Changes in transorgan electric potential inChenopodium rubrum during the course of photoperiodic flower induction

Electrophysiological processes were investigated in reception organs of photoperiodism in a model short-day plant,Chenopodium rubrum L. (selection 374), within the inductive cycle for flowering. Transorgan (surface) electric potential (E_tr) was measured as a potential difference between the first leaf surface and the roots of an intact plant, and between the surface of an excised leaf and the petiole base. The time-course of E_tr in intact plants showed irregular, or partially regular, oscillations within both phases of the inductive cycle and under continuous light. The highest amplitudes were during the postinductive light period. E_tr in excised leaves behaved practically in the same way as in intact plants. The E_tr oscillations were localized in leaves. In general, no electrophysiological changes were found in the reception organs within the inductive cycle which could be correlated with the formation and transport of floral stimulus, or with the attainment of an induced state. The results indirectly support the idea that the floral stimulus is chemical in nature.     

Effect of varying lengths of inductive and supplementary non-inductive photoperiods on vegetative growth and flowering of Impatiens balsamina

The photoperiodic requirement for flowering in Impatiens balsaminachanges with the length of the photoperiod. Floral buds wereinitiated with two 8 hr but with four 15 hr photoperiods andflowers opened with four 8 hr but twenty-eight 15 hr photoperiods.A part of the photoperiodic requirement for floral inductionin this plant can be substituted by LDs containing 4 or morehours of darkness (10). It indicates the identical nature ofthe floral stimulus produced during the dark period, whetherit forms a part of the inductive or non-inductive cycles. Theeffect of these supplementary non-inductive photoperiodic cyclesin causing floral bud initiation also depends on the lengthof the first inductive obligatory cycle. More floral buds andflowers were produced on plants exposed to 15 hr than 8 hr photoperiods,probably due to the higher number of leaves that were producedunder the former condition of weaker induction. The shorterthe dark period in the photoperiodic cycle, the weaker the induction,the slower the rate of extension growth but the more differentiationof leaves. ¹ Present address: Department of Biology, Guru Nanak Dev University,Amritsar-143005, India. (Received November 9, 1977; )     

The association of carbohydrate changes in the shoot tip of cauliflower with flowering

Changes in levels of sugars and starch in the shoot tip of cauliflower, Brassica oleracea L. var. botrytis D. C. cv. Main Crop were studied during periods of growth which were inductive or non-inductive to flowering. Flowering was induced by growing plants for 2 weeks under 16 hr of light at 5°. During this period of floral induction there was a significant increase in sugar and starch content compared to that in vegetative plants grown at 20 to 26°. Sugar and starch content did not increase and flowering was prevented when light and CO₂ were excluded during growth at 5°. A 3-day dark period at 20° or a high temperature treatment at 33° with light following growth at 5° reduced the carbohydrate level and prevented flowering.     

The comparison between membrane and transorgan electric potentials inChenopodium rubrum: The methods

A new method of simultaneous measurement of membrane (E_m) and transorgan (E_tr)electric potential in intactChenopodium rubrum plants and changes in E_m and E_tr under various experimental conditions are described. E_m was measured in mesophyll leaf cells, and E_tr in the same plant as a potential difference between a first pair leaf, tip and roots. The two potentials differed distinctly, E_m averaging-156mV and E_tr -5mV. But the,-changes in E_m and E_tr had approximately the same magnitude and time-course after changing light and darkness or KCl concentration in solution flowing around the leaf or during the photoperiodic induetive cycle. In relation to the same electrode connection they had an opposite polarity. The nature of E_tr and its relation to E_m are discussed. The measurement of E_tr is much more stable and simpler than that of E_m. The similarity of E_m and E_tr time-courses justifies the use of E_tr measurement in electrophysiological studies as a replacement for E_m measurements especially in intact plants.     

The effect of red light on the level of free IAA

Red light (660 nm) break which cancels the floral inductive effect of the dark period brought about a transitional rise in the level of free IAA in the shoot ofChenopodium rubrum. The higher content of IAA was then found in treated plants at the beginning of the following photoperiod, too. The red light treatment did not change the phase of endogenous rhythm fluctuations of free IAA.     

