Maximum Rate Of Photosynthesis Research Articles

Photosynthesis and growth rates of Laurencia brongniartii J. Agardh (Rhodophyta, Ceramiales) in preparation for cultivation

Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (Pmax) was 4.41 μmol photon m−2 s−1 at 26 °C and 4.07 μmol photon m−2 s−1 at 28 °C. Saturating irradiance occurred at 95 μmol photon m−2 s−1 at 26 °C and 65 μmol photon m−2 s−1 at 28 °C. In contrast, growth experiments at 41.7 μmol photon m−2 s−1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day−1 at 28 °C and 25 μmol photon m−2 s−1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation.

Effect of a Short Photoinhibition Stress on Photosynthesis, Chlorophyll a Fluorescence, and Pigment Contents of Different Maize Cultivars. Can a Rapid and Objective Stress Indicator be Found?

The effect of a short cold stress in combination with photoinhibition stress, similar to a low temperature and a high irradiance situation during early morning in the spring time, was examined on four maize cultivars common for Belgium, that differ in early vigour. After 1 h of 2 °C and 500 μmol(photon) m-2 s-1, quantum efficiency and maximum photosynthesis rate at saturating irradiance decreased on average by 11 and 8 %, respectively. For one cultivar, Magister, the decrease was the largest: by 23 and 10 %, respectively. For this cultivar it was combined with a decrease of the water vapour conductance after the stress. The decrease of Fv/F0 due to the cold/light stress was dependent on the cold tolerance (early vigour) of the cultivars. Fv/F0 changed with -45.5 and -40.2 % for the cultivars Ardiles and Banguy, respectively (cultivars with a less good early vigour) in comparison to -36.3 and -35.9 % for Fjord and Magister, which have a good early vigour. Also the ratio of total chlorophylls/total carotenoids changed in dependence on cold tolerance of the cultivars. For more cold tolerant cultivars, the relative amount of total carotenoids (x+c) was higher, indicating a higher protective state. Both the parameter Fv/F0 and the ratio of total chlorophylls to total carotenoids can be used to differentiate the cold tolerant cultivars from the cold non-tolerant ones. Fv/F0 has the advantage because its resolving power is larger and the measurement is less expensive than determination of the pigment ratio.

Response of total night‐time respiration to differences in total daily photosynthesis for leaves in a Quercus rubra L. canopy: implications for modelling canopy CO2 exchange

AbstractMeasurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night‐time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 μmol m−2 s−1) were approximately four times higher than those in the lower leaves (4.3 μmol m−2 s−1) and maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m−2 s−1) were about half those in the upper leaves. There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, Vcmax, were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, Jmax (P>0.05), and light use efficiency, ɛ (P<0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO2 exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process‐based approach linking respiration to photosynthesis when modelling long‐term carbon exchange in forest ecosystems.

Light acclimation of photosynthesis in three charophyte species

The main aim of this study was to investigate if the charophyte species Chara baltica, Chara canescens (two populations from the Baltic Sea (BS) and the Gulf of Korinth, Greece (GK)), and Lamprothamnium papulosum exhibit different acclimation capacities to irradiance. Growth, photosynthesis and pigment content were examined in the laboratory under six irradiance conditions (35–500 μmol photons m −2 s −1). Growth experiments showed increasing growth rates from 35 μmol photons m −2 s −1 (∼10 mg fresh weight (FW)) up to 70 μmol photons m −2 s −1 (∼20 mg FW) in C. baltica, from 35 μmol photons m −2 s −1 (∼15 mg FW) up to 380 μmol photons m −2 s −1 (∼145 mg FW) in C. canescens (BS), and up to the highest growth irradiance in algae of L. papulosum (35 μmol: ∼5 mg FW; 500 μmol: ∼20 mg FW). The species were tested for their ability to acclimate to different growth irradiances ( E g) by calculating P max (maximum photosynthesis rate at saturating irradiances), α (the efficiency of light utilization at limiting irradiance), and E k (the light saturation point of photosynthesis, P max/ α). All species exhibited increasing P max with increasing E g. Whereas both populations of C. canescens increased α with increasing E g, L. papulosum and C. baltica did not acclimate α at all. E k, the irradiance at which photosynthesis ceased to be light-limited, was constant for all Chara species within the range of irradiances tested. Chl a/Chl b ratios of all species were constant over the whole range of E g. Chl a/carotenoid ratios were constant in C. baltica, whereas Chl a/carotenoid ratios in L. papulosum and C. canescens (BS) decreased from 250 and 70 μmol photons m −2 s −1 upwards, respectively. Pigmentation analysis showed that Chl a/carotenoid acclimation was mainly caused by species-specific capacity to raise the content of lutein and carotene ( C. canescens (BS), C. canescens (GK)) and xanthophyll cycle pigments (XCP; L. papulosum). The non-photochemical quenching (NPQ) capacities of L. papulosum, C. canescens (BS), and C. canescens (GK) were dependent from preacclimation status of algae, whereas NPQ of C. baltica was independent from growth irradiance. Our results indicate that C. baltica and C. canescens (BS) were light saturated within the chosen irradiances, whereas C. canescens (GK) and L. papulosum did not reach their limits of high-light acclimation. The photosynthetic pigments lutein, α- and β-carotene are suggested to act as photo-protective pigments in L. papulosum and C. canescens.

