Exposure times in rapid light curves affect photosynthetic parameters in algae

Short-and long-duration light curves were applied to four macroalgae (Ulva sp., Codium fragile, Ecklonia radiata and Lessonia variegata), and two microalgal species (Chlorella emersonii and Chaetoceros muellerii). With increasing light curve duration, the maximal relative electron transport rate increased by a factor of three in E. radiata, and by factors of 1.25 and 1.23 in C. emersonii and L. variegata, respectively, but did not change in C. fragile and Ch. muellerii. The light saturation point E_k increased by 26 μmol photons m⁻² s⁻¹ in C. emersonii and 20 μmol photons m⁻² s⁻¹ in Ch. muellerii and E. radiata with elevated light curve exposure times. Oscillatory patterns of the continuous fluorescence readings reflect accumulation of Q_A. Continuous fluorescence values increased, or decreased, by approximately 10% within light curve increments. However, oscillations of 25% were not uncommon, which shows that cells are changing their photo-physiological response state during steady light conditions. Increasing dark acclimation times prior to light curve application lowered maximal relative electron transport rates in the C. emersonii (from 28 ± 1.7 to 25 ± 1.2 for 15 and 95 min dark acclimation in short-duration light curves respectively). This effect was counterbalanced by longer light curve application. It can therefore be concluded that manipulation of light exposure and dark incubation prior to the experiment affects the photosynthetic response, presumably due to different activation states of photosynthetic and photoprotective mechanisms. The highly species-specific photo-response patterns imply that a common rapid light curve protocol will generate artefacts in some species.     

Light-dependent phosphate uptake of a submersed macrophyte Myriophyllum spicatum L.

The uptake kinetics of phosphate (Pi) by Myriophyllum spicatum was determined from adsorption and absorption under light and dark conditions. Pi uptake was light dependent and showed saturation following the Michaelis-Menten relation (in light: V = 16.91 × [Pi](1.335 + [Pi]), R² = 0.90, p < 0.001; in the dark: V = 5.13 × [Pi](0.351 + [Pi]), R² = 0.77, p < 0.001). Around 77% of the loss of Pi in the water column was absorbed into the tissue of M. spicatum, and only 23% was adsorbed on the surface of the plant shoots. Our study shows that M. spicatum shoots have a much higher affinity (in light: 3.9 μmol g⁻¹ dw h⁻¹ μM⁻¹; in the dark: 3.7 μmol g⁻¹ dw h⁻¹ μM⁻¹) and V_max (maximum uptake rate, shoot light) for Pi uptake than many other aquatic macrophytes (in light: 0.002-0.23 μmol g⁻¹ dw h⁻¹ μM⁻¹; in the dark: 0.002-0.19 μmol g⁻¹ dw h⁻¹ μM⁻¹), which may provide a competitive advantage over other macrophytes across a wide range of Pi concentrations.     

Bicarbonate use in three aquatic plants

Our study aimed to test the ability of aquatic plants to use bicarbonate when acclimated to three different bicarbonate concentrations. To this end, we performed experiments with the three species Ceratophyllum demersum, Egeria densa, Lagarosiphon major to determine photosynthetic rates under varying bicarbonate concentrations. We measured bicarbonate use efficiency, photosynthetic performance and respiration. For all species, our results revealed that photosynthetic rates were highest in replicates grown at low alkalinity. Thus, E. densa had approx. five times higher rates at low (264 ± 15 μmol O₂ g⁻¹ DW h⁻¹) than at high alkalinity (50 ± 27 μmol O₂ g⁻¹ DW h⁻¹), C. demersum had three times higher rates (336 ± 95 and 120 ± 31 μmol O₂ g⁻¹ DW h⁻¹), and L. major doubled its rates at low alkalinity (634 ± 114 and 322 ± 119 μmol O₂ g⁻¹ DW h⁻¹). Similar results were obtained for bicarbonate use efficiency by E. densa (136 ± 44 and 43 ± 10 μmol O₂ mequiv. L⁻¹ g⁻¹ DW h⁻¹) and L. major (244 ± 29 and 82 ± 24 μmol O₂ mequiv. L⁻¹ g⁻¹ DW h⁻¹). As to C. demersum, efficiency was high but unaffected by alkalinity, indicating high adaptation ability to varied alkalinities. A pH drift experiment supported these results. Overall, our results suggest that the three globally widespread worldwide species of our study adapt to low inorganic carbon availability by increasing their efficiency of bicarbonate use.     