Cellular and morphological changes at the terminal shoot apex of the short-day plant Pharbitis nil during the transition to flowering

Changes in morphology and measurements of cell doubling time were recorded for the first time in the terminal shoot apex of the short-day plant, Pharbitis nil Chois. ( Ipomea nil L.) cv. Violet, undergoing the floral transition. A treatment comprising 48 h darkness given to 4-day-old plants resulted in 100% flowering at the shoot terminal meristem. An inhibitory treatment comprising two 5 min red night-breaks during the 48 h dark period was used to discriminate between events essential for flowering, and those changes resulting from shifts from light to darkness and vice versa. Morphology was studied using both light microscopy and scanning electron microscopy. Cell doubling times were measured using the colchicine accumulation of metaphases method. An increase in the rate of primordial initiation, a change in the divergence angle and a change in phyllotaxis occurred during the floral transition. Moreover, the apex widened and flattened following the inductive dark treatment; the cell doubling time decreased in the peripheral zone and increased in the central zone of these pre-floral meristems.     

Involvement of indole-3-acetic acid in the circadian growth of the first internode of Arabidopsis

The extension rate of the first inflorescence node of Arabidopsis was measured during light/dark or continuous light exposure and was found to exhibit oscillations which showed a circadian rhythmicity. Decapitation induced a strong inhibition of stem extension. Subsequent application of IAA restored growth and the associated extension–rate oscillations. In addition, IAA treatments, after decapitation, re-established the circadian rhythmicity visible in the intact plants during free run. This indicates that the upper zone of the inflorescence has a major influence on the extension rate of floral stems and implies a role for auxin. Application of N-(1-naphthyl)phthalamic acid, an IAA transport inhibitor, to an intact floral stem inhibited growth and the rhythmicity in the extension rate oscillations, indicating that IAA polar transport may play a role in the dynamics of stem elongation. Furthermore, IAA-aspartate application, after decapitation, did not restore growth and rhythmicity. Nevertheless, biochemical analysis of IAA and IAA-aspartate demonstrated circadian fluctuations of the endogenous levels of both compounds. These observations suggest that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of Arabidopsis floral stems. Received: 21 September 1998 / Accepted: 23 January 1999     

Flowering response of Lemna perpusilla 6746 to a single dark period

Lemna perpusilla 6746 is induced to flower by a single longdark period, but the floral buds once formed disappear afterseveral days under 5000 lux/25?C. Such regression of floralbuds is prevented by lowering the light intensity or temperature,but if the light intensity and/or temperature are lowered beyondcritical levels, new floral buds form. If the cultures are subjectedto 100 lux/20?C, neither regression nor new formation of floralbuds occurs. Under such conditions, the number of floral frondsreaches maximum about 6 days after the inductive dark periodand reamins unchanged for at least 10 days, while the percentageof floral fronds rapidly decreases thereafter, owing to thedilution by newly developed vegetative fronds. When the cultures are subjected to various lengths of a singledark period (25?C) followed by 100 lux/20?C, flowering responsesrepresented by the number of floral fronds per flask show rhythmicfluctuation with a cycle length of about 24 hr. Similar rhythmicresponse is observed when a brief light interruption is givenat different times during a single long dark period. (Received December 2, 1974; )     

Stimulation of the Flowering of Pharbitis nil Chois. by Gibberellin A3 : Time Dependent Action at the Apex

Gibberellin A₃ (GA₃) stimulated flowering when it was appliedto the shoot apex of seedlings of Pharbitis nil, dwarf strainKidachi; but, not when it was applied to the cotyledons. GA₃applied to the plumule before or shortly after the start ofan inductive dark period promoted both flowering and shoot elongation;but, the later the time of application during the dark periodless the promotion of flowering, although marked promotion ofshoot elongation always took place. The variation with time in the response of flowering to GA₃indicates that early floral processes at the apex are stimulatedby GA₃, but that subsequent processes are insensitive to it.The early processes of floral stimulus produced by a 16 hr inductivedark period probably are completed within 20 hr at 28°Cafter the end of the dark period. At low temperatures, suchas 15 and 20°C, early floral processes continued for morethan 40 hr. When cotyledons were removed at various times, the export ofthe floral stimulus to the shoot apex was apparent within hoursof the generation of the floral stimulus in the cotyledons,which started with the passage of the critical 9-hr dark period. (Received February 18, 1981; Accepted March 24, 1981)     

Properties of phosphoenolpyruvate carboxylase in rapidly prepared,desalted leaf extracts of the Crassulacean acid metabolism plant Mesembryanthemum crystallinum L.

