Characteristics of thermoluminescence bands of intact leaves and isolated chloroplasts in relation to the watersplitting activity in photosynthesis

Plant materials (intact leaves, chloroplasts or subchloroplast particles) preilluminated at a low temperature (e.g. −60°C) were rapidly cooled to −196°C and then the luminescence emitted from the sample on raising the temperature was measured as a function of temperature, by means of a sensitive photo-electron counting technique. Mature spinach leaves showed five luminescence bands at different temperatures which were denoted as Z_v, A, B₁, B₂ and C bands. The A, B₁, B₂ and C bands appeared at constant temperatures, −10, +25, +40 and +55°C, respectively, being independent of the illumination temperature, but the Z_v band appeared at a variable temperature slightly higher than the illumination temperature. The B₁ and B₂ bands were absent in the thermoluminescence profiles of samples devoid of the oxygenevolving activity, such as heat-treated spinach leaves, wheat leaves greened under intermittent illumination and photosystem-II particles prepared with Triton X-100. It was deduced that these luminescence bands arise from the energy stored by the electron flow in photosystem II to evolve oxygen, and other bands were ascribed to charge-separation in some other sites not related to the oxygen evolving system.     

A new EPR signal associated with photosystem 1 of algal and plant photosynthesis

A new low temperature electron paramagnetic resonance (EPR) signal with a g-value of 1.97 was found in Photosystem-1 particles from a blue-green alga, Anacystis nidulans, illuminated at room temperature. A similar signal was also found in spinach Photosystem-1 particles treated with thiophenol to decrease interference from a signal due to Center A. In the dark, the signal appeared only when the Anacystis particles were at redox potentials lower than -0.5 volts where Centers A and B were also reduced. The signal is most likely due to another iron-sulfur cluster, tentatively designated as Center C. Center C could be photoreduced at low temperatures like Center A when Centers A and B were partially reduced prior to illumination, indicating possible close association of these centers in Photosystem 1 of green plant and algal photosynthesis.     

Molecular heterogeneity of ferredoxin-NADP+ reductase from spinach leaves

Highly purified ferredoxin-NADP+ reductase from spinach leaves showed at least eight different protein bands in the electrofocused gel. All of them were catalytically active and were adsorbed on a ferredoxin-Sepharose 4B affinity column. The N-terminal amino acid sequence of the main component species was analyzed by the automatic Edman degradation method. It was found that when the reductase was stored at 4 degrees C, new protein bands appeared in isoelectric focusing and sodium dodecyl sulfate polyacrylamide gel electrophoreses, but the appearance of the bands was suppressed by the addition of a protease inhibitor, diisopropyl fluorophosphate. This indicates that the molecular heterogeneity of the reductase may result from the digestion with a protease present in spinach leaves.     

Effect of complete extraction and re-addition of manganese on thermoluminescence of pea Photosystem II preparations

Thermoluminescence of Photosystem II particles isolated from pea chloroplasts using digitonin and Triton X-100 was measured after 1 min illumination at a certain temperature (T(ex)) followed by illumination during cooling (40 Cdeg/min) to a lower temperature. Glow curves of the particles are characteristic of the photosynthetic oxygen-evolving material studied earlier. Complete (more than 95%) removal of Mn from the Photosystem II particles abolishes thermoluminescence bands around 0° C, related to the oxygen-evolving system, but the thermoluminescence bands peaking around -30°C (TL(-30)), -55°C (TL_ (-55)) and between-68 and -85° C, depending on Tex(TLv), remain unaltered. The bands are characterized by different dependence on T,x. The TL(-30), TL(-55) and TL v bands can also be observed in the glow curve of isolated pea and spinach chloroplasts. Re-addition of MnCI (2) (2 μM, corresponding to nearly 4 Mn atoms per reaction center of Photosystem II) to the Mn-depleted particles does not reactivate the thermoluminescence bands around 0° C. However, it does lead to suppression of TL(-30) accompanied by parallel activation of TL(-55), revealing competition of the TL (-30) and TL(-55) for charges generated by the reaction center. These data, as well as the results on the effect of inhibitors and electron donors to Photosystem II, show that positive charges contributing to the TL(-30), TL (-55) and TL v thermoluminescence bands are located on secondary electron donors of Photosystem II which do not require Mn and are located closer to the reaction center than the Mn-containing, water-oxidizing enzyme.     

