PROMOTION OF PLASTID PIGMENT ACCUMULATION IN WATER STRESSED WHEAT LEAF SECTIONS BY HORMONE TREATMENT

Chlorophyll (Chl) and carotenoid accumulations are markedly reduced in detached wheat leaves after 20-hr exposure to light when subjected to mild (-7 bar) osmotic stress prior to illumination. The treatment of N⁶-benzyladenine (BA) in combination with gibberellic acid (GA) stimulated the recovery of plastid pigment accumulations in mildy stressed detached leaves. The combination treatment of BA plus IAA increased pigment accumulations only to the level stimulated by BA treatment alone. Mild water deficits increased the Chl a/b ratio and reduced the Chl/carotenoid ratio in wheat sections. Growth promoting hormones generally lowered the Chl a/b ratio and increased the Chl/carotenoid ratio in mildly stressed detached leaves. Abscisic acid (ABA) reduced plastid pigment concentrations in greening tissue and increased the Chl a/b ratio while decreasing the Chl/carotenoid ratio. In detached leaves stressed to -14 bars, BA in combination with GA stimulated only partial recovery of the plastid pigment accumulations and improved slightly the Chl a/b and Chl/carotenoid ratios.     

Effect of RotyIenchulus reniformis on Reflectance of Cotton Plant Leaves

Differences between light reflectance from leaves of cotton (Gossypiurn hirsutum) plants grown with a low- or no-nematode (Rotylenchulus reniformis) population (nonstressed), and from leaves grown with a high nematode population (stressed) were measured in field and greenhouse experiments. Reflectance was measured spectrophotometrically in the laboratory on single leaves and spectroradiometrically in the field on plant canopies. Nematode-stressed cotton plants were stunted with fewer, smaller, and darker-green leaves than nonstressed plants. Over the 0.5- to 2.5-/μm waveband, stressed leaves had lower reflectance than nonstressed leaves of the same chronological age for both field- and greenhouse-grown plants. Reflectance differences between stressed and nonstressed leaves in the visible (0.5 to 0.75 μm), near-infrared (0.75 to 1.35 μm) and infrared water absorption (1.35 to 2.5 μm) regions were primarily caused by differences in leaf chlorophyll concentration, mesophyll structure, and water content, respectively. Results indicate the potential for remotely sensing nematode-infested plants to distinguish them from normal plants.     

Role of ethylene in the senescence of detached rice leaves

The role of ethylene in the senescence of detached rice leaves in relation to their changes in 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production was studied. In freshly excised rice leaf segments, ACC level and ethylene production rates were very low. Following incubation, the rates of ethylene production increased and reached a maximum in 12 h, and subsequently declined. The rise of ethylene production was associated with a 20- to 30-fold increase in ACC level.

Ethylene seems to be involved in the regulation of the senescence of detached rice leaves. This conclusion was based on the observations that (a) maximum ethylene production preceded chlorophyll degradation, (b) ACC application promoted chlorophyll degradation, (c) inhibitors of ethylene production and ethylene action retarded chlorophyll degradation, and (d) various treatments such as light, cycloheximide, α,α-dipyridyl, Ni²⁺, and cold temperature, which retarded chlorophyll degradation, also inhibited ethylene production.

Abscisic acid promoted senescence but significantly decreased ethylene production, whereas benzyladenine retarded senescence but promoted ethylene production. This is interpreted to indicate that abscisic acid treatment increased the tissue sensitivity to ethylene, whereas benzyladenine treatment decreased it.

     