Properties of phosphoenolpyruvate (PEP) carboxylase, obtained from leaves of Mesembryanthemum crystallinum L. performing Crassulacean acid metabolism (CAM), were determined at frequent time points during a 12-h light/12-h dark cycle. Leaf extracts were rapidly desalted and PEP carboxylase activity as a function of PEP concentration, malate concentration, and pH was measured within 2 min after homogenization of the tissue. Maximum velocity of PEP carboxylase was similar in the light and dark at pH 7.5 and pH 8.0. However, PEP carboxylase had as much as a 12-fold lower K _m for PEP and as much as a 20-fold higher K _i for malate during the dark than during the light periods, the magnitude of these differences being dependent on the assay pH. Assuming that enzyme properties immediately after isolation reflect the approximate state of the enzyme in vivo, these differences in enzyme properties reduce the potential for CO₂ fixation via PEP carboxylase in the light. A small decrease in cytoplasmic pH in the light would greatly magnify the above differences in day/night properties of PEP carboxylase, because the sensitivity of PEP carboxylase to inhibition by malate increased with decreasing pH. Properties of PEP carboxylase were also studied in plants exposed to short-term perturbations of the normal 12-h light/12-h dark cycle (e.g., prolonged light period, prolonged dark period). Under all light/dark regimes, there was a close correlation between change in properties of PEP carboxylase and changes of the tissue from acidification to deacidification, and vice versa. Changes in properties of PEP carboxylase were not merely light/dark phenomena because they were also observed in plants exposed to continuous light or dark. the data indicate that, during CAM, PEP carboxylase exists in two stages which differ in their capacity for net malate synthesis. The physiologically-active state is distinguished by a low K _m for PEP and a high K _i for malate and favors malate synthesis. The physiologically-inactive state has a high K _m for PEP and a low K _i for malate and exists during periods of deacidification and other periods lacking synthesis of malic acid.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPC PEP carboxylase - RuBP ribulose 1,5-bisphosphate - RH relative humidity     

Promotory effect of GA13 on flowering ofAmaranthus — a short day plant

Abstact The three plant types ofAmaranthus namely,A. caudatus f.albiflorus, A. caudatus f.caudatus andA. tricolor var.tristis are qualitative short day plants with critical photoperiods 16.0, 15.5 and 15.0 h, respectively. Gibberellins A₃, A₄₊₇ and A₁₃ affect extension growth, leaf differentiation and floral induction differently. Thus, in all the three plant types ofAmaranthus, whereas, GA₃ and G₄₊₇ enhanced extension growth, GA₁₃ was completely ineffective under both, 24- and 8-h photoperiods. None of the three gibberellins could affect the leaf differentiation. In all the three plant types, flowering was promoted by GA₁₃ and not by other gibberellins tried. GA₁₃ caused promotion was manifested in two manners, firstly by lowering the critical dark period requirement in each inductive cycle, and secondly by shortening the total period taken for the initiation of inflorescence primordia under inductive photoperiods. The floral induction by gibberellins inAmaranthus is contrary to the gibberellin-anthesin concept of Chailakhyan. It is suggested that gibberellins other than GA₃ may be playing an important role in floral morphogenesis of short day plants.     

Changes in the transpiration rate of barley plants infected with powdery mildew in the light and in the dark

Changes in the transpiration rate of intact spring barley plants, cv. “Slovensky dunajsky trh”, were studied separately in the light and in the dark under controlled temperature and illumination, after the infection withErysiphe graminis DC, during an 8 day period of the development of the fungus. In the first stage of pathogenesis, the fungus diminishes water output from the host plants in the light. An opposite phenomenon can be observed in the dark; water output from infected plants in the dark increases sharply mainly in the stage of advanced fructification. Thus, the fungus considerably diminishes the ratio of water output from the host plants in the light to that in the dark.     

Effect of cyanide in dark and light on the membrane potential and the ATP level of young and mature green tissues of higher plants

The effect of CN⁻ and N₂ on the electrical membrane potential (E_m) was compared with that of CN⁻ on the ATP levels in cotyledons of Gossypium hirsutum and in Lemna gibba L. In mature cotton tissue, CN⁻ depolarized E_m to the energy-independent diffusion potential (E_D) in the dark. In the light E_m recovered transiently. The same was observed in leaves of Nicotiana, Avena, Impatiens, Kalanchoë, and in Lemna. In contrast, in young cotton cotyledons and tobacco leaves and, to a large extent, in +sucrose-grown Lemna, E_m was depolarized to E_D also in the light in a similar way as in the dark.

