MEASURING RATES OF SYMBIOTIC NITROGEN FIXATION BY ALNUS TENUIFOLIA

Rates of nitrogen fixation were determined for alder (Abuts tenuifolia Nutt.) with the C₂H₂ assay and verified by the ¹⁵N method. Rates of C₂H₂ reduction obtained varied between 0.0342 and 10.9 μmoles per gram of nodule tissue (fresh weight) per hour with a seasonal average of 2.70 μmoles g^-1 hr^-1. Expressed as nitrogen fixation rates these are equivalent to 0.0114, 3.63, and 0.9 μmoles g^-1 hr^-1 of elemental nitrogen, respectively. Nitrogen fixation (¹⁵N) rates averaged 2.20 μmoles N g^-1 hr^-1 with a standard deviation of 1.14. The influence of pC₂H₂, pO₂, temperature during incubation, length of incubation time, presence vs. absence of N₂ in the incubation atmosphere, and amount of nodule biomass per sample were investigated.     

PHOTOSYNTHESIS AND RESPIRATION OF ARCEUTHOBIUM TSUGENSE (LORANTHACEAE)

Dark respiration rates of the aerial shoots of Arceuthobium tsugense, obtained by manometric and IRGA techniques, show production of CO₂ to range between 155–300 μl CO₂ g^-1hr^-1 with evidence of seasonal variation. Experiments with ¹⁴CO₂ indicate that the aerial shoots are capable of some photosynthetic CO₂ fixation, with 10–15 % of the available ¹⁴C incorporated by the plant tissue in one hr. Biochemical characterization of the products of photosynthesis reveals that 80–90 % of the incorporated ¹⁴C is ethanol soluble. Ten percent of the ethanol soluble fraction is lipoidal in nature, the rest is H₂O soluble. IRGA experiments indicate an apparent photosynthetic CO₂ fixation capacity of 80–90 μl CO₂ g^-1hr^-1, or 25–30% of the amount of CO₂ produced by respiration. The significance of these findings is discussed with respect to nutrition of the parasite and effects upon the host.     

Modulation of photosynthesis and respiration in guard and mesophyll cell protoplasts by oxygen concentration

Stomatal movement is an energetic oxygen-requiring process. In the present study, the effect of oxygen concentration on mitochondrial respiratory activity and red-light-dependent photosynthetic oxygen evolution by Vicia faba and Brassica napus guard cell protoplasts was examined. Comparative measurements were made with mesophyll cell protoplasts isolated from the same species. At air saturated levels of dissolved oxygen in the protoplast suspension media, respiration rates by mesophyll protoplasts ranged from 6 to 10μmoles O₂ mg^?1 chl h^?1, while guard cell protoplasts respired at rates of 200–300 μmoles O₂ mg chl^?1 h^?1, depending on the species. Lowering the oxygen concentration below 50–60 mmol m^?3 resulted in a decrease in guard cell respiration rates, while rates by mesophyll cell protoplasts were reduced only at much lower concentrations of dissolved oxygen. Rates of photosynthesis in mesophyll cell protoplasts isolated from both species showed only a minor reduction in activity at low oxygen concentrations. In contrast, photosynthesis by guard cell protoplasts isolated from V. faba and B. napus decreased concomitantly with respiration. Oligomycin, an inhibitor of oxidative phos-phorylation, reduced photosynthesis in mesophyll cell protoplasts by 27–46% and in guard cell protoplasts by 51–58%. The reduction in both guard cell photosynthesis and respiration following exposure to low oxygen concentrations suggest close metabolic coupling between the two activities, possibly mediated by the availability of substrate for respiration associated with photosynthetic electron transport activity and subsequent export of redox equivalents.     