Spectral effects of supplementary lighting on the secondary metabolites in roses,chrysanthemums, and campanulas

To investigate the effect of the light spectrum on photosynthesis, growth, and secondary metabolites Rosa hybrida ‘Scarlet’, Chrysanthemum morifolium ‘Coral Charm’, and Campanula portenschlagiana ‘BluOne’ were grown at 24/18 °C day/night temperature under purpose-built LED arrays yielding approximately 200 μmol m⁻² s⁻¹ at plant height for 16 h per day. The four light treatments were (1) 40% Blue/60% Red, (2) 20% Blue/80% Red, (3) 100% Red, and (4) 100% White (Control). The plant height was smallest in 40% Blue/60% Red in roses and chrysanthemums, while the biomass was smallest in the white control in roses and in 100% Red in chrysanthemums. The total biomass was unaffected by the spectrum in campanulas, while the leaf area was smallest in the 40% Blue/60% Red treatment. In 100% Red curled leaves and other morphological abnormalities were observed. Increasing the blue to red ratio increased the stomatal conductance though net photosynthesis was unaffected, indicating excess stomatal conductance in some treatments. With higher blue light ratio all phenolic acids and flavonoids increased. In view of the roles of these secondary metabolites as antioxidants, anti-pathogens, and light protectants, we hypothesize that blue light may predispose plants to better cope with stress.     

Prevention of floret abscission for Agapanthus praecox requires an adequate supply of carbohydrate to the developing florets

Vase solutions of 2.5 or 5% (w/v) sucrose markedly reduced the abscission of all open florets and buds on cut Agapanthus praecox stems. Pulsing cut stems with these solutions was not as effective as continuous treatments. Pulse treatments with 10% sucrose for 4–24 h were relatively ineffective at reducing floret and bud abscission but longer pulses of 48 h reduced abscission. The relative ineffectiveness of sucrose pulse treatments appeared to be due to the low water uptake of the stems (1–3 ml/day). Reducing the number of florets and buds on an inflorescence reduced abscission of florets and buds, and increasing stem length from 25 cm to 50 cm decreased bud abscission. This reduction in abscission is possibly due to the increased availability of assimilates for the remaining buds and florets or reduced competition for assimilates. Exogenous ethylene treatments (9 µl/L for up to 24 h) had no effect on abscission, although STS treatment (4 mM, 4 h) significantly reduced floret abscission when stems were held in vase solutions of sucrose. We conclude that postharvest floret abscission in A. praecox is influenced primarily by the availability of assimilates to the developing florets.     

Diurnal courses of net photosynthesis and photosystem II quantum efficiency of submerged Lagarosiphon major under natural light conditions

An optode device for net-photosynthesis measurements, based on oxygen-depending quenching of fluorescence from O₂-specific sensors, and PAM fluorometry have been used to study diurnal courses of net-photosynthesis and the F_v/F_m ratio of the submerged plant Lagarosiphon major. Plants were pre-cultivated and studied in large mesocosm flow-through outdoor tanks under 50% and 80% shade cloth, respectively. Growth under the different shade cloths resulted in similar light compensation points (∼20 μmol photons m⁻² s⁻¹), but strongly different light saturation levels, with about 150 μmol m⁻² s⁻¹ for plants grown under 80% shade cloth and about 350 μmol m⁻² s⁻¹ for plants grown under 50% shade cloth. Plants under both growth conditions showed a transient reduction of the maximum F_v/F_m value in the afternoon (down to 70% of the morning control values under 80% shade cloth and down to 85% under 50% shade cloth), which was not accompanied by a reduction of the net photosynthetic rate. This indicated that the fluorescence parameter F_v/F_m must not be a reliable indicator of the rate of photosynthesis under all conditions. The new photo-optical device became evidenced as a valuable tool not only for laboratory experiments, but also for field studies of gas exchange of submerged plants.     

