Maximum Quantum Yield Of Photochemistry Research Articles

Influence of Low Light on the Growth and Development of <i>Vallisneria natans</i> Seedlings

To research influence of low light on macrophyte growth, seedlings of Vallisneria natans were transplanted into the waters with 5%, 1%, 0.5% and 0.1% full sunlights, respectively. The results show that germination rates are significantly inhibited with the experiment. In first 20 days leaf formation of new plants aren’t inhibited, and their leaf elongation are promoted. Since then, them of old plants and new plants are remarkably inhibited with the experiment. Though maximum photochemical quantum yield (Fv/Fm), relative electron transport rate (ETR) and non-photochemical quenching (NPQ) of old plants reduce significantly on low light conditions, but V. natans still hsa a certain degree of photoasynthetic capacity on the 80th day. Fresh weight ratio of roots and stems increase with decreasing light intensity. However, it exhibits an opposite trend for leaves. Thus, V. natans has a strong ability of low-light (≤1% full sunlight) tolerance.

Xanthophyll cycle induction by anaerobic conditions under low light in Chlamydomonas reinhardtii

The effect of anaerobiosis on the induction of the xanthophyll cycle was investigated in Chlamydomonas reinhardtii. The results showed that, anaerobiosis obtained by either sulfur starvation or by bubbling nitrogen in the culture grown in complete medium induced the xanthophyll cycle even when cultures were exposed to low light conditions. The zeaxanthin content reached 35 mmol mol−1 Chl a, after 110 h in anaerobic sulfur-starved cultures, and 30 mmol mol−1 Chl a within 24 h in sulfur replete cultures bubbled with nitrogen. Both starved and non-starved cultures grown under aerobic conditions, did not exhibit any sizeable increase in the zeaxanthin content. Chlorophyll fluorescence measurements revealed a decrease in the maximum photochemical quantum yield of PSII (Fv/Fm) by more than 50 %. The chlorophyll fluorescence kinetics (OJIP) analysis showed a strong rise at the J-step indicating a strong reduction of QA. Our findings demonstrated that anaerobiosis in low light exposed cultures induced the xanthophyll cycle through a strong increase of the level of plastoquinone pool reduction, which was associated to the formation of a trans-thylakoid membranes proton gradient, while in dark anaerobic cultures, no appreciable induction of xanthophyll cycle could be observed, despite the sizeable increase in non–photochemical quenching.

Osmotic stress in Arctic and Antarctic strains of the green alga Zygnema (Zygnematales, Streptophyta): Effects on photosynthesis and ultrastructure

The osmotic potential and effects of plasmolysis on photosynthetic oxygen evolution and chlorophyll fluorescence were studied in two Arctic Zygnema sp. (strain B, strain G) and two Antarctic Zygnema sp. (strain E, strain D). Antarctic strain D was newly characterized by rbcL sequence analysis in the present study. The two Antarctic strains, D and E, are most closely related and may represent different isolates of the same species, in contrast, strain B and G are separate lineages. Incipient plasmolysis in the cells was determined by light microscopy after incubating cells in sorbitol solutions ranging between 200mM and 1000mM sorbitol for 3, 6 and 24h. In Zygnema strain B and G incipient plasmolysis occurred at ∼600mM sorbitol solution (720mOsmolkg−1, ψ=−1.67MPa) and in strains D and E at ∼300mM (318mOsmolkg−1, ψ=−0.8MPa) sorbitol solution. Hechtian strands were visualized in all plasmolysed cells, which is particularly interesting, as these cells lack pores or plasmodesmata. Ultrastructural changes upon osmotic stress were a retraction of the condensed cytoplasm from the cell walls, damages to chloroplast and mitochondrial membranes, increasing numbers of plastoglobules in the chloroplasts and membrane enclosed particles in the extraplasmatic space. Maximum photosynthetic rates (Pmax) in light saturated range were between 145.5μmol O2 h−1mg−1Chl a in Zygnema G and 752.9μmol O2 h−1mg−1Chl a in Zygnema E. After incubation in 800mM sorbitol for 3h Pmax decreased to the following percentage of the initial values: B: 16.3%, D: 16.8%, E: 26.1% and G: 35.0%. Osmotic stress (800mM sorbitol) decreased maximum photochemical quantum yield of photosystem II (Fv/Fm) when compared to controls. Maximum values of relative electron transport rates of photosystem II (rETRmax) decreased after incubation in 400mM sorbitol in Zygnema D and E, while they decreased in Zygnema B and G only after incubation in 800mM sorbitol. The kinetics of the rETR curves were similar for the Arctic strains Zygnema B and G, but distinct from the Antarctic strains Zygnema D and E, which were similar when compared with each other. This suggests that the investigated Arctic Zygnema sp. strains might be better adapted to tolerate osmotic water stress than the investigated strains from the Antarctic.

