Inhibition of oxygen evolution in chloroplasts isolated from leaves with low water potentials

Chloroplasts were isolated from pea and sunflower leaves having various water potentials. Oxygen evolution by the chloroplasts was measured under identical conditions for all treatments with saturating light and with dichloroindophenol as oxidant. Evolution was inhibited when leaf water potentials were below -12 bars in pea and -8 bars in sunflower and the inhibition was proportional to leaf water potential below these limits. Inhibition was more severe in sunflower than in pea chloroplasts. In sunflower, it could be detected after 5 minutes of leaf desiccation, and, up to 1 hour, the effect was independent of the duration of low leaf water potential.     

Photosynthesis: new light on biological oxygen production

In oxygenic photosynthesis, a highly oxidising chlorophyll species strips electrons out of two water molecules, generating molecular oxygen as a waste product. A recent study has provided new insights into the structure of the molecular machinery responsible for biological oxygen production.     

Nitrate photo-assimilation in tomato leaves under short-term exposure to elevated carbon dioxide and low oxygen

The role of photorespiration in the foliar assimilation of nitrate (NO₃^–) and carbon dioxide (CO₂) was investigated by measuring net CO₂ assimilation, net oxygen (O₂) evolution, and chlorophyll fluorescence in tomato leaves (Lycopersicon esculentum). The plants were grown under ambient CO₂ with ammonium nitrate (NH₄NO₃) as the nitrogen source, and then exposed to a CO₂ concentration of either 360 or 700 µmol mol^?1, an O₂ concentration of 21 or 2%, and either NO₃^– or NH₄⁺ as the sole nitrogen source. The elevated CO₂ concentration stimulated net CO₂ assimilation under 21% O₂ for both nitrogen treatments, but not under 2% O₂. Under ambient CO₂ and O₂ conditions (i.e. 360 µmol mol^?1 CO₂, 21% O₂), plants that received NO₃^– had 11–13% higher rates of net O₂ evolution and electron transport rate (estimated from chlorophyll fluorescence) than plants that received NH₄⁺. Differences in net O₂ evolution and electron transport rate due to the nitrogen source were not observed at the elevated CO₂ concentration for the 21% O₂ treatment or at either CO₂ level for the 2% O₂ treatment. The assimilatory quotient (AQ) from gas exchange, the ratio of net CO₂ assimilation to net O₂ evolution, indicated more NO₃^– assimilation under ambient CO₂ and O₂ conditions than under the other treatments. When the AQ was derived from gross O₂ evolution rates estimated from chlorophyll fluorescence, no differences could be detected between the nitrogen treatments. The results suggest that short‐term exposure to elevated atmospheric CO₂ decreases NO₃^– assimilation in tomato, and that photorespiration may help to support NO₃^– assimilation.     

Oxygen transport by low and normal oxygen affinity hemoglobin vesicles in extreme hemodilution

The oxygen transport capacity of phospholipid vesicles encapsulating purified Hb (HbV) produced with a Po(2) at which Hb is 50% saturated (P 50 ) of 8 (HbV(8)) and 29 mmHg (HbV(29)) was investigated in the hamster chamber window model by using microvascular measurements to determine oxygen delivery during extreme hemodilution. Two isovolemic hemodilution steps were performed with 5% recombinant albumin (rHSA) until Hct was 35% of baseline. Isovolemic exchange was continued using HbV suspended in rHSA solution to a total [Hb] of 5.7 g/dl in blood. P(50) was modified by coencapsulating pyridoxal 5'-phosphate. Final Hct was 11% for the HbV groups, with a plasma [Hb] of 2.1 +/- 0.1 g/dl after exchange with HbV(8) or HbV(29). A reference group was hemodiluted to Hct 11% with only rHSA. All groups showed stable blood pressure and heart rate. Arterial oxygen tensions were significantly higher than baseline for the HbV groups and the rHSA group and significantly lower for the HbV groups compared with the rHSA group. Blood pressure was significantly higher for the HbV(8) group compared with the HbV(29) group. Arteriolar and venular blood flows were significantly higher than baseline for the HbV groups. Microvascular oxygen delivery and extraction were similar for the HbV groups but lower for the rHSA group (P < 0.05). Venular and tissue Po(2) were statistically higher for the HbV(8) vs. the HbV(29) and rHSA groups (P < 0.05). Improved tissue Po(2) is obtained when red blood cells deliver oxygen in combination with a high- rather than low-affinity oxygen carrier.     