Horizontal or vertical photobioreactors? How to improve microalgae photosynthetic efficiency

The productivity of a vertical outdoor photobioreactor was quantitatively assessed and compared to a horizontal reactor. Daily light cycles in southern Spain were simulated and applied to grow the microalgae Chlorella sorokiniana in a flat panel photobioreactor.The maximal irradiance around noon differs from 400 μmol photons m⁻² s⁻¹ in the vertical position to 1800 μmol photons m⁻² s⁻¹ in the horizontal position. The highest volumetric productivity was achieved in the simulated horizontal position, 4 g kg culture⁻¹ d⁻¹. The highest photosynthetic efficiency was found for the vertical simulation, 1.3 g of biomass produced per mol of PAR photons supplied, which compares favorably to the horizontal position (0.85 g mol⁻¹) and to the theoretical maximal yield (1.8 g mol⁻¹). These results prove that productivity per unit of ground area could be greatly enhanced by placing the photobioreactors vertically.     

Nitrogen nutrition of Canna indica: Effects of ammonium versus nitrate on growth, biomass allocation, photosynthesis, nitrate reductase activity and N uptake rates

The effects of inorganic nitrogen (N) source (NH₄⁺, NO₃⁻ or both) on growth, biomass allocation, photosynthesis, N uptake rate, nitrate reductase activity and mineral composition of Canna indica were studied in hydroponic culture. The relative growth rates (0.05-0.06 g g⁻¹ d⁻¹), biomass allocation and plant morphology of C. indica were indifferent to N nutrition. However, NH₄⁺ fed plants had higher concentrations of N in the tissues, lower concentrations of mineral cations and higher contents of chlorophylls in the leaves compared to NO₃⁻ fed plants suggesting a slight advantage of NH₄⁺ nutrition. The NO₃⁻ fed plants had lower light-saturated rates of photosynthesis (22.5 μmol m⁻² s⁻¹) than NH₄⁺ and NH₄⁺/NO₃⁻ fed plants (24.4-25.6 μmol m⁻² s⁻¹) when expressed per unit leaf area, but similar rates when expressed on a chlorophyll basis. Maximum uptake rates (V_max) of NO₃⁻ did not differ between treatments (24-35 μmol N g⁻¹ root DW h⁻¹), but V_max for NH₄⁺ was highest in NH₄⁺ fed plants (81 μmol N g⁻¹ root DW h⁻¹), intermediate in the NH₄NO₃ fed plants (52 μmol N g⁻¹ root DW h⁻¹), and lowest in the NO₃⁻ fed plants (28 μmol N g⁻¹ root DW h⁻¹). Nitrate reductase activity (NRA) was highest in leaves and was induced by NO₃⁻ in the culture solutions corresponding to the pattern seen in fast growing terrestrial species. Plants fed with only NO₃⁻ had high NRA (22 and 8 μmol NO₂⁻ g⁻¹ DW h⁻¹ in leaves and roots, respectively) whereas NRA in NH₄⁺ fed plants was close to zero. Plants supplied with both forms of N had intermediate NRA suggesting that C. indica takes up and assimilate NO₃⁻ in the presence of NH₄⁺. Our results show that C. indica is relatively indifferent to inorganic N source, which together with its high growth rate contributes to explain the occurrence of this species in flooded wetland soils as well as on terrestrial soils. Furthermore, it is concluded that C. indica is suitable for use in different types of constructed wetlands.     

