Photophysiology of turion germination in Spirodela polyrhiza (L.) Schleiden. X. Role of nitrate in the phytochrome-mediated response

It was shown previously that light-dependent germination of turions of Spirodela polyrhiza (Lemnaceae) is mediated by the photoreceptor phytochrome [Appenroth & Augsten (1990) Photochemistry and Photobiology 52, 61–65]. In the present study, we found that this photoresponse depends on nitrate in the surrounding medium both during after-ripening (under natural conditions occurring in winter) and during germination after light-induction (in spring). The action of nitrate in the germination response is neither related to the induction of nitrate reductase nor to the rate of uptake of ¹⁵NO₃^?. Moreover, two-factor analysis (phytochrome, nitrate) revealed a multiplicative coaction, i.e. independent action of both factors in mediation of germination. The notion that nitrate is a nutritional prerequisite in phytochrome-mediated germination of turions, is supported by the following facts: (1) Nitrate-requirement during germination was strongly increased by nitrate starvation during after-ripening prior to germination. (2) Ammonium could substitute for nitrate. (3) Nitrate uptake by the turions was unaffected by phytochrome and very pronounced even at low concentrations (0.07 mol m^?3) in the medium. With regard to the phytochrome-induced chain of events, it is concluded that nitrate is a prerequisite during a specific developmental phase. Nitrate is not a regulatory element within the chain. In an ecological sense, however, nitrate contents of the aquatic system regulate the germination of turions.     

Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. II. Ultrastructure of the turion; a stereological analysis

Abstract The ultrastructural features of the abscisic-acid-induced turion of Spirodela polyrrhiza are briefly described and a comparison between turion and vegetative frond tissue was made by stereological analysis. The turion is characterized by its small size, reniform shape, and dark-brown coloration; the mesophyll is undifferentiated and totally lacking the substantial acrenchyma development found in the vegetative frond. The turion cells have a smaller vacuole and a denser cytoplasm than the cells of the vegetative frond. Stereological analysis showed that the tissues differed quantitatively only in three main respects: air space formation, vacuolation, and starch and cell wall material accumulation. During development, it is suggested that the cells of the turion, while reaching the same final size as the vegetative frond cells, accumulate numerous starch grains, thick cell walls, and large deposits of tannins and anthocyanin pigment at the expense of the vacuolar expansion characteristic of the normal maturity programme. Certain features of the turion ultrastructure indicate a differential cell sensitivity to ABA.     

Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. I. Production and development of the turion

Abstract The production, growth, and development of the abscisic-acid-induced turion (a small dormant bud) of Spirodela polyrrhiza were investigated. Addition of ABA to a culture of S. polyrrhiza resulted in growth inhibition at concentrations as low as 10⁻⁶molm⁻³, growth being completely arrested at 10⁻² mol m. Over a single order of magnitude range around I0⁻⁴molm⁻³, ABA also induced the production of turions. The range of turion-producing concentrations of ABA was found to be much narrower than previously reported, turion production having a clearly defined threshold, optimum, and upper limit. The possibility that growth inhibition and turion formation are integrally linked aspects of a single response is discussed. Only primordia ≤0.7 mm long at the time of ABA addition could be induced to develop into turions and the events leading to turion formation were found to be reversible up to 72 h in ABA . It is concluded that in terms of turion formation there is a sensitivity window to abscisic acid lasting some 4–20h in the normal developmental life of frond cells. Providing cells experience the appropriate signal in this sensitivity window they initiate a new programme which eventually leads to turion formation. Microscopical analysis showed that the cells within this sensitivity window were still actively dividing. It is suggested that the developmental switch-over to rapid cell expansion and separation marks the end of this ABA sensitivity window.     

Low‐molecular weight carbohydrates modulate dormancy and are required for post‐germination growth in turions of Spirodela polyrhiza

The aquatic duckweed Spirodela polyrhiza propagates itself vegetatively by forming turions – bud‐like perennation organs – in the autumn, which spend the winter on the bottom of ponds and then germinate in the following spring and proliferate on the water surface. Newly formed turions usually require a period of cold after‐ripening and light to germinate effectively, but an ample supply of exogenous sugar can lead to germination even in the dark and independent of after‐ripening. The results of the present study indicate that the availability of readily metabolised carbohydrates is a determining factor for turion germination. Freshly harvested turions do not contain soluble, low‐molecular weight carbohydrates at a level sufficient to allow germination to take place, but after‐ripened turions do. Augmentation of the soluble carbohydrate content during after‐ripening derives from gradual breakdown of reserve starch of the turions. The long time required for any germination to be observed in turions incubated in darkness and the limited frequency of germination in the dark (about 50% of turion population), even with an ample external sugar, supply emphasise that both after‐ripening and light are essential for ensuring rapid germination and subsequent frond proliferation at an ecologically appropriate time. The carbohydrate supply required for rapid proliferation of the fronds produced at germination is provided by the rapid light‐induced breakdown of turion reserve starch.     

