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The kinetics of in vivo chlorophyll fluorescence of photosystem II (PS II) was measured at room temperature and 77 K during frost hardening of seedlings of Scots pine (Pinus sylvestris L.), and after exposure of frost-hardened shoots to sub-freezing temperatures. A more pronounced decrease in variable fluorescence yield for the upper exposed than for the lower shaded surface of the needles suggested that some photoinhibition occurred during prolonged frost hardening at 50 μmol photons m[SUP-2] s[SUP-1] and 4°C. Reversible inhibition of photosynthesis after exposure to sub-freezing temperatures was initially manifested as an increase of steady-state energy-dependent fluorescence quenching (q[SUBE]) and a reduction in the rate of O[SUB2] evolution. Further inhibition after treatment at still lower temperatures caused a progressive decline of steady-state photochemical quenching (q[SUBQ]) and the rate of O[SUB2] evolution, whereas q[SUBE] remained high. This implies an inactivation of enzymes in the photosynthetic carbon reduction cycle decreasing the consumption of ATP and NADPH, which is likely to cause an increase of membrane energization and a reduction of the primary electron acceptor (Q[SUBA]) of PS II. Alternatively, the changes in q[SUBQ] and q[SUBE] might be attributed to an inhibition of photophosphorylation. Severe, irreversible damage to photosynthesis resulted in a suppression of q[SUBE] and of variable fluorescence yield, probably because the photochemical efficiency of PS II was impaired. Changes in the fast fluorescence kinetics at room temperature after severe freezing damage were interpreted as an inhibition of the electron flow from Q[SUBA] to the plastoquinone pool. It is suggested that irreversible freezing injury to needles of frost-hardened P. sylvestris causes damage to the Q[SUBB]-protein. [ABSTRACT FROM AUTHOR]

Quantum yields of photosynthetic CO2 uptake by Pinus sylvestris (L.) shoots were measured at temperatures between -2°C and 35°C from September 1984 to September 1985. The ratio of variable to peak fluorescence of photosystem II (Fv/Fp) was also measured. Quantum yield measured at 25°C varied with time from a low winter value of 0.017 to a high summer value of 0.057. This variation was strongly correlated to variation in Fv/Fp, (r2 = 0.91). The response of quantum yield to temperature changed with season. During winter, quantum yield was essentially constant between 0°C and 35°C. The constancy above 5°C was associated with a strong increase in intercellular space CO2 (Ci) with temperature. In June, the quantum yield peaked at 5 °C, decreased sharply below 5 °C, and was rather constant between 25 °C and 35 °C. This insensitivity to increased temperature above 25 °C was attributed to a large increase in Ci. In contrast, by September, the quantum yield was less sensitive to temperature below 5 °C and more sensitive above 25 °C, despite an unchanged Ci response with increasing temperatures as compared with June. In August, quantum yields were lowered at 0, 5 and 15 °C, apparently as a result of high carbohydrate levels in the leaves. Overall, the results suggest that there are sites other than in photosystem 11 or at ribulose bisphosphate carboxylase/oxygenase at which the quantum yield of photosynthetic CO2 uptake is affected. Possible causes for the changes in efficiency are discussed. [ABSTRACT FROM AUTHOR]

Scots pine (Pinus sylvestris L.) is generally resistant to chilling temperatures. Porphyrin metabolism under low temperature stress was studied in etiolated seedlings of Scots pine. Low temperatures affect porphyrin accumulation in at least 3 different temperature sensitive sites: 1) the light activated accumulation of 5 -aminolevulinic acid, a porphyrin precursor, 2) the metabolism of 5 -aminolevulinic acid to form porphyrins and 3) a preferential accumulation of chlorophyll a over chlorophyll b. The temperature sensitivity of pine is compared to maize (Zea mays L.), a chilling sensitive plant. [ABSTRACT FROM AUTHOR]

