THE EFFECT OF LIGHT INTENSITY AND SUCROSE FEEDING ON THE FINE STRUCTURE IN CHLOROPLASTS AND ON THE CHLOROPHYLL CONTENT OF ETIOLATED LEAVES

Changes in the fine structure of proplastids of etiolated leaves exposed to various conditions of light and darkness for 24 and 48 hours were investigated, and the chlorophyll content of the leaves so treated was determined in vivo. The light treatments were given while the leaves were floated on tap water or on a 0.2 M sucrose solution. Leaves floated on water under low light intensity (2 foot-candles) were low in chlorophyll and contained plastids with concentric rows of vesicles. Transferring the leaves back to darkness resulted in the disappearance of the concentric rigs and re-formation of vesicular centers together with straight rows of vesicles and tubules, evenly spaced throughout the stroma. Chloroplasts of leaves floated on a sucrose solution under low light showed large vesicular centers together with stacks of rows of elongated tubules. The same chloroplast structure was found in leaves floated on a sucrose solution in the dark, after having been exposed to weak light for 24 hours. Chlorophyll content in these leaves was the same as in leaves floated on water under high light intensity, where the chloroplasts had normal grana and lamellae. The effect of the investigated factors on plastid development is discussed.     

The effect of light wavelength on the susceptibility of plants to virus infection

Plants exposed for 24–72 h to light of different wavelengths differed in their subsequent susceptibility to virus infection. French bean leaves were less susceptible to infection by tobacco necrosis virus and Nicotiana glutinosa leaves were less susceptible to infection by tobacco mosaic virus when previously exposed to blue or red light than when exposed to green light. These differences were most pronounced at low energy levels. Leaves exposed to each kind of light were less susceptible than those kept in darkness.     

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 irradiation and growth regulators on degradation processes in detached soybean leaves

Changes in soluble protein profile and chlorophyll (Ch1) content in detached soybean leaves incubated in darkness or light were delayed by application of benzyladenine or indole-3-acetic acid and enhanced by abscisic acid. The degradation in light differed significantly from the degradation in darkness. Chl and proteins were lost at a higher rate in darkness than in light.     

Oak seedlings grown in different light qualities

Oak seedlings (Quercus robur L.) were germinated in darkness for 3 weeks and then given continuous long wavelength far-red light (LFR; wavelengths longer than 700 nm). A control group of seedlings was kept in darkness. After 2 additional weeks the chlorophyll formation ability in red light was examined in the different seedlings. The stability of the protochlorophyll(ide) and chlorophyll(ide) forms to high intensity red irradiation was also measured. Oak seedlings grown in darkness accumulated protochlorophyll(ide) (6 μg per g fresh matter). Absorption spectra and fluorescence spectra indicated the presence of more protochlorophyll(ide)_628–632 than protochlorophyllide_650–657. The level of protochlorophyll(ide) was higher in leaves of plants cultivated in LFR light (13 μg per g fresh matter) than in leaves of dark grown plants. 12% of the protochlorophyll(ide) was esterified in both cases. The level of protochlorophyll(ide)_628–632 in LFR grown oaks varied with the age of the leaves, being higher in the older (basal) leaves, but also in the very youngest (top-most) leaves. The ability of the leaves to form photostable chlorophyll in red light showed a similar age dependence, being low in rather young and in older leaves. A low ability to form photostable chlorophyll thus appears to be correlated with a high content of protochlorophyll(ide)_628–632. Upon irradiation only the protochlorophyllide_650–657 was transformed to chlorophyllide. After this phototransformation the chlorophyllide peak at 684 nm shifted to 671 nm within about 30 min in darkness. This shift took place without any accompanying change in photostability of the chlorophyll(ide). Upon irradiation with strong red light a similar shift took place within one minute. This indicates that the chlorophyllide after phototransformation was rather loosely bound to the photoreducing enzyme. The development towards photostable chlorophyll forms consists of three phases and is discussed.     