Quality control of Photosystem II under light stress – turnover of aggregates of the D1 protein in vivo

When photodamaged under excessive light, the D1 protein is digested and removed from Photosystem (PS) II to facilitate turnover of the protein. In vitro studies have shown that part of the photodamaged D1 protein forms aggregates with surrounding polypeptides before being digested by a protease(s) in the stroma [Yamamoto Y (2001) Plant Cell Physiol 42: 121–128]. The aim of this study was to examine whether light-induced aggregation of the D1 protein also occurs in vivo. The following results were obtained: (1) PS II activity in spinach leaves was significantly inhibited by weak illumination (light intensity, 20–100 μE m⁻² s⁻¹), as monitored by chlorophyll fluorescence Fv/Fm, when the leaves were kept at higher temperatures (35–40 °C); (2) aggregation of the D1 protein, as well as cleavage of the protein, was detected in thylakoids isolated from spinach leaves that had been subjected to heat/light stress; (3) aggregates of the D1 protein disappeared after incubation of the leaves at 25 °C in the dark or under illumination with weak light. Since it is dependent on the presence of oxygen, aggregation of the D1 protein is probably induced by reactive oxygen species produced in thylakoids upon illumination at elevated temperatures. Consistent with this notion, singlet oxygen production in thylakoid samples under illumination was shown to be stimulated significantly at higher temperatures.     

The role of electron transport in determining the temperature dependence of the photosynthetic rate in spinach leaves grown at contrasting temperatures

The temperature response of the uncoupled whole-chain electron transport rate (ETR) in thylakoid membranes differs depending on the growth temperature. However, the steps that limit whole-chain ETR are still unclear and the question of whether the temperature dependence of whole-chain ETR reflects that of the photosynthetic rate remains unresolved. Here, we determined the whole-chain, PSI and PSII ETR in thylakoid membranes isolated from spinach leaves grown at 30 degrees C [high temperature (HT)] and 15 degrees C [low temperature (LT)]. We measured temperature dependencies of the light-saturated photosynthetic rate at 360 microl l(-1) CO2 (A360) in HT and LT leaves. Both of the temperature dependences of whole-chain ETR and of A360 were different depending on the growth temperature. Whole-chain ETR was less than the rates of PSI ETR and PSII ETR in the broad temperature range, indicating that the process was limited by diffusion processes between the PSI and PSII. However, at high temperatures, whole-chain ETR appeared to be limited by not only the diffusion processes but also PSII ETR. The C3 photosynthesis model was used to evaluate the limitations of A360 by whole-chain ETR (Pr) and ribulose bisphosphate carboxylation (Pc). In HT leaves, A360 was co-limited by Pc and Pr at low temperatures, whereas at high temperatures, A360 was limited by Pc. On the other hand, in LT leaves, A360 was solely limited by Pc over the entire temperature range. The optimum temperature for A360 was determined by Pc in both HT and LT leaves. Thus, this study showed that, at low temperatures, the limiting step of A360 was different depending on the growth temperature, but was limited by Pc at high temperatures regardless of the growth temperatures.     

Experimental and theoretical study of temperature dependence of steady-state parameters of delayed luminescence induction in leaves of higher plants

The temperature dependence of delayed luminescence of Hibiscus rosa sinensis leaves was studied in the temperature range from -25 degrees C to degrees C. Temperature dependence of the steady-state delayed luminescence intensity has two maxima, at -10 and at +35 degrees C. For the theoretical modeling, the mathematical model of plant photosynthesis developed earlier was used. The temperature dependence of the delayed fluorescence induction was obtained by introducing into the model the temperature dependences for 12 rate constants derived from the data on the effect of temperature on different stages of photosynthesis. The theoretical temperature dependence of the steady-state delayed luminescence was shown to have the same shape as the experimental one.     