In Lemna grown without sucrose, the energy-dependent component of E_m was only partially depolarized by CN⁻ in dark or light. Cyanide plus salicylhydroxamic acid completely reduced E_m to E_D, abolished respiration and photosynthesis, and severely diminished the ATP level. This suggests the operation of a CN⁻-insensitive respiration in uninjured Lemna. The initial CN⁻-induced decay of the ATP level in cotton and Lemna was more rapid than the decay of E_m. CN⁻-induced oscillations of the ATP level were followed by similar but slower oscillations of E_m. This supports the view of a general dependence of E_m on ATP. Discrepancies between inhibitor-induced changes of E_m and ATP levels are suggested to result from additional regulation of E_m by the cytoplasmatic pH value.

A comparison of E_D in young and mature cotton cotyledons in the dark and in the light suggests that in growing young cotyledons the different effect of CN⁻ in the light is due to a less effective photosynthesis together with high mitochondrial respiration. In Lemna and in mature cotton tissue, E_m in the light is maintained by noncyclic photophosphorylation and photosystem II, which is only partly inhibited by CN⁻, thus resulting in an incomplete depolarization and recovery of E_m. Complete inhibition of photosynthetic O₂ evolution and membrane depolarization by CN⁻ plus salicylhydroxamic acid are suggested to result from photooxidation.

     

Changes in the transpiration rate of barley plants cv. “Profesor Schiemann” infected with powdery mildew in light and in darkness

The transpiration rate of intact spring barley plants, cv. Profesor Schiemann resistant to powdery mildew was studied separately in light and in darkness. At the first stage of pathogenesis the fungus temporarily reduced the transpiration rate of the host plants in light (E ₁). The transpiration rate in darkness (E _d) was not influenced in this period and only in the period of formation of necrotic spotsE _d in the host plants was somewhat higher than that in the control plants.E ₁ of the host plants gradually reached the level of the control plants in this period. The fungus considerably reduced the rationE ₁/E _d. The differences in this ratio between infected and control plants increased in the susceptible cultivar but were moderated in the course of necrobiosis.     

Changes of the chlorophyll fluorescence induction kinetics of C3 and CAM plants during day/night cycles

The induction kinetics of the 680 nm chlorophyll fluorescence were measured on attached leaves of Kalanchoë daigremontiana R. Hamet et Perr. (CAM plant), Sedum telephium L. and Sedum spectabile Bor. (C₃ plant in spring, CAM plant in summer) and Raphanus sativus L. (C₃ plant) at three different times during a 12/12h day/night cycle. During the fluorescence transient the fluorescence intensity at the O, P and T-level (f_O, f_max, f_st,) was different for the plant species tested; this may be due to their different leaf structure, pigment composition and organization of their photosystems. The kinetics of the fluorescence induction depended on the time of preillumination or dark adaptation during the light/dark cycle but not on the type of primary CO₂ fixation mechanism (C₃ and CAM). For dark adapted leaves measured either at the end of the dark phase or after dark adaptation of plants taken from the light phase a higher P-level fluorescence, a higher variable fluorescence (P-O) and a larger complementary area were found than for leaves of plants taken directly from the light phase. This indicates the presence of largely oxidized photosystem 2 acceptor pools during darkness. During the light phase the fluorescence decline after the P-level was faster than during the dark phase; from this we conclude that the light adaptation of the photosynthetic apparatus (state 1→ state 2 transition, Δ pH) during the induction period proceeded faster in plants taken from the light phase than in plants taken from the dark phase.     

Promotion and Inhibition of Inflorescence Growth by Long Days in Short-day Plants

Results of previous investigators have indicated that long periodsof light intercalated between inductive short-day cycles havean inhibitory effect on inflorescence growth in short-day plants.The present experiments show that such light periods can eitherpromote or inhibit inflorescence growth in Xanthium pemtsylvanicumand Chenopodium amaranticolor depending on their previous degreeof induction. Intercalated light exerts an inhibitory influence on the inductiveprocesses occurring during the dark period which follows itwhen unifoliate Xanthium plants have been previously exposedto not more than one short day and when fully foliated Chenopodiumplants have been previously exposed to not more than one ortwo short days. When plants are more strongly induced initially,an intercalated light period has a very marked promoting effecton the dark period succeeding it. In Xanthium this stimulatoryeffect increases with the duration of the light period up toan optimum of approximately 80 hours. It is suggested on the basis of available evidence that thepromotive effect of such intercalated light possibly affectsthe sensitivity of the apex to inductive stimuli and that itsinhibitory effect acts on the inductive processes occurringin the leaves.     