Possible mechanisms of light-induced chlorophyll degradation in senescing leaves of Hydrilla verticillata

Light treatment markedly accelerated the chlorophyll loss in senescing leaves of Hydrilla verticillata [(L.f.) Royle] as compared to dark treatment, whereas such acceleration could not be observed in senescing spinach (Spinacia oleracea L.) leaves. The light-induced cholorophyll loss in Hydrilla was retarded slightly by chloramphenicol and markedly by cycloheximide. Catalase (EC 1.11.1.6) activity did not change appreciably in Hydrilla leaves either in light or in darkness, while in spinach it declined markedly in the dark, and light retarded such decline. Peroxidase activity in Hydrilla showed faster increase in light than in darkness, while in spinach it increased only in light during senescence. The activity of phenol(pyrogallol)-specific peroxidase increased markedly in light, and that of ascorbate-specific peroxidase decreased slightly both in light and darkness during senescence of Hydrilla leaves. This rise in phenolspecific peroxidase activity was prevented by cycloheximide treatment. Pretreatment of Hydrilla leaves with monophenol (2,4-dichlorophenol) and o-diphenol (hydroquinone) accelerated and retarded, respectively, the light-induced cholorophyll loss. Pretreatment of Hydrilla leaves with H₂O₂ augmented the chlorophyll loss more markedly in light than in darkness. The endogenous level of H₂O₂ increased more in light than in dark during senescence of Hydrilla leaves. Treatment of Hydrilla leaves with 3-(3.4-dichlorophenyl)-l,l-dimethylurea. a photosystem II inhibitor, prevented both light-induced rise in H₂O: level and chlorophyll loss, but it was without effect in the dark. Retardation of light-induced chlorophyll loss occurred during senescence of Hydrilla leaves when light was given in different photoperiods in a 24-h daily cycle for 6 days instead of as continuous irradiance. There was a negative correlation between the length of the photoperiod and the extent of cholorophyll loss.     

Apple leaf senescence: leaf disc compared to attached leaf

Attached apple leaves (Pyrus malus L., Golden Delicious) began to lose protein in early August as the first sign of senescence. Apple leaf discs prepared from samples before early August gained protein for up to 7 days after detachment. After early August, the loss of protein from leaf discs was no greater than the loss from attached leaves in 7 days. The loss of chlorophyll from leaf discs began over 2 months before attached leaves began to lose chlorophyll naturally and before leaf discs lost protein. Leaf discs from presenescent leaves did not senesce significantly faster when maintained in darkness instead of 12 hours of light. In general, the loss of protein and chlorophyll from apple leaf discs after 7 days was much less than for most other leaf types studied.     

Metabolism of Oat Leaves during Senescence : VII. The Interaction of Carbon Dioxide and Other Atmospheric Gases with Light in Controlling Chlorophyll Loss and Senescence

In air largely freed from CO₂, senescence of isolated oat (Avena sativa cv Victory) seedling leaves is no longer prevented by white light; instead, the leaves lose both chlorophyll and protein as rapidly as in the dark. Senescence in light is also accelerated in pure O₂, but it is greatly delayed in N₂; 100% N₂ preserves both protein and chlorophyll in light and in darkness. In light in air, most of the compounds tested that had previously been found to delay or inhibit senescence in darkness actually promote the loss of chlorophyll, but they do not promote proteolysis. Under these conditions, proteolysis can therefore be separated from chlorophyll loss. But in light minus CO₂, where chlorophyll loss is rapid in controls, two of these same reagents prevent the chlorophyll loss. Unlike the many reagents whose action in light is thus the opposite of that in darkness, abscisic acid, which promotes chlorophyll loss in the dark, also promotes it in light with or without CO₂. Kinetin, which prevents chlorophyll loss in the dark, also prevents it in light minus CO₂. In general, therefore, the responses to light minus CO₂ are similar to the responses to darkness, and (with the exception of abscisic acid and kinetin) opposite to the response to light in air.     

