exchanges between the ecosystem and atmosphere, we measured CO₂ and water vapour fluxes over an introduced Brachiara brizantha pasture located in the Cerrado region of Central Brazil. Measurements using eddy covariance technique were carried out in field campaigns during the wet and dry seasons. Midday CO₂ net ecosystem exchange rates during the wet season were ?40 μmol m^?2 s^?1, which is more than twice the rate found in the dry season (?15 μmol m^?2 s^?1). This was observed despite similar magnitudes of irradiance, air and soil temperatures. During the wet season, inferred rates of canopy photosynthesis did not show any tendency to saturate at high solar radiation levels, with rates of around 50 μmol m^?2 s^?1 being observed at the maximum incoming photon flux densities of 2200 μmol m^?2 s^?1. This contrasted strongly to the dry period when light saturation occurred with 1500 μmol m^?2 s^?1 and with maximum canopy photosynthetic rates of only 20 μmol m^?2 s^?1. Both canopy photosynthetic rates and night‐time ecosystem CO₂ efflux rates were much greater than has been observed for cerrado native vegetation in both the wet and dry seasons. Indeed, observed CO₂ exchange rates were also much greater than has previously been reported for C₄ pastures in the tropics. The high rates in the wet season may have been attributable, at least in part, to the pasture not being grazed. Higher than expected net rates of carbon acquisition during the dry season may also have been attributable to some early rain events. Nevertheless, the present study demonstrates that well‐managed, productive tropical pastures can attain ecosystem gas exchange rates equivalent to fertilized C₄ crops growing in the temperate zone.     

Field Measurements of Photosynthesis of Umbilicarious Lichens in Winter

The photosynthetic performance of Lasallia pustulata and Umbilicaria spodochroa was measured both in the field on granitic rock in southern Norway and in the laboratory under controlled conditions of light and temperature. In the field thallus temperatures varied between ? 2 and + 5 °C during the daylight period in January 1994. In situ water contents were between 50 and 400% d.wt. in L. pustulata and between 100 and 500% d.wt. in U. spodochroa. The lichens were active during the whole period of investigation. Photosynthetic rates reached 13.03 μmol CO₂ g Chl^?1 s^?1 in L. pustulata and 5.56 μmol CO₂ g Chl^?1 s^?1 in U. spodochroa. Ice formation on the thallus surface did not impede CO₂ exchange. Light was mainly the limiting factor, as could be confirmed by the laboratory experiments. In general, habitat conditions never provided optimum photosynthetic rates but photosynthetic carbon balance was positive during 4 of the 5 days investigated. The coldest day was photosynthetically almost as efficient as the warmest day during this period.     

CO2 uptake and fluorescence responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO2 concentrations

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol^?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m^?2 day^?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m^?2 day^?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol^?1 was 25% of that under 740 μmol CO₂ mol^?1. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m^?2 day^?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.