Effects of internal conductance on the temperature dependence of the photosynthetic rate in spinach leaves from contrasting growth temperatures

The photosynthetic rate may be strongly limited by internal conductance from the intercellular airspace to the chloroplast stroma (g(i)). However, the effects of growth and leaf temperature on g(i) are still unclarified. In this work, we determined the temperature dependence of g(i) in spinach leaves grown at 30/25 degrees C (high temperature; HT) and 15/10 degrees C (low temperature; LT), using the concurrent measurements of the gas exchange rate and stable carbon isotope ratio. Moreover, we quantified the effects of g(i) on the temperature dependence of the photosynthetic rate. We measured g(i) and the photosynthetic rate at a CO(2) concentration of 360 microl l(-1) under saturating light (A(360)) at different leaf temperatures. The optimum temperature for A(360) was 28.5 degrees C in HT leaves and 22.9 degrees C in LT leaves. The optimum temperatures for g(i) were almost similar to those of A(360) in both HT and LT leaves. There was a strong linear relationship between A(360) and g(i). The photosynthetic rates predicted from the C(3) photosynthesis model taking account of g(i) agreed well with A(360) in both HT and LT leaves. The temperature coefficients (Q(10)) of g(i) between 10 and 20 degrees C were 2.0 and 1.8 in HT and LT leaves, respectively. This suggests that g(i) was determined not only by physical diffusion but by processes facilitated by protein(s). The limitation of the photosynthetic rate imposed by g(i) increased with leaf temperature and was greater than the limitation of the stomatal conductance at any temperature, in both HT and LT leaves. This study suggests that g(i) substantially limits the photosynthetic rate, especially at higher temperatures.     

Compared thermoluminescence characteristics of pea thylakoids studied in vitro and in situ (in leaves). The effect of photoinhibitory treatments

Characteristics of thermoluminescence (TL) glow curves were studied in thylakoids (isolated from pea leaves) or in intact pea leaves after an exposure to very high light for 2 min in the TL device. The inhibition of photosynthesis was detected as decreases of oxygen evolution rates and/or of variable fluorescence.In thylakoids exposed to high light, then dark adapted for 5 min, a flash regime induced TL glow curves which can be interpreted as corresponding to special B bands since: 1) they can be fitted by a single B band (leaving a residual band at –5°C) with a lower activation energy and a shift of the peak maximum by –5 to –6°C and, 2) the pattern of oscillation of their amplitudes was normal with a period of 4 and maxima on flashes 2 and 6. During a 1 h dark adaptation, no recovery of PS II activity occurred but the shift of the peak maximum was decreased to –1 to –2°C, while the activation energy of B bands increased. It is supposed that centers which remained active after the photoinhibitory treatment were subjected to reversible and probably conformational changes.Conversely, in intact leaves exposed to high light and kept only some minutes in the dark, TL bands induced by a flash regime were composite and could be deconvoluted into a special B band peaking near 30°C and a complex band with maximum at 2–5°C. In the case of charging bands by one flash, this low temperature band was largely decreased in size after a 10 min dark adaptation period; parallely, an increase of the B band type component appeared. Whatever was the flash number, bands at 2–5°C were suppressed by a short far red illumination given during the dark adaptation period and only remained a main band a 20°C; therefore, the origin of the low temperature band was tentatively ascribed to recombinations in centers blocked in state S₂Q_A ^–Q_B ^2–. In vivo, the recovery of a moderately reduced state in the PQ pool, after an illumination, would be slow and under the dependence of a poising mechanism, probably involving an electron transfer between cytosol and chloroplasts or the so-called chlororespiration process.Abbreviations E_a- activation energy - FR- far-red - MV- methylviologen - pBQ- p-benzoquinone - PQ- plastoquinone - PS II- Photosystem II - Q_A- primary quinone electron acceptor of PS II - Q_B- secondary quinone electron acceptor of PS II - TL- thermoluminescence     