CYTOCHROME f LOSS IN ASTAXANTHIN-ACCUMULATING RED CELLS OF HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE): COMPARISON OF PHOTOSYNTHETIC ACTIVITY, PHOTOSYNTHETIC ENZYMES, AND THYLAKOID MEMBRANE POLYPEPTIDES IN RED AND GREEN CELLS

Photosynthetic activity, chloroplast enzymes, and poly-peptides were compared in green and red (ketocarotenoid-containing) cultures of the microalga Haematococcus pluvialis Flotow. Green cultures, grown at 80 μmol pho-tons.m^-2. s^-1 in an acetate-containing medium, had a mean generation time of 27 h. Ketocarotenoid accumulation was induced by transfer of green cultures to PO₄-deficient medium and exposure to 250 μmol photons.m^-2. s^-1. Under these conditions, there was no increase in cell number, and the cultures turned red. Relative amounts of enzymes and thylakoid polypeptides in red and green cells were ascertained by immunoprobing with standardization on a chlorophyll (Chl) basis. In red cultures, the level of cytochrome f was greatly decreased (< 1% of green cell level), which is expected to greatly impair the linear electron flow from photosystem (PS) II to PS I. Also, the levels of apoproteins in red cells, namely, of CPI, D2, CP47, LHC I, and ribulose-1, 5-bisphosphate carboxylase were reduced to 15, 18, 29, 48, and 80%, respectively, of those in green cells. Only adenosine triphosphate syn-thase exhibited no significant change in the two types of cultures. The respiration rate of red cultures was much higher (100 μmoles O₂. mg Chl^-1.h^-1) than that of green cells (16 μmoles O₂. mg Chl^-1.h^-1). Conversely, net O₂ evolution (at P_max in green cultures was 80 μmoles O₂. mg Chl^-1.h^-1 but was —40 μmoles O₂. mg Chl^-1.h^-1 in red cultures. PS II activity was demonstrated in broken cells of both green and red cultures, showing activity of 40 and 15 μmoles DCPIP-mg Chl^-1.h^-1 (with DPC as electron donor), respectively. In contrast, PS I activity measured by the Mehler reaction showed that red rather than green cells had a greater activity (64 vs. 46 μmoles O₂. mg Chl^-1.h^-1, respectively). Thus, in spite of the decline of O₂ evolution in red cells, the photosystems were still functional. We postulate that the decline of O₂, evolution in red cells is largely attributable to an increase in the respiration rate and the impairment of linear electron flow from PS II to PS I and, to some extent, to a decrease in components of the photosystems.     

ALGAL BIOMASS AND PRIMARY PRODUCTION WITHIN A TEMPERATE ZONE SANDSTONE

The use of dimethyl sulfoxide (DMSO) to extract chlorophyll a and ^l4C-labelled photosynthate from endolithic algae of sparsely vegetated, cold temperate grasslands on the Colorado Plateau in Arizona has yielded the first estimates of biomass and photosynthesis for this unusual community. These subsurface microorganisms are found widespread in exposed Coconino Sandstone, a predominant formation in this cold temperate region. The endolithic community in Coconino Sandstone, composed primarily of coccoid blue-green and coccoid/sarcinoid green algae, yielded a biomass value (as chlorophyll a content) of 87 mg m^-2 rock surface area and a photosynthetic rate of 0.37 mg CO₂ dm ² hr^-1 or 0.48 mg CO₂ mg^-1 chl a hr^-1. The endolithic algal community contributes moderate biomass (5–10%) and substantial photosynthesis (20–80%) to the sparse grassland ecosystem.     

Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 μeinsteins m⁻²sec⁻¹. In air containing 21% O₂, velvetleaf and cotton grown at 750 μeinsteins m⁻²sec⁻¹ had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm⁻²hr⁻¹, respectively. Maximum rates for leaves grown at 320 and 90 μeinsteins m⁻²sec⁻¹ were 15.3 and 10.3 mg of CO₂ dm⁻²hr⁻¹ in velvetleaf and 12 and 6.7 mg of CO₂ dm⁻²hr⁻¹ in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.     

