Showing papers on "Photosynthesis published in 1976"

Comparison of the photosynthetic characteristics of three submersed aquatic plants.

Abstract: Light- and CO(2)-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 mumol O(2)/mg Chl.hr at 30 C. At air levels of CO(2), the rates were less than 5% of those achieved by terrestrial C(3) plants. The low photosynthetic rates correlated with low activities of the carboxylation enzymes. In each species, ribulose 1,5-diphosphate carboxylase was the predominant carboxylation enzyme. The apparent K(m)(CO(2)) values for photosynthesis were 150 to 170 mum at pH 4, and 75 to 95 mum at pH 8. The K(m)(CO(2)) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 mum at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5, 35.0, and 28.5 C, respectively. The apparent ability of each species to use HCO(3) (-) ions for photosynthesis was similar, but at saturating free CO(2) levels, there was no indication of HCO(3) (-) use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO(2). With saturating free CO(2) (0.5 mm), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C(3) plants, was still in excess of that required to support HCO(3) (-) utilization.Hydrilla and Ceratophyllum had CO(2) compensation points of 44 and 41 mul/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO(2) compensation points under 1% O(2) indicated that some "dark" respiration occurred in the light. The inhibition of photosynthesis by O(2) was less than with terrestrial C(3) plants. Glycolate oxidase activity was 12.3 to 27.5 mumol O(2)/mg Chl.hr, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600 to 700 mueinsteins/m(2).sec in each species grown under full sunlight. Hydrilla had the lowest light compensation point, and required the least irradiance to achieve the half-maximal photosynthetic rate.Field measurements in a Hydrilla mat indicated that in the afternoon, free CO(2) dropped to zero, and O(2) rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO(2) was highest and O(2) and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.

Toxicity of ammonia to algae in sewage oxidation ponds.

Abstract: Ammonia, at concentrations over 2.0 mM and at pH values over 8.0, inhibits photosynthesis and growth of Scenedesmus obliquus, a dominant species in high-rate sewage oxidation ponds. Photosynthesis of Chlorella pyrenoidosa, Anacystis nidulans, and Plectonema boryanum is also susceptible to ammonia inhibition. Dark respiration and cell morphology were unaffected by any combination of pH and ammonia concentrations tested, thus limiting the apparent effect to inhibition of the normal function of the chloroplasts. Methylamine had the same effect as ammonia, and its penetration into the cells was found to be pH dependent. Therefore, the dependence of toxicity of amines to algae on pH apparently results from the inability to penetrate the cell membrane in the ionized form. When operated at 120-h detention time of raw wastewater, the high-rate oxidation pond maintained a steady state with respect to algal growth and oxygen concentration, and the concentration of ammonia did not exceed 1.0 mM. Shifting the pond to 48-h detention time caused an increase in ammonia concentration in the pond water to 2.5 mM, and the pond gradually turned anaerobic. Photosynthesis, which usually elevates the pH of the pond water to 9.0 to 10.0, could not proceed beyond pH 7.9 because of the high concentration of ammonia, and the algal population was washed out and reduced to a concentration that could maintain a doubling time of 48 h without photosynthesis bringing the pH to inhibitory levels. Under these conditions, the pH of the bond becomes a factor that limits the operational efficiency of the oxidation pond.

Rates of growth, respiration and photosynthesis of unicellular algae as related to cell size—a review1,2

Abstract: SUMMARY The decline of growth rate with increasing species size of unicellular algae grown under uniform conditions is quantified by applying to published data the equation, growth, (cell · time)−1= a (cell carbon)b where a and b are coefficients. The degree of size-dependence might be highest under optimal conditions of growth where b is 0.75. Respiration rate is shown to decline with size in the same manner. It is postulated that gross photosynthesis and processes underlying growth are similarly size-dependent. Growth, efficiency (net over gross photosynthesis) cannot be shown to be size-dependent. Cell size, expressed as carbon, is proposed as a scaling factor in comparative algal physiology.