Thermoluminescence Investigation of Low Temperature Stress in Maize

The thermoluminescence (TL) emission of photosynthesising materials originates from the recombination of charge pairs created by a previous excitation. Using a recently described TL set-up the effect of chilling stress on TL bands occurring at positive temperatures (AG, B, and HTL) was investigated in intact leaves. The far-red irradiation of leaves at low, but non-freezing temperatures induced a TL band peaking at around 40–45 °C (AG band), together with a B band peaking between 20 and 35 °C. Low temperature stress first caused a downshift and a temporary increase in the AG band after 4 h at 0 °C in the light, then a decrease in the AG and B TL bands after 1 d at 0 °C in the light. This decrease was less pronounced in cold-tolerant genotypes and in those grown at acclimating temperatures. Furthermore, an additional band appeared above 80 °C after severe cold stress. This band indicates the presence of lipid peroxides. Thus TL is a useful technique for studying the effects of low temperature stress.     

Light-dependent development of thermoluminescence, delayed emission and fluorescence variation in dark-grown spruce leaves

Thermoluminescence profiles of spruce leaves grown under various light or dark conditions were measured after excitation at a low temperature (−70 to −20 °C) by 1-min illumination with red light, and the following results were obtained. Mature spruce leaves showed five thermoluminescence bands at −30, −5, +20, +40 (or +35) and +70 °C (denoted as Z_v, A, B₁, B₂ and C bands, respectively), but dark-grown spruce leaves with a similar chlorophyll content showed only two bands, at −30 and +70 °C (the Z_v and C bands) and were devoid of the three other bands (the A, B₁ and B₂ bands). On exposure of the dark-grown leaves to continuous red light, the A, B₁ and B₂ bands were rapidly developed, and the development was accompanied by enhancement of delayed emission, fluorescence variation and the Hill activity (photoreduction of 2,6-dichlorophenolindophenol with water as electron donor). It was demonstrated that the dark-grown spruce leaves are devoid of the water-splitting system in Photosystem II, and that the latent water-splitting activity is rapidly photoactivated by exposure of the leaves to continuous red light. These results on the gymnosperm spruce leaves, in which greening proceeds in complete darkness, being independent of the development of the water-splitting system in light, were discussed in relation to previous observations on angiosperm leaves, in which both greening and the activity generation proceed in the light.     

Comparison of survival of Campylobacter jejuni in the phyllosphere with that in the rhizosphere of spinach and radish plants

Campylobacter jejuni has been isolated previously from market produce and has caused gastroenteritis outbreaks linked to produce. We have tested the ability of this human pathogen to utilize organic compounds that are present in leaf and root exudates and to survive in the plant environment under various conditions. Carbon utilization profiles revealed that C. jejuni can utilize many organic acids and amino acids available on leaves and roots. Despite the presence of suitable substrates in the phyllosphere and the rhizosphere, C. jejuni was unable to grow on lettuce and spinach leaves and on spinach and radish roots of plants incubated at 33 degrees C, a temperature that is conducive to its growth in vitro. However, C. jejuni was cultured from radish roots and from the spinach rhizosphere for at least 23 and 28 days, respectively, at 10 degrees C. This enteric pathogen also persisted in the rhizosphere of spinach for prolonged periods of time at 16 degrees C, a temperature at which many cool-season crops are grown. The decline rate constants of C. jejuni populations in the spinach and radish rhizosphere were 10- and 6-fold lower, respectively, than on healthy spinach leaves at 10 degrees C. The enhanced survival of C. jejuni in soil and in the rhizosphere may be a significant factor in its contamination cycle in the environment and may be associated with the sporadic C. jejuni incidence and campylobacteriosis outbreaks linked to produce.     