No photoperiodoc control of the formation of turions in eight clones of Spirodela polyrhiza

The influence of daily photoperiod (8, 16, 24 h) on eight clones of Spirodela polyrhiza was tested in two different nutrient media. The number of vegetative fronds and resting turions formed after 50 days of cultivation were scored. The specific turion yield (STY; number of turions formed per vegetative frond) was used to evaluate the effectiveness of turion formation of the tested clones. All clones formed turions in both nutrient media. The STY varied substantially between the different clones, ranging from 0.22 +/- 0.03 (clone SC from Cuba) to 3.9 +/- 0.3 (clone 9256 from Finland) in continuous light. The STY increased with increasing duration of the photoperiod. This increase may have been due to the extended period of photosynthesis rather than that of a photoperiodic long-day response. Shorter photoperiods did not stimulate turion formation in any of the clones. S. polyrhiza is a day-neutral plant with respect to turion formation, as noted previously (Appenroth et al. 1990. Annals of Botany 66: 163-168). In accordance with this conclusion, no correlation was detected between the STY and the latitude at which the clones occur naturally. Environmental factors other than shortening of photoperiods seem to be effective in signalling seasonal changes of growth conditions in advance to S. polyrhiza.     

Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. IV. Comparative ion flux characteristics of the turion and the vegetative frond and the effect of ABA during early turion development

Abstract Using the method of compartmental analysis, the ion fluxes and compartment concentrations of Ca²⁺, K⁺ and Cl^- have been compared in the untreated vegetative frond and the abscisic acid (ABA) induced turion of Spirodela polyrrhiza. The ABA-induced turion is characterized by reduced Ca²⁺ exchange across the tonoplast and low vacuolar Ca²⁺ concentration relative to the vegetative frond. In addition the turion exhibits a higher plasmalemma flux with a correspondingly high Ca²⁺ concentration in the cytoplasm. The concentration of K⁺ and Cl^- is much lower in the cytoplasm of the ABA-induced turion than in the vegetative frond with the influx/efflux ratio at both the plasmalemma and the tonoplast being less than 1, a finding exhibited also in dormant storage tissue. Treatment of vegetative fronds with ABA for 18 h resulted in a reduced K⁺ plasmalemma efflux relative to untreated vegetative fronds and a concomitant increase in the cytoplasmic concentration. There was no rapid effect of ABA on Ca²⁺, K⁺ or Cl^- fluxes through either membrane. These results are consistent with the notion that drastic changes in ion fluxes and concentrations in the turion are a secondary consequence of ABA-induced development, possibly due to prior regulation by ABA of enzymes inherent to processes involved in membrane transport.     

Differences in demographic parameters among electrophoretic clones of Daphnia obtusa Kurz (Crustacea: Cladocera)

In an 18-month electrophoretic study, a Daphnia obtusa population that recently colonized Lake Orta (a large, severely polluted lake in Northern Italy) showed no genetic variation.Populations from other localities were sampled. From clones established by single females, genotype differences were determined by two life table experiments (using Lake Orta and Lake Maggiore water as a culture medium).As expected, in the Lake Orta water the colonizing clone (LO) had longer survival, produced more newborns and had a larger rate of increase than the other clones. In the Lake Maggiore water the LO clone still fared better than the other clones.     

