Effects of spermidine on chlorophyll content,photosynthetic activity and chloroplast ultrastructure in the dark and under light

Spermidine prevented the loss of chlorophyll from detached komatsuna (Brassica campestris cv. komatsuna) leaves in the dark but caused a striking reduction in light. It also inhibited photochemical activity in leaves incubated in the dark and light. Striking changes were seen in the chloroplast ultrastructure in light; the envelope was lacking, grana were dilated and thylakoids were distributed throughout the cytoplasm.     

外源Ca^2＋对离体菠菜叶片衰老的影响

以叶绿素、蛋白质含量为衰老指标,研究了Ca~(2+)在离体菠菜叶片衰老过程的效应。同时,采用同位素示踪技术研究了光照对离体菠菜叶片吸收,累积Ca~(2+)的影响;实验结果表明,0—10mmol/L Ca~(2+)能使菠菜叶片维持较高含量的叶绿素及各种蛋白质组分。在光、暗条件下,叶片均能吸收外源Ca~(2+),而光照能促进叶片对Ca~(2+)的吸收,暗处理3天的叶片,尚能在分离的叶绿体组分中发现~(45)Ca~(2+),占叶片总放射强度的0.5%。     

Stress Tolerance of Photosystem II in Vivo: Antagonistic Effects of Water, Heat, and Photoinhibition Stresses

The in vivo photochemical activity of photosystem II was inferred from modulated chlorophyll fluorescence and photoacoustic measurements in intact leaves of several plant species (Lycopersicon esculentum Mill., Solanum tuberosum L., Solanum nigrum L.) exposed to various environmental stresses (drought, heat, strong light) applied separately or in combination. Photosystem II was shown to be highly drought-resistant: even a drastic desiccation in air of detached leaf samples only marginally affected the quantum yield for photochemistry in photosystem II. However, water stress markedly modified the responses of photosystem II to superimposed constraints. The stability of photosystem II to heat was observed to increase strongly in leaves exposed to water stress conditions: heat treatments (e.g. 42°C in the dark), which caused a complete and irreversible inhibition of photosystem II in well-watered (tomato) leaves, resulted in a small and fully reversible reduction of the photochemical efficiency of photosystem II in drought-stressed leaves. In vivo photoacoustic data indicated that photosystem I was highly resistant to both heat and water stresses. When leaves were illuminated with intense white light at 25°C, photoinhibition damage of photosystem II was more pronounced in water-stressed leaves than in undesiccated controls. However, in nondehydrated leaves, photoinhibition of photosystem II was strongly temperature dependent, being drastically stimulated at high temperatures above 38 to 40°C. As a consequence, when exposed to strong light at high temperature, photosystem II photochemistry was significantly less inhibited in dehydrated leaves than in control well-hydrated leaves. Our results demonstrate the existence of a marked antagonism between physicochemical stresses, with water stress enhancing the resistance of photosystem II to constraints (heat, strong light at high temperature) that are usually associated with drought in the field.     

Chlorophyll Fluorescence and Photon Yield of Oxygen Evolution in Iron-Deficient Sugar Beet (Beta vulgaris L.) Leaves

The response of sugar beet (Beta vulgaris L.) leaves to iron deficiency can be described as consisting of two phases. In the first phase, leaves may lose a large part of their chlorophyll while maintaining a roughly constant efficiency of photosystem II photochemistry; ratios of variable to maximum fluorescence decreased by only 6%, and photon yields of oxygen evolution decreased by 30% when chlorophyll decreased by 70%. In the second phase, when chlorophyll decreased below a threshold level, iron deficiency caused major decreases in the efficiency of photosystem II photochemistry and in the photon yield of oxygen evolution. These decreases in photosystem II photochemical efficiency were found both in plants dark-adapted for 30 minutes and in plants dark-adapted overnight, indicating that photochemical efficiency cannot be repaired in that time scale. Decreases in photosystem II photochemical efficiency and in the photon yield of oxygen evolution were similar when measurements were made (a) with light absorbed by carotenoids and chlorophylls and (b) with light absorbed only by chlorophylls. Leaves of iron-deficient plants exhibited a room temperature fluorescence induction curve with a characteristic intermediate peak I that increases with deficiency symptoms.     