A study on the shade tolerance ofMuehlewbeckia complera

Muehlewbeckia complera was introduced to China in 2002 as indoor-hanging ornamental foliage plant. The experiment of the shade tolerance for this species was carried out in different light intensities (0.14–946.00 μmol·m−2·s−1). After 40 days in experimental areas, leaf photosynthentic characteristics indexes ofM. complera in different photosynthesis active radiation (PAR) were measured with LI-COR6400 apparatus, such as the light compensation point, light saturation point, and maximum net photosynthesis rate, at the same time, the increments of total leaf area and leaf amount were measured. The results showed that the optimum light intensity range forM. complera was from 9.26 μmol·m−2·s−1 to 569.00 μmol·m−2·s−1 (463–28150 lx, relative humidity (RH) for 46–60%, temperature at 16–22°C). Under this condition, leaf photosynthetic efficiency was tiptop. AlthoughM. complera belonged to the moderate sun-adaptation plant species, the plant growth was inhibited when PAR increased to the level of 569.000 μmol·m−2·s−1 or above.M. complera could sprout new leaves in photosynthesis active radiation of 0.16–19.22 μmol·m−2·s−1 (8–961 lx), or 10 μmol·m−2·s−1 for above 6 h.

Interaction of UV-Radiation and Inorganic Carbon Supply in the Inhibition of Photosynthesis: Spectral and Temporal Responses of Two Marine Picoplankters

The effect of ultraviolet radiation (UVR) on inhibition of photosynthesis was studied in two species of marine picoplankton with different carbon concentration mechanisms: Nannochloropsis gaditana Lubian possesses a bicarbonate uptake system and Nannochloris atomus Butcher a CO2 active transport system. Biological weighting functions (BWFs) for inhibition of photosynthesis by UVR and photosynthesis vs irradiance (PI) curves for photosynthetically active radiation (PAR) were estimated for both species grown with an enriched CO2 supply (high dissolved inorganic carbon [DIC]: 1% CO2 in air) and in atmospheric CO2 levels (low DIC: 0.03% CO2). The response to UVR and PAR exposures was different in each species depending on the DIC treatment. Under PAR exposure, rates of maximum photosynthesis were similar between treatments in N. gaditana. However, the cultures growing in high DIC had lower sensitivity to UVR than the low DIC cultures. In contrast, N. atomus had higher rates of photosynthesis under PAR exposure with high DIC, but the BWFs were not significantly different between treatments. The results suggest that one or more processes in N. gaditana associated with HCO3- transport are target(s) for UV photodamage because there was relatively less UV inhibition of the high DIC-grown cultures in which inorganic carbon fixation is supplied by passive CO2 diffusion. Time courses of photochemical efficiency in PAR, during UV exposure and during subsequent recovery in PAR, were determined using a pulse amplitude modulated fluorometer. The results were consistent with the BWFs. In all time courses, a steady state was obtained after an initial decrease, consistent with a dynamic balance between damage and repair as found for other phytoplankton. However, the relationship of response to exposure showed a steep decline in activity that is consistent with a constant rate of repair. A novel feature of a model developed from a constant repair rate is an explicit threshold for photosynthetic response to UV.