Clonal structure and patterns of allozyme diversity in the rare endemic Cycladenia humilis var. jonesii (Apocynaceae)

The rare endemic Cycladenia humilis var. jonesii (Jones cycladenia) has low levels of sexual reproduction. Enzyme electrophoresis was used to explore possible causes of low seed set and high fruit abortion by assessing the clonal structure and genetic diversity in populations. The seven populations studied were composed of multiple, highly interdigitated clones; thus low fruit set is not likely to be due to a scarcity of mates. Genotype frequencies did not differ significantly from Hardy-Weinberg proportions, suggesting that populations are not highly inbred. Jones cycladenia exhibited high levels of genetic diversity at both the population level (A = 1.7; P = 37; He = 0.14) and the taxon level (A = 2.7; P = 60) in comparison to other plants. These data suggest that genetic drift is unlikely to have left this taxon genetically depauperate. Furthermore, we detected little divergence among geographically disjunct populations of Jones cycladenia (FST = 0.10). In comparison, Jones cycladenia populations were highly differentiated from a population of the taxon's close relative, C. h. var. humilis (mean genetic identity = 0.76). Our study suggests that other reasons for low fruit set in Jones cycladenia, such as resource or pollinator limitation, or genetic load, should be explored in future research.     

Independent control of organogenesis and shoot tip abortion are key factors to developmental plasticity in kiwifruit (Actinidia)

BACKGROUND AND AIMS: In kiwifruit (Actinidia), the number of nodes per shoot is highly variable and is influenced by genotype and environmental conditions. To understand this developmental plasticity, three key processes were studied: organogenesis by the shoot apical meristem during shoot growth; expansion of phytomers; and shoot tip abortion. METHODS: Studies were made of organogenesis and shoot tip abortion using light and scanning electron microscopy. The effect of temperature on shoot growth cessation was investigated using temperature indices over the budbreak period, and patterns of shoot tip abortion were quantified using stochastic modelling. KEY RESULTS: All growing buds began organogenesis before budbreak. During shoot development, the number of phytomers initiated by the shoot apical meristem is correlated with the number of expanding phytomers and the mean internode length. Shoot tip abortion is preceded by growth cessation and is not brought about by the death of the shoot apical meristem, but occurs by tissue necrosis in the sub-apical zone. For most genotypes studied, the probability of shoot tip abortion is higher during expansion of the preformed part of the shoot. Lower temperatures during early growth result in a higher probability of shoot tip abortion. CONCLUSIONS: Organogenesis and shoot tip abortion are controlled independently. All buds have the potential to become long shoots. Conditions that increase early growth rate postpone shoot tip abortion.     

Effect of kinetin and abscisic acid on the distribution of 2-amino (1-14C) isobutyric acid in leaves of Zea mays

2-Amino (1-¹⁴C) isobutyric acid (AIB) was applied to the leaf tips of thirteen-day-old primary leaves of Zea mays L. cv. Goldprinz. Simultaneously, the leaves were treated in the basal region with solutions of kinetin (KN), abscisic acid (ABA) and mixtures of both hormones. The distribution of the radioactive material was determined after 72 h. Treatment with KN caused an accumulation of radioactive material at the point of application (KN spot). In comparison to controls, the leaf zones between the KN spot and the leaf tip contained lower amounts of radioactive material. Treatment with ABA caused a pattern of distribution opposite to that induced by KN. The ABA point of application (ABA spot) was depleted of radioactive material, whereas the leaf area between the ABA spot and the leaf tip showed enhanched levels of radioactive AIB. The effect of ABA was reduced by application of ABA and KN in equimolar amounts. The results are discussed in relation to the possible role of ABA in hormone-directed transport processes.     

Clonal and fine-scale genetic structure in populations of a restricted Korean endemic, Hosta jonesii (Liliaceae) and the implications for conservation

BACKGROUND AND AIMS: In plant populations the magnitude of spatial genetic structure of apparent individuals (including clonal ramets) can be different from that of sexual individuals (genets). Thus, distinguishing the effects of clonal versus sexual individuals in population genetic analyses could provide important insights for evolutionary biology and conservation. To investigate the effects of clonal spread on the fine-scale spatial genetic structure within plant populations, Hosta jonesii (Liliaceae), an endemic species to Korea, was chosen as a study species. METHODS: Using allozymes as genetic markers, spatial autocorrelation analysis of ramets and of genets was conducted to quantify the spatial scale of clonal spread and genotype distribution in two populations of H. jonesii. KEY RESULTS: Join-count statistics revealed that most clones are significantly aggregated at < 3-m interplant distance. Spatial autocorrelation analysis of all individuals resulted in significantly higher Moran's I values at 0-3-m interplant distance than analyses of population samples in which clones were excluded. However, significant fine-scale genetic structure was still observed when clones were excluded. CONCLUSIONS: These results suggest that clones enhance the magnitude of spatial autocorrelation due to localized clonal spread. The significant fine-scale genetic structure detected in samples excluding clones is consistent with the biological and ecological traits exhibited by H. jonesii including bee pollination and limited seed dispersal. For conservation purposes, genetic diversity would be maximized in local populations of H. jonesii by collecting or preserving individuals that are spaced at least 5 m apart.     