Concentration quenching in chlorophyll

Abstract: IN the primary process of plant photosynthesis it is generally accepted that efficient energy migration occurs between about 300 molecules of chlorophyll a, with subsequent light collection by a chemical trap. Solutions of chlorophyll in vitro, whether in fluid solvents1, monolayers2,3, multilayers4,5 or, as shown recently in our laboratory, in rigid matrices of PMMA and in bilayer lipid vesicles, exhibit the phenomenon of concentration quenching of the excited state at concentrations much lower than those which are present in the chloroplast. At a chlorophyll concentration of 10−1 M, which is comparable with that in the chloroplast, none of the in vitro systems has a fluorescent yield as high as is found in vivo, especially when the photochemical traps are closed. To try to understand the apparent absence of concentration quenching in vivo, we have re-examined its mechanism in vitro, and conclude that each chlorophyll molecule in the light-collecting system must be separated from other chlorophyll molecules, so as to prevent trap formation by orbital overlap and so that the minimum distance, averaged over all orientations in a random array, is 10 A.

Compartmentation and Transport in C4 Photosynthesis

Abstract: The C4 dicarboxylic acid pathway of photosynthetic carbon assimilation (the C4 pathway) is a complex biochemical and physiological elaboration of the common photosynthetic carbon reduction cycle (PCR cycle, C3 pathway; Bassham and Calvin, 1962). We define the C4 pathway as the complete reaction sequence in which CO2 is transferred via the C-4 carboxyl of C4 acids to the reactions of the PCR cycle and there reduced to the level of carbohydrate (Hatch and Slack, 1970; Hatch et al., 1971; Black, 1973; Hatch, 1976a). The distinctive biochemical features of this process are the carboxylation and associated reactions leading to the synthesis of C4 acids, and those concerned with the subsequent decarboxylation of these C4 acids to supply CO2 for the PCR cycle. Unlike the PCR cycle, in which carboxylation and carbon reduction is restricted to the chloroplast, the C4 pathway involves the operation of reactions in cytoplasm, mitochondria, and chloroplasts, and the transport of intermediates between intracellular compartments. In this sense it may be compared with another well-established elaboration of the PCR cycle, the glycolate pathway of photorespiration (see Chap. II,5). However, an additional and distinctive feature of the C4 pathway is the mandatory exchange of photosynthetic intermediates between adjacent cells. These exchanges constitute one of the most rapid and complex forms of symplastic transport known.

The concept of light intensity adaptation in marine phytoplankton: Some experiments with Phaeodactylum tricornutum

Abstract: The historical background on adaptation of algae to various light intensities is analysed It is argued that there is little evidence to suggest that previous growth at low light intensities enhances the ability of an alga to utilize these low light levels Rather, the published evidence suggests that the most general response to growth at sub-optimal light intensities is a reduced ability to utilize saturating levels The present experiments with Phaeodactylum tricornutum Bohlin have tested this concept of light intensity adaptation Changing photosynthetic abilities during batch growth depended on the light intensity used for growth and these changes affected interpretations of the data When measurements were made intensities appeared to photosynthesize (at all intensities) better than did those grown at higher light levels When the changes during batch growth were taken into account, or when the alga was grown in turbidostat cultures, a different picture was obtained Growth at reduced light intensities was accompanied by (a) increased chlorophyll content, (b) decreased rate of light-saturated photosynthesis expressed on a chlorophyll, cell number or cell protein basis, and (c) decreased activity of RuDP carboxylase One result suggested that growth at a suboptimal light intensity did enhance the ability to utilize lower light levels The light-saturation curve of cells grown in batch culture at 07 klux showed higher slopes at the low light intensities than did those grown at 12 klux This was most marked when photosynthesis was expressed per cell, but was also apparent when it was put on a per chlorophyll basis

Photosynthetic rates of benthic marine algae in relation to light intensity and seasonal variations

Abstract: The effect of light intensity on rate of photosynthesis was measured at irregular intervals over a 12-month period for 22 benthic marine algae in the western Baltic Sea. In most species photosynthesis (mg O2·g dry weight-1·h-1) was highest in spring and summer, corresponding to the seasonal growth pattern of the algae. In winter all the species showed adaptation of the light compensation point. Highest productivity was shown by algae which are: short-lived annual species rather than perennials, eulittoral rather than sublittoral, and which possess sheet-like or filamentous thalli rather than coarsely branched forms. These factors are clearly inter-related.