Assimilate partitioning in leaves of the raffinose-storing herb Lamium album L.: the effects of altering source-sink balance

In Lamium album, sucrose and raffinose-family oligosaccharides are the major products of photosynthesis that are stored in leaves. Using gas analysis and 14CO2 feeding, we compared photosynthesis and the partitioning of recently-fixed carbon in plants where sink activity was lowered by excision of flowers and chilling of roots with those where sink activity was not modified. Reduction in sink activity led to a reduction in the maximum rate of photosynthesis, to retention of fixed carbon in source leaves and to the progressive accumulation of raffinose-family oligosaccharides. This ultimately affected the extractable activities of invertase and sucrose phosphate synthase. At the end of the light period, invertase activity was significantly higher in treated plants. By contrast sucrose phosphate synthase activity was significantly lower in treated plants. We propose that reducing sink activity in L. album is associated with a shift in metabolism away from starch and sucrose synthesis and towards sucrose catabolism, galactinol utilisation and the synthesis of raffinose-family oligosaccharides.

Photosynthetic and growth responses of three freshwater algae to phosphorus limitation and daylength

Summary1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., green alga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown under contrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] and phosphorus (P) regimes (P‐sufficient versus 1 μm P). The rates of growth and photosynthesis, as well as growth efficiencies and pigment concentrations, were compared among treatments.2. The growth and photosynthetic parameters of the three species depended on both P status and daylength in a species‐specific way. The responses to P limitation depended on daylength and, conversely, the responses to daylength depended on P status.3. Growth rates and the maximum rates of photosynthesis (Pmax) of all species decreased under P limitation under both light regimes. However, the decrease of Pmax because of P limitation was greater under long daylength. The Pmax of the green alga S. schroeteri decreased the most (ca. sixfold) under P limitation compared with the other two species. The photosynthesis saturation parameter Ik also decreased under P limitation; the decline was significant in Nitzschia and Sphaerocystis. P‐limitation significantly increased photoinhibition (β) in Nitzschia and Sphaerocystis, but not in Phormidium. The excess photochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesis rate at the growth irradiance), characterising the ability to utilise fluctuating light, was significantly lower under P limitation.4. The growth efficiency (growth rate normalised to daylength) declined with increasing daylength in all species. Under short daylength the cyanobacterium Phormidium had the lowest growth efficiency of the three species.5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis was significantly higher under short daylength, but only under P‐sufficient conditions. In Nitzschia, under short daylength, P‐limitation significantly decreased cellular chlorophyll concentration. In contrast, P‐limitation increased cellular chlorophyll concentration in Sphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the green alga also declined under short daylength and under P‐limited conditions.

Use of light and inorganic carbon acquisition by two morphotypes of Zostera noltii Hornem

The affinity for dissolved inorganic carbon (DIC) and the mechanisms to use HCO 3 − as a source of DIC for photosynthesis were investigated in two morphotypes of Zostera noltii Hornem. Both morphotypes were collected at Ria Formosa lagoon (Southern Portugal) at two different levels in the intertidal. Affinity for DIC at saturating photon fluence rate (PFR), estimated as photosynthetic conductance for CO 2 ( g p(CO 2) ), was reduced by 75% in the Z. noltii plants adapted to shade conditions (lower intertidal) in comparison to the sun morphotype (45×10 −6 and 182×10 −6 m s −1, respectively), indicating that the plants acclimated to sun conditions (higher intertidal) had a higher capacity to use HCO 3 − as DIC source for photosynthesis. Since external carbonic anhydrase activity was negligible and a large inhibitory effect was produced by Tris buffer addition, this HCO 3 − use was attributed to the operation of H + ATPases creating low pH zones in periplasmic space. The photosynthetic CO 2-flux supported for this mechanism was calculated to be 53 μmol O 2 m −2 s −1 in sun morphotype, about 80% out of maximum photosynthesis rate. In order to determine the possible photosynthetic energy cost of the HCO 3 − use, the effect of decreasing light on photosynthetic rates and g p(CO 2) was estimated. Photosynthetic conductance decreased in both morphotypes at non-saturating PFR. This dependence of g p(CO 2) on PFR indicated the existence of a positive interactive effect between DIC and PFR which was more pronounced in the shade morphotype since the ascending slope of O 2 evolution vs. PFR curves at limiting PFRs was reduced from 7.2 to 2.3 mmol O 2 mol photon −1 at 4 and 0.5 mol m −3 of DIC, respectively.