Seedlings of Betula pendula were grown in a controlled environment chamber at quantum flux densities of 50, 250 and 600 μmol m-2 s-1. The relationship between the flux densities of absorbed CO2 and quanta was determined for shoots of whole seedlings. Rates of both light-saturated and in situ (measured under the growing conditions) net photosynthesis were determined and the photosynthetic quantum yields under light-limiting conditions were calculated. Anatomical leaf characteristics, chlorophyll contents and sizes and densities of the photosynthetic units (chlorophyll/P700) were determined. Chloroplasts were isolated and their rates of 2,6-dichlorophenol indophenol photoreduction were measured together with their pool sizes of the electron transport carriers plastoquinone and cytochrome f. Although acclimated to different quantum flux densities, the three birch populations showed the same quantum yield of net photosynthesis. This was approximately 0.028 in normal air (21.2 kPa oxygen) and about 0.040 when photorespiration was largely inhibited in 2.0 kPa oxygen. In addition, the in situ net photosynthesis rates were limited by the absorbed quantum flux density for low, intermediate and high light grown seedlings. It was concluded that birch acclimated to the three light regimes at different levels of organization (metabolic and anatomical). Thus, the quanta which were absorbed in situ could be transferred into chemical equivalents at an optimal and constant efficiency. The use of different reference bases for expressing rates of net photosynthesis are also discussed. [ABSTRACT FROM AUTHOR]

Net photosynthesis, photosynthetic electron transport, and leaf area density of photosynthetic units have been studied in developing, mature, and old leaves of seedlings of Betula pendula. The photosynthetic quantum yield under light-limiting conditions and the leaf area related rate of light-saturated net photosynthesis were lower in developing than in mature and old leaves. Developing leaves also had more oxygen inhibition of photosynthesis, a lower pool size of plastoquinone in the electron transport chain, a lower chlorophyll content and a lower leaf area density of photosynthetic units than mature and old leaves. The photosynthetic properties of the oldest leaves resulted partly from acclimation to shade and partly from a different ontogeny to that of younger leaves. [ABSTRACT FROM AUTHOR]

An apparatus is described which consists of an assimilation chamber mounted in the centre of a light-integrating Ulbricht sphere, irradiated through two fibre-optic light pipes. This arrangement provides totally diffuse radiation in the sphere. The quantum flux density in the sphere is measured by an integrating quantaspectrometer connected to die sphere by eight fibre-optic light pipes. The quantaspectrometer gives the spectral and the total quantum flux density in the sphere. By measuring the quantum flux density in the sphere before and after the insertion of a plant the number of absorbed quanta per unit time and plant area can be determined. In addition, the spectral distribution of the absorbed quantum flux density is obtained. CO2-assimilation per unit time and plant area is determined in the same apparatus. The totally diffuse radiation within the sphere minimizes mutual shading between branches and leaves. Hence, well-defined light-response-curves of photosynthesis can be obtained by plotting the flux density of CO2-assimilation as a function of the absorbed quantum flux density, and the quantum yields of intact plants can be calculated. Examples of photosynthetic quantum yield determinations and quanta absorption spectra are given for some plant species. [ABSTRACT FROM AUTHOR]

Photoinhibition of photosynthesis and its recovery were studied in intact barley (Hordeum vulgare L. cv. Gunilla) leaves grown in a controlled environment by exposing them to two temperatures, 5 and 20°C, and a range of photon flux densities in excess of that during growth. Additionally, photoinhibition was examined in the presence of chloramphenicol (CAP, an inhibitor of chloroplast protein synthesis) and of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Susceptibility to photoinhibition was much higher at 5 than at 20°C. Furthermore, at 20°C CAP exacerbated photoinhibition strongly, whereas CAP had little additional effect (10%) at 5°C. These results support the model that net photoinhibition is the difference between the inactivation and repair of photosystem II (PSII); i.e. the degradation and synthesis of the reaction centre protein, D1. Furthermore, the steady-state extent of photoinhibition was strongly dependent on temperature and the results indicated this was manifested through the effects of temperature on the repair process of PSII. We propose that the continuous repair of PS II at 20°C conferred at least some protection from photoinhibition. At 5°C the repair process was largely inhibited, with increased photoinhibition as a consequence. However, we suggest where repair is inhibited by low temperature, some protection is alternatively conferred by the photoinhibited reaction centres. Providing they are not degraded, such centres could still dissipate excitation energy non-radiatively, thereby conferring protection of remaining photochemically active centres under steady-state conditions. A fraction of PS II centres were capable of resisting photoinhibition when the repair process was inhibited by CAP. This is discussed in relation to PSII heterogeneity. Furthermore, the repair process was not apparently activated within 3 h when barley leaves were transferred to photoinhibitory light conditions at 20°C. [ABSTRACT FROM AUTHOR]

We provide here an overview of the remarkable life and outstanding research of David (Dave) Charles Fork (March 4, 1929-December 13, 2021) in oxygenic photosynthesis. In the words of the late Jack Edgar Myers, he was a top 'photosynthetiker'. His research dealt with novel findings on light absorption, excitation energy distribution, and redistribution among the two photosystems, electron transfer, and their relation to dynamic membrane change as affected by environmental changes, especially temperature. David was an attentive listener and a creative designer of experiments and instruments, and he was also great fun to work with. He is remembered here by his family, coworkers, and friends from around the world including Australia, France, Germany, Japan, Sweden, Israel, and USA., (© 2022. The Author(s), under exclusive licence to Springer Nature B.V.)