Effects of temperature and irradiance on photosystem activity during Alhagi sparsifolia leaf senescence

During the period of senescence of desert plant Alhagi sparsifolia Shap. the maximum photochemical quantum yield measured as variable to maximum fluorescence ratio (Fv/Fm) remained relatively high, although the number of active reaction centres per cross section (RCs) decreased significantly. The efficiency of electron acceptors beyond the primary quinone acceptor (QA) decreased. The effect of temperature and irradiance on photosystem activity was maximum after 6 d. Our results suggest that: 1) the down-regulation of photosystem activity was due to the decline of both RCs and electron acceptance between plastoquinone (PQ) and cytochrome (cyt) b6/f; 2) photosystem activity presented negative correlation with daily mean temperature, and 3) reduction of daily sunshine period and increase of temperature at noon can stimulate the speed of senescence.

Photosynthetic performance, lipid production and biomass composition in response to nitrogen limitation in marine microalgae

Increasing energy prices demand a renewable, carbon neutral, transport fuel that is environmentally and commercially sustainable. The interest in the production of microalgae as biofuels is increasing due to their high oil content, rapid biomass production and small foot print. In this research, marine microalgae Dunaliella tertiolecta (Chlorophyceae) and Thalassiosira pseudonana (Bacillariophyceae) were incubated in nitrogen (N)-replete medium, and then transferred to N-free medium for 15 and 11 days, respectively. Fluorescence induction and relaxation (FIRe) fluorometry and Fourier transform infrared spectroscopy (FTIR) were used to monitor the photosynthetic performance, lipid production and metabolic responses to changing N availability. Growth rates of D. tertiolecta and T. pseudonana were 0.84 ± 0.16 d(-1) and 1.21 ± 0.09 d(-), respectively in N-replete medium. Upon transfer to N-free medium. The growth rates of T. pseudonana declined rapidly, while D. tertiolecta continued to grow for 5 days in N-free medium before growth declined slowly. The maximum quantum yield of photochemistry (F(v)/F(m)) remained high initially for D. tertiolecta but decreased immediately after transfer to N-free media for T. pseudonana. The functional absorption cross section for PSII (σ(PSII)) increased, the time constant for Q(A) reoxidation (τ(Qa)) and connectivity factor (p) decreased in parallel to the nutritional status of the microalgae. The relative protein and lipid content varied in response to N limitation, but carbohydrates did not change. Based on FTIR, D. tertiolecta and T. pseudonana produced 20-26% lipid when most stressed. The combination of photosynthetic efficiency and biomass composition monitoring provided evidence that metabolic strategies to changing nutrient status are species-specific.

