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.     

Co‐action of temperature and phosphate in inducing turion formation in Spirodela polyrhiza (Great duckweed)*

Increased phosphate concentration, higher temperature and addition of glucose all increased the number of fronds and turions of the duckweed Spirodela polyrhiza formed under in vitro conditions. Increasing the number of turions by increasing the plant biomass does not mean that the developmental process (switch of the programme of the primordia from vegetative fronds toward resting turions) has been specifically influenced. The specific turion yield (STY; number of turions formed by one frond) and the time of onset of turion formation have been used as more specific measures of turion induction. At more than 30 µm initial phosphate the STY was increased by lower temperature (15 °C) and became independent of the phosphate concentration. Between 10 and 30 µm and at higher temperatures (25 °C) the STY was increased by lower phosphate levels. The stimulatory effect of lower temperature was more pronounced than that of lower phosphate concentrations. Decreased phosphate concentration highly accelerated the formation of the first turions. The influence of low temperature was small at lower phosphate concentration but became dominant at higher concentrations (especially in autotrophic cultures). Low phosphate levels (e.g. 10 µm ) and low temperatures (e.g. 15 °C) both represent specific turion‐inducing factors having significant interactive effects. In S. polyrhiza, these signals may replace the interactive effects of photoperiods and low temperature known from other hydrophytes in turion induction under natural conditions.     

TURION FORMATION AND GERMINATION IN SPIRODELA POLYRHIZA

Three clones of Spirodela polyrhiza L. (Schleid.) formed dormant bodies called turions. A clone from Puerto Rico did not form turions under all conditions tried. In those clones producing turions, formation was stimulated by the addition of sucrose (10–50 mM) to the nutrient solution. Increased levels of Ca(NO₃)₂ plus sucrose stimulated turion production. In the absence of NO₃^–, Ca⁺⁺ was more effective than K⁺ in stimulating turion formation. Turion buoyancy was not light dependent, nor was it promoted by sucrose. Normal turions required light for germination, whereas sucrose-induced turions germinated in the dark. Dark germination was not promoted by either Ca⁺⁺ or K⁺. Sucrose stimulation of turion formation and subsequent promotion of dark germination was attributed to metabolic rather than osmotic effects. One hundred mM sucrose concentrations inhibited turion buoyancy and germination. Turions formed one primary abscission layer which separated them from the stolon and the mother frond. Subepidermal idioblasts appeared to seal the stolon stump after separation.     

Turion formation and behaviour inSpirodela polyrhiza at two levels of phosphate supply

The clone SJ ofSpirodela polyrhiza (L.) Schleiden forms turions under various nutritive conditions. As compared to 1500 μmol 1⁻¹ phosphate (P+), growth and frond yield of mixotropic cultures decreased, when 60 μol 1⁻¹ phosphate (P-) were available. By contrast, P-conditions increased the number, individual size, dry matter content, and total turion yield (mg turions per ml of the nutrient medium) of P-turions as compared to P+ ones. Germination behaviour of P-turions is characterized by fairly low zero levels in the controls, and by low heterogeneity in individual size as well as in the response patterns concerning the influence of light and/or phytoactive substances. P-turions from youngSpirodela cultures are extremely dormant. However, they undergo an after-ripening process if kept inside ageing cultures.     

The clonal dependence of turion formation in the duckweed Spirodela polyrhiza—an ecogeographical approach

Formation of turions, the vegetative perennation organs, plays an important role in the survival strategy of Spirodela polyrhiza (L.) Schleiden. Turion formation [quantified as number of turions formed per frond; specific turion yield (SY)] was investigated in 27 clones collected from a wide geographical range. The Pearson correlation was tested with (1) duration of growing season (monthly average temperature of ≥10°C), (2) relative growth rate of the fronds, (3) longitude and latitude, and (4) several climatic parameters, in all possible single and multiple regressions. All single coefficients of determination were below 0.10. The highest correlation (R² = 0.61; adjusted for the number of explaining variables 0.54) was found in a multiple linear regression with the following five parameters: average temperatures over the year and during the growing season, duration of the growing season and precipitation over the year and during the growth period. All these parameters were shown to have significant contributions. This equation was used successfully to predict the SY of five newly isolated clones. Finally, on the basis of all 32 clones the following conclusions were drawn: The mean annual temperature has the highest impact. It is suggested that lower temperatures decrease the survival rate of turions and that adaptation refers to increasing SY. The different levels of SY in the clones (ranging from SY = 0.22 to 5.9) were detected even after several years of in vitro cultivation. It is therefore assumed that these adaptations to the climatic conditions are genetically determined.     