Photosynthesis at low water potentials

Abstract: Low leaf water potentials result in large reductions in photosynthesis. In higher plants, the reductions are caused both by decreases in the photosynthetic activity of a unit of leaf and in the production of new leaf surface. Photosynthetic activity declines because of decreased stomatal opening and the inhibition of chloroplast activity, either of which may control photosynthesis depending on which is more limiting at low leaf water potentials. The production of new leaf area is highly sensitive to water deficits and is usually reduced before photosynthetic activity decreases. This may be attributed to the high responsiveness of leaf enlargement to turgor, which expands the cells. When low leaf water potentials are prolonged, leaf senescence often occurs and the quantity of existing leaf area may decline. There is evidence that translocation is less sensitive than photosynthesis to low leaf water potentials. Consequently, grain yield, which depends on both photosynthesis and translocation, is more likely to be limited by photosynthesis than translocation. Since substantial translocation to the grain may occur from parts of the plant other than the leaves during desiccation, the total photosynthate accumulated during the growing season is more important than that produced during the grain-filling period alone when plants have had low water potentials.

Changes in photosynthetic pigment concentration in seaweeds as a function of water depth

Abstract: We conducted a study of the relationship between changes in photosynthetic pigment content and water depth in Great Harbor near Woods Hole, Massachusetts, USA, on the green algae Ulva lactuca and Codium fragile and the red algae Porphyra umbilicalis and Chondrus crispus. A calibrated underwater photometer equipped with spectral band filters measured light attenuation by the water column. The depth required for a 10-fold diminution of photon flux was 3.6, 5.3, 6.0 and 6.0 m for red, blue, yellow and green light, respectively. Seaweeds were attached to vertically buoyed lines and left to adapt for 7 days; then, with their positions reversed, they were allowed to readapt for 7 days. All species showed greater photosynthetic pigment content with increased depth. Further, the ratio of phycobiliproteins and chlorophyll b to chlorophyll a increased with depth. Changes in pigment content were reversible and occurred in the absence of cell division. There was a net loss of pigments near the surface (high irradiance), and subsequent synthesis when seaweeds were transferred to a position deep in the water column (low irradiance). In contrast, seaweeds which were found in intertidal habitats changed only their pigment concentration, and not pigment ratio, a phenomena analogous to higher plant sun and shade adaptation. Therefore, seaweeds modify their photon-gathering photosynthetic antennae to ambient light fields in the water column by both intensity adaptation and complementary chromatic adaptation.

Effects of translocation and assimilate demand on photosynthesis

Abstract: Net carbon dioxide uptake by a photosynthesizing primary leaf of bean plants, Phaseolus vulgaris cv. Black Valentine, was measured during treatments designed to alter export from the leaf. Removal of shoot apices lessened sink demand while removal of all source leaves except the one being observed increased sink demand. Export from the leaf under study was lessened by chilling the primary leaf petiole and node to 2 °C. No adjustments in the rate of net photosynthesis were observed during the 33-h period after any of the treatments.The results of this study are in general agreement with previous reports in the literature. After modification of sink demand or of export by experimental manipulation of plants, a period of 2 or 3 days is usually required for adjustment of net photosynthesis rate. Rapid changes in net photosynthesis rate are generally the result of concomitant changes in morphology or metabolism during the course of plant development. The results of this work, and of others in the literature, i...