Antioxidant action on the level of reactive oxygen species in normal and transformed fibroblasts

The level of reactive oxygen species (ROS) in normal (3T3) and transformed (3T3-SV40) murine fibroblasts treated with antioxidants for 15 min was studied using a carboxy-H₂DCFDA fluorescent probe. It was shown that N-acetylcysteine (NAC) decreased the ROS level in both cell types. Antioxidant alpha-lipoic acid (ALA) and its reduced form dihydrolipoic acid (DHLA) caused a pro-oxidant effect. ALA and DHLA in the concentration range from 0.1–1.25 mM increased the ROS level in a dose-dependent manner in both cell types. The ability of ALA and DHLA to activate hydrogen peroxide production is discussed.     

Photosynthesis and nitrogen relationships in leaves of C3 plants

Summary The photosynthetic capacity of leaves is related to the nitrogen content primarily bacause the proteins of the Calvin cycle and thylakoids represent the majority of leaf nitrogen. To a first approximation, thylakoid nitrogen is proportional to the chlorophyll content (50 mol thylakoid N mol^-1 Chl). Within species there are strong linear relationships between nitrogen and both RuBP carboxylase and chlorophyll. With increasing nitrogen per unit leaf area, the proportion of total leaf nitrogen in the thylakoids remains the same while the proportion in soluble protein increases. In many species, growth under lower irradiance greatly increases the partitioning of nitrogen into chlorophyll and the thylakoids, while the electron transport capacity per unit of chlorophyll declines. If growth irradiance influences the relationship between photosynthetic capacity and nitrogen content, predicting nitrogen distribution between leaves in a canopy becomes more complicated. When both photosynthetic capacity and leaf nitrogen content are expressed on the basis of leaf area, considerable variation in the photosynthetic capacity for a given leaf nitrogen content is found between species. The variation reflects different strategies of nitrogen partitioning, the electron transport capacity per unit of chlorophyll and the specific activity of RuBP carboxylase. Survival in certain environments clearly does not require maximising photosynthetic capacity for a given leaf nitrogen content. Species that flourish in the shade partition relatively more nitrogen into the thylakoids, although this is associated with lower photosynthetic capacity per unit of nitrogen.     

Photomorphogenesis inPhycomyces: Fluence-response curves and action spectra

Blue light regulates vegetative reproduction inPhycomyces blakesleeanus Bgff. by inhibiting the development of microphores and stimulating that of macrophores. Fluence-response curves were obtained at twelve different wavelengths. Each response exhibits a two-step (biphasic) dependence on fluence, as if it resulted from the addition of two separate components with different thresholds, midpoints, and amplitudes. The absolute threshold is close to 10 photons·m². The threshold fluence of the low-intensity component is about 10⁴ times smaller than that of the high-intensity component. The action spectra for each of the two components of the two responses share general similarities, but exhibit significant differences that might be taken to favour four separate photosystems. Additional complexity is indicated by the wavelength dependence of the saturation levels.     

Photosynthesis or persistence: nitrogen allocation in leaves of evergreen and deciduous Quercus species

Photosynthetic nitrogen use efficiency (PNUE, photosynthetic capacity per unit leaf nitrogen) is one of the most important factors for the interspecific variation in photosynthetic capacity. PNUE was analysed in two evergreen and two deciduous species of the genus Quercus. PNUE was lower in evergreen than in deciduous species, which was primarily ascribed to a smaller fraction of nitrogen allocated to the photosynthetic apparatus in evergreen species. Leaf nitrogen was further analysed into proteins in the water‐soluble, the detergent‐soluble, and the detergent‐insoluble fractions. It was assumed that the detergent‐insoluble protein represented the cell wall proteins. The fraction of nitrogen allocated to the detergent‐insoluble protein was greater in evergreen than in deciduous leaves. Thus the smaller allocation of nitrogen to the photosynthetic apparatus in evergreen species was associated with the greater allocation to cell walls. Across species, the fraction of nitrogen in detergent‐insoluble proteins was positively correlated with leaf mass per area, whereas that in the photosynthetic proteins was negatively correlated. There may be a trade‐off in nitrogen partitioning between components pertaining to productivity (photosynthetic proteins) and those pertaining to persistence (structural proteins). This trade‐off may result in the convergence of leaf traits, where species with a longer leaf life‐span have a greater leaf mass per area, lower photosynthetic capacity, and lower PNUE regardless of life form, phyllogeny, and biome.     