Mapping Phytoplankton Community Physiology on a River Impacted Continental Shelf: Testing a Multifaceted Approach

Seasonal phosphorus limitation occurs on the Louisiana continental shelf as a result of high nitrogen loads in the spring and early summer. Prior studies have assessed such nutrient limitation by laborious and time-consuming nutrient analyses, enzyme assays, and nutrient addition bioassays. We undertook surface (0.5–1 m) mapping of fast repetition rate fluorescence (FRRF) parameters to assess nutrient limitation in real time on the Louisiana continental shelf and Mississippi River plume from 29 June to 08 July, 2002 in an effort to further understand phytoplankton productivity in this region, as well as to better inform effective nutrient management strategies. Surface nutrient concentrations (NO 3 − , NO 2 − , NH 4 + , PO 4 3− ), chlorophyll a biomass, alkaline phosphatase (AP) activity, and four FRRF parameters: the maximum quantum yield of photochemistry (F v /F m ), the functional absorption cross section for PSII, the time constant for Q A reoxidation, and the connectivity factor, were measured during continuous underway mapping. Results from traditional methods to assess phytoplankton nutrient stress indicated widespread phosphorus limitation from the Mississippi River plume to the Atchafalaya River, manifested as high inorganic N/P ratios and elevated AP activities associated with phytoplankton biomass. The FRRF data indicated complex patterns of phytoplankton physiology that were likely driven by the rapidly changing conditions in local surface waters and heterogeneous phytoplankton community structure. Correlations of nutrient data and enzyme assays with FRRF parameters were significant but low, potentially due to differences in the manner and time scale with which nutrient limitation affects the different techniques used, indicating that further work is needed to interpret FRRF parameters in large, heterogeneous environments such as estuaries and continental shelves.

Differences in chlorophyll fluorescence parameters and water content in heteromorphic leaves of Populus euphratica from Inner Mongolia, China

We studied three typical heteromorphic leaves of Populus euphratica trees growing in the Wuhai region of Inner Mongolia, China, i.e., lanceolate, broad-ovate and dentate broad-ovate leaves and mainly focused on the changes in chlorophyll fluorescence parameters and free water and bound water content. The results show that the values of F m (maximal fluorescence yield), F v/F m (maximum photochemical quantum yield of PSII) and F v/F 0 (potential quantum efficiency of PSII) of lanceolate leaves were the least on young trees, while these parameters were the least on the ovate leaves of old trees. Compared with young trees, the free water content of heteromorphic leaves of old trees increased significantly, i.e., by 78.94% in lanceolate leaves and in the leaves of broad-ovate and dentate broad-ovate by 10.99% and 10.60%, respectively. Correlation analysis showed that free water content is significantly related to F v/F m and F v/F 0 in young trees, while the relationship of total water content with F v/F m and F v/F 0 is positive in old trees.

Growth and physiological state of beech seedlings grown in a nursery in different light conditions

Seedlings of European beech of two populations (from the 4<sup>th</sup> and 7<sup>th</sup> forest altitudinal zone) were grown in a shaded and unshaded plastic greenhouse. The objective was to compare seedling growth and the function of assimilatory organs and to determine their reactions after transfer to different light conditions.Seedlings grown in the unshaded plastic greenhouse (the sun variant) were taller and stronger at the end of the first growing season and had the higher weight and volume of shoots and root systems than seedlings grown in the shade. A higher number of leaves, larger total leaf area and higher dry matter of leaves per 1 plant were determined in seedlings grown in the sun. The average area of one leaf was larger in seedlings grown in the shade. The higher photosynthetic electron transport rate (ETR) determined from the light curves of chlorophyll fluorescence in seedlings grown in the sun was apparently connected with the higher photosynthetic rate and more intensive growth of these seedlings. The transfer of seedlings from full sun to shade resulted only in small changes in chlorophyll fluorescence (Fv/Fm, ETR). On the contrary, the transfer of seedlings from the shaded plastic greenhouse to the sun induced photoinhibition leading to a significant reduction in the maximum quantum yield of photochemistry Fv/Fm and in the photosynthetic electron transport rate (ETR).