Chloroplastic regulation of apoplastic alpha-amylase activity in pea seedlings

Photobleaching of pea (Pisum sativum L.) seedling leaves by treatment with norflurazon (San 9789) and 7 days of continuous white light caused a 76- to 85-fold increase in the activity of the primary α-amylase, a largely apoplastic enzyme, over normally greening seedlings. Levels of chlorophyll were near zero and levels of plastid marker enzyme activities were very low in norflurazon-treated seedlings, indicating severe photooxidative damage to plastids. As levels of norflurazon or fluence rates were lowered, decreasing photobleaching of tissues, α-amylase activity decreased. Levels of leaf β-amylase and starch debranching enzyme changed very little in norflurazon-treated seedlings. Infiltration extraction of leaves of norflurazon-treated and normally greening seedlings indicated that at least 57 and 62%, respectively, of α-amylase activity was in the apoplast. α-Amylase activity recovered from the apoplast of photobleached leaves of norflurazon-treated seedlings was 18-fold higher than that for green leaves. Inhibitors of photosynthesis (DCMU and atrazine) and an inhibitor of chlorophyll accumulation that does not cause photooxidation of plastid components (tentoxin) had little effect on levels of α-amylase activity, indicating norflurazon-caused loss of chlorophyll and lack of photosynthesis did not cause the large induction in α-amylase activity. An inhibitor of both abscisic acid and gibberellin synthesis (paclobutrazol [PP333]) and an analog of norflurazon which inhibits photosynthesis but not carotenoid synthesis (San 9785) caused only moderate (about five-fold) increases in α-amylase activity. Lincomycin and chloramphenicol increased α-amylase activity in light grown seedings to the same magnitude as norflurazon, indicating that the effect of norflurazon is probably through the destruction of plastid ribosomes. It is proposed that chloroplasts produce a negative signal for the regulation of the apoplastic α-amylase in pea.     

THE ULTRASTRUCTURAL DEVELOPMENT OF THE DIMORPHIC PLASTIDS OF ZEA MAYS L.

The development of the dimorphic chloroplasts of Zea mays L. in adult foliage leaves is described, and a method of correlating ultrastructural stages by means of leaf chlorophyll is presented. In addition, the developmental changes in chlorophyll a/b ratio are discussed. Both the mesophyll and the bundle sheath plastids contain small grana at the earliest stages of plastid development. As the plastids enlarge, the mesophyll grana stacks increase in both length of the appressed membrane and in the number of thylakoids per granum. Initially, the grana stacks in the bundle sheath plastids also enlarge, but as the plastids approach full size, most of the membrane appression is lost. However, the remaining areas of appression in the bundle sheath plastids show an increase in the number of thylakoids in each small granum.     

Specific Features of Plastid Pigment Apparatus and Photosynthesis in the Leaves of Ephemeroid and Summer Plants as Related to Photoinhibition

The state of the pigment apparatus and potential photosynthesis (PP) was compared in the leaves of plants falling into two ecological groups, ephemeroids (three species) and summer plants (two species). For the first time, the organization of the plastid pigment apparatus was investigated in ephemeroids using the data on chlorophyll and carotenoid distribution between the major photosynthetic pools. The molar ratio between xanthophylls and chlorophyll in the light-harvesting complex of plastids in the ephemeroids (0.5 to 0.6) considerably exceeded that in the summer plants (0.3–0.4). By using salicylaldoxime, an inhibitor of the reverse reaction of the violaxanthin cycle, we were able to calculate the active pool of violaxanthin on its way to zeaxanthin. This pool was shown to amount to 85% of the sum total of xanthophylls of the violaxanthin cycle in the ephemeroid leaf plastids as compared to 60% in the summer species. Thus, potentially, the photosynthetic apparatus in the ephemeroid leaves is better provided with the pigments essential for photoprotective function and for maintaining a high photosynthetic rate under early spring conditions. Under chilling temperatures of 5–10°C and full insolation, PP in ephemeroids was as high as in the summer plants at 20°C.     

Plastid differentiation during androgenesis in albino and non-albino producing cultivars of barley (Hordeum vulgare L.)

In order to better understand androgenic albinism in barley, we compared plastid differentiation during anther culture in two cultivars, an albino (spring cultivar Cork) and a non-albino (winter cultivar Igri) producing cultivar. The ultrastructure of plastids and the relative amount of DNA containing plastids were followed in both cultivars during the androgenic process and correlated with the proportion of regenerated chlorophyllous plantlets. For androgenesis, anthers were collected at the uninucleate stage, during mid- or late-microspore vacuolation. At this stage DNA was detected in 15.3 ± 2. 7% of microspore plastid sections in the winter cultivar Igri, compared to 1.7 ± 0.5% in the spring cultivar Cork. In the winter cultivar Igri, starch was broken down after anther pretreatment but plastids divided rapidly during anther culture and thylakoids developed in the stroma. Prior to regeneration, plastids contained 2.0 ± 0.2 thylakoids per plastid and starch represented 26.1 ± 3.3% of the plastid volume. In the spring cultivar Cork, plastids followed a different developmental pathway. After anther pretreatment, microspore plastids differentiated exclusively into amyloplasts, accumulating starch and losing their thylakoids as well as their capacity to divide. This developmental pattern became progressively more marked, so that by the end of anther culture plastids contained 0.5 ± 0.4 thylakoids per plastid and starch represented up to 90.3 ± 4.3% of plastid volume. Following androgenesis, the response was similar in both cultivars except that the winter cultivar Igri provided 87.8% of chlorophyllous plantlets compared to 99.7% albino plantlets in the cultivar Cork. The results presented here suggest that the exclusive regeneration of albino plantlets in the spring cultivar Cork may be due to degradation of microspore plastid DNA during early pollen development, preventing the plastids from differentiating into chloroplasts under culture conditions. Received: 13 March 2000 / Revision accepted: 6 June 2000     