Independent fluctuations of malate and citrate in the CAM species Clusia hilariana Schltdl. under low light and high light in relation to photoprotection

Clusia hilariana Schltdl. is described in literature as an obligate Crassulacean acid metabolism (CAM) species. In the present study we assessed the effect of irradiance with low light (LL, 200 μmol m⁻² s⁻¹) and high light (HL, 650–740 μmol m⁻² s⁻¹), on the interdependency of citrate and malate diurnal fluctuations. In plants grown at HL CAM-type oscillations of concentration of citrate and malate were obvious. However, at LL daily courses of both acids do not seem to indicate efficient utilization of these compounds as CO₂ and NADPH sources. One week after transferring plants from LL to HL decarboxylation of malate was accelerated. Thus, in the CAM plant C. hilariana two independent rhythms of accumulation and decarboxylation of malate and citrate take place, which appear to be related to photosynthesis and respiration, respectively. Non photochemical quenching (NPQ) of photosystem II, especially well expressed during the evening hours was enhanced. Exposure to HL for 7 d activated oxidative stress protection mechanisms such as the interconversion of violaxanthin (V), antheraxanthin (A) and zeaxanthin (Z) (epoxydation/de-epoxydation) measured as epoxydation state (EPS). This was accompanied by a slight increase in the total amount of these pigments. However, all these changes were not observed in plants exposed to HL for only 2 d. Besides violaxanthin cycle components also lutein, which shows a small, but not significant increase, may be involved in dissipating excess light energy in C. hilariana.     

Physiological responses to flooding and light in two tree species native to the Amazonian floodplains

In Amazonian floodplains, plant survival is determined by adaptations and growth strategies to effectively capture sunlight and endure extended periods of waterlogging. By measuring gas exchange, quantum efficiency of photosystem 2 (PSII), and growth parameters, we investigated the combined effects of flooding gradients and light on two common evergreen floodplain tree species, the light-tolerant Cecropia latiloba and the shade-tolerant Pouteria glomerata. Individual plants were subjected to different combinations of light and flooding intensity in short-term and long-term experiments. Plants of C. latiloba lost all their leaves under total submersion treatments (plants flooded to apex and with reduced irradiance) and showed highest maximum assimilation rates (A_max) in not flooded, high light treatments (6.1 μmol CO₂ m⁻² s⁻¹). Individuals of P. glomerata showed similar patterns, with A_max increasing from 1.9 μmol CO₂ m⁻² s⁻¹ under total flooding to 7.1 μmol CO₂ m⁻² s⁻¹ in not flooded, high light treatments. During the long-term flooding experiment, quantum efficiency of PSII (F_v/F_m) of C. latiloba was not affected by partial flooding. In contrast, in P. glomerata F_v/F_m decreased to values below 0.73 after 120 days of total flooding. Moreover, total submergence led P. glomerata to reduce significantly light saturation point (LSP), as compared to C. latiloba. For both species morphological adjustments to long-term flooding, such as the production of adventitious roots, resulted in reduced total biomass, relative growth rate (RGR) and leaf mass ratio (LMR). Growth increase in C. latiloba seemed to be more limited by low-light than by flooding. Therefore, the predominant occurrence of this species is in open areas with high light intensities and high levels of inundation. In P. glomerata flooding induced high reductions of growth and photosynthesis, whereas light was not limiting. This species is more abundant in positions where irradiance is reduced and periods of submergence are slightly modest. We could show that the physiological requirements are directly responsible for the flooding (C. latiloba) and shade (P. glomerata) tolerance of the two species, which explains their local distribution in Amazonian floodplain forests.     

Characterization of functional intermediates of endoglucanase from Aspergillus aculeatus during urea and guanidine hydrochloride unfolding

Low concentrations of urea and GuHCl (2 M) enhanced the activity of endoglucanase (EC 3.1.2.4) from Aspergillus aculeatus by 2.3- and 1.9-fold, respectively. The K_m values for controls, in the presence of 2 M urea and GuHCl, were found to be 2.4 ± 0.2 × 10⁻⁸ mol L⁻¹, 1.4 ± 0.2 × 10⁻⁸ mol L⁻¹, and 1.6 ± 0.2 × 10⁻⁸ mol L⁻¹, respectively. The dissociation constant (K_d) showed changes in the affinity of the enzyme for the substrate with increases in the K_cat suggesting an increased turnover number in the presence of urea and GuHCl. Fluorescence studies showed changes in the microenvironment of the protein. The increase in the activity of this intermediate state was due to conformational changes accompanied by increased flexibility at the active site.     