The expression of the hydroxyproline-rich glycopeptide systemin precursor A in response to (a)biotic stress and elicitors is indicative of its role in the regulation of the wound response in tobacco (<Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.)

Two hydroxyproline-rich glycopeptide systemin (TobHS) precursor proteins known as preproTobHypSys-A and B were recently discovered in tobacco (Nicotiana tabacum L.) [Pearce et al. in Nature 411:817–820, 2001]. In this work, the effect of elicitors, insect damage, and abiotic stress on the expression of preproTobHypSys-A ppTobHS-A) in tobacco plants was evaluated. Foliar application of methyl jasmonate preferentially induced the systemic expression of ppTobHS-A in leaves phyllotactically one position above-treated leaves. Abscisic acid strongly induced ppTobHS-A, but water-stress did not. Mechanical wound-induction of ppTobHS-A in young plantlets was rapidly (1 h) and simultaneously detected in wounded and upper unwounded leaves, whereas in older plants induction was slow (12 h) and localized. ppTobHS-A was induced in plants infested with Bemisia tabaci or damaged by herbivory with Manduca sexta larvae. Compared to mechanical wounding, larval herbivory induced a stronger and more stable expression of ppTobHS-A. Moreover, exposure to Manduca-damaged plants induced its expression in neighboring intact plants. In most treatments, the expression patterns of ppTobHS-A coincided with those of selected wound-responsive (WR) genes (e.g., PIOX, NtPI-I, TPI). This correlation was tighter in the wounded and MeJA-treated leaves, whereas in distal, undamaged leaves, it appeared to depend on the type of WR gene examined and on the type of damage sustained by the plant. These results are consistent with the perceived role of the TobHS in defense signaling.     

Control by phytochrome of the synthesis of protein related to senescence in chloroplasts of barley (Hordeum vulgare)

Protein synthesis has been measured in chloroplast isolated from detached leaves of barley ( Hordeum vulgare L. cv. Hassan). The effects of hormone and light treatments of the leaves on chloroplast protein synthesis have been compared with effects on other senescence symptoms. Interruption of the dark with red light retards senescence and increases chloroplast protein synthesis. The effect of red light was reversed by far-red light. Red light did not act additively with kinetin, and it competed with ethylene and abscisic acid, accelerators of senescence, which decreased protein synthesis. In contrast to the interruption of the dark with red light, continuous light decreased chloroplast protein synthesis. It may be concluded effects on chloroplast protein synthesis. The retardation of senescence by continuous light is not necessarily related to P_u Instead, energy provided by photosynthesis may be an important factor.     

Differences in the location of subcotyledonary buds among Epilobium angustifolium L., E. dodonaei Vill. and E. fleischeri Hochst. (Onagraceae) and effects on architecture and population structure

STÖCKLIN, J., 1992. Differences in the location of subcotyledonary buds among Epilobium angustifolium L., E. dodonaei Vill. and E. fleischeri Hochst. (Onagraceae) and effects on architecture and population structure . Morphological and architectural features in the two closely related pioneer plant species Epilobium dodonaei and E. fleischeri are examined in cultivation and in the field and compared with E. angustifolium. In E. angustifolium , the aerial shoot system is renewed every year from buds on horizontal roots and results in a horizontal spread and a clonal growth form. In E. dodonaei , bud formation is restricted to the hypocotyl and in larger plants to the transitional region between root and shoot. Consequently this species shows no vegetative mobility and develops a shrub-like habit. The alpine E. fleischeri combines the habit of E. angustifolium and E. dodonaei and may either develop successive generations of shoots from the transitional region between root and shoot and/or exploit new areas by horizontal roots and the formation of shoots from root buds. The simple difference in the location of renewal buds is accentuated by cumulative growth. The study shows that E. dodonaei and E. fleischeri , which often are considered as subspecies, are separated by fundamental differences in their architectural models. The ecological and demographic implications of these differences are discussed.     