Photosynthetic activity of far-red light in green plants

We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, Q(A). The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8+/-0.5 kJ mol(-1) and 12.5+/-0.7 kJ mol(-1) have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol(-1) between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants.     

Optimization and evolution of light harvesting in photosynthesis: the role of antenna chlorophyll conserved between photosystem II and photosystem I

The efficiency of oxygenic photosynthesis depends on the presence of core antenna chlorophyll closely associated with the photochemical reaction centers of both photosystem II (PSII) and photosystem I (PSI). Although the number and overall arrangement of these chlorophylls in PSII and PSI differ, structural comparison reveals a cluster of 26 conserved chlorophylls in nearly identical positions and orientations. To explore the role of these conserved chlorophylls within PSII and PSI we studied the influence of their orientation on the efficiency of photochemistry in computer simulations. We found that the native orientations of the conserved chlorophylls were not optimal for light harvesting in either photosystem. However, PSII and PSI each contain two highly orientationally optimized antenna chlorophylls, located close to their respective reaction centers, in positions unique to each photosystem. In both photosystems the orientation of these optimized bridging chlorophylls had a much larger impact on photochemical efficiency than the orientation of any of the conserved chlorophylls. The differential optimization of antenna chlorophyll is discussed in the context of competing selection pressures for the evolution of light harvesting in photosynthesis.     

Enhanced Thermal Energy Dissipation Depending on Xanthophyll Cycle and D1 Protein Turnover in Iron-Deficient Maize Leaves Under High Irradiance

Pigment contents of chloroplasts and net photosynthetic rate were dramatically reduced in maize leaves suffering from iron deficiency. However, the reduction in photosynthesis was probably not caused by decreased contents of chlorophylls and carotenoids and by photon absorption; the primary limiting factor for photosynthesis may rather be the decrease of electron transport activity in photosystem 1. Iron-deficient leaves suffered serious acceptor-side photoinhibition, and more than 60 % of absorbed photons were dissipated, while less than 40 % was used in photochemical reaction. Thermal energy dissipation depending on xanthophyll cycle and D1 protein turnover was enhanced when acceptor-side photoinhibition occurred in iron-deficient maize leaves.     

Control of development of amylolytic and proteolytic activities in cotyledons of germinating black gram seeds

The effect of polyamines and related metabolites on several parameters of leaf senescence was followed in detached radish ( Raphanus sativus L. var. radicular cv. "Giant Butter") leaves floated on test solutions in darkness. Leaf senescence was accompanied by a marked loss of chlorophyll, which started at 24–48 h of incubation. The polyamines, spermine and spermidine, and the diamines, putrescine and cadaverine, were highly effective in arresting chlorophyll loss over a period of at least 96 h. l -arginine, and especially l -ornithine, were less active. Polyaminens prevented the marked chlorophyll loss in dark-incubated leaves, but did not compensate for the moderate chlorophyll loss when the leaves were aged in light. Polyamines were also highly effective in retarding earlier events of leaf senescence, prior to chlorophyll loss: both protein degradation and ribonuclease activity were inhibited by spermidine. Chlorophyll and protein loss in dark-or light-incubated suspensions of either "intact" or disrupted chloroplasts was not affected by polyamines. – It is concluded that polyamines are highly effective in preventing chlorophyll loss from detached leaves, possibly by controlling early senescence-linked events which occur in darkness rather than by direct inhibition of chlorophyll degradation.     