Effects of UV radiation and nitrate limitation on the production of biogenic sulfur compounds by marine phytoplankton

We tested the effects of UV radiation (UVR) and nitrate limitation on the production of dimethylsulfide (DMS), particulate dimethylsulfoniopropionate (DMSPp), and particulate dimethylsulfoxide (DMSOp) in natural seawater from the Gulf of Mexico and in phytoplankton cultures. DMS/Chl a ratios in PAR-only and PAR + UV-exposed seawater were 0.44–2.0 and 0.46–1.9 nmol DMS μg⁻¹ Chl a, respectively, whereas the ratios in cultures of Amphidinium carterae were 1.0–2.2 nmol μg⁻¹ in PAR-exposed samples and 0.91–2.1 nmol μg⁻¹ in PAR + UV-exposed samples. These results suggested that UVR did not substantially affect DMS/Chl a ratios in seawater and A. carterae culture samples. Similarly, UVR had no significant effect on DMSOp/Chl a in seawater samples (0.83–1.6 nmol DMSO μg⁻¹ Chl a for PAR + UV vs. 0.70–1.5 nmol μg⁻¹ for PAR-exposed seawater samples, respectively) or in A. carterae cultures (0.20–1.3 and 0.19–0.88 nmol DMSO μg⁻¹ Chl a in PAR + UV- and PAR-exposed cultures, respectively). In an experiment with the diatom, Thalassiosira oceanica, the culture was grown in high nitrate (30 μM) or low nitrate (6 μM) media and exposed to PAR-only or PAR + UV. The low nitrate, PAR-only samples showed an increase of intracellular dimethylsulfoniopropionate (DMSP) concentration from 2.1 to 15 mmol L⁻¹ in 60 h, but the increase occurred only after cultures reached the stationary phase. Cultures of T. oceanica grown under UVR had lower growth rates than those under PAR-only (μ′ = 0.17 and 0.32 d⁻¹, respectively) and perhaps did not experience severe nitrate limitation even in the low nitrate treatment. These results suggest that the elevated UVR in low nitrate environments could result in reduction of DMSP in some species, whereas DMSP concentrations would not be affected in eutrophic areas.     

Multiple bud cultures of 'Barhee' date palm (Phoenix dactylifera) and physiological status of regenerated plants

Adventitious bud clusters of date palm ‘Barhee’ were successfully established from juvenile leaves (<1 cm) using reduced amounts of 2,4-D (0.2 mg L⁻¹) to limit the risk of somaclonal variation. An average of 8.4 adventitious buds per explant were obtained. Histological examination showed that the superficial cell layers of leaves had the highest caulogenic capacity. High sucrose concentration (70 g L⁻¹) was used for the conversion of initial buds to multiple bud clusters. The promoting effect of temporary immersion on shoot proliferation was found to be significant when compared to cultivation on solid media. Elongation of shoots was also better using a thin film of PGR-free liquid medium instead of a solid medium. Anatomical observations indicated that roots from vitroplants were potentially functional at various developmental stages. However, only 12-month-old vitroplants were found to be physiologically able to control transpirational vapor loss. Additionally, the photochemical activity of photosystem II in these vitroplants was close to that measured in plants that were already acclimatized. As a result, 83.3% of regenerated plants were successfully acclimatized. No phenotypic variation was observed among more than 500 adventitious bud-derived plants. All regenerants survived after field transplantation. We found that the production of adventitious bud clusters in small bioreactors was able to provide an efficient micropropagation system for date palm cv. ‘Barhee’. An in vitro hardening step was a prerequisite for the successful transfer of vitroplants in soil.     