Transfer conductance in second growth Douglas‐fir (Pseudotsuga menziesii (Mirb.)Franco) canopies

ABSTRACTThe internal conductance from intercellular spaces to the sites of carboxylation (gi) has only been measured in a few tree species and not in conifers, despite the fact it may impose a large limitation on photosynthesis. The present study provides the first estimates of gi for a coniferous species, and examines variation in gi with height and its relationships to anatomical, biochemical and physiological traits. Measurements were made on upper and lower canopy current‐year needles of 50‐year‐old Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco). Needle thickness and specific leaf area decreased by 30% from the top to bottom of the canopy. These anatomical/morphological changes were accompanied by modest variation in allocation of N to chlorophyll and the chlorophyll a/b ratio. Allocation of N to Rubisco did not vary with height, but the ratio of Rubisco to chlorophyll did owing to the aforementioned changes in allocation to chlorophyll. The value of gi was estimated in one tree from concurrent measurements of carbon isotope discrimination and net photosynthesis. To examine the variation in gi among trees a second independent method based on day respiration and the difference between the chloroplastic and intercellular photocompensation points (photocompensation point method) was used. Estimates of gi obtained by the two methods agreed well with values varying between 0.14 and 0.20 mol m−2 s−1. It is estimated that gi limits photosynthesis by approximately 20% as compared to an approximately 30% stomatal limitation (under well‐watered conditions). The value of gi scaled approximately with maximum rates of photosynthesis, which were significantly greater in upper canopy needles. Nevertheless, gi did not vary significantly with canopy height, owing to greater variability in gi than photosynthesis.

Influence of temperature on the relationship between oxygen- and fluorescence-based estimates of photosynthetic parameters in a marine benthic diatom (Cylindrotheca closterium)

In this paper we investigate the temperature sensitivity of the photosynthetic process of the benthic diatom Cylindrotheca closterium grown in light-limited turbidostat cultures at two different growth rates. Photosynthesis was measured as the rate of oxygen evolution and as the photosystem II (PSII) electron transport rate (ETR). The photosynthetic efficiency (a), as measured by both methods, was rather insensitive to temperature, and decreased significantly only at the extreme temperatures used (5 and 35°C). The maximum PSII quantum efficiency (Fv/Fm) showed a small but significant trend of reduction with increasing temperature. However, the maximum rate of photosynthesis (PB max and ETRmax) was extremely temperature sensitive. The effect of temperature on the relationship between PB and ETR was limited to the most extreme temperatures investigated; deviations from linearity were most extreme at 5°C and different conversion factors were observed at 5 and 35°C. A short-term change in temperature (10–30°C), as might be experienced during emersion on a European tidal flat, will not significantly affect the relationship between PB and ETR. However, care should be taken when using a single conversion factor between PB and ETR at the extremes of the temperature range. We have also shown that algal absorption measurements are important for correct calculation of ETR. The facts that different species seem to have different conversion factors and that changing environmental conditions will affect the absorption capacity and growth rate of the microphytobenthos (MPB) community suggest that it is wise to perform further calibrations of the relationship in the field before use in primary production modelling. Variable fluorescence measurements are quick and non-invasive and, with knowledge of the absorption properties of the MPB community, allow the quantification of photosynthetic parameters across large areas. Hence they are potentially useful for improving our estimates of ecosystem scale primary production.

Diffusive boundary layers and photosynthesis of the epilithic algal community of coral reefs

The effects of mass transfer resistance due to the presence of a diffusive boundary layer on the photosynthesis of the epilithic algal community (EAC) of a coral reef were studied. Photosynthesis and respiration of the EAC of dead coral surfaces were investigated for samples from two locations: the Gulf of Aqaba, Eilat (Israel), and One Tree Reef on the Great Barrier Reef (Australia). Microsensors were used to measure O2 and pH at the EAC surface and above. Oxygen profiles in the light and dark indicated a diffusive boundary layer (DBL) thickness of 180–590 μm under moderate flow (~0.08 m s−1) and >2,000 μm under quasi-stagnant conditions. Under light saturation the oxygen concentration at the EAC surface rose within a few minutes to 200–550% air saturation levels under moderate flow and to 600–700% under quasi-stagnant conditions. High maximal rates of net photosynthesis of 8–25 mmol O2 m−2 h−1 were calculated from measured O2 concentration gradients, and dark respiration was 1.3–3.3 mmol O2 m−2 h−1. From light–dark shifts, the maximal rates of gross photosynthesis at the EAC surface were calculated to be 16.5 nmol O2 cm−3 s−1. Irradiance at the onset of saturation of photosynthesis, E k, was <100 µmol photons m−2 s−1, indicating that the EAC is a shade-adapted community. The pH increased from 8.2 in the bulk seawater to 8.9 at the EAC surface, suggesting that very little carbon in the form of CO2 occurs at the EAC surface. Thus the major source of dissolved inorganic carbon (DIC) must be in the form of HCO3 −. Estimates of DIC fluxes across the DBL indicate that, throughout most of the daytime under in situ conditions, DIC is likely to be a major limiting factor for photosynthesis and therefore also for primary production and growth of the EAC.