An examination of photosynthetic production, excretion of photosynthetic products, and heterotrophic utilization of dissolved organic compounds with reference to results from a coastal subtropical sea

Abstract: The rate of primary production, excretion of photosynthetic products and turnover of glucose and amino acids was measured at a station in a coastal region in the Bahamas. Over the depths 0 to 50 m, total photosynthetic rates varied from 1.7 to 12.7 μgC fixed 1-1day-1, averaging 4.3. The extent of extracellular photosynthetic products ranged from undetectable to 23%, averaging 6.9%. Neither the field data nor studies with axenic cultures of Dunaliella tertiolecta, Skeletonema costatum, and Monochrysis lutheri showed any evidence of an increase in the percentage excretion at low population densities or low photosynthetic rates. Rates of amino acid turnover varied from 21 to 168% day-1, and that of glucose from 8.3 to 41% day-1. Light seems to have little effect on the uptake and respiration of these substrates by the planktonic population. There was a significant relationship between the fraction of the substrate used for respiration and that retained by the cell. On average, 42% of the glucose taken up was respired and 21% of the amino acid mixture. Tentative calculations suggest that the production of dissolved organic material as extracellular photosynthetic products would be insufficient to supply the heterotrophic population, and it was concluded that some other route(s) must be of major importance.

Nitrogen nutrition, leaf resistance, and leaf photosynthetic rate of the rice plant

Abstract: The evidence that indicates a close association between nitrogen nutrition and the Photosynthetic rate of single rice leaves (Table 1) shows that the photosynthetic rate is related to the nitrogen content on a dry weight basis or on a leaf area basis, Photosynthesis of single leaves can be analyzed in terms of a series of diffusion resistances, i.e., stomatal, mesophyll, and carboxylation (1, 3, 6). Nitrogen nutrition may affect either one of these or more than two at the same time. Nitrogen nutrition affects mainly the mesophyll resistance, and the stomatal resistance to a lesser extent, in cotton, beans, and maize (10) while it affects both the stomatal and mesophyll resistance in sugar beet (8).

The path of carbon in photosynthesis by marine phytoplankton12

Abstract: SUMMARY Using cultures of a number of different marine algae (diatoms Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin, chrysophyte Isochrysis galbana Parke, green flagellate Dunaliella tertiolecta Butcher, dinoflagellate Gonyaulax tamarensis Lebour) the short-term, pattern of 14CO2 assimilation has been investigated. In all except D. tertiolecta the labelling of amino acids and intermediates of the tricarboxylic acid (Krebs) cycle was significantly heavier than that of sugar phosphates. Over periods of 30–120 s labelling in amino acids and Krebs cycle intermediates accounted for 41–95% of the 14C fixed (depending on the alga). Over shorter times (< 10 s) the pattern in the 2 diatoms showed significant labelling of C4 acids (and related com-pounds) and little labelling of sugar phosphates. The reverse wits seen with D. tertiolecta. Also, in the 2 diatoms and in G. tamarensis significant inhibition of photosynthesis by oxygen could only be achieved with 100% oxygen; atmospheric levels having little effect. Parallel measurements of 2 carboxylating enzymes showed that ribulose-1,5-diphosphate carboxylase (RuDPCase) was significantly greater than phospho (enol)pyruvate carboxylase (PEPCase) activity only in the green flagellate. It is suggested that photosynthesis in marine diatoms depends on an active PEPCase utilizing bicarbonate as a substrate and that a less active RuDPCase utilizes CO2. In D. tertiolecta the pattern more closely resembles that of a “Calvin (C3)” plant. The dinoflagellate and the chrysophyte appeared to show a mixed C3 and C4 photosynthesis.

Photosynthesis, Dark Respiration, and Growth of Rumex patientia L. Exposed to Ultraviolet Irradiance (288 to 315 Nanometers) Simulating a Reduced Atmospheric Ozone Column.

Abstract: Net photosynthesis, dark respiration, and growth of Rumex patientia L. exposed to a ultraviolet irradiance (288-315 nanometers) simulating a 0.18 atm·cm stratospheric ozone column were determined. The ultraviolet irradiance corresponding to this 38% ozone decrease from normal was shown to be an effective inhibitor of photosynthesis and leaf growth. The repressive action on photosynthesis accumulated through time whereas leaf growth was retarded only during the initial few days of exposure. Small increases in dark respiration rates occurred but did not continue to increase with longer exposure periods. A reduction in total plant dry weight and leaf area of approximately 50% occurred after 22 days of treatment, whereas chlorophyll concentrations remained unaltered.