Seasonal rhythmics of the leucoplast structure in bulb scales of Scilla sibirica L

A study was made of seasonal changes in plastids of ground tissue cells of bulb scales in early-spring ephemeroid Scilla sibirica L. In summer, plastids are represented by typical amyloplasts, with their main volume (97.0 +/- 4.3%) being occupied by one large starch grain. The volume fraction of plastid stroma is at its minimum. The stroma contains small plastoglobuli and no thylakoids. The same structure is characteristic of plastids in October. However, no starch is found in December, when some thylakoids are seen at the plastid periphery. In the early spring (March), when leaves still remain below the ground, the volume fraction of starch grains is 53.0 +/- 2.2%. In the stroma some structures superficially similar to those of microtubuli are revealed. The thylakoid system is fairly well developed, some of thylakoids being concentrically arranged. Some electron-opaque material is seen in the thylakoid lumen. Many plastids are sheathed with elements of the smooth endoplasmic reticulum. Based on the analysis of these and literature data, a conclusion is made that plastids of bulb scales not only store starch, but also seemingly participate in phytohormone biosynthesis.     

Aspects of the biochemistry and ultrastructure of a cytoplasmically inherited plastid defect (Dp1) of tobacco

Dpl, a cytoplasmically inherited plastid defect of Nicotiana tabacum L., has been further characterized by pigment and ribulose diphosphate carboxylase (RuDPCase) assays and electron microscopy. RuDPCase activity was reduced in defective plastids to 20–67% of that in normal chloroplasts. The chlorophyll content was reduced to 5% or less of that in normal chloroplasts. Leaf areas with only defective plastids were very light green for several days after the leaf began to expand but eventually turned white. This loss of chlorophyll was correlated with a reduction in internal plastid lamellae, but there was much less reduction in RuDPCase activity. The presence of cells with both mutant and normal plastids indicate that the plastid and not some other cytoplasmic factor was the site of the controlling unit.Scientific Paper No. 3812, College of Agriculture, Washington State University, Pullman, Projects 1916 and 1920. Supported in part by funds provided for medical and biological research by Washington State Initiative Measure 171.     

Chloroplast development in low light-grown barley seedlings

Segments of 7-d low light-grown barley laminae cut at 0.5 cm intervals up from the intercalary meristem were examined ultrastructurally and biochemically. The different regions upwards showed the succession of plastid development in light-grown tissues of eoplasts, amyloplasts, amoeboid, immature and mature plastids as described by Whatley (1977). Semi-crystalline bodies were detected in all of them. The eoplast-amyloplast regions are characterised by a greater proportion of mitochondria and high levels of ATP and 3-phosphoglyceric acid, together with low levels of inorganic phosphate conducive to the activation of ADP glucose pyrophosphorylase. The amoeboid and immature plastid regions have higher levels of inhibitory phosphate and starch breakdown may be responsible for the release of metabolites and energy for development. Segments containing amoeboid and immature plastids also have reduced levels of ATP (and 3-phosphoglyceric acid) as photosynthetic components are synthesised. Using ultrastructural assessments of areas of thylakoids, first -carotene and violaxanthin, followed by chlorophyll a and lutein and, lastly, chlorophyll b are concentrated in the developing lamellar systems of the immature and mature chloroplasts. The formation of additional membraneous material which spreads these pigment systems over a greater thylakoid area within the plastids is the final stage of plastid morphogenesis in low light-grown seedlings.Abbreviations Chl chlorophyll - 3-PGA 3 phosphoglyceric acid     

Nodule Activity and Allocation of Photosynthate of Soybean during Recovery from Water Stress