Interrelations between sulfate-reducing and methane-producing bacteria in bottom deposits of a fresh-water lake. III. Experiments with 14C-labeled substrates

An ecological substrate relationship between sulfate-reducing and methane-producing bacteria in mud of Lake Vechten has been studied in experiments using ¹⁴C-labeled acetate and lactate as substrates. Fluoroacetate strongly inhibited the formation of ¹⁴CO₂ from [U-¹⁴C]-acetate and β-fluorolactate gave an inhibition of similar magnitude of the breakdown of [U-¹⁴C]-l-lactate to ¹⁴CO₂ thus confirming earlier results on the specific action of these inhibitors. The turnover-rate constant of l-lactate was 2.37 hr^-1 and the average l-lactate pool size was 12.2 μg per gram of wet mud, giving a turnover rate of 28.9 μg of lactate/gram of mud per hr. The turnover-rate constant of acetate was 0.35 hr^-1 and the average pool size was 5.7 μg per gram of wet mud, giving a rate of disappearance of 1.99 μg of acetate/gram of mud per hr. Estimations of the acetate turnover rate based upon the formation of ¹⁴CO₂ from [U-¹⁴C]-acetate or [1-¹⁴C]-acetate yielded figures of the same magnitude (range 0.45 to 1.74). These and other results suggest that only a portion of the lactate dissimilated is turned over through the acetate pool. The ratio of ¹⁴CO₂/¹⁴CH₄ produced from [U-¹⁴C]-acetate by mud was 1.32; indicating that 0.862 moles of CH₄ and 1.138 moles of CO₂ are formed per mole of acetate. From the rate of disappearance of acetate (0.027 μmoles/gram wet mud per hr) and the rate of methane production (0.034 μmoles/gram wet mud per hr), it may be concluded that acetate is an important precursor of methanogenesis in mud (approximately 70%). A substrate relationship between the two groups of bacteria is likely since ¹⁴CH₄ was formed from [U-¹⁴C]-l-lactate.     

EVOLUTION OF C3 AND C4 PLANTS ALONG AN ENVIRONMENTAL MOISTURE GRADIENT: PATTERNS OF PHOTOSYNTHETIC DIFFERENTIATION IN HAWAIIAN SCAEVOLA AND EUPHORBIA SPECIES

The endemic Hawaiian species of Scaevola and Euphorbia grow in a wide variety of native habitats and exhibit a wide range of variation in photosynthetic responses. Light-saturated photosynthetic capacities range from 12.0 to 24.7 μmol CO₂ m^−-2 s^−-1 in the Scaevola species and from 18.2 to 51.4 μmol CO₂ m^−-2 s^−-1 in the Euphorbia species. Within each genus, differences in light-saturated photosynthetic capacity are paralleled by differences in mesophyll and leaf conductances to CO₂. Within each habitat, the C₄ Euphorbia species exhibits a significantly higher photosynthetic capacity and a significantly higher mesophyll conductance than the corresponding C₃ Scaevola species. These differences are greatest in the dry scrub habitat and least in the wet forest habitat. One photosynthetic characteristic that exhibits little variation among the species within each genus, yet that exhibits a consistently large difference between the species within each habitat, is photosynthetic water-use efficiency. The C₄ Euphorbia species possess water-use efficiencies that are 2–3½ times as high as those of the C₃ Scaevola species, regardless of whether these species are native to very dry or very wet habitats. At present, the ecological significance of this large inherent difference in photosynthetic water-use efficiency is unknown. Indeed, it appears that neither photosynthetic pathway has imposed any major inherent constraints on the ability of the Scaevola and Euphorbia species to diversify into a wide variety of habitats.     

EFFECTS OF LIGHT AND TEMPERATURE ON NET PHOTOSYNTHESIS AND DARK RESPIRATION OF GAMETOPHYTES AND EMBRYONIC SPOROPHYTES OF MACROCYSTIS PYRIFERA1