Far-red induced changes of the protochlorophyllide components in wheat leaves

Biosynthesis of chlorophyll is partly controlled by the phytochrome system. In order to study the effects of an activated phytochrome system on the protochlorophyllide (PChlide) biosynthesis without accompanying phototransformation to chlorophyll, wheat seedlings (Triticum aestivum L. cv. Starke II Weibull) were irradiated with long wavelength far-red light of low intensity. Absorption spectra were measured in vivo after different times in the far-red light or in darkness. The relationship between the different PChlide forms, the absorbance ratio _{650nm636 nm} changed with age in darkness, and the change was more pronounced when the leaves were grown in far-red light. Absorption spectra of dark-grown leaves always showed a maximum in the red region at 650 nm. For leaves grown in far-red light the absorption at 636 nm was high, with a maximum at the 5 day stage where it exceeded the absorption at 650 nm. At the same time there was a maximum in the total amount of PChlide accumulated in the leaves, about 30% more than in leaves grown in darkness. But the amount of the directly phototransformable PChlide, mainly PChlide_650–657, was not increased. The amount of PChlide_628–632, or more probably the amount of (PChlide_628–632, + PChlide _636–657) was thus higher in young wheat leaves grown in far-red light than in those grown in darkness. After the 5 day stage the absorption at 636 nm relative to 650 nm decreased with age, and at the 8 day stage the spectra were almost the same in both types of leaves. Low temperature fluorescence spectra of the leaves also showed a change in the ratio between the different PChlide forms. The height of the fluorescence peak at 632 nm relative to the peak at 657 nm was higher in leaves grown in far-red light than in dark-grown leaves. – After exposure of the leaves to a light flash, the half time for the Shibata shift was measured. It increased with age both for leaves grown in darkness and in far-red light; but in older leaves grown in far-red light (7–8 days) the half time was slightly longer than in dark-grown leaves. – The chlorophyll accumulation in white light as well as the leaf unrolling were faster for leaves pre-irradiated with far-red light. The total length of the seedlings was equal or somewhat shorter in far-red light, but the length of the coleoptile was markedly reduced from 8.1 ± 0.1 cm for dark-grown seedlings to 5.2 ± 0.1 cm for seedlings grown in far-red light.     

Photodynamic injury to heated leaves

Summary The leaves of Tradescantia fluminensis Vell, were kept in light and darkness after short-term heating (5 min and 10 min) at different temperatures. In light temperature causing injury was 10° lower than in darkness. A considerable destruction of chlorophyll occurred when the heated leaves were kept in light. If the light intensity was 4,000 lux or even lower the damage to cells was not accompanied by bleaching of chlorophyll. Light produced no effect on unheated leaves. In variegated white-green leaves of Chlorophytum elatum R. Br. light injured only green parts of leaf blades. The minimal light intensity which brought about injury of Tradescantia leaves in experimental conditions was 1,000 lux. Light of the same intensity accelerated death of heated isolated leaves of Cucumis sativus L.Light damage to Tradescantia leaves occurred when the action of light was accompanied by that of high temperature.In an atmosphere of nitrogen the injurious effect of light sharply decreased.It is suggested that the injury of heated leaves in light is caused by photooxydation which is sensitized by chlorophyll and occurs at the expense of photochemical energy which is not used in photosynthesis. Photosynthesis itself is repressed by heating.     

Pigment formation in potato tubers (Solanum tuberosum) exposed to light followed by darkness

Potato tubers ( Solanum tubersoum cvs Bintje and King Edward). never exposed to light, lack chlorophyllous pigments. Continuous irradiation results in chlorophyll (Chl) formation and induces the ability for protochlorophyll (Pchl) formation when the tubers are brought back to darkness. Pigment synthesis takes place in both blue and red light, but blue light is more effective than red in starting the greening process. The pigment formation is strongest in the layers just below the periderm with a steep gradient inwards. Small amounts of Chl formed after irradiation. slowly fade away during extended darkness. However, the Chl formed after long time of irradiation is remarkably stable. Irradiated potatoes, placed in darkness, form Pchl with a fluorescence emission peak at 633 nm. A maximal level is reached after ca 7 days. Resolution of the Pchl spectrum suggests the presence of small amounts of a pigment with an emission maximum at around 642 nm. No sign of the Pchl with emission maximum at 657 nm, which dominates in etiolated leaves, is found. A faint Chl fluorescence indicates that some Pchl, probably the 642 nm form, is phototransformed into Chl in weak light. The Chl formation in the potato tuber is discussed in relation to that of roots and leaves.     