Characteristics of chlorophyll fluorescence and antioxidative system in super-hybrid rice and its parental cultivars under chilling stress

Characteristics of chlorophyll fluorescence and antioxidative system were investigated in rice (Oryza sativa L.) super-hybrid Liangyoupeijiu (LYPJ), maternal cultivar Peiai64s, and paternal cultivar indica rice 9311 under chilling stress. During 6-d chilling treatment, chlorophyll content of all three genotypes was gradually declined. However, the decrease in photosystem 2 (PS 2) maximum photochemical efficiency (Fv/Fm) and quantum yield of PS 2 (ΦPS2) was less expressive in LYPJ than in parental cultivars The electrolyte leakage and malondialdehyde content in all cultivars increased after chilling treatment, but LYPJ exhibited the least increasing tendency. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) were higher in LYPJ than in parental cultivars. The results demonstrated that tolerance to chilling stress in LYPJ might be adopted mostly from its maternal cultivar.

Seasonal acclimation of photosystem II in Pinus sylvestris. I. Estimating the rate constants of sustained thermal energy dissipation and photochemistry

Acclimation of the partitioning of absorbed light energy in Photosystem II (PSII) between photochemical and non-photochemical processes includes short-term adjustments that are rapidly reversed in the dark and seasonal acclimation processes that are unaffected by dark acclimation. Thus, by using dark-acclimated leaves to study the seasonal acclimation of PSII, the confounding effect of short-term adjustments is eliminated. The maximum quantum yield of photochemistry, estimated by chlorophyll fluorescence analysis as F(v)/F(m), where F(v) = (F(m) - F(o)), and F(m) and F(o) are maximum and minimum chlorophyll fluorescence, respectively, has been widely used to follow the seasonal acclimation of PSII, because it is measured in dark-acclimated leaves. Seasonal changes in F(v)/F(m) can be caused by adjustments in either the photochemical capacity in PSII, or the capacity of thermal dissipation in PSII, or both. However, there is a lack of chlorophyll fluorescence parameters that can distinguish between these processes. In this study, we introduce two new parameters: the rate constants of sustained thermal energy dissipation (k(NPQ)) and of photochemistry (k(P)). We estimated k(NPQ) and k(P) from dark-acclimated F(o) and F(m) measured during spring recovery of photosynthesis in Scots pine (Pinus sylvestris L.) trees. We suggest that k(NPQ) and k(P) be used to study the mechanisms underlying the observed seasonal acclimation in PSII, because these parameters provide quantitative data that complement and extend F(v)/F(m) measurements.

Eutrophication‐induced phosphorus limitation in the Mississippi River plume: Evidence from fast repetition rate fluorometry

We assessed nutrient limitation in the Mississippi River plume and Louisiana continental shelf during the summer of 2002 (04‐08 July). We measured nutrient concentrations, alkaline phosphatase (AP) activities, chlorophyll a (Chl a) concentrations, and four fast repetition rate fluorescence (FRRF) parameters: the maximum quantum yield of photochemistry in photosystem II (PSII), Fv:Fm; the functional absorption cross section for PSII, σPSII; the time for photosynthetic electron transport on the acceptor side of PSII, τQa; and the connectivity factor, p, in 24‐h‐long nutrient addition bioassays near the Mississippi River delta. Low phosphorus (P) concentrations, elevated inorganic nitrogen‐to‐phosphorus ratios, high AP activities, and Chl a increases in response to P additions in the bioassays all indicated phosphorus limitation that was confirmed by the response of FRRF parameters. This is the first study to use FRRF to confirm results from basic oceanographic methods to demonstrate phosphorus limitation in a marine setting. Fv:Fm and p responded positively to phosphorus addition, while σPSII and τQa decreased in the same treatments. When nitrate alone was added, none of the measured parameters differed significantly from the control. We therefore suggest that FRRF can be used to rapidly detect phosphorus limitation in marine ecosystems.