The rates of net photosynthesis as a function of irradiance and temperature were determined for gametophytes and embryonic sporophytes of the kelp, Macrocystis pyrifera (L.) C. Ag. Gametophytes exhibited higher net photosynthetic rates based on oxygen and pH measurements than their derived embryonic sporophytes, but reached light saturation at comparable irradiance levels. The net photosynthesis of gametophytes reached a maximum of 66.4 mg O₂ g dry wt^?1 h^?1 (86.5 mg CO₂ g dry wt^?1 h^?1), a value approximately seven times the rate reported previously for the adult sporophyte blades. Gametophytes were light saturated at 70 μE m^?2 s^?1 and exhibited a significant decline in photosynthetic performance at irradiances 140 μE m^?1 s^?1. Embryonic sporophytes revealed a maximum photosynthetic capacity of 20.6 mg O₂ g dry wt^?1 h^?1 (25.3 mg CO₂ g dry wt^?1 h^?1), a rate about twice that reported for adult sporophyte blades. Embryonic sporophytes also became light saturated at 70 μE m^?2 s^?1, but unlike their parental gametophytes, failed to exhibit lesser photosynthetic rates at the highest irradiance levels studied; light compensation occurred at 2.8 μE m^?2 s^?1. Light-saturated net photosynthetic rates of gametophytes and embryonic sporophytes varied significantly with temperature. Gametophytes exhibited maximal photosynthesis at 15° to 20° C, whereas embryonic sporophytes maintained comparable rates between 10° and 20° C. Both gametophytes and embryonic sporophytes declined in photosynthetic capacity at 30° C. Dark respiration of gametophytes was uniform from 10° to 25° C, but increased six-fold at 30° C; the rates for embryonic sporophytes were comparable over the entire range of temperatures examined. The broader light and temperature tolerances of the embryonic sporophytes suggest that this stage in the life history of M. pyrifera is well suited for the subtidal benthic environment and for the conditions in the upper levels of the water column.     

Photosynthetic studies with mesophyll protoplasts from Notonia grandiflora, a crassulacean acid metabolism plant

A method is described for rapid enzymatic isolation of mesophyll protoplasts and cells from the crassulacean acid metabolism (CAM) plant Notonia grandiflora DC. The mesophyll protoplasts exhibited high rates of ¹⁴CO₂ fixation both in the light (45 μmol of CO₂ fixed mg^?1 Chl h^?1) and in the dark (20 μmol of CO₂ fixed mg^?1 Chl h^?1). The protoplasts also showed O₂ evolution (40 μmol of O₂ evolved mg^?1 Chl h^?1) without added bicarbonate. Exogenously added bicarbonate had no stimulating effect on the O₂ evolution. Analyses of early photosynthetic products in the light showed the formation of both C₃ and C₄ acids. Aspartate was found to be a predominant photosynthate.     

Nitrite assimilation and amino nitrogen synthesis in isolated spinach chloroplasts

The assimilation of nitrite leading to de novo synthesis of amino nitrogen in a chloroplast-enriched fraction isolated from freshly harvested young spinach (Spinacia oleracea L.) leaves was demonstrated. The preparations showed approximately 55% intact chloroplasts as determined by light scattering properties and fixed CO₂ at rates of approximately 100 μmoles hr⁻¹ mg chlorophyll⁻¹.     

C(4) Photosynthesis: Light-dependent CO(2) Fixation by Mesophyll Cells, Protoplasts, and Protoplast Extracts of Digitaria sanguinalis

Mesophyll cells, protoplasts, and protoplast extracts of Digitaria sanguinalis were used for comparative studies of light-dependent CO₂ fixation. CO₂ fixation was low without the addition of organic substrates. Pyruvate, oxaloacetate, and 3-phosphoglycerate induced relatively low rates (10 to 90 μmoles/mg chlorophyll·hr) of CO₂ fixation when added separately. However, a highly synergistic relationship was found between pyruvate + oxaloacetate and pyruvate + 3-phosphoglycerate for inducing light-dependent CO₂ fixation in the mesophyll preparations. Highest rates of CO₂ fixation were obtained with protoplast extracts. Pyruvate, in combination with oxaloacetate or 3-phosphoglycerate induced light-dependent rates from 150 to 380 μmoles of CO₂ fixed/mg chlorophyll·hr which are equivalent to or exceed reported rates of whole leaf photosynthesis in C₄ species. Concentrations of various substrates required to give half-maximum velocities of CO₂ fixation were determined, with the protoplast extracts generally saturating at the lowest substrate concentrations. Chloroplasts separated from protoplast extracts showed little capacity for CO₂ fixation. The results suggest that CO₂ fixation in C₄ mesophyll cells is dependent on chloroplasts and extrachloroplastic phosphoenolpyruvate carboxylase.     