The flexible interrelation between AOX respiratory pathway and photosynthesis in rice leaves.

Alternative respiratory pathway was investigated in rice seedlings grown under total darkness, light/dark cycle, or continuous light. The capacity of the alternative pathway was relatively higher in leaves that had longer light exposure. An analysis of rice AOX1 multigene family revealed that AOX1c, but not AOX1a and AOX1b, had a light-independent expression. The alternative oxidase (AOX) inhibitor, salicylhydroxamic acid (SHAM, 1mM), inhibited nearly 68% of the capacity of the alternative pathway in leaves grown under different light conditions. The plants grown under different light periods were treated with SHAM and then were exposed to illumination for 4h. The transition from dark to 4h of light stimulated the capacity of alternative pathway in etiolated rice seedlings and in those grown under light/dark cycle, whereas the capacity of the alternative pathway was constant in seedlings grown under continuous light with additional 4h of illumination. Etiolated leaves did not show any CO(2) fixation after 4h of illumination, and the increase in chlorophyll content was delayed by the SHAM pretreatment. When seedlings grown under light/dark cycle were moved from dark and exposed to 4h of light, increases in chlorophyll content and CO(2) fixation rate were reduced by SHAM. Although these parameters were stable in plants grown under continuous light, SHAM decreased CO(2) fixation rate but not the chlorophyll content. These results indicate that the role and regulation of AOX in light are determined by the developmental stage of plant photosynthetic apparatus.     

CYCLING OF STOMATAL APERTURE IN LEAVES OF PLANTS WITH CRASSULACEAN ACID METABOLISM UNDER CONSTANT CONDITIONS

When leaves of plants with C₃ metabolism are detached and held in darkness, they senesce and the stomata close. Because the relation of senescence and stomatal closure is very close, if not actually causal, the question arose as to whether in the leaves of plants with Crassulacean acid metabolism whose stomata open at night the relationship to senescence would be reversed. Detached leaves of four species of Hoya, floated on water in constant darkness or constant light, were found to show no large differences in stomatal aperture (measured as diffusion resistance) between those in the light or dark, but the aperture changed in a regular circadian rhythm. In some leaves the rhythm was simple, in others the peak showed small secondary peaks, but in all cases the values were nearly the same in the light as in the dark, throughout the cycle. Previous culture of the intact plants under normal day/night conditions gave results similar to those with plants that had had prolonged culture under constant light or darkness. In those cases when the stomata were more open in the dark, the chlorophyll content was greater than when the stomata were more open in the light; but when they were more open in the light, the chlorophyll content showed little difference between light and dark. When the leaves had only their petioles in water they showed greater senescence in the light than in the dark, and the stomata were more tightly closed in the light, especially at the apical ends. All four species of Hoya gave similar results. We deduce that senescence of these leaves is modified by stomatal aperture, and generally in the same direction as in C₃ leaves, but that in continuous light or darkness the primary control over the aperture is the endogenous cycle.     

Haem and chlorophyll formation in etiolated and greening leaves of barley

The amounts of protochlorophyllide (P650) and protohaem were measured in ageing dark-grown barley leaves. Maximum amounts of P650 and protohaem were found in 6- to 8-day-old material after which P650 declined rapidly and protohaem more slowly. In leaves exposed to light maximum chlorophyll was produced in 6-day-old material with progressively less the older the leaves. Haem concentrations increased in seedlings of all ages exposed to light. A lag phase was observed for both chlorophyll and haem formation in leaves given a light treatment. Haem, however, showed a slight yet sig nificant decline as chlorophyll production commenced. The results indicate that chlorophyll and haem synthesis share a common pool of δ-aminolae vulinic acid (ALA). At a certain stage of development, the magnesium porphyrin pathway diverts precursors away from haem synthesis. It is only when the ALA synthesising system is well developed that the production of ALA can satisfy pathways to both haem and chlorophyll. The observed changes in haem under certain conditions suggest that, as in animal systems, haem levels may regulate porphyrin formation (chlorophylls) by controlling the supply of ALA.     