Cryopreservation of in vitro-grown apical meristems of Lilium by droplet-vitrification

In this study, apical meristems from adventitious buds of three lily (Lilium L.) cultivars were successfully cryopreserved by droplet-vitrification. The most effective techniques were as follows. Excised meristems from in vitro plantlets which had been sub-cultured for about 2 months were cold-hardened at 4 °C for 1 week, precultured on MS medium supplemented with 0.3 mol L⁻¹ sucrose for 2 days, osmoprotected in loading solution for 20-40 min at room temperature and then soaked in PVS2 solution for 90-120 min at 0 °C, frozen in microdroplets of vitrification solution placed on aluminium foils, which were immersed rapidly in liquid nitrogen. The meristems were then rapidly rewarmed by dilution solution, transferred to regeneration medium and stored in the dark for two weeks at 20 °C, and then cultured under white fluorescent light at an intensity of 2000 lux, with a 16 h photoperiod at 20 °C. The highest post-thaw survival percentages of three cultivars ‘Siberia’ (Lilium × siberia), Lilium lancifolium Thunb. and ‘Snow Queen’ Lilium × longiflorum were 65.0%, 83.8% and 43.3%, and regeneration percentages were 62.0%, 67.6% and 35.0%, respectively. The study demonstrated that cryopreservation by droplet-vitrification increased survival and regeneration percentages of certain lily cultivars compared with vitrification. Thus to cryopreserve lily meristems, droplet-vitrification method is preferable to the vitrification method.     

Responses of Gmelina arborea, a tropical deciduous tree species, to elevated atmospheric CO2: Growth, biomass productivity and carbon sequestration efficacy

The photosynthetic response of trees to rising CO₂ concentrations largely depends on source-sink relations, in addition to differences in responsiveness by species, genotype, and functional group. Previous studies on elevated CO₂ responses in trees have either doubled the gas concentration (>700 μmol mol⁻¹) or used single large addition of CO₂ (500-600 μmol mol⁻¹). In this study, Gmelina arborea, a fast growing tropical deciduous tree species, was selected to determine the photosynthetic efficiency, growth response and overall source-sink relations under near elevated atmospheric CO₂ concentration (460 μmol mol⁻¹). Net photosynthetic rate of Gmelina was ∼30% higher in plants grown in elevated CO₂ compared with ambient CO₂-grown plants. The elevated CO₂ concentration also had significant effect on photochemical and biochemical capacities evidenced by changes in F_V/F_M, ABS/CSm, ET₀/CSm and RuBPcase activity. The study also revealed that elevated CO₂ conditions significantly increased absolute growth rate, above ground biomass and carbon sequestration potential in Gmelina which sequestered ∼2100 g tree⁻¹ carbon after 120 days of treatment when compared to ambient CO₂-grown plants. Our data indicate that young Gmelina could accumulate significant biomass and escape acclimatory down-regulation of photosynthesis due to high source-sink capacity even with an increase of 100 μmol mol⁻¹ CO₂.     

Luminostat operation: a tool to maximize microalgae photosynthetic efficiency in photobioreactors during the daily light cycle?

The luminostat regime has been proposed as a way to maximize light absorption and thus to increase the microalgae photosynthetic efficiency within photobioreactors. In this study, simulated outdoor light conditions were applied to a lab-scale photobioreactor in order to evaluate the luminostat control under varying light conditions. The photon flux density leaving the reactor (PFD_out) was varied from 4 to 20 μmol photons m⁻² s⁻¹and the productivity and photosynthetic efficiency of Chlorella sorokiniana were assessed.Maximal volumetric productivity (1.22 g kg⁻¹ d⁻¹) and biomass yield on PAR photons (400-700 nm) absorbed (1.27 g mol⁻¹) were found when PFD_out was maintained between 4 and 6 μmol photons m⁻² s⁻¹. The resultant photosynthetic efficiency was comparable to that already reported in a chemostat-controlled reactor. A strict luminostat regime could not be maintained under varying light conditions. Further modifications to the luminostat control are required before application under outdoor conditions.     