Correlation of Changes in Pigment Content with Photosynthetic Capacity of Seaweeds as a Function of Water Depth

Abstract: We conducted a study of the relationship between changes in photosynthetic pigment content and photosynthetic capacity as a function of water depth in Great Harbor near Woods Hole, Massachusetts, USA, on the green algae Ulva lactuca and Codium fragile and the red algae Porphyra umbilicalis and Chondrus crispus. Seaweeds were attached to vertically buoyed lines at 0.5 and 10 m and were allowed to adapt to the ambient light field. All species showed greater pigment content with depth, and the ratio of accessory pigments to chlorophyll a increased with depth. Seaweed samples from 0.5 and 10 m were placed in tandem pairs of stoppered bottles and hung at prescribed depths. The rates of O2 evolution were calculated from changes in dissolved O2 content, both as a function of biomass and chlorophyll a concentration. Our results indicate that intensity and/or chromatic adaptation enhance the photosynthetic capacity of a seaweed in limiting light conditions. The strategy of seaweeds in manipulating their photon-gathering antennae is not to maximize photosynthetic rate, but rather to optimize the photosynthetic rate. They can change pigment rations, or simply increase the total amount of pigment, or both. Further, if a seaweed is optically thick, as are Codium fragile and Chondrus crispus, it does not matter what color it is. We conclude that the red algae are phylogenetically no better adapted to utilize the ambient light at great depth than their green counterparts. The ambient light conditions alone do not determine the limit for the vertical distribution of the red algae relative to the green algae.

Role of cyclic electron transport in photosynthesis as measured by the photoinduced turnover of P700 in vivo.

Abstract: The light-induced turnover of P700 was measured spectrophotometrically in a wide variety of algae and some photosynthetic mutants. Analysis of the postillumination recovery of P700+ revealed that the apparent first-order rate constant for reduction via the cyclic pathway was much lower that that via the noncyclic pathway. After activation of photosystems 1 and 2 the half-time for reduction of P700+ was 5-20 ms, whereas after activation of primarily photosystem 1 a longer half-time of ca. 150 ms was observed. The extent of the photooxidation of P700 was the same in both regimes of illumination. The longer half-time was also noted after inhibition of photosystem 2 by 3-(3,4-dichlorophenyl)-1,1-dimethylurea or mild heat shock and in mutant algae known to lack a functional photosystem 2. No signal was observed in mutants lacking P700 itself but those strains lacking either plastocyanin or cytochrome f were capable of a very slow turnover (reduction t 1/2 greater than 500 ms at room temperature). This very slow turnover was not affected by carbonyl cyanide m-chlorophenylhydrazone or the plastoquinone antagonist, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, indicating that the pathway for reduction of P700+ in these mutants is not energy linked and does not utilize the intersystem electron transport chain. The slow, 150 ms, reduction of P700+ due to cyclic flow was not observed when cells were engaged in photosynthesis at high-light intensities. The data are interpreted as evidence for the involvement of the total functional pool of P700 in both electron transport pathways, and we suggest that cyclic electron transport does not contribute to photosynthesis in oxygen-evolving autotrophs.

Effect of Zinc Nutrition on Photosynthesis and Carbonic Anhydrase Activity in Cotton

Abstract: Photosynthesis, respiration, carbonic anhydrase activity and chlorophyll concentrations were correlated with zinc nutrition in cotton (Gossypium hirsutum L.). The critical zinc level during early plant growth was 13 μg/g dry weight in recently matured leaves (blade three). Photosynthesis and chlorophyll concentration required a minimum Zn of 13 and 14 μg/g dry weight, respectively, in blade three for maximum activity and synthesis. Respiration was not influenced by zinc status. Carbonic anhydrase activity increased curvilinearly as zinc status improved from deficiency to adequacy.