Photosynthesis in the basal growing zone of barley leaves

Cell proliferation, elongation, determination and differentiation mainly take place in the basal 5 mm of a barley leaf, the so-called basiplast. A considerable portion of cDNAs randomly selected from a basiplast cDNA library represented photosynthetic genes such as CP29, RUBISCO-SSU and type I-LHCP II. Therefore, we became interested in the role of the basiplast in establishing photosynthesis. (1) Northern blot analysis revealed expression of photosynthetic genes in the basiplast, although at a low level. Analysis of basiplasts at different developmental stages of the leaves revealed maximal expression of photosynthetic genes during early leaf development. The activity of these genes shows that plastid differentiation involves the development of the photosynthetic apparatus even at this early state of leaf cell expansion. (2) This conclusion was supported by the fact that chlorophylls and carotenoids are synthesized in the basiplast. The qualitative pattern of pigment composition was largely similar to that of fully differentiated green leaves. (3) The transition from proplastids to chloroplasts progressed in the basal 5 mm of the leaf, so that the number of grana lamellae per thylakoid stack increased with distance from the meristem from zero to about five. (4) Photosynthetic function was studied by chlorophyll a-fluorescence measurements. In dark-adapted 8-day-old primary leaves, the fluorescence ratio (F_P-F_o)/F_P was little decreased in basiplasts as compared to leaf blades. During steady state photosynthesis, the ratio (F_M-F_o)/F_M was high in leaf blade (0.5), but low in the sheath (0.25) and in the basiplast (0.18), indicating the existence of functional, albeit low light-adapted chloroplasts in the basiplast. (5) Further on, chlorophyll a fluorescence analysis in relation to seedling age revealed efficient photosynthetic performance in the basiplast of 3- to 6-day-old seedlings which later-on differentiates into leaf blade as compared to the basiplast of 7- to 12-day-old seedlings which develops into leaf sheath and finally ceases to grow. The leaf age dependent changes in basiplast photosynthesis were reflected by changes in pigment contents and LHCP II expression both of which also revealed a maximum in the basiplast of 4-day-old seedlings.Abbreviations bas 1 basiplast-associated gene 1 encoding a peroxide reductase - cab chlorophyll a/b binding protein - CP 29 29 kDa chlorophyll binding protein - DIG digoxigenin - EMIP epidermal major intrinsic protein - LHCP II light harvesting complex of Photosystem II - LSU large subunit of Rubisco - NPQ non photochemical chlorophyll a fluorescence quenching - PSI/PS II Photosystem I/II - PQ photochemical chlorophyll a fluorescence quenching - Rubisco Ribulose-1,5-bisphosphate carboxylase - SSU small subunit of Rubisco     

Photosynthesis and chlorophyll content in different areas of fodder cabbage leaves

Byla stanovena intensita fotosynthesy a obsah chlorofylu v terěících z r?znych okrsk? ?epele listu krmné kapusty odr. Coulet de Flandre. Obsah chlorofylu na jednotku plochy byl vy??í v apikální ?ásti list? ne? v ?ásti basální, vy??í ve st?edové ne? v okrajové ?ásti listu. Nebyly nalezeny podstatné rozdily v intensitě fotosynthesy ve vzorcích z r?znych ?ástí listu. Intensita fotosynthesy není p?ímo úměrná váze su?iny daného vzorku, a?koliv vàha su?iny na jednotku plochy je podstatně vy??í v apikálni ne? v basální ?ásti listové plochy.     

Photosynthesis in frost-hardened and frost-stressed leaves of Hedera helix L.

Abstract The purpose of this study was to determine the respective extents to which winter reduction of photosynthetic capacity in ivy (Hedera helix L.) is caused by direct frost injury to the photosynthetic apparatus and by preceding protoplasmic changes connected with the acquisition of frost tolerance. Potted juvenile ivy plants were placed in the open under natural weather conditions whilst others were hardened under controlled conditions and subjected to the desired frost stress. Low non-freezing temperatures induced frost tolerance in ivy leaves down to about – 12°C (50% injury = TL₅₀) without impairing net photosynthetic rate as measured under standard conditions (20°C, light saturation, natural CO₂ level; = Standard-F_n. Only if the leaves froze (below ? 3°C to ?4°C) was a reversible inhibition of Standard-F_n observed. As long as the temperatures did not fall below approximately ?8°C the inhibition was small and Standard-F_n reached about 80–90% of the control. In this case the stomatal opening narrowed, giving a poorer supply of CO₂ to the mesophyll cells. Maximal frost tolerance (TL_5O?20°C to ?24°C) developed only with severe frosts below about ? 10°C. After such frosts, Standard-F_n was reduced to less than 20% of the control. The dependence of the rate of net photosynthesis on the internal CO₂ concentration showed a lower initial slope, thus indicating disturbances of chloroplast functions. However, neither in outdoor plants nor in those artificially frosted at – 20°C could there be found an appreciable inhibition of the electron transport capacity from H₂O to dichlorophenol indophenol or of ribulose bisphosphate carboxylase. If intact, severely frosted ivy plants were then held at higher temperatures (20/15°C), Standard-F_n recovered completely in approximately 10 d. Furthermore, following a frost period with temperatures down to ?12°C, mild weather caused a distinct improvement in Standard-F_n in outdoor plants, and there was no loss of maximum frost tolerance. Thus it can be concluded that the inhibition of Standard-F_n after severe frosts is not due to the development of maximal frost tolerance, but rather may be attributed to frost damage to the photosynthetic apparatus.     