Effect of Leaf Detachment on Chlorophyll Fluorescence during Chilling Experiments

The effect of leaf detachment on chlorophyll fluorescence was analyzed for Zea mays, Cucumis sativus, Phaseolus vulgaris, and Echinochloa crus-galli. Results clearly indicate that detachment hastens the decrease in chlorophyll fluorescence during the course of chilling experiments. For maize and bean, the activity of photosystem II of chloroplasts isolated from detached leaves is lower than that of chloroplasts isolated from attached leaves. There are also large differences in ionic loss between detached and attached leaves of barnyard grass which could correlate with changes in leaf water status. The detached leaves lost some 50% of their total ionic content. Finally, detachment alters the ranking of the species with regard to their chilling tolerance.     

Photosynthetic activity of far-red light in green plants

We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, Q_A. The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8 ± 0.5 kJ mol⁻¹ and 12.5 ± 0.7 kJ mol⁻¹ have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol⁻¹ between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants.     

Studies on the Mechanism of Photoinhibition in Higher Plants: I. EFFECTS OF HIGH LIGHT INTENSITY ON CHLOROPLAST ACTIVITIES IN CUCUMBER ADAPTED TO LOW LIGHT

Cucumber plants (Cucumis sativus L.), grown at low quantum flux density (120-150 microeinsteins per square meter per second) were photoinhibited by a three-hour exposure in air to ten times the light intensity experienced during growth. Chloroplasts were isolated from photoinhibited and control leaves and the following activities determined: O₂ evolution in the presence of ferricyanide, photosystem I activity, noncyclic and cyclic photophosphorylation, and light-induced proton uptake. Chlorophyll and chloroplast absorbance spectra, and chloroplast fluorescence were also measured. It was found that photosystem II electron transport and non-cyclic photophosphorylation were inhibited by about 50%, while cyclic photophosphorylation was less inhibited and photosystem I electron transport and light-induced proton uptake were unaffected. Electron transport to methylviologen could not be fully restored by electron donation to photosystem II. Chloroplast fluorescence induction at room temperature was strongly reduced following photoinhibition. There was no difference in the absorption spectra of the extracted chlorophylls from control and photoinhibited chloroplasts, but an increase of the absorption in the blue wavelength region was observed in the photoinhibited chloroplasts. It is suggested that high light stress does not result in alteration of the membrane properties, as is the case in low-temperature stress for example, but affects directly the photosynthetic reaction centers, primarily of photosystem II.     

Ultra-structural and functional changes in the chloroplasts of detached barley leaves senescing under dark and light conditions

Changes in the chloroplast ultra-structure and photochemical function were studied in detached barley (Hordeum vulgare L. cv. Akcent) leaf segments senescing in darkness or in continuous white light of moderate intensity (90 mumol m-2 s-1) for 5 days. A rate of senescence-induced chlorophyll degradation was similar in the dark- and light-senescing segments. The Chl a/b ratio was almost unchanged in the dark-senescing segments, whereas in the light-senescing segments an increase in this ratio was observed indicating a preferential degradation of light-harvesting complexes of photosystem II. A higher level of thylakoid disorganisation (especially of granal membranes) and a very high lipid peroxidation were observed in the light-senescing segments. In spite of these findings, both the maximal and actual photochemical quantum yields of the photosystem II were highly maintained in comparison with the dark-senescing segments.     

Proline enhances primary photochemical activities in isolated thylakoid membranes of Brassica juncea by arresting photoinhibitory damage.

The presence of L-proline in the reaction mixture enhances the photosystem II (H2O----DCPIP) and whole chain (H2O----MV) catalysed electron transport activities of thylakoids isolated from the cotyledonary leaves of Brassica juncea seedlings raised in the absence and the presence of NaCl. The extent of stimulation in activities was higher in the thylakoids of NaCl raised plants than the controls. The extent of proline mediated stimulation was seen even in the presence of uncoupler NH4Cl suggesting that this stimulation is not due to uncoupling. However, photosystem I (DCPIPH2----MV) catalysed photoreaction remained almost insensitive to proline. The presence of proline in the incubation medium brought about a significant reduction in the time dependent loss in photochemical activity of thylakoids exposed to strong light suggesting that proline prevents photoinhibitory loss in chloroplast activity. Also, proline brought about a considerable reduction in the production of lipid peroxidation linked maiondialdehyde during strong illumination. We suggest that proline protects the components involved in water oxidation capacity by reducing the production of free radicals and/or scavenging the free radicals and thereby reducing thylakoid lipid peroxidation.     