THE ROLE OF PHYTOPLANKTON SIZE ON PHOTOCHEMICAL RECOVERY DURING THE SOUTHERN OCEAN IRON EXPERIMENT1

Phytoplankton primary productivity in the Southern Ocean is controlled by complex interactions among iron, light, and grazing. This project interfaced with the Southern Ocean iron experiment (SOFeX) that created two iron‐enriched patches north and south of the Polar Front each with distinct silicic acid concentrations. We used pulse amplitude modulated fluorometry and measured the recovery of the maximum quantum yield of photochemistry (Fv/Fm) for three size fractions (whole, <5, <20 μm) and light adapted quantum yield (ΔF/F′m) for single phytoplankton cells. The rates of recovery from iron stress were found to be unrelated to average cell size for both size‐fractioned and single‐celled measurements. The smallest cells appeared to exhibit more severe iron stress at the onset of the experiment than the larger taxa. The largest response detected in regression parameters was that of the pennate diatoms, which took only ∼3.4 days to reach the maximum quantum yield, whereas the centric diatom Asteromphalus sp. reached maximum ΔF/F′m after ∼10.4 days. The north patch measurements showed a different response; the smallest cells never reached maximum ΔF/F′m, whereas the size fraction containing the largest cells did. Single‐celled measurements made nearly 30 days after the initial iron enrichment suggested that diatoms were experiencing either silicic acid or iron limitation, whereas measurements of Phaeocystis sp. did not. These data represent the first study of in situ recovery rates of PSII for groups of diatoms, and may help elucidate the mechanisms of species change in response to environmental perturbation.

Diel `tuning' of coral metabolism: physiological responses to light cues

Hermatypic-zooxanthellate corals track the diel patterns of the main environmental parameters - temperature, UV and visible light - by acclimation processes that include biochemical responses. The diel course of solar radiation is followed by photosynthesis rates and thereby elicits simultaneous changes in tissue oxygen tension due to the shift in photosynthesis/respiration balance. The recurrent patterns of sunlight are reflected in fluorescence yields, photosynthetic pigment content and activity of the two protective enzymes superoxide dismutase (SOD) and catalase (CAT), enzymes that are among the universal defenses against free radical damage in living tissue. All of these were investigated in three scleractinian corals: Favia favus, Plerogyra sinuosa and Goniopora lobata. The activity of SOD and CAT in the animal host followed the course of solar radiation, increased with the rates of photosynthetic oxygen production and was correlated with a decrease in the maximum quantum yield of photochemistry in Photosystem II (PSII) (DeltaF'/F(m)'). SOD and CAT activity in the symbiotic algae also exhibited a light intensity correlated pattern, albeit a less pronounced one. The observed rise of the free-radical-scavenger enzymes, with a time scale of minutes to several hours, is an important protective mechanism for the existence and remarkable success of the unique cnidarian-dinoflagellate associations, in which photosynthetic oxygen production takes place within animal cells. This represents a facet of the precarious act of balancing the photosynthetic production of oxygen by the algal symbionts with their destructive action on all living cells, especially those of the animal host.

Solar UV-B radiation, associated with ozone depletion, inhibits the Antarctic terrestrial microalga, Stichococcus bacillaris

This study shows that increased UV-B arising from stratospheric ozone depletion over Antarctica reduced cell viability and the maximum quantum yield of photochemistry (Fv/Fm) in a unicellular terrestrial microalga, Stichococcus bacillaris. In the UV waveband, rates of Fv/Fm decline increased with decreasing wavelength. Photosynthetically active radiation (PAR) also reduced Fv/Fm, though less than UV radiation. Further experiments under different ozone column thicknesses showed a significantly greater decline in cell viability and Fv/Fm under ozone depletion compared with non-depleted conditions. The inhibitory effects of ambient solar radiation suggest that S. bacillaris is unlikely to inhabit soil surfaces, but colonises shaded areas beneath soil surface particles. During periods of ozone depletion, increases in the ratio of UV-B:PAR may reduce the thickness of the sub-surface zone where light conditions are suitable for colonisation by this alga.