Contents of Volume

The affinity for dissolved inorganic carbon (DIC) and the mechanisms to use HCO3− as a source of DIC for photosynthesis were investigated in two morphotypes of Zostera noltii Hornem. Both morphotypes were collected at Ria Formosa lagoon (Southern Portugal) at two different levels in the intertidal. Affinity for DIC at saturating photon fluence rate (PFR), estimated as photosynthetic conductance for CO2 (gp(CO2)), was reduced by 75% in the Z. noltii plants adapted to shade conditions (lower intertidal) in comparison to the sun morphotype (45×10−6 and 182×10−6 m s−1, respectively), indicating that the plants acclimated to sun conditions (higher intertidal) had a higher capacity to use HCO3− as DIC source for photosynthesis. Since external carbonic anhydrase activity was negligible and a large inhibitory effect was produced by Tris buffer addition, this HCO3− use was attributed to the operation of H+ ATPases creating low pH zones in periplasmic space. The photosynthetic CO2-flux supported for this mechanism was calculated to be 53 μmol O2 m−2 s−1 in sun morphotype, about 80% out of maximum photosynthesis rate. In order to determine the possible photosynthetic energy cost of the HCO3− use, the effect of decreasing light on photosynthetic rates and gp(CO2) was estimated. Photosynthetic conductance decreased in both morphotypes at non-saturating PFR. This dependence of gp(CO2) on PFR indicated the existence of a positive interactive effect between DIC and PFR which was more pronounced in the shade morphotype since the ascending slope of O2 evolution vs. PFR curves at limiting PFRs was reduced from 7.2 to 2.3 mmol O2 mol photon−1 at 4 and 0.5 mol m−3 of DIC, respectively.

Oryzacystatin I expression in transformed tobacco produces a conditional growth phenotype and enhances chilling tolerance

A recent strategy for pest control in plants has involved transformation with genes encoding cysteine proteinase inhibitors (cystatins). Little is known, however, about the effects of constitutive cystatin expression on whole plant physiology. The present study using oryzacystatin I (OC-I) expression in transformed tobacco was designed to resolve this issue and also to test the effects on abiotic stress tolerance. All transformed plants expressing OC-I showed a conditional phenotype. A marked effect on stem elongation was observed in plants grown under low light intensities. After 7 weeks of growth at low light, the plants expressing OC-I were smaller with fewer expanded leaves and a slightly lower total biomass than empty vector controls or wild type plants. Maximal rates of photosynthesis (A(max)) were also decreased, the inhibitory effect being greatest in the plants with highest OC-I expression. After 12 weeks of growth at low light, however, the plants expressing OC-I performed better in terms of shoot biomass production, which was nearly double that of the empty vector or wild type controls. All plants showed similar responses to drought, however photosynthesis was better protected against chilling injury in plants constitutively expressing OC-I. Photosynthetic CO(2) assimilation was decreased in all plants following exposure to 5 degrees C, but the inhibition was significantly less in the OC-I expressing plants than in controls. The transformed tobacco plants expressing OC-I therefore show a phenotype-environment interaction with important implications for biotechnological applications.

Dual role of cysteine 172 in redox regulation of ribulose 1,5-bisphosphate carboxylase/oxygenase activity and degradation.