Induction of ethylene synthesis and lipid peroxidation in damaged or TMV-infected tobacco leaf tissues by light

The effect of light on ethylene and ethane production in damaged leaf tissues was investigated. When whole leaves of tobacco cv. Samsun NN were damaged with liquid nitrogen, the ethylene formation was the highest, if 100?% of leaves were injured and were kept in the light, the lowest when leaves after 100?% injury were kept in darkness. Ethane production (lipid peroxidation) could be detected only in damaged, but not in control leaves, and was much higher in light than in darkness. In addition, there was a strong degradation of chlorophyll of damaged leaves kept in light. In light aminoethoxy-vinylglycine (AVG) inhibited ethylene formation in control, non-damaged whole leaves effectively, but in leaves with 100?% damage the inhibitory effect was much weaker and similar to the effect of propyl gallate (PG), a free radical scavenger. Both AVG and PG treatments decreased ethylene formation by control leaf discs and discs with 100?% damage. Ethane production was significantly inhibited by PG and slightly by AVG in the case of 100?% damage. Tiron, another free radical scavenger gave similar results on leaf discs as PG did. Paraquat (methylviologen, Pq), as a photosynthesis inhibiting and reactive oxygen species (ROS) producing herbicide produced a large amount of ethylene and ethane in light but very small amount in darkness. In accordance, tobacco mosaic virus (TMV) infection on the necrotic host resulted in significantly larger amount of ethylene and ethane formation in light than in darkness. We conclude that ethylene and ethane production of damaged plant tissues is strongly induced by light and ROS that are involved in this induction.     

叶黄素循环和D1蛋白周转在毛竹叶片光保护中的作用

利用叶绿素荧光技术,对强光胁迫下以及叶黄素循环抑制剂-二硫苏糖醇(DTT)和D1蛋白合成抑制剂-硫酸链霉素(SM)处理后毛竹(Phyllostachys edulis (Carr.) Lehaie)的光抑制特征进行研究。结果显示:在夏季中午强光或人为强光胁迫下,毛竹叶片最大光化学效率Fv/Fm均显著降低;在下午光强减弱或黑暗、弱光条件下,Fv/Fm可有效恢复。DTT和SM均可抑制毛竹叶片非光化学淬灭(NPQ),且DTT效果明显优于SM。另外,在强光下,DTT和SM处理均能使毛竹叶片Fv/Fm、实际光化学效率Y(Ⅱ)和光化学淬灭qP等荧光参数下降幅度增大。研究结果表明毛竹叶片具有完善的光破坏防御机制,NPQ与叶黄素循环和D1蛋白周转紧密关联,在叶片光保护机制中具有重要作用。     

The Spectral Response of Light Dependent Chlorophyll b Formation

Dark grown seedlings of barley will obtain a high ratio chlorophyll a/chlorophyll b when exposed to intermittent light (1 min of incandescent light or one electronic flash every hour). Such material has been exposed to monochromatic light of different wavelengths for 1 h. The ratio a/b gets a minimum in light of 670 nm, indicating the highest rate of chlorophyll b formation at this wavelength. The possibility is discussed that the light absorber (and also precursor) could be a short-lived chlorophyll a form, existing prior to the forms in the Shibata-shift. Chlorophyll b formation in darkness is discussed from the findings that the rate of formation of chlorophyll b is higher in intermittent light than it should be, calculated from the rate in continuous light, where the saturation intensity is rather low.     