Responses of Colobanthus quitensis (Kunth) Bartl. to high light and low temperature

Antarctic plants are exposed to high light and low temperature during their natural growing season. Therefore, the maintenance of an oxidative status compatible with normal metabolism is an important determinant for their survival. We hypothesized that Colobanthus quitensis, the only dicotyledonous plant from the Antarctic, has efficient energy-dissipating mechanisms, and/or high antioxidant capacity. A short time treatment (24 h) of high light and/or low temperature reduced maximum photochemical quantum yield (Fv/Fm), photochemical quenching (qP), and the quantum yield of PSII (ΦPSII). Concomitantly, an increase in the nonphotochemical quenching (qN) was observed. Superoxide dismutase (SOD) activity increased after cold acclimation. High light and low temperature did not significantly affect SOD, ascorbate peroxidase (APx), and glutathione reductase (GR) in nonacclimated plants. Total soluble antioxidants and carotenoids decreased after cold acclimation. Thus, it is likely that the main mechanism of C. quitensis to cope with energy imbalance is heat dissipation, as evidenced by the great increase in qN.

Photobehavior of stony corals: responses to light spectra and intensity

Tentacle expansion and contraction were investigated in four zooxanthellate coral species and one azooxanthellate coral (Cladopsammia gracilis). Favia favus, Plerogyra sinuosa and Cladopsammia gracilis expand their tentacles at night, while tentacles in Goniopora lobata and Stylophora pistillata are expanded continuously. Light at wavelengths in the range 400-520 nm was most effective in eliciting full tentacle contraction in F. favus and in P. sinuosa. Higher light intensities in the range 660-700 nm also caused tentacle contractions in F. favus. Tentacles in C. gracilis did not respond to light. Zooxanthellar densities in tentacles were significantly higher in G. lobata, which has continuously expanded tentacles, than in F. favus and P. sinousa, where tentacles are expanded at night. Photosynthetic efficiency in F. favus and P. sinuosa was lower in specimens with contracted tentacles. However, in the dark, no differences were found in the maximum quantum yield of photochemistry in PSII (Fv/Fm) of the expanded versus the contracted tentacles of any of the four species. This work suggests that species whose tentacles remain continuously expanded have either dense algal populations in their tentacles, as in G. lobata, or minute tentacles, like S. pistillata. Dense algal populations in tentacles allow harvesting of light while small tentacles do not scatter light or shade zooxanthellae in the underlying body of the polyp.

Response of two Antarctic bryophytes to stratospheric ozone depletion

AbstractWe report a study which measured changes to the radiative environment arising from stratospheric O3 depletion at Rothera Point on the western Antarctic Peninsula (67°S, 68°W) and subsequent associations between these changes and the pigmentation and maximum quantum yield of photochemistry (Fv/Fm) of two Antarctic bryophytes, the liverwort Cephaloziella varians and the moss Sanionia uncinata. We found a strong relationship between O3 column depth and the ratio of UV‐B to PAR irradiance (FUV‐B/FPAR) recorded at ground level. Weaker, but significant, associations were also found between O3 column depth and noon irradiances and daily doses of unweighted and biologically effective UV‐B radiation received at ground level. Regression analyses indicated that FUV‐B/FPAR and daily dose of unweighted UV‐B were best predictors for concentrations of total carotenoids and UV‐B screening pigments extracted from bryophyte tissues. Concentrations of these pigments were loosely but significantly positively associated with O3‐dependent irradiance parameters. HPLC analyses of carotenoids also suggested that both species increased the synthesis of neoxanthin during periods of O3 depletion. Violaxanthin, lutein, zeaxanthin and b,bββ‐carotene concentrations were also apparently influenced by O3 reduction, but not consistently across both bryophyte species. Concentrations of chlorophylls a and b were apparently unaffected by O3 depletion.No direct associations between Fv/Fm and O3‐dependent irradiance parameters were found. However stepwise multiple regression analyses suggested that the production of UV‐B screening pigments conferred protection from elevated FUV‐B/FPAR on Fv/Fm in both species and that carotenoids conferred protection on Fv/Fm in Sanionia. Our data suggest that changes to the radiative environment associated with stratospheric O3 depletion influence the pigmentation of two Antarctic bryophytes, but that Fv/Fm is unaffected, at least in part because of rapid synthesis of protective pigments.