Alkylation and oxidation of cysteine residues significantly decrease the catalytic activity and stimulate the degradation of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO). We analyzed the role of vicinal cysteine residues in redox regulation of RuBisCO from Synechocystis sp. strain PCC 6803. Cys172 and Cys192, which are adjacent to the catalytic site, and Cys247, which cross-links two large subunits, were replaced by alanine. Whereas all mutant cells (C172A, C192A, C172A-C192A, and C247A) and the wild type grew photoautotrophically at similar rates, the maximal photosynthesis rates of C172A mutants decreased 10 to 20% as a result of 40 to 60% declines in RuBisCO turnover number. Replacement of Cys172, but not replacement of Cys192, prominently decreased the effect of cysteine alkylation or oxidation on RuBisCO. Oxidants that react with vicinal thiols had a less inhibitory effect on the activity of either the C172A or C192A enzyme variants, suggesting that a disulfide bond was formed upon oxidation. Thiol oxidation induced RuBisCO dissociation into subunits. This effect was either reduced in the C172A and C192A mutant enzymes or eliminated by carboxypentitol bisphosphate (CPBP) binding to the activated enzyme form. The CPBP effect presumably resulted from a conformational change in the carbamylated CPBP-bound enzyme, as implied from an alteration in the electrophoretic mobility. Stress conditions, provoked by nitrate deprivation, decreased the RuBisCO contents and activities in the wild type and in the C192A and C247A mutants but not in the C172A and C172A-C192A mutants. These results suggest that although Cys172 does not participate in catalysis, it plays a role in redox regulation of RuBisCO activity and degradation.

Expression of a cyanobacterial sucrose-phosphate synthase from Synechocystis sp. PCC 6803 in transgenic plants.

Sucrose-phosphate synthase (SPS) from the cyanobacterium Synechocystis sp. PCC 6803 lacks all of the Ser residues known to be involved in the regulation of higher plant SPS by protein phosphorylation. The Synechocystis SPS is also not allosterically regulated by glucose 6-phosphate or orthophosphate. To investigate the effects of expressing a potentially unregulated SPS in plants, the Synechocystis sps gene was introduced into tobacco, rice and tomato under the control of constitutive promoters. The Synechocystis SPS protein was expressed at a high level in the plants, which should have been sufficient to increase overall SPS activity 2-8-fold in the leaves. However, SPS activities and carbon partitioning in leaves from transgenic and wild-type plants were not significantly different. The maximal light-saturated rates of photosynthesis in leaves from tomato plants expressing the Synechocystis SPS were the same as those from wild-type plants. Tomato plants expressing the maize SPS showed 2-3-fold increases in SPS activity, increased partitioning of photoassimilate to sucrose and up to 58% higher maximal rates of photosynthesis. To investigate the apparent inactivity of the Synechocystis SPS the enzyme was purified from transgenic tobacco and rice plants. Surprisingly, the purified enzyme was found to have full catalytic activity. It is proposed that some other protein in plant cells binds to the Synechocystis SPS resulting in inhibition of the enzyme.

Metabolic Engineering of Starch for Enhanced Plant Productivity and Yields

Plant productivity and yield depend on the capacity of leaves to fix CO2 during photosynthesis and on the capacity of sink tissues such as developing seeds or fruits to convert the fixed carbon into dry matter. The capacity to convert triose-phosphate to sucrose or starch in the leaf can limit the maximal rate of photo synthesis, and we have demonstrated that photosynthetic capacity and growth correlate with the capacity to make leaf starch. The findings presented here reveal that in Arabidopsis thaliana with increased activity of the major starch regulatory enzyme, ADPglucose pyrophosphorylase (AGPase), there is a greater capacity for starch biosynthesis compared to wildtype and a starch-deficient mutant. This resulted in increased CO2 as similation due to suppression of photosynthetic feedback and in higher starch turnover rates in these plants. Collectively these effects translated to increased growth and seed biomass. Previous studies from our laboratory showed that rice is limited by the capacity to convert triose-P to carbohydrates under ordinary environ mental conditions. Therefore, productivity may benefit by increasing the conversion of triose-P into leaf starch through increased AGPase activity.

Effects of CO2 concentrations on the freshwater microalgae, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus (Chlorophyta)

In order to investigate the possible impacts of increased atmospheric CO2 levels on algal growth and photosynthesis, the influence of CO2 concentration was tested on three planktonic algae (Chlamydomonas reinhardtii, Chlorella pyrenoidosa, and Scenedesmus obliquus). Increased CO2 concentration enhanced significantly the growth rate of all three species. Specific growth rates reached maximal values at 30, 100, and 60 μM CO2 in C. reinhardtii, C. pyrenoidosa, and S. obliquus, respectively. Such significant enhancement of growth rate with enriched CO2 was also confirmed at different levels of inorganic N and P, being more profound at limiting levels of N inC. pyrenoidosa and P in S. obliquus. The maximal rates of net photosynthesis, photosynthetic efficiency and light-saturating point increased significantly (p < 0.05) in high-CO2-grown cells. Elevation of the CO2 levels in cultures enhanced the photoinhibition of C. reinhardtii, but reduced that of C. pyrenoidosa and S. obliquus when exposed to high photon flux density. The photoinhibited cells recovered to some extent (from 71% to 99%) when placed under dim light or in darkness, with better recovery in high-CO2-grownC. pyrenoidosa and S. obliquus. Although pH and pCO2 effects cannot be distinguished from this study, it can be concluded that increased CO2 concentrations with decreased pH could affect the growth rate and photosynthetic physiology of C. reinhardtii, C. pyrenoidosa, and S. obliquus.