The Relationship Between the Rates of Carbon Transport and of Photosynthesis in Tomato Leaves

Abstract: The rate of carbon transport based on the carbon balance over a 6 -h period from a mature tomato leaf was measured over a range of net photosynthetic rates from 0-1 to 4-9 mg C dm-2 h_1 under light flux densities from 4 to 140 W m-2. A proportional relationship was demonstrated between the rate of carbon transport and carbon fixation when the carbon fixation rate was higher than 2 mg C dm-2 h_1. Below a carbon fixation rate of 1 mg C dm-2 h-1, the rate of carbon export was maintained at 1 mg C dm-2 h_1 at the expense of the breakdown of starch. A highly significant correlation was observed between sucrose concentration and the rate of carbon transport. The sucrose concentration in the leaf appears to be the factor controlling carbon export.

Biochemical differentiation of plastids and other organelles in rye leaves with a high-temperature-induced deficiency of plastid ribosomes.

Abstract: 1. In developing rye (Secale cereale L.) leaves the formation of plastidic ribosomes was selectively prevented in light as well as in darkness, when the seedlings were grown at an elevated temperature of 32° instead of 22° where normal development ocurred. Plastid ribosome deficient parts of lightgrown leaves were chlorotic at 32°. — 2. At both temperatures the leaves contained under all conditions (light or dark, on H2O or nutrient solution) equal or very similar amounts of total amino nitrogen. In light, the contents of total protein and dry weight were lower at 32° than at 22°, especially when the plants were grown on nutrient solution. — 3. Mitochondrial marker enzymes had normal or even higher activities in 32°-grown leaves. Respiration rates were similar for segments of leaves grown on water in light either at 32° or at 22° but by 20–30% lower for 32°-grown plants when they had been raised in darkness or on nutrient solution. In contrast to 22°-grown tissue, respiration of 32°-grown leaf segments was rather insensitive to KCN. Comparative inhibitor studies indicated the presence of both the cyanide-sensitive and the cyanide-insensitive pathway of respiration in 32°-grown leaves. — 4. Leaf microbody marker enzymes were present in leaves grown at 32°. From chlorotic parts of 32°-light-grown leaves a typical microbody fraction was isolated on sucrose densitygradients. — 5. Leaves of seedlings grown at 32° contained only very low levels of ribulosediphosphate carboxylase activity and of fraction I protein. Photosynthetic 14CO2-fixation of such leaves was only a few per cent of that observed in normal leaves, and no photosynthetic oxygen evolution was observed in chlorotic leaf segments. However, ten other soluble enzymes which are exclusively or partially localized in chloroplasts reached high activities under all conditions at 32° (Table 4). — 6. From chlorotic parts of 32°-light-grown leaves as well as from etiolated 32°-grown leaves a fraction of intact plastids was isolated and purified by sucrose gradient centrifugation which contained several soluble chloroplast enzymes. From the results we conclude that cytoplasmic protein synthesis must contribute a functional chloroplast envelope including the mechanism for the recognition and uptake of chloroplast proteins which are synthesized on cytoplasmic ribosomes.

Effects of simulated acid rain on phaseolus vulgaris L. (fabaceae)

Abstract: A B S T R A C T Experiments were performed to determine the effects of simulated acid rain on Phaseolus vulgaris L. At pH values below 3, plants exhibited a failure to attain normal height, had necrotic and wrinkled leaves, excessive and adventitious budding, and premature abscission of primary leaves. Histologically, leaves had smaller cells, less intercellular space, and smaller starch granules within the chloroplasts. Respiration rates of the treated plants increased only slightly at low pH values. Apparent rates of photosynthesis, however, increased dramatically. Both carbohydrate production and root biomass were reduced by low pH treatments, and application of Congo red indicator to the acid-treated leaf tissue showed that the cell contents were acidified to a pH of below 4.0.