COMPARATIVE PHYSIOLOGICAL STUDY OF MARINE DIATOMS AND DINOFLAGELLATES IN RELATION TO IRRADIANCE AND CELL SIZE. I. GROWTH UNDER CONTINUOUS LIGHT1,2

Cell division rates and chlorophyll a and protein contents for ten diatom and dinoflagellate species were measured. Species were chosen to include a wide range of cell size in terms of both cell volume and cell protein: from 0.004 ng protein/cell for a small Chaetoceros sp. to 2.2 ng protein/cell for Prorocentrum micans Ehrenberg. Experiments were conducted in batch or semi-continuous cultures at 21 C under continuous illumination from 8–256 μEin ^.m^-2'^.s^-1. Light saturation of cell division occurred at 32–80 μEin m^-1 s^-1 for all species, with no observable difference between the two phylogenetic groups. When the light-saturated cell division rates were plotted against cell size as protein/cell, the diatoms and dinoflagellates fell on two separate lines with the diatoms having higher rates. Chl a /protein ratios (μg/μg) decreased with increasing irradiance. The diatoms had higher chl a per unit protein. The relationship between cell division rate and the chl a/protein ratio is discussed.     

PHOTOACCLIMATION OF PHOTOSYNTHESIS IRRADIANCE RESPONSE CURVES AND PHOTOSYNTHETIC PIGMENTS IN MICROALGAE AND CYANOBACTERIA1

The photosynthesis‐irradiance response (PE) curve, in which mass‐specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light‐limited rate to chl a yields the chl a‐specific initial slope (α^chl). This is proportional to the light absorption coefficient (a^chl), the proportionality factor being the photon efficiency of photosynthesis (φ_m). Thus, α^chl is the product of a^chl and φ_m. In microalgae α^chl typically shows little (<20%) phenotypic variability because declines of φ_m under conditions of high‐light stress are accompanied by increases of a^chl. The variation of α^chl among species is dominated by changes in a^chl due to differences in pigment complement and pigment packaging. In contrast to the microalgae, α^chl declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light‐saturated photosynthesis (P_m) is limited by a factor other than the rate of light absorption. Normalizing P_m to organic carbon concentration to obtain P_m^C allows a direct comparison with growth rates. Within species, P_m^C is independent of growth irradiance. Among species, P_m^C covaries with the resource‐saturated growth rate. The chl a:C ratio is a key physiological variable because the appropriate currencies for normalizing light‐limited and light‐saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a:C is reduced to about 40% of its maximum value at an irradiance that supports 50% of the species‐specific maximum growth rate and light‐harvesting accessory pigments show similar or greater declines. In the steady state, this down‐regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light‐limited region for growth. The reason for down‐regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.     

PHOTOSYNTHESIS,DARK RESPIRATION AND DESICCATION RESISTANCE OF THE INTERTIDAL SEAWEEDS HESPEROPHYCUS HARVEYANUS AND PELVETIA FASTIGIATA F. GRACILIS12