Over-reduced states of the Mn-cluster in cucumber leaves induced by dark-chilling treatment

Oxygen evolution is inhibited when leaves of chilling-sensitive plants like cucumber are treated at 0 degrees C in the dark. The activity is restored by moderate illumination at room temperature. We examined the changes in the redox state of the Mn-cluster in cucumber leaves in the processes of dark-chilling inhibition and subsequent light-induced reactivation by means of thermoluminescence (TL). A TL B-band arising from S(2)Q(B)(-) charge recombination in PSII was observed upon single-flash illumination of untreated leaves, whereas four flashes were required to yield the B-band after dark-chilling treatment for 24 h. This three-step delay indicates that over-reduced states of the Mn-cluster such as the S(-2) state were formed during the treatment. Fitting analysis of the flash-number dependence of the TL intensities showed that the Mn-cluster was more reduced with a longer period of the treatment and that S(-3) was the lowest S-state detectable in the dark-chilled leaves. Measurements of the Mn content by atomic absorption spectroscopy showed that Mn atoms were gradually released from PSII during the dark-chilling treatment but re-bound to PSII by illumination at 30 degrees C. Thus, dark-chilling inhibition of oxygen evolution can be ascribed to the disintegration of the Mn-cluster due to its over-reduction. The observation of the S(-3) state in the present in vivo system strongly suggests that S(-3), which has been observed only by addition of exogenous reductants into in vitro preparations, is indeed a redox intermediate of the Mn-cluster in the processes of its disintegration and photoactivation.     

Photobleaching of photosynthetic pigments in spinach thylakoid membranes. Effect of temperature, oxygen and DCMU

The time dependence of photobleaching of photosynthetic pigments under high light illumination of isolated spinach thylakoid membranes at 22 and 4 degrees C was investigated. At 22 degrees C, the bleaching at 678, 472 and 436 nm was prominent but lowering the temperature up to 4 degrees C during illumination prevented the pigments from bleaching almost completely. The accelerating effect on pigment photobleaching by the presence of 3-(3,4 dichlorophenyl)-1,1-dimethyl-urea)-(DCMU), a well-known inhibitor of the electron transport and known to prevent photosystem I (PSI) and photosystem II (PSII) against photoinhibitory damage, was also suppressed at low temperature. At 22 degrees C in the presence and absence of DCMU, the decrease of the absorption at 678 and 472 nm was accompanied by a shift to the shorter wavelengths. To check the involvement of reactive oxygen species in the process, pigment photobleaching was followed in anaerobiosis. The effects of the three different environmental factors--light, temperature and DCMU--on the dynamics of photobleaching are discussed in terms of different susceptibility of the main pigment-protein complexes to photoinhibition.     

An increase in unsaturation of fatty acids in phosphatidylglycerol from leaves improves the rates of photosynthesis and growth at low temperatures in transgenic rice seedlings

The level of cis-unsaturated fatty acids in phosphatidylglycerol (PG) from rice leaves was genetically altered from 19.3% in the wild-type to 29.4 and 32.0% in T1 plants segregated with cDNAs for glycerol-3-phosphate acyltransferase of chloroplasts (GPAT; EC 2.3.1.15) from Arabidopsis (+AGPAT plant) and spinach (+SGPAT plant), respectively; and to 21.4% in a non-transformant segregated from +SGPAT plants (-SGPAT plant). In all these plants, O2 evolution from leaves was similar at 25 degrees C and was impaired to a similar extent at 5 and 11 degrees C. However, in parallel with the levels of cis-unsaturated fatty acids in PG, +AGPAT and +SGPAT plants showed less impaired rates of O(2) evolution from leaves than the wild-type and -SGPAT plants at 14 and 17 degrees C. In agreement with this, the fresh weight of 14-day-old seedlings increased to 571 + or - 18, 591 + or - 23, 687 + or - 32 and 705 + or - 31 mg in the wild-type, -SGPAT, +AGPAT and +SGPAT plants, respectively, after 6 weeks at 17/14 degrees C (day/night). These results demonstrate the practical importance of the present technology with GPAT in improvement of the chilling sensitivity of crops.     