Main Conclusion: Decreased PG constrains PSI activity due to inhibition of transcript and polypeptide abundance of light-harvesting and reaction center polypeptides generating a reversible, yellow phenotype during cold acclimation of pgp1. Cold acclimation of the Arabidopsis pgp1 mutant at 5 °C resulted in a pale-yellow phenotype with abnormal chloroplast ultrastructure compared to its green phenotype upon growth at 20 °C despite a normal cold-acclimation response at the transcript level. In contrast, wild type maintained its normal green phenotype and chloroplast ultrastructure irrespective of growth temperature. In contrast to cold acclimation of WT, growth of pgp1 at 5 °C limited the accumulation of Lhcbs and Lhcas assessed by immunoblotting. However, a novel 43 kD polypeptide of Lhcb1 as well as a 29 kD polypeptide of Lhcb3 accumulated in the soluble fraction which was absent in the thylakoid membrane fraction of cold-acclimated pgp1 which was not observed in WT. Cold acclimation of pgp1 destabilized the Chl-protein complexes associated with PSI and predisposed energy distribution in favor of PSII rather than PSI compared to the WT. Functionally, in vivo PSI versus PSII photochemistry was inhibited in cold-acclimated pgp1 to a greater extent than in WT relative to controls. Greening of the pale-yellow pgp1 was induced when cold-acclimated pgp1 was shifted from 5 to 20 °C which resulted in a marked decrease in excitation pressure to a level comparable to WT. Concomitantly, Lhcbs and Lhcas accumulated with a simultaneous decrease in the novel 43 and 29kD polypeptides. We conclude that the reduced levels of phosphatidyldiacylglycerol in the pgp1 limit the capacity of the mutant to maintain the structure and function of its photosynthetic apparatus during cold acclimation. Thus, maintenance of normal thylakoid phosphatidyldiacylglycerol levels is essential to stabilize the photosynthetic apparatus during cold acclimation., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

To finish this special issue, some friends, colleagues and students of Prof. Chow (Emeritus Professor, the Research School of Biology, the Australian National University) have written small tributes to acknowledge not only his eminent career but to describe his wonderful personality., (© 2021. Springer Nature B.V.)

High latitude forests will experience large changes in temperature and CO 2 concentrations this century. We evaluated the effects of future climate conditions on 2 dominant boreal tree species, Pinus sylvestris L. and Picea abies (L.) H. Karst, exposing seedlings to 3 seasons of ambient (430 ppm) or elevated CO 2 (750 ppm) and ambient temperatures, a + 4 °C warming or a + 8 °C warming. Pinus sylvestris responded positively to warming: seedlings developed a larger canopy, maintained high net CO 2 assimilation rates (A net ), and acclimated dark respiration (R dark ). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of A net decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of R dark was weak. Elevated CO 2 tended to exacerbate these effects in warm-grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO 2 treatment produced fewer buds than they had 3 years earlier. Treatments had little effect on leaf and wood anatomy. Our results highlight that species within the same plant functional type may show opposite responses to warming and imply that Picea abies may be particularly vulnerable to warming due to low plasticity in photosynthetic and respiratory metabolism., (© 2018 John Wiley & Sons Ltd.)

Bean plants (Phaseolus vulgaris L. cv. Scarlett), germinated in darkness for I week, were transferred to light (200 μmol m -2 s -1 ) and cultivated for I week in a complete nutrient solution. After this period, cadmium ions in the form of CdSO 4 were added at the concentrations of 0.10.20 and 50 μM. The effects of this metal on the properties of photosystem II photochemistry were studied by means of modulated fluorescence analysis. Steady state photochemical quenching. non-photochemical quenching and terminal fluorescence were determined in control and cadmiumtreated plants. We postulate that, during short term exposure of plants to cadmium in the early stages of growth, the Calvin cycle reactions are more likely than photosystem II to be the primary target of the toxic influence of cadmium. The reduced demand for ATP and NADPH upon Calvin cycle inhibition causes a down-regulation of photosystem II photochemistry and of the yield of linear electron transport.