Strong light elevates thermotolerance of photosynthetic apparatus and the content of membranes and polar lipids in wheat leaves

The influence of excess irradiance on resistance of wheat (Triticum aestivum L.) photosynthetic apparatus to heating in darkness and in the light was investigated and compared with changes in leaf cell ultra-structure and composition of cell lipids and fatty acids. The leaves of 14- to 16-day-old plants grown at low irradiance (about 20 W/m²) were exposed for 1 h to irradiance of 370 or 600 W/m² PAR. Using infrared gas analysis, we found that the preexposure of leaves to excess irradiation elevated resistance of apparent photosynthesis to 10-min heat treatment at 40–45°C. The rate of Hill reaction (reduction of 2,6-dichlorophenolindophenol by isolated chloroplasts) was higher for leaves heated at high irradiance than for leaves heated in darkness. During illumination of leaves with strong light, mesophyll cells became more abundant in mitochondria and peroxysomes, as well as in cisternae of endoplasmic reticulum and Golgi complex. The chloroplast thylakoids and grana became more extensive and numerous. At the same time, the leaf content of main classes of membrane glycerolipids increased in parallel with the increase in the phospholipid/glycolipid and lipid/chlorophyll ratios. The unsaturation index of fatty acids of membrane lipids increased because of the elevated content of linolenic acid. Thus, excessive light (not fully utilized in photosynthesis) induced in wheat leaves a series of nonspecific adaptive changes that were similar to those occurring under the action of other environmental factors, such as heat shock, cooling, salinity, and osmotic stresses.     

The relation between nitrogen deficiency and leaf senescence

Because the "mobilization" of nitrogen resulting from nutritional nitrogen deficiency is also prominent during leaf senescence, the characteristics of these two syndromes were compared. Oat plants ( Avena sativa L. cv. Victory) were raised on a nutrient solution, complete except for nitrogen supply (i.e., with only the seed protein as nitrogen source), and the senescence of their leaves was compared with that of controls grown on a full nutrient solution. The N-deficient plants flowered after forming only 4 leaves and each set a single seed. The nitrogen lack affected the content of chlorophyll somewhat more than the content of the amino acids or protein nitrogen. However, spraying the plants with kinetin solution was able to retain 20–30% of the chlorophyll and protein. During senescence, the chlorophyll appears to be less stable in the N-deficient leaves than in the controls, while the protein is somewhat more stable than in the controls. Also, when the detached leaves from N-deficient plants senesced in white light or in darkness, kinetin delayed their senescence almost as effectively as that of control leaves. Most strikingly, the stomata of N-deficient leaves after detachment and floating on water were largely closed in light, just as in senescence, but could be partially induced to open by kinetin treatment. Since stomatal closure has earlier been shown to cause senescence, the characteristic syndrome of foliar nitrogen deficiency is concluded to be partly that of senescence.     

High salt stress in wheat leaves causes retardation of chlorophyll accumulation due to a limited rate of protochlorophyllide formation

When exposed to salt stress, leaves from dark-grown wheat seedlings ( Triticum aestivum , cv. Giza 168) showed reduced accumulation of chlorophyll during irradiation. To elucidate the mechanism behind salt-influenced reduction of chlorophyll biosynthesis, we have investigated the effect of salt stress on the spectral forms of Pchlide, the phototransformation of Pchlide to Chlide, the Shibata shift, the regeneration of Pchlide and the accumulation of Pchlide from 5-aminolevulinic acid (ALA). We found that the phototransformation of Pchlide to Chlide was not affected by salt stress. The blue shift (Shibata shift) of newly formed Chlide was delayed both after flash irradiation and in continuous light. The reformation of Pchlide in darkness after a flash irradiation or after a period of 3-h irradiation was retarded in the salt-treated leaves. However, after a 20-h dark period, Pchlide was reformed even in salt-treated leaves but the formation of short-wavelength Pchlide was suppressed. Compared to controls, salt treatment also reduced the amount of Pchlide accumulated in leaves floated on ALA. The increase in the low temperature fluorescence emission spectrum at 735 nm, which occurred gradually during several hours of irradiation with continuous light in control leaves, was completely suppressed in salt-treated leaves. It is concluded that salt stress inhibits chlorophyll accumulation partly by reducing the rate of porphyrin formation but, as discussed, also by a possible reduction in the formation of chlorophyll-binding proteins.