Functional intermediates in the reaction of membrane-bound cytochrome oxidase with oxygen.

Flash photolysis of the membrane-bound cytochrome oxidase/carbon monoxide compound in the presence of oxygen at low temperatures and in the frozen state leads to the formation of three types of intermediates functional in electron transfer in cytochrome oxidase and reduction of oxygen by cytochrome oxidase. The first category (A) does not involve electron transfer to oxygen between -125 degrees and -105 degrees, and includes oxy compounds which are spectroscopically similar for the completely reduced oxidase (Cu1+alpha3(2+)-O2) or for the ferricyanide-pretreated oxidase (Cu2+alpha3(3+)-O2). Oxygen is readily dissociated from compounds of type A. The second category (B) involves oxidation of the heme and the copper moiety of the reduced oxidase to form a peroxy compound (Cu2+alpha 3(3+)-O2=or Cu2+alpha3(2+)-O2H2) in the temperature range from -105 degrees to -60 degrees. Above -60 degrees, compounds of type B serve as effective electron acceptors from cytochromes a, c, and c1. The third category (C) is formed above -100 degrees from mixed valency states of the oxidase obtained by ferricyanide pretreatment, and may involve higher valency states of the heme iron (Cu2+alpha3(4+)-O2=). These compounds act as electron acceptors for the respiratory chain and as functional intermediates in oxygen reduction. The remarkable features of cytochrome oxidase are its highly dissociable "oxy" compound and its extremely effective electron donor reaction which converts this rapidly to tightly bound reduced oxygen and oxidized oxidase.     

Light- and temperature-induced changes in the distribution of excitation energy between Photosystem I and Photosystem II in spinach leaves

The influence of light quality and temperature on the distribution of the absorbed quanta between Photosystem I (PS I) and Photosystem II (PS II) in spinach leaves has been studied from the characteristics of chlorophyll fluorescence at 77 K. Leaves were preilluminated at different temperatures with either PS I light (to establish State 1) or with PS II light (to establish State 2), then cooled to 77 K and measured for fluorescence. In State 1, energy distribution appeared to be unaffected by temperature. A transition to State 2 resulted in an increase in PS I fluorescence and a decrease in the PS II fluorescence, indicating that a larger fraction of energy becomes redistributed to PS I. However, the extent of this redistribution varied: it was only small at 5°C to 20°C, but it largely increased at temperatures exceeding 20°C. This variation in the extent was related to a change in the mechanism of the state transition: at 15°C only the ‘initial’ distribution of energy was affected, while at 35°C an additional increase in the spill-over constant, k_{T (II → I)}, was included. It is assumed that under physiological conditions k_{T (II → I)} is under the control of temperature rather than of light quality, whereby in leaves adapted to high physiological temperatures, the probability of energy spill-over from closed PS II centres to PS I is enhanced. In darkened leaves, the spill-over constant has been manipulated by preincubation at different temperatures. Then, the light-induced ‘energization’ of thylakoid membranes has been tested by measuring the light-induced electrochromic absorbance change at 515 nm (and light-induced light-scattering changes) in these leaves. The flash-induced 515 nm signal as well as the initial peak during a 1 s illumination were not affected by energy distribution. However, the amplitude of the pseudo-steady-state signal (as established during 1 s illumination) was considerably enhanced in leaves in which a larger fraction of the absorbed energy is distributed to PS I at the expense of PS II excitation. The results have been interpreted in such a way that an increase in energy spill-over from PS II to PS I favours a cyclic electron transport around PS I. It is discussed that changes in energy distribution (via spill-over) may serve to maintain a suitable balance between non-cyclic and cyclic electron transport in vivo.     