Photosynthesis impairment in cassava leaves in response to nitrogen deficiency

Plants of cassava (Manihot esculenta Crantz cv. Cigana Preta) grown in a sand root medium were watered with nutrient solutions containing either 3 mM nitrate (low N) or 12 mM nitrate (high N). Chlorophyll concentration, chlorophyll a/b ratio, stomatal conductance, photorespiration rate and net carbon assimilation rate (on an area and a mass basis, but not on a chlorophyll basis) all decreased in low-N plants as compared with high-N ones. By contrast, photosynthetic nitrogen-use efficiency increased in low-N plants. As indicated by chlorophyll a fluorescence data, these plants exhibited increases in both excitation pressure on Photosystem II and thermal energy dissipation, with a corresponding decrease in quantum yield of electron transport, when contrasted with high-N plants. This decrease paralleled an unchanged maximal Photosystem II photochemical efficiency, suggesting a down-regulation of the Photosystem II photochemistry. It is proposed that decline in biochemical capacity for carboxylation, rather than stomatal limitation or electron transport, were the major constraints associated to the reduced photosynthetic rates induced by nitrogen deficiency in cassava plants.     

Photosynthesis in intact leaves of C3 plants: Physics,physiology and rate limitations

The instantaneous rate of photosynthetic CO₂ assimilation in C₃ plants has generally been studied in model systems such as isolated chloroplasts and algae. From these studies and from theoretical analyses of gas exchange behavior it is now possible to study the biochemistry of photosynthesis in intact leaves using a combination of methods, most of which are nondestructive. The limitations to the rate of photosynthesis can be divided among three general classes: (1) the supply or utilization of CO₂, (2) the supply or utilization of light, and (3) the supply or utilization of phosphate. The first limitation is most readily studied by determining how the CO₂ assimilation rate varies with the partial pressure of CO₂ inside the leaf. The second limitation can be studied by determining the quantum requirement of photosynthesis. The third limitation is most easily detected as a loss of O₂ sensitivity of photosynthesis. Measurement of fluorescence from intact leaves can give additional information about the various limitations. These methods are all non-destructive and so can be observed repeatedly as the environment of a leaf is changed. In addition, leaves can be quick-frozen and metabolite concentrations then measured to give more information about the limitations to intact leaf photosynthesis rates. In this review the physics and biochemistry of photosynthesis in intact C₃ leaves, and the interface between physiology and photosynthesis—triose phosphate utilization—are discussed.     

A library of action spectra for erythema and pigmentation

The first action spectra for erythema and delayed pigmentation in human skin were determined 90 years ago by Karl Hausser and Wilhelm Vahle in Germany, and since then a number of studies have been undertaken to redefine these action spectra. In this paper we give an overview of the action spectra for erythema and pigmentation that have been published during this 90-yr period, as well as indicating their uncertainties and shortcomings.     

After effect of heat shock on induction of fluorescence and low temperature fluorescence spectra of wheat leaves

The dynamics of restoration of variable and millisecond delayed fluorescence of chlorophyll a, as well as low-temperature fluorescence spectra at 77 K in leaves of 11-day-old wheat seedlings subjected to heat shock (41.5 degrees C, 20 min) and cultivated for 4 days under white light of different intensity was investigated. A comparative analysis of changes in variable, delayed, and low-temperature fluorescence of chlorophyll a depending on light intensity and the proteolitic activity in leaf homogenates during the restoration of the seedlings was carried out. It follows from the data that the restoration of fluorescence includes several phases and is stimulated by light. A possible mechanism of restoration of fluorescence and photochemical activity of photosystem II is discussed.