Nodulated soybean plants (Glycine max [L.] Merr. cv Ransom) in a growth-chamber study were subjected to a leaf water potential (Ψ_w) of −2.0 megapascal during vegetative growth. Changes in nonstructural carbohydrate contents of leaves, stems, roots, and nodules, allocation of dry matter among plant parts, in situ specific nodule activity, and in situ canopy apparent photosynthetic rate were measured in stressed and nonstressed plants during a 7-day period following rewatering. Leaf and nodule Ψ_w also were determined. At the time of maximum stress, concentration of nonstructural carbohydrates had declined in leaves of stressed, relative to nonstressed, plants, and the concentration of nonstructural carbohydrates had increased in stems, roots, and nodules. Sucrose concentrations in roots and nodules of stressed plants were 1.5 and 3 times greater, respectively, than those of nonstressed plants. Within 12 hours after rewatering, leaf and nodule Ψ_w of stressed plants had returned to values of nonstressed plants. Canopy apparent photosynthesis and specific nodule activity of stressed plants recovered to levels for nonstressed plants within 2 days after rewatering. The elevated sucrose concentrations in roots and nodules of stressed plants also declined rapidly upon rehydration. The increase in sucrose concentration in nodules, as well as the increase of carbohydrates in roots and stems, during water stress and the rapid disappearance upon rewatering indicates that inhibition of carbohydrate utilization within the nodule may be associated with loss of nodule activity. Availability of carbohydrates within the nodules and from photosynthetic activity following rehydration of nodules may mediate the rate of recovery of N₂-fixation activity.     

Dynamics of localization and protein composition of plastid nucleoids in light-grown pea seedlings

Summary Localization and protein composition of plastid nucleoids was analyzed in light-grown pea seedlings at various stages of leaf development. In young plastids of unopened leaf buds, nucleoids were abundant and localized in the periphery of plastids, whereas, in mature leaves, chloroplasts contained nucleoids within narrow spaces restricted by thylakoids or grana. The migration of nucleoids into the interior of plastids preceded the formation of grana, and hence, the maturation of the photosynthetic apparatus. The protein composition of nucleoids was considerably different in young plastids and mature chloroplasts. Polypeptides with a molecular mass of 70–100 kDa predominated in the nucleoids of young plastids, whereas polypeptides with molecular mass of 20–30 kDa were abundant in the nucleoids of mature chloroplasts. Immuno-blot analysis with antibodies against the nucleoids of young plastids identified various polypeptides that were significantly more abundant in the nucleoids of young plastids than in the nucleoids of mature chloroplasts. These results demonstrate that plastid nucleoids are subject to dynamic changes in both localization and composition during the normal development of chloroplasts in the light.Abbreviations DAPI 4,6-diamidino-2-phenylindol - DiOC₆ 3,3-dihexyloxacarbocyanine iodide     

High salt stress induces swollen prothylakoids in dark-grown wheat and alters both prolamellar body transformation and reformation after irradiation

High salinity causes ion imbalance and osmotic stress in plants. Leaf sections from 8-d-old dark-grown wheat (Triticum aestivum cv. Giza 168) were exposed to high salt stress (600 mM) and the native arrangements of plastid pigments together with the ultrastructure of the plastids were studied using low-temperature fluorescence spectroscopy and transmission electron microscopy. Although plastids from salt-treated leaves had highly swollen prothylakoids (PTs) the prolamellar bodies (PLBs) were regular. Accordingly, a slight intensity decrease of the short-wavelength protochlorophyllide (Pchlide) form was observed, but no change was found in the long-wavelength Pchlide form emitting at 656 nm. After irradiation, newly formed swollen thylakoids showed traversing stromal strands. The PLB dispersal was partly inhibited and remnants of the PLBs formed an electron-dense structure, which remained after prolonged (8 h) irradiation. The difference in fluorescence emission maximum of the main chlorophyll form in salt-stressed leaves (681 nm) and in control leaves (683 nm) indicated a restrained formation of the photosynthetic apparatus. Overall chlorophyll accumulation during prolonged irradiation was inhibited. Salt-stressed leaves returned to darkness after 3 h of irradiation had, compared with the control, a reduced amount of Pchlide and reduced re-formation of regular net-like PLBs. Instead, the size of the electron-dense structures increased. This study reports, for the first time, the salt-induced swelling of PTs and reveals traversing stromal strands in newly formed thylakoids. Although the PLBs were intact and the Pchlide fluorescence emission spectra appeared normal after salt stress in darkness, plastid development to chloroplasts was highly restricted during irradiation.     