Thermoluminescence study of photosystem II activity in Haberlea rhodopensis and spinach leaves during desiccation

Thermoluminescence glow curve parameters were used to access the functional features of PS II in the Balkan endemic Haberlea rhodopensis. This representative of the higher desiccation-tolerant plants is unique for the European flora. An unusual high temperature of TL emission from Haberlea leaves after excitation by one flash at 5 degrees C was observed. The position of the main TL B band (S (2)Q (B)(-)) was at 45 - 47 degrees C, while this temperature was 30 - 32 degrees C in drought-sensitive mesophytic spinach. Consistent with the up-shift in TL emission, the lifetime of the S (2) state was also increased, showing a stabilization of charge storage in PS II complex in this resurrection plant. In addition, a part of PS II centres was less susceptible to DCMU. We consider the observed unusual TL characteristics of Haberlea rhodopensis reflect some structural modifications in PS II (especially in D1 protein), which could be related to the desiccation tolerance of this plant. This suggestion was supported by the different manner in which dehydration affected the TL properties in desiccation-tolerant Haberlea and desiccation-sensitive spinach plants.     

Studies on ferrochelatase. The enzymic formation of haem in proplastids, chloroplasts and plant mitochondria

1. Ferrochelatase was demonstrated in the chloroplasts and proplastids isolated from the primary leaves of beans (a dicotyledon) and oats (a monocotyledon). It was also detected in chloroplasts from etiolated bean seedlings made green by illumination before being harvested. The specific activities of the three types of bean organelles are similar, as are the specific activities of the oat proplastids and chloroplasts. 2. Chloroplasts from young spinach leaves also contain ferrochelatase; these chloroplasts were tested for their ability to form magnesium tetrapyrroles and found unable to catalyse the insertion of Mg(2+) into mesoporphyrin IX. 3. Ferrochelatase was also detected in potato tuber mitochondria. 4. Ferrochelatase activity in these plant preparations is much less stable on storage than similar preparations from bacteria and animal tissues. 5. Temperature affects the activities of spinach chloroplast ferrochelatase and rat liver ferrochelatase differently. Activity of the chloroplast enzyme increases as the temperature rises from 20.6 degrees to 26 degrees , but becomes increasingly inactivated as the temperature rises further to 38 degrees . The initial velocity of the mammalian enzyme, however, increases as the temperature rises from 25.8 degrees to 65 degrees , but the enzyme is inactivated after several minutes at 65 degrees .     

Temperature acclimation of photosynthesis and related changes in photosystem II electron transport in winter wheat

Winter wheat (Triticum aestivum L. cv Norin No. 61) was grown at 25 degrees C until the third leaves reached about 10 cm in length and then at 15 degrees C, 25 degrees C, or 35 degrees C until full development of the third leaves (about 1 week at 25 degrees C, but 2-3 weeks at 15 degrees C or 35 degrees C). In the leaves developed at 15 degrees C, 25 degrees C, and 35 degrees C, the optimum temperature for CO(2)-saturated photosynthesis was 15 degrees C to 20 degrees C, 25 degrees C to 30 degrees C, and 35 degrees C, respectively. The photosystem II (PS II) electron transport, determined either polarographically with isolated thylakoids or by measuring the modulated chlorophyll a fluorescence in leaves, also showed the maximum rate near the temperature at which the leaves had developed. Maximum rates of CO(2)-saturated photosynthesis and PS II electron transport determined at respective optimum temperatures were the highest in the leaves developed at 25 degrees C and lowest in the leaves developed at 35 degrees C. So were the levels of chlorophyll, photosystem I and PS II, whereas the level of Rubisco decreased with increasing temperature at which the leaves had developed. Kinetic analyses of chlorophyll a fluorescence changes and P700 reduction showed that the temperature dependence of electron transport at the plastoquinone and water-oxidation sites was modulated by the temperature at which the leaves had developed. These results indicate that the major factor that contributes to thermal acclimation of photosynthesis in winter wheat is the plastic response of PS II electron transport to environmental temperature.