Rates of net photosynthesis and dark respiration were determined under submersed and emerged conditions for Hesperophycus harveyanus S. & G. and Pelvetia fastigiata f. gracilis (Decne.) S. & G. Both species exhibited submersed photosynthesis-light relationships and dark respiration rates similar to those established for other closely related intertidal, fucoids. Maximal net photosynthesis of H. harveyanus (0.21 mmol O₂ g dry wt.^-1· h^-1; 0.18 mmol CO₂ g dry wt.^-1· h^-1) was similar to that of P. fastigiata f. gracilis (0.17 mmol. O₂ g dry wt.^-1· h^-1; 0.14 mmol CO₂ g dry wt. ^-1· h^-1). Light saturation occurred between 150 and 250 μE · m^-2· s^-1 for H. harveyanus and between 75 and 150 μE · m^-2· s^-1 for P. fastigiata f. gracilis; photon flux densities required for compensation were 6.4 and 9.2 μE · m^-2· s^-1, respectively. Photoinhibition was not observed for either species. The light-saturated, submersed net photosynthetic performances of both species varied significantly with temperature. Greatest photosynthetic rates were obtained at 23° C for H. harveyanus and at 18° C for P. fastigiata f. gracilis. Under emersed conditions, the maximal net photosynthetic rate and the photon flux densities required for saturation were greater for H. harveyanus (0.08 mmol CO₂ g dry wt.^-1· h^-1; 260 to 700 μE · m^-2· s^-1) than for P. fastigiata f. gracilis (0.02 mmol CO₂g dry wt.^-1· h^-1; 72 to 125 μE · m^-2· s^-1). However, for both species, emersed photosynthetic rates were much lower (14–44%) than those obtained under submersed conditions. Desiccation negatively influenced emersed photosynthesis, of both species, but H. harveyanus thalli contained more water when fully hydrated and lost water more slowly during dehydration, thus suggesting greater photosynthetic potential during field conditions of emersion.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).     

Markedly low requirement of added CO2 for photosynthesis by mesophyll protoplasts of pea (Pisum sativum): possible roles of photorespiratory CO2 and carbonic anhydrase

Mesophyll protoplasts of pea required only 74.1 μM CO₂ for maximal photosynthesis, unlike chloroplasts, which required up to 588 μM CO₂. Such a markedly low requirement for CO₂ could be because of an internal carbon source and/or a CO₂ concentrating mechanism in mesophyll protoplasts. Ethoxyzolamide (EZA), an inhibitor of internal carbonic anhydrase (CA) suppressed photosynthesis by mesophyll protoplasts at low CO₂ (7.41 μM) but had no significant effect at high CO₂ (741 μM). However, acetazolamide, another inhibitor of CA, did not exert as much dramatic effect as EZA. Three photorespiratory inhibitors, aminoacetonitrile or glycine hydroxamate (GHA) or aminooxyacetate inhibited markedly photosynthesis at low CO₂ but not at high CO₂. Inhibitors of glycolysis or tricarboxylic acid cycle (NaF, sodium malonate) or phosphoenolpyruvate carboxylase (3,3‐dichloro‐2‐dihydroxy phosphinoyl‐methyl‐2‐propenoate) had no significant effect on photosynthesis. The CO₂ requirement of protoplast photosynthesis and the sensitivity of photosynthesis to EZA were much higher at low oxygen (65 nmol ml^?1) than that at normal oxygen (212 nmol ml^?1). In contrast, the inhibitory effect of photorespiratory inhibitors on protoplast photosynthesis was similar in both normal and low oxygen medium. The marked elevation of glycine/serine ratio at low O₂ or in presence of GHA confirmed the suppression of photorespiratory decarboxylation by GHA. While demonstrating interesting difference between the response of protoplasts and chloroplasts to CO₂, we suggest that photorespiration could be a significant source of CO₂ for photosynthesis in mesophyll protoplasts at limiting CO₂ and at atmospheric levels of oxygen. Obviously, carbonic anhydrase is essential to concentrate or retain CO₂ in mesophyll cells.     

Growth characteristics of Nostoc flagelliforme at intermittent elevated CO2 concentrations

Algal cultivation is a potential candidate for CO₂ mitigation. CO₂ plays important roles in mass cultivation of algae, including supplying carbon source and adjusting medium pH. To assess the possibility of using edible cyanobacterium Nostoc flagelliforme as carbon storage device, the growth characteristics of N. flagelliforme batch cultured under elevated CO₂ concentrations (0, 2.5, 5, 20, and 40%) were investigated in this study. Results showed that the net photosynthetic rate, efficiency and carbon sequestration rate at 20% CO₂ were increased at a maximum of 121 μmol O₂ (mg chla)^?1 h^?1 8.40% and 0.17 g CO₂ L^?1 day^?1, and increased by 0.42, 1.03 and 1.13 folds compared with that of the control, respectively. Higher CO₂ concentration resulted in the declines in photosynthetic rate, efficiency and carbon sequestration rate because of medium pH reduction. Accordingly, the dry cell weight, amount of exopolysaccharides and protein content of N. flagelliforme cells at 20% CO₂ were obtained at a maximum of 1.45 g L^?1, 54.98 mg L^?1 and 57.75%, increased by 0.93, 0.29 and 0.8 folds compared with that of the control, respectively. These results provided important information for CO₂ mitigation by N. flagelliforme and would shed more light on elucidating the mechanisms of CO₂ tolerance in cyanobacterium.     