Photosynthetic Activity and Acclimation Ability of Pea Plants to Low and High Temperature Treatment as Studied by Means of Chlorophyll Fluorescence

Summary The functional activity of the photosynthetic apparatus of pea plants ( Pisum sativum L.) growing at lower (4 °C and 10 °C) and higher temperatures (35 °C and 40 °C) than control plants (23 °C) and their acclimation ability to these conditions were investigated by means of chlorophyll (Chi) fluorescence. A temperature treatment at 10 °C did not significantly influence PS2 activity of pea plants. Treatment at 40 °C led, however, to a serious injury of the photosynthetic apparatus as seen in a sharp increase of the ground Chi fluorescence FO and a decrease of the variable Chi fluorescence F v and the ratios F v /F m F v /F o and the Rfd-values. At 4 °C a reduced photosynthetic activity was detectable as seen mainly in a decrease of the maximum (F m ) and variable Chi fluorescence (F v ) and in reduced Rfd-values, whereas F o did not change. An exposure of pea plants to 35 °C did not alter the Chi fluorescence ratios F v /F m or F v /F o but decreased the Rfd-values with increasing time of treatment. This demonstrates that at 35 °C the maximum photochemical quantum yield, as indicated by the ratios F v /F m and F v /F o cannot be maintained under steady state conditions of photosynthesis and thus causes a decline of Rfd-values. The changes in Chi fluorescence parameters at 4 °C and 35 °C were reversible; thus these two temperatures seem to set the temperature limits for studies on the cold and heat acclimation mechanisms of pea plants.

Inhibition and acclimation of photosynthesis to heat stress is closely correlated with activation of ribulose-1,5-bisphosphate Carboxylase/Oxygenase

Increasing the leaf temperature of intact cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) plants caused a progressive decline in the light-saturated CO2-exchange rate (CER). CER was more sensitive to increased leaf temperature in wheat than in cotton, and both species demonstrated photosynthetic acclimation when leaf temperature was increased gradually. Inhibition of CER was not a consequence of stomatal closure, as indicated by a positive relationship between leaf temperature and transpiration. The activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is regulated by Rubisco activase, was closely correlated with temperature-induced changes in CER. Nonphotochemical chlorophyll fluorescence quenching increased with leaf temperature in a manner consistent with inhibited CER and Rubisco activation. Both nonphotochemical fluorescence quenching and Rubisco activation were more sensitive to heat stress than the maximum quantum yield of photochemistry of photosystem II. Heat stress led to decreased 3-phosphoglyceric acid content and increased ribulose-1, 5-bisphosphate content, which is indicative of inhibited metabolite flow through Rubisco. We conclude that heat stress inhibited CER primarily by decreasing the activation state of Rubisco via inhibition of Rubisco activase. Although Rubisco activation was more closely correlated with CER than the maximum quantum yield of photochemistry of photosystem II, both processes could be acclimated to heat stress by gradually increasing the leaf temperature.

Phytoplankton photosynethetic characteristics from fluorescence induction assays of individual cells

Saturating‐flash fluorescence techniques, which can provide information about the physiological state of phytoplankton, at present measure bulk water samples and so provide “averaged” values for all the fluorescent particles present. In analyzing natural samples, however, more detailed information about the distribution of photosynthetic characteristics among different cell types and(or) individual cells is desirable. Therefore we developed two methods for applying a “pump‐during‐probe” technique on a cell‐by‐cell basis. We used either an epifluorescence microscope or a flow cytometer to make time‐resolved measurements of the increase in chlorophyll fluorescence induced by a rectangular excitation pulse of 100‐µs duration. We used a biophysical model of fluorescence induction to obtain information about the quantum yield of photochemistry in photosystem 2 (PS2) and the functional absorption cross‐section for PS2. For several species (including the smallest phytoplankton, Prochlorococcus, which are 0.7 µm in diameter), the maximum quantum yield of photochemistry in PS2 obtained by averaging data from many individual cells agreed well with estimates derived from bulk measurements of DCMU enhancement of Chl fluorescence.