The effect of alkalinity on photosynthesis–light curves and inorganic carbon extraction capacity of freshwater macrophytes

The influence of variable bicarbonate (HCO 3 −) alkalinity was assessed on the photosynthetic performance of four submerged macrophyte species of contrasting growth form and distribution but all reportedly capable of HCO 3 − use: Hydrilla verticillata, Egeria densa, Potamogeton schweinfurthii and Potamogeton lucens. Whole-shoot photosynthesis–irradiance curves and pH-drift capacity were measured at two alkalinities, a low and a high concentration (0.5 and 2.0 meq l −1 HCO 3 −). E. densa, H. verticillata and P. lucens had higher gross maximum photosynthesis ( P max) and respiration rates at the higher HCO 3 − availability. The species also differed significantly in P max and respiration for both alkalinities, with P max estimates ranging between 3 and 22 mg O 2 g AFDW −1 h −1. Half-saturation constant estimates ( K 1/2) did not differ between species or treatments, but were comparatively low (<10 μmol PAR m −2 s −1). Incorporation of a light inhibition term did not improve the explained variation of the fitted curves significantly. Overall, respiration rates were high (2–13 mg O 2 g AFDW −1 h −1), largely due to the high incubation temperature employed (25 °C). In the pH-drift experiment H. verticillata reached a pH above 9.5 in both alkalinities after 48 h, which is higher than all the other species tested. In the higher alkalinity pH-drift experiment, only E. densa failed to raise the pH. All species raised the pH more rapidly in the low alkalinity medium probably due to its weaker buffering capacity. Final (96 h) maximum pHs were also higher in low alkalinity. Across species, however, no correlations existed between any of the P– I curve parameters including net photosynthesis at 200 μmol PAR m −2 s −1 and carbon extraction capacity expressed as drifted pH after 48 h. In parallel growth experiments, H. verticillata maintained growth even at 3 meq l −1 (RGR=0.06 per day), whereas in E. densa RGR declined rapidly beyond 1 meq l −1, which is consistent with the pH-drift experiments.

INTERSPECIFIC COMPETITION AND RESOURCE PULSE UTILIZATION IN A COLD DESERT COMMUNITY

In desert ecosystems a large proportion of water and nitrogen is supplied in rain-induced pulses. It has been suggested that competitive interactions among desert plants would be most intense during these pulse periods of high resource availability. We tested this hypothesis with three cold desert shrub species of the Colorado Plateau (Gutierrezia sarothrae, Atriplex confertifolia, and Chrysothamnus nauseosus), which differ in their dis- tribution of functional roots. In a three-year field study we conducted a neighbor removal experiment in conjunction with simulated 25-mm precipitation events and the addition of a nitrogen pulse in either spring or summer. We measured predawn water potential (P), gas exchange, leaf 8'5N, carbon isotope discrimination (A), and growth of target plants for the duration of the study. We found that G. sarothrae used resource pulses to a larger extent than A. confertifolia, which has more functional roots at depth. In all species, the addition of a water or nitrogen pulse did not significantly affect maximal rates of photosynthesis or branch growth. Contrary to our initial hypothesis, we did not find that pulse use was reduced by the presence of neighboring plants. Nevertheless, there was strong evidence for com- petitive interactions, which were more likely mediated by water at depth than by nitrogen. In the more deep-rooted species A. confertifolia, neighbor removal affected P, gas exchange, A, percentage of carbon, and growth. G. sarothrae, which has a much smaller proportion of roots at depth, was less affected by the removal of neighboring shrubs, and not at all when only predominantly shallow-rooted herbaceous species were removed. These results suggest that shrubs in this cold desert community may primarily compete for water in deeper soil layers, where water depletion is slow and dominated by plant water uptake. There appeared to be little competition for water in shallow soil layers, where depletion is fast and dominated by evaporation.