Watering converts a CAM plant to daytime CO 2 uptake

Abstract: THREE different photosynthetic options have been identified in plants1,2: (1) most plants have the reductive pentose phosphate or C3 pathway, where CO2 is incorporated into ribulose-1,5-diphosphate (RuDP) to yield two molecules of 3-phosphoglyceric acid, a three-carbon compound; (2) the C4 mode, where the first photosynthetic products are four-carbon dicarboxylic acids like oxaloacetate and malate formed following CO2 incorporation into phosphoenolpyruvate (PEP); and (3) crassulacean acid metabolism (CAM), found in many succulent plants growing in arid regions. In the last, stomatal opening and net CO2 uptake occur at night, CO2 being incorporated by way of PEP carboxylase into organic acids. The tissue acidity decreases as the organic acids are decarboxylated during the day, when the internally released CO2 is prevented from leaving by the closed stomata. The water vapour concentration difference between the tissue and ambient air is less at night, and thus the night-time stomatal opening of CAM plants leads to overall water conservation. For example, the water lost per CO2 fixed averages about sixfold higher for C4 plants and tenfold higher for C3 ones than for CAM plants in natural conditions2. The net daily CO2 uptake by CAM plants is less than for C3 or C4 plants, so CAM plants tend to be relatively slow growing.

The interaction of components controlling net phytoplankton photosynthesis in a well‐mixed lake (Lough Neagh, Northern Ireland)

Abstract: Summary The homogeneous distribution of the phytoplankton in a shallow (mean depth 8·6 m) unstratified lake, L. Neagh, Northern Ireland, facilitated the study of the interaction of components controlling gross photosynthesis per unit area. These included the photosynthetic capacity, the phytoplankton content of the euphotic zone, and a logarithmic function describing the effective radiation input. These factors were analysed for two sites, the open lake and Kinnego Bay, which respectively had standing crops of up to 90 and 300 mg chlorophyll a m−3 and maximum daily rates of gross integral photosynthesis of 11·7 and 15·6 g O2 m−2 day−1. Values are reduced by the high contribution to light attenuation by non-algal sources, which increases at low standing crops particularly in winter, when values of integral photosynthesis decrease to 0·5 g O2 m−2 day−1. This relative change is the result of self-shading behaviour of the phytoplankton altering the crop content of the euphotic zone at different population densities. Changes in the irradiance function, incorporating day length, are largely responsible for the changes in daily rates of integral gross photosynthesis; as daily irradiance is also a determinant of water temperature, it exerts further influence through the photosynthetic capacity which was strongly correlated with temperature. Much of the gain in gross photosynthesis resulting from higher photosynthetic capacity may not be reflected in a higher net column photosynthesis, because of the greater proportional rise in respiration with temperature. The balance in the water column between respiration losses and photosynthetic input may frequently alter since the ratio of illuminated to dark zones is between 1/4 to 1/5 in the open lake, and small shifts in any of the controlling features may result in conditions unfavourable for growth. This is analysed especially for the increase of diatoms in spring, when small modifications of the underwater light field can delay growth.

Low Temperature Photosynthesis in Successional Winter Annuals

Abstract: Winter annuals usually germinate in summer and fall, overwinter as vegetative rosettes, and then dominate 1—yr fields, suppressing spring—germinated summer annuals. Net photosynthetic response to light and temperature of Erigeron canadensis, E. annuus, Rorippa sessiliflora, Capsella bursa—pastoris, and Lactuca scariola were measured three times during the year. Optimum temperature for photosynthesis shifted from ° 25 degrees—30 degrees C during the summer to 15 degrees C during winter. Absolute rates of photosynthesis at 15 degrees C during winter approached maximum summer rates indicating temperature compensation. In the field winter photosynthesis is maximized because (1) rosette leaf temperatures, at air temperatures of 0 degrees —10 degrees C, were up to 10 degrees C above air temperature, (2) low light compensation point of photosynthesis shifts form 75 μE m(—2) s(—1) at 25 degrees C to 18 μE m(—2) s(—2) at 5 degrees C, (3) start up time for photosynthesis after dark period was only a few minutes and maximum photosynthesis was reached within 12 min, and (4) temperature compensation for photosynthesis approached 100% in a number of individuals. Photosynthesis over winter gives the species competitive advantage and may be critical to the dominance of winter annuals in succession during spring and summer.