Fluctuations of Uridine Incorporation in the Shoot Apex ofChenopodium rubrum L. during Photoperiodic Induction

Uridine incorporation into the shoot apex of the short-day plantChenopodium rubrum was investigated during a 16 h period of darkness and the following transfer to light. Uridine incorporation during this single inductive cycle was compared to incorporation under non-inductive conditions of continuous light. After transfer of the plants from light to darkness RNA synthesis was reduced to about half after the first two hours. This occurred not only when the plants were precultivated in continuous light but also after an interruption of the dark period by light for 31/2 h. The low level of uridine incorporation was maintained for the whole duration of the dark period. Incorporation regained its initial level after exposure of the plants to light irrespective of the duration of the preceding dark period. After this immediate rise of uridine incorporation in plants transferred from darkness to light a slight temporary decrease was observed in light. In darkness the decrease of incorporation into the nucleoli was still more marked than the reduction of overall incorporation. After the termination of the dark period incorporation into the nucleolus rose slowly and extranucleolar incorporation was relatively enhanced during the first 10 h of light in induced plants. The fluctuations of RNA synthesis observed in the shoot apex during photoperiodic treatment may be regarded as a necessary condition for the transition from the vegetative to the reproductive state.     

Superoxide dismutase activity in C<Subscript>3</Subscript> and C<Subscript>3</Subscript>/CAM intermediate species of <Emphasis Type="Italic">Clusia</Emphasis>

The C₃-CAM intermediate Clusia minor L. and the C₃ obligate Clusia multiflora H.B.K. plants were exposed for 7 d to a combination of drought stress and high irradiance of about 1200 μmol m⁻² s⁻¹ for 12 h per day. In both species under these conditions a strong decrease in stomatal conductance was observed at dawn and dusk. Changes in stomatal behaviour of C. minor were accompanied by only a low nocturnal accumulation of malate and citrate. Thus, in C. minor drought stress applied in combination with high irradiance limited CAM expression, and possibly this is the main reason why C. minor prefers semi-shaded sites in the field. The mitochondrial MnSOD, in both well watered and stressed plants of two species showed strong diurnal oscillations with maximum activity at dusk. These oscillations can be explained by the engagement of mitochondria in dissipation of an excess of reducing equivalents. In plants which are able to carry out CAM metabolism tricarboxylic acid cycle is expected to be down regulated in the dark period to prevent breakdown of the entire malate and citrate.     

Spectral Dependence of Night-break Effect on Photoperiodic Floral Induction in Lemna paucicostata 441

A single dark period of longer than 8 hr induced flowering inLemna paucicostata 441 cultured in E medium. Monochromatic lightsof 450, 550, 650 and 750 nm with a half-power bandwidth of 9nm given for 10 min at the 8th hour of a 14-hr dark period inhibitedflowering. The fluence rates required for 50% inhibition were10, 0.5, 0.1 and 3 µmol m^–2. sec^–1, respectively.When applied between the 4th and the 10th hour of the dark period,lights of 450, 550 and 650 nm were inhibitory showing a maximumeffect at the 8th hour. But 750-nm light completely inhibitedflowering when applied at any time during the first 8 hr ofthe dark period. The inhibitory effect of 750-nm light givenat the beginning of the dark period was totally reversed bya subsequent exposure to 650-nm light, and the fluence-responsecurves for the effect of 750-nm light given at the 0, 4th and8th hour were essentially the same. This suggests that the presenceof P_FR is crucial for the floral initiation throughout the first8 hr of the inductive dark period. The role of phytochrome inthe photoperiodic flower induction of L. paucicostata is discussed. (Received January 4, 1982; Accepted March 19, 1982)     

Phytochrome-mediated effects of near-ultraviolet radiation in the induction of flowering in etiolated Lemna paucicostata T-101, a short-day plant

Induction of flowering of etiolated Lemna paucicostata Hegelm. T-101, a short-day plant, was inhibited by far-red (FR) or blue light (BL) applied at the beginning of a 72-h inductive dark period which was followed by two short days. In either case the inhibition was reversed by a subsequent exposure of the plants to near-ultraviolet radiation (NUV), with a peak of effectiveness near 380 nm. Inhibition by BL or FR and its reversion by NUV are repeatable, i.e., NUV is acting in these photoresponses like red light although with much lower effectiveness. Thus, it is considered that NUV acts through phytochrome and no specific BL and NUV photoreceptor is involved in photocontrol of floral induction on this plant.Abbreviations BL blue light - FR far-red light - NUV near ultraviolet radiation - P red-absorbing form of phytochrome - P_fr far-red absorbing form of phytochrome - R red light