Plastid differentiation and chlorophyll biosynthesis in different leaf layers of white cabbage (Brassica oleracea cv. capitata)

The contents of protochlorophyllide, protochlorophyll and chlorophyll together with the native arrangements of the pigments and the plastid ultrastructure were studied in different leaf layers of white cabbage (Brassica oleracea cv. capitata) using absorption, 77 K fluorescence spectroscopy and transmission electron microscopy. The developmental stage of the leaves was determined using the differentiation of the stoma complexes as seen by scanning electron microscopy and light microscopy. The pigment content showed a gradual decrease from the outer leaf layer towards the central leaves. The innermost leaves were in a primordial stage in many aspects; they were large but had typical proplastids with few simple inner membranes, and contained protochlorophyllide and its esters in a 2 : 1 ratio and no chlorophyll. Short‐wavelength, not flash‐photoactive protochlorophyllide and/or protochlorophyll forms emitting at 629 and 636 nm were dominant in the innermost leaves. These leaves also had small amounts of the 644 and 654 nm emitting, flash‐photoactive protochlorophyllide forms. Rarely prolamellar bodies were observed in this layer. The outermost leaves had the usual characteristics of fully developed green leaves. The intermediary layers contained chlorophyll a and chlorophyll b besides the protochlorophyll(ide) pigments and had various intermediary developmental stages. Spectroscopically two types of intermediary leaves could be distinguished: one with only a 680 nm emitting chlorophyll a form and a second with bands at 685, 695 and 730 nm, corresponding to chlorophyll–protein complexes of green leaves. In these leaves, a large variety of chloroplasts were found. The data of this work show that etioplasts, etio‐chloroplasts or chloro‐etioplasts as well as etiolated leaves do exist in the nature and not only under laboratory conditions. The specificity of cabbage leaves compared with those of dark‐grown seedlings is the retained primordial or intermediary developmental stage of leaves in the inner layers for very long (even for a few month) period. This opens new developmental routes leading to formation of specially developed plastids in the various cabbage leaf layers. The study of these plastids provided new information for a better understanding of the plastid differentiation and the greening process .     

Assembly of the barley light-harvesting chlorophyll a/b proteins in barley etiochloroplasts involves processing of the precursor on thylakoids

A barley gene encoding the major light-harvesting chlorophyll a/b-binding protein (LHCP) has been sequenced and then expressed in vitro to produce a labelled LHCP precursor (pLHCP). When barley etiochloroplasts are incubated with this pLHCP, both labelled pLHCP and LHCP are found as integral thylakoid membrane proteins, incorporated into the major pigment-protein complex of the thylakoids. The presence of pLHCP in thylakoids and its proportion with respect to labelled LHCP depends on the developmental stage of the plastids used to study the import of pLHCP. The reduced amounts of chlorophyll in a chlorophyll b-less mutant of barley does not affect the proportion of pLHCP to LHCP found in the thylakoids when import of pLHCP into plastids isolated from the mutant plants is examined. Therefore, insufficient chlorophyll during early stages of plastid development does not seem to be responsible for their relative inefficiency in assembling pLHCP. A chase of labelled pLHCP that has been incorporated into the thylakoids of intact plastids, by further incubation of the plastids with unlabelled pLHCP, reveals that the pLHCP incorporated into the thylakoids can be processed to its mature size. Our observations strongly support the hypothesis that after import into plastids, pLHCP is inserted into thylakoids and then processed to its mature size under in vivo conditions.     

Phytotoxicity of selected herbicides to mung bean (Phaseolus aureus Roxb.)

Mung bean (Phaseolus aureus Roxb.) is grown after harvest of wheat during the fallow period. Herbicides such as metsulfuron, atrazine and isoxaflutole are recommended to control weeds in wheat–rice cropping system including weeds of fallow crop. The effects of three herbicides with different modes of action—atrazine, photosystem II inhibitor; metsulfuron, acetolactate synthase inhibitor; and isoxaflutole, 4-hydroxyphenylpyruvatedioxygenase inhibitor—on shoot height, chlorophyll concentrations and cellular damage in herbicide-treated mung bean were studied. While isoxaflutole inhibited shoot growth and chlorophyll concentration of mung bean, atrazine and metsulfuron did not cause reduction in the shoot growth of mung bean. Metsulfuron (226, 452, 1356 and 2260 μg/kg soil) and isoxaflutole (452, 1356 and 2260 μg/kg soil) in soil reduced the concentration of leaf chlorophyll of mung bean compared to the control. Atrazine in soil did not affect the total chlorophyll concentration of mung bean leaves. Electron micrographs showed that untreated mung bean had elongated chloroplasts, thylakoids organized as intact grana, distinct starch grains and a small number of plastoglubuli. Mesophyll cells of atrazine-treated mung bean leaves had swollen chloroplasts and thylakoids with disorganized grana. Leaves of metsulfuron-treated mung bean had swollen chloroplasts with a large number of starch grains. Starch grains were not observed in leaves of mung bean treated with either atrazine or isoxaflutole. Complete disruption of thylakoids was observed in isoxaflutole-treated mung bean leaves. Leaves of atrazine-treated mung bean showed detached microfibrils along with distorted and degenerated secondary walls. Metsulfuron-treated mung bean leaves showed aggregated microfibrils with completely dissolved secondary walls, while isoxaflutole-treated leaves had completely degenerated secondary walls with complete loss of microfibrils. We conclude that isoxaflutole at higher doses, influence mung bean at the morphological, physiological and cellular levels.     