CARBON SKELETON SOURCES FOR AMMONIUM ASSIMILATION IN N-SUFFICIENT AND N-LIMITED CELLS OF CYANIDIUM CALDARIUM (RHODOPHYTA)1

The possible origin of carbon skeletons for ammonium assimilation in Cyanidium caldarium (Tilden) Geitler was investigated. N-sufficient cells assimilated ammonium at a rate of 182 ± 18 μmol·mL packed cell volume (pcv)^-1· h^-1. Removal of CO₂ or darkening almost immediately prevented ammonium assimilation. N-limited cells in light assimilated ammonium at a rate of 493 ± 45 μmol · mL pcv^-1· h^-1 in the presence of CO₂ and at a lower rate of 168 ± 17 μmol · mL pcv^-1· h^-1 in the absence of CO₂. In darkness they assimilated ammonium at a rate of 293 ± 29 μmol · mL pcv^-1 h^-1 in the presence of CO₂, only 60% of the assimilation rate in light. In the absence of CO₂, ammonium was assimilated at a similar rate of 325 ± 14 μmol · mL pcv^-1· h^-1. Under the latter conditions, however, assimilation was inhibited after 40 min and ceased after 70 min; it resumed upon resupply of CO₂. We suggest that N-sufficient cells of C. caldarium obtain carbon skeletons for ammonium assimilation exclusively by photosynthetic reactions. Upon N-limitation they develop the ability, apparently through derepression or activation of regulatory enzyme system(s), to obtain a consistent quantity of additional carbon skeletons and ATP from mobilization of carbon reserves. This enables the N-limited cell to assimilate ammonium not only in light but also in darkness, and at a higher rate than N-sufficient cells. The fact that ammonium assimilation in light occurs at a higher rate than in darkness suggests that ammonium assimilation in light is the sum of both light and dark ammonium assimilation, which implies separate metabolic reactions for the two processes. These results suggest the existence of two distinct and differently controlled pathways in N-limited cells, but not in N-sufficient cells, through which carbon skeletons for ammonium assimilation originate. An important role for dark CO₂ fixation in dark or light ammonium assimilation is also indicated.     

THE RELATIONSHIP BETWEEN THE THICKNESS OF DECIDUOUS LEAVES AND THEIR MAXIMUM PHOTOSYNTHETIC RATE

Mc Clendon , John H. (U. Delaware, Newark.) The relationship between the thickness of deciduous leaves and their maximum photosynthetie rate. Amer. Jour. Bot. 49(4): 320–322. Illus. 1962.—Data of Willstätter and Stoll (25°, saturating light and CO₂) are used to show that it is useful to plot photosynthetie rate per unit area as a function of the density thickness (g/cm²; fresh wt) of the leaves. Data for 23 species were plotted together. If P is photosynthetie rate, T the total density thickness, E the density thickness of the epidermis, a linear relation is found using the maximum values of P, such that P = R (T - E), where R is about 30 (mg CO₂) (hr)^-1 (g F.W.)^-1. Using this expression, 15 species were represented by values of R over 23, while very young leaves, aurea² leaves and a few others fell in the range 8–16. A mean value of E was taken, from other data, to be about 3 × 10^-3 g/cm², while T ranged from 10 to 40 × 10^-3 g/cm², and P ranged from 0.2 to 0.8 (mg CO₂) hr^-1 cm^-2 for “normal” leaves.