Analysis of an abscisic acid (ABA)-responsive gene promoter belonging to the Asr gene family from tomato in homologous and heterologous systems

Asr is a family of genes that maps to chromosome 4 of tomato. Asr2, a recently reported member of this family, is believed to be regulated by abscisic acid (ABA), stress and ripening. A genomic Asr2 clone has been fully sequenced, and candidate upstream regulatory elements have been identified. To prove that the promoter region is functional in vivo, we fused it upstream of the β-glucuronidase (GUS) reporter gene. The resulting chimeric gene fusion was used for transient expression assays in papaya embryogenic calli and leaves. In addition, the same construct was used to produce transgenic tomato, papaya, tobacco, and potato plants. Asr2 upstream sequences showed promoter function in all of these systems. Under the experimental conditions tested, ABA stimulated GUS expression in papaya and tobacco, but not in tomato and potato systems. Received: 24 March 1997 / Accepted: 26 November 1997     

Influences of Temperature and Desiccation on Breaking Diapause in the Gemmules of Eunapius fraqilis (Leidy)

Summary

Most specimens of Eunapius fraqilis in Gallup Pond (Connecticut) apparently formed gemmules in September and then died. The gemmules were initially in a protracted diapause which was gradually broken during the winter. They germinated in the pond between late March and late April. Gemmules collected during the fall did not germinate during four months when they were kept at 20°C. Other gemmules from the same collections, which were kept at 5°C for several months before they were placed at 20°C, exhibited good germination. Finally, gemmules that did not germinate following a cold treatment could be activated by drying them at 20°C for seven days and then maintaining them in pond water at 20°C. These results suggest that low temperature and desiccation are effective in breaking gemmule diapause of Eunapius fraqilis.     

Species Differences in the Selective Inhibition of Monoamine Oxidase (1-methyl-2-phenylethyl)hydrazine and its Potentiation by Cyanide

The potentiating effects of cyanide on the inhibition of rat liver mitochondrial monoamine oxidase-A & B and of ox liver mitochondrial MAO-B by pheniprazine [(1-methyl-2-phenylethyl)hydrazine] has been studied. Pheniprazine was shown to behave as a mechanism-based MAO inhibitor. For rat liver MAO-B, the initial non-covalent step was characterized by dissociation constant (K _i) of 2450 nM and the first-order rate constant (k ₊₂) for the covalent adduct formation was 0.16 min⁻¹. As a reversible inhibitor it was selective towards rat liver MAO-A (K _i = 420 nM) but the rate of irreversible inhibition of that enzyme was considerably slower (k ₊₂ = 0.06 min⁻¹). MAO-B from ox liver more closely resembled MAO-A from the rat in sensitivity to reversible inhibition by pheniprazine (K _i = 450 nm) but it was closer to rat liver MAO-B in rate of irreversible inhibition (k ₊₂ = 0.29 min⁻¹). The K _i values were significantly decreased in the presence of KCN but there was little effect on the k ₊₂ values. However, sensitivities of the different enzymes to KCN varied widely and considerably higher concentrations of KCN were required for this effect to be apparent with the rat liver mitochondrial MAO-A than with MAO-B from rat and ox liver. The kinetic behaviour of cyanide activation was consistent with partial (non-essential) competitive activation in all cases. Special issue dedicated to Dr. Moussa Youdim.     

Androgen receptors are required for full masculinization of the ventromedial hypothalamus (VMH) in rats

The ventromedial hypothalamus (VMH) is one of several sexually dimorphic nuclei that regulate mating behavior, and is rich in steroid hormone receptors and aromatase activity. We looked at the contribution of the androgen receptor (AR) to the volume of the VMH in rats by measuring each of the four subdivisions of the VMH in 90 day old male, female, and XY male rats carrying a mutant AR allele (tfm), which renders animals largely unresponsive to androgens. Confirming published reports, total VMH volume was greater in wild-type males than in females (P<0.01). The mean total volume of the VMH in TFM males was intermediate, but not significantly different from either females or males (Ps>0.10). The sex difference in VMH volume was primarily accounted for by the ventrolateral subdivision (VMHvl), which in both females and TFM males was significantly smaller than in wild-type males (Ps<0.005). There was no significant sex difference in the volume of the other three subdivisions of the VMH. Neuronal somata were larger in males than females in VMHvl, central VMH (VMHc) and the dorsomedial VMH (VMHdm), with TFM males having feminine neuronal somata in the VMHdm and VMHc. These data suggest that AR plays a role during sexual differentiation of the VMH, imparting its greatest effect in the VMHvl. ARs may regulate aromatase expression or activity to affect estrogen receptor activation, or may act independently of estrogen receptors to influence VMH morphology.