Amylases in Pea Tissues with Reduced Chloroplast Density and/or Function

Pea (Pisum sativum L.) tissues with reduced chloroplast density (e.g. petals and stems) or function (i.e. senescent leaves and leaves darkened for prolonged periods) were surveyed to determine whether tissues with genetically or environmentally reduced chloroplast density and/or function also have significantly different amylolytic enzyme activities and/or isoform patterns than leaf tissues with totally competent chloroplasts. Native PAGE followed by electrophoretically blotting through a starch or β-limit dextrin containing gel and KI/I₂ staining revealed that the primary amylases in leaves, stems, petals, and roots were the primarily vacuolar β-amylase (EC 3.2.1.2) and the primarily apoplastic α-amylase (EC 3.2.1.1). Among tissues of light grown pea plants, petals contained the highest levels of total amylolytic (primarily β-amylase) activity and considerably higher ratios of β- to α-amylase. In aerial tissues there was an inverse relationship between chlorophyll and starch concentration, and β-amylase activity. In sections of petals and stems there was a pronounced inverse relationship between chlorophyll concentration and the activity of α-amylase. Senescing leaves of pea, as determined by age, and protein and chlorophyll content, contained 3.8-fold (fresh weight basis) and 32-fold (protein basis) higher α-amylase activity than fully mature leaves. Leaves maintained in darkness for 12 days displayed a 14-fold (fresh weight basis) increase in α-amylase activity over those grown under continuous light. In senescence and prolonged darkness studies, the α-amylase that was greatly increased in activity was the primarily apoplastic α-amylase. These studies indicate that there is a pronounced inverse relationship between chloroplast function and levels of apoplastic α-amylase activity and in some cases an inverse relationship between chloroplast density and/or function and vacuolar β-amylase activity.     

Evolution de la structure plastidiale et des reserves polysaccharidiques au cours de la déshydratation des gamétophytes feuillés du Polytrichum formosum Hedw. (Polytrichales)

Abstract

The effects of a 24 h dehydration period on plastid ultrastructure and their starch content have been studied on the leafy gametophyte of a poikilohydrous moss Polytrichum formosum Hedw. It has been shown that desiccation induces a swift resorption of the starch grains in the plastids of the meristematic cells of the shoot apex without any major disorganisation of the thylakoids. However starch grains are preserved in the dehydrated tissues of the adult leaves. The starch hydrolysis is continued with a marked increase of the sucrose concentration to this may be attributed an osmoregulatory role in the first stages of the water loss. Upon rehydration the plastid ultrastructure is entirely restored with new starch inclusions appearing in less than 4 h.     

Control of chlorophyll production in rapidly greening bean leaves

The possible involvement of nucleic acid and protein synthesis in light-regulated chlorophyll formation by rapidly greening leaves has been studied.

Removing leaves from illumination during the phase of rapid greening results in a reduction in the rate of pigment synthesis; cessation occurs within 2 to 4 hours. Etiolated leaves which exhibit a lag in pigment synthesis when first placed in the light do not show another lag after a 4 hour interruption of illumination during the phase of rapid greening.

Actinomycin D, chloramphenicol, and puromycin inhibit chlorophyll synthesis when applied before or during the phase of rapid greening. Application of δ-amino-levulinic acid partially relieves the inhibition by chloramphenicol.

It is suggested that light regulates chlorophyll synthesis by controlling the availability of δ-aminolevulinic acid, possibly by mediating the formation of an enzyme of δ-aminolevulinate synthesis. This process may result from gene activation or derepression; the involvement of RNA synthesis of some sort is suggested by the inhibitory effect of actinomycin D on chlorophyll production by rapidly greening leaves.