The effect of light on chlorophyll loss in senescing leaves of sycamore (Acer pseudoplatanus L.)

Breakdown of chlorophylls in attached senescing sycamore leaves held in darkness was significantly less over a 14-d period than that occurring in leaves exposed to natural light. Chlorophyll a declined more rapidly than chlorophyll b in both situations, the stability of the latter being particularly increased in darkness. The differences between dark-maintained leaves and those exposed to light with respect to soluble protein, cytoplasmic RNA, and free amino-nitrogen were much less marked. The data indicate that chlorophyll loss during senescence is, at least in part, the result of a direct photochemical degradation of the pigment.     

Response of soybean photosynthesis and chloroplast membrane function to canopy development and mutual shading

The effect of natural shading on photosynthetic capacity and chloroplast thylakoid membrane function was examined in soybean (Glycine max. cv Young) under field conditions using a randomized complete block design. Seedlings were thinned to 15 plants per square meter at 20 days after planting. Leaves destined to function in the shaded regions of the canopy were tagged during early expansion at 40 days after planting. To investigate the response of shaded leaves to an increase in available light, plants were removed from certain plots at 29 or 37 days after tagging to reduce the population from 15 to three plants per square meter and alter the irradiance and spectral quality of light. During the transition from a sun to a shade environment, maximum photosynthesis and chloroplast electron transport of control leaves decreased by two- to threefold over a period of 40 days followed by rapid senescence and abscission. Senescence and abscission of tagged leaves were delayed by more than 4 weeks in plots where plant populations were reduced to three plants per square meter. Maximum photosynthesis and chloroplast electron transport activity were stabilized or elevated in response to increased light when plant populations were reduced from 15 to three plants per square meter. Several chloroplast thylakoid membrane components were affected by light environment. Cytochrome f and coupling factor protein decreased by 40% and 80%, respectively, as control leaves became shaded and then increased when shaded leaves acclimated to high light. The concentrations of photosystem I (PSI) and photosystem II (PSII) reaction centers were not affected by light environment or leaf age in field grown plants, resulting in a constant PSII/PSI ratio of 1.6 ± 0.3. Analysis of the chlorophyll-protein composition revealed a shift in chlorophyll from PSI to PSII as leaves became shaded and a reversal of this process when shaded leaves were provided with increased light. These results were in contrast to those of soybeans grown in a growth chamber where the PSII/PSI ratio as well as cytochrome f and coupling factor protein levels were dependent on growth irradiance. To summarize, light environment regulated both the photosynthetic characteristics and the timing of senescence in soybean leaves grown under field conditions.     

The stimulating effect of a cold dark-pretreatment on the etioplast/chloroplast transformation of angiosperms. III. Involvement of the excision factor and of cold-retarded aging processes?

Our question is whether the stimulating effect of a cold dark-pretreatment on the process of de-etiolation in primary leaves of wheat seedlings under subsequent continuous white light is essentially mediated by the retarding effect of highly lowered temperatures on the following processes: aging and/or senescence, realization of the so-called excision factor in detached leaves, decrease of the cytokinin level in detached leaves. The strong stimulating effect of a cold dark-preatreatment remains inspite of progressive aging in parts of the leaves and a strong decrease of the capability of chlorophyll accumulation in detached in contrast to attached leaves. The strong stimulatory effect of a cold dark-pretreatment is not diminished by application of cytokinin or gibberellic acid. The stimulating effect of a cold dark-pretreatment is detectable over several days under continuous light, but it is lost during a warm dark-phase of a few hours duration between the cold dark-pretreatment and the white light phase.     