High irradiance improves ammonium tolerance in wheat plants by increasing N assimilation

Ammonium is a paradoxical nutrient ion. Despite being a common intermediate in plant metabolism whose oxidation state eliminates the need for its reduction in the plant cell, as occurs with nitrate, it can also result in toxicity symptoms. Several authors have reported that carbon enrichment in the root zone enhances the synthesis of carbon skeletons and, accordingly, increases the capacity for ammonium assimilation. In this work, we examined the hypothesis that increasing the photosynthetic photon flux density is a way to increase plant ammonium tolerance. Wheat plants were grown in a hydroponic system with two different N sources (10 mM nitrate or 10 mM ammonium) and with two different light intensity conditions (300 μmol photon m⁻² s⁻¹ and 700 μmol photon m⁻² s⁻¹). The results show that, with respect to biomass yield, photosynthetic rate, shoot:root ratio and the root N isotopic signature, wheat behaves as a sensitive species to ammonium nutrition at the low light intensity, while at the high intensity, its tolerance is improved. This improvement is a consequence of a higher ammonium assimilation rate, as reflected by the higher amounts of amino acids and protein accumulated mainly in the roots, which was supported by higher tricarboxylic acid cycle activity. Glutamate dehydrogenase was a key root enzyme involved in the tolerance to ammonium, while glutamine synthetase activity was low and might not be enough for its assimilation.     

Chlorophyll fluorescence in the leaves of Tradescantia species of different ecological groups: Induction events at different intensities of actinic light

Chlorophyll fluorescence analysis is one of the most convenient and widespread techniques used to monitor photosynthesis performance in plants. In this work, after a brief overview of the mechanisms of regulation of photosynthetic electron transport and protection of photosynthetic apparatus against photodamage, we describe results of our study of the effects of actinic light intensity on photosynthetic performance in Tradescantia species of different ecological groups. Using the chlorophyll fluorescence as a probe of photosynthetic activity, we have found that the shade-tolerant species Tradescantia fluminensis shows a higher sensitivity to short-term illumination (≤20 min) with low and moderate light (≤200 μE m⁻² s⁻¹) as compared with the light-resistant species Tradescantia sillamontana. In T. fluminensis, non-photochemical quenching of chlorophyll fluorescence (NPQ) and photosystem II operational efficiency (parameter Φ_PSII) saturate as soon as actinic light reaches ≈200 μE m⁻² s⁻¹. Otherwise, T. sillamontana revealed a higher capacity for NPQ at strong light (≥800 μE m⁻² s⁻¹). The post-illumination adaptation of shade-tolerant plants occurs slower than in the light-resistant species. The data obtained are discussed in terms of reactivity of photosynthetic apparatus to short-term variations of the environment light.     

Purification and characterization of two extracellular endochitinases from Massilia timonae

Two extracellular chitinases (designated as Chi-56 and Chi-64) produced by Massilia timonae were purified by ion-exchange chromatography, ammonium sulfate precipitation, and gel-filtration chromatography. The molecular mass of Chi-56 was 56 kDa as determined by both SDS-PAGE and gel-filtration chromatography. On the other hand, Chi-64 showed a molecular mass of 64 kDa by SDS-PAGE and 28 kDa by gel-filtration chromatography suggesting that its properties may be different from those of Chi-56. The optimum temperature, optimum pH, pI, K_m, and V_max of Chi-56 were 55 °C, pH 5.0, pH 8.5, 1.1 mg mL⁻¹, and 0.59 μmol μg⁻¹ h⁻¹, respectively. For Chi-64, these values were 60 °C, pH 5.0, pH 8.5, 1.3 mg mL⁻¹, and 1.36 μmol μg⁻¹ h⁻¹, respectively. Both enzymes were stimulated by Mn²⁺ and inhibited by Hg²⁺, and neither showed exochitinase activity. The N-terminal sequences of Chi-56 and Chi-64 were determined to be Q-T-P-T-Y-T-A-T-L and Q-A-D-F-P-A-P-A-E, respectively.