Catalase, Peroxidase, and Polyphenoloxidase Activities during Rice Leaf Senescence

The activities of catalase, peroxidase, and polyphenoloxidase were studied in attached and detached rice (Oryza sativa L. cv. Ratna) leaves. Catalase activity decreased while peroxidase and polyphenoloxidase activities increased during senescence of both attached and detached rice leaves. Kinetic (5 mum) and benzimidazole (1 mm), which are known to delay the senescence of detached rice leaves, retarded the decrease of catalase activity during detached leaf senescence. On the other hand, these chemicals accelerated the increase of peroxidase and polyphenoloxidase activities over the water control. Total phenolics accumulated in detached and darkened rice leaves, but in attached leaf senescence in light no accumulation of phenolics was observed.     

The interplay between cytokinins and light during senescence in detached Arabidopsis leaves

Light and cytokinins are known to be the key players in the regulation of plant senescence. In detached leaves, the retarding effect of light on senescence is well described; however, it is not clear to what extent is this effect connected with changes in endogenous cytokinin levels. We have performed a detailed analysis of changes in endogenous content of 29 cytokinin forms in detached leaves of Arabidopsis thaliana (wild‐type and 3 cytokinin receptor double mutants). Leaves were kept under different light conditions, and changes in cytokinin content were correlated with changes in chlorophyll content, efficiency of photosystem II photochemistry, and lipid peroxidation. In leaves kept in darkness, we have observed decreased content of the most abundant cytokinin free bases and ribosides, but the content of cis‐zeatin increased, which indicates the role of this cytokinin in the maintenance of basal leaf viability. Our findings underscore the importance of light conditions on the content of specific cytokinins, especially N⁶‐(Δ²‐isopentenyl)adenine. On the basis of our results, we present a scheme summarizing the contribution of the main active forms of cytokinins, cytokinin receptors, and light to senescence regulation. We conclude that light can compensate the disrupted cytokinin signalling in detached leaves.     

Photosynthetic Responses of Leaves to Water Stress, Expressed by Photoacoustics and Related Methods : II. The Effect of Rapid Drought on the Electron Transport and the Relative Activities of the Two Photosystems

The effect of rapid dehydration of detached tobacco leaves (Nicotiana tabacum L.) on the photochemical apparatus of photosynthesis was studied in vivo by a combination of methods: photoacoustics, chlorophyll a fluorescence, and cytochrome f difference spectroscopy. It was shown that the inhibition of gross O₂ evolution was mainly caused by inactivation of PSII: (a) The saturation curve of cytochrome-f photooxidation by farred (>710 nanometers) light was resistant to the stress, leading to the conclusion that photosystem I (PSI) was largely unaffected by the stress. (b) The extent of the chlorophyll a variable fluorescence arising from photosystem II (PSII) decreased with the progression of the stress, but was largely unaffected when the leaf was preincubated with electron donors to PSII, such as hydroxylamine. It is concluded that the drought damage to PSII occurred on the photooxidative side. Despite the extensive inhibition of PSII and the relative preservation of PSI, the apparent PSII/PSI activity balance was somewhat larger in stressed leaves than in the control, as indicated by photoacoustic measurements of Emerson enhancement. These measurements were performed continuously under conditions which favor transitions to either state 1 or 2, showing that the transition to state 2 was considerably inhibited. Simultaneous measurements of chlorophyll fluorescence induction at 680 and 730 mm at room temperature were also used to probe changes in energy distribution between PSII and PSI and indicated that the transition from a dark adapted state to state 2 was also affected in water-stressed leaves. The saturation curve of the far-red light effect in Emerson enhancement was not changed by the stress, giving another independent evidence for the drought resistance of PSI activity. This apparent preservation of the imbalance in photochemical activities in favor of PSII, despite the fact that PSII is strongly inhibited, and PSI is not, supports a previous suggestion that the electron transfer between the two photosystems is not random but that a large extent of PSII and PSI units are specifically linked.