Turion formation in Spirodela polyrhiza: The environmental signals that induce the developmental process in nature

The formation of turions of Spirodela polyrhiza is induced by a large number of environmental signals, already investigated under laboratory conditions. To get more close‐to‐nature experimental conditions, chemical composition and temperature of the water were measured during the growing season in 2002 and 2003 in a pond near Jena (50°52′N, 11°42′E). Whereas the concentrations of nitrate and sulphate (both in the millimolar range) remained fairly constant that of phosphate decreased from approximately 13 µM at the beginning of the season to 2 µM at the time of onset of turion formation (17 August in 2002, 21 July 2003). This concentration was used in experiments under controlled conditions together with the other outdoor data (day temperature, lower night temperature and photoperiod) in subsequent experiments to investigate their role in the induction of turion formation. The concentration of the nutrient media were kept constant. The following conclusions were drawn. (1) Low phosphate concentration appears to be the decisive factor in inducing turion formation. Growing fronds take up phosphate, and turion formation is then induced towards the end of the season. (2) Lower temperatures during the day (18 vs 25°C) and especially during the night (18 vs 15°C) evidently enhance the effect of the turion‐inducing factor phosphate by increasing the yield. (3) At much higher anthropogenic phosphate concentrations low temperature takes over the function of inducing turion formation. (4) Whereas much lower concentrations act directly to induce the formation of turions regardless of the season.     

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.     

Light-induced degradation of storage starch in turions of Spirodela polyrhiza depends on nitrate

Light induces both the germination of turions of the duckweed Spirodela polyrhiza and the degradation of the reserve starch stored in the turions. The germination photoresponse requires nitrate, and we show here that nitrate is also needed for the light-induced degradation of the turion starch. Ammonium cannot substitute for nitrate in this regard, and nitrate thus acts specifically as signal to promote starch degradation in the turions. Irradiation with continuous red light leads to starch degradation via auto-phosphorylation of starch-associated glucan, water dikinase (GWD), phosphorylation of the turion starch and enhanced binding of alpha-amylase to starch granules. The present study shows that all of these processes require the presence of nitrate, and that nitrate exerts its effect on starch degradation at a point between the absorption of light by phytochrome and the auto-phosphorylation of the GWD. Nitrate acts to coordinate carbon and nitrogen metabolism in germinating turions: starch will only be broken down when sufficient nitrogen is present to ensure appropriate utilization of the released carbohydrate. These data constitute the first report of control over the initiation of reserve starch degradation by nitrate.     

Phytochrome-induced transmissible signal elicits germination response in turions of Spirodela polyrhiza

The role of cell competence, including the spatiotemporal aspect of phytochrome-induced long-distance signal transmission, was investigated in turions of Spirodela polyrhiza (L.) Schleiden. Irradiation of the dorsal surface of the turions triggered a significant germination response, while identical treatment of the ventral surface was less effective. Red-light (R) microbeam irradiation of a subregion (ca. 1 μm²) of the dorsal surface could induce the germination response. Therefore, photoactivation of phytochrome in a single cell or few cells is sufficient to trigger the photomorphogenetic response. The ultimate response occurs at the proximal end of the turion by way of growth and emergence of the frond primordia about 1.3 mm away from the microbeam-irradiated distal cell(s). This photoinduction was reversible by a pulse of far-red light (FR) given less than 24 h after R microbeam irradiation. Microsurgical separation of distal (irradiated) and proximal (primordium-bearing) halves of the turions following microbeam irradiation further revealed that the light-induced transmissible signal can be intercepted and that it required more than 48 h to traverse one half distance of the turions. Based on the kinetics of the signal transmission, the possible involvement of light scattering, light piping, or transfer of electrophysiological signals can be excluded. Taken together, the results indicate that a transmissible signal is generated by the irradiated cell(s) and propagated across to the non-irradiated cells, leading to induction of the photomorphogenetic response.     

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.     

Changes in vectors of endogenously generated ion currents in light-induced germination of turions of Spirodela polyrrhiza

Nondormant turions of Spirodela polyrhiza (L.) Schleiden were utilized to investigate endogenous ion currents in light-induced germination and early growing processes of higher plants. A small outward current was detected at the ventral side of the turions near the pocket containing the most developed sprout primordium. After a light pulse, the direction of the endogenous current changed from outward to inward. These ion currents are most likely conditioned by unspecific diffusion of cations (probably H+) inside the cell. Three-day-old sprouts of Spirodela showed the highest inward current near the sprout base which decreases toward its edge. Newly formed sprouts demonstrated a strong gravity effect (bending), which was preceded by a lowering of the Z-component of vectors close to the sprout base after a change of the turion fixation.     

Phytochrome Control of Turion Formation in Spirodela polyrhiza L. Schleiden

Turion yield in Spirodela polyrhiza, strain SJ, is increasedby increasing the daily light period. This effect is more pronouncedin autotrophic than in mixotrophic conditions. Night-break irradiation(15 mins) increased turion yield by 150 % under the conditionsof an 8-h daily light period. Besides the effect of night-breakirradiation, end-of-day far-red irradiation decreased turionyield with increasing photoperiod, whereas end-of-day red irradiationwas without any effect. This demonstrates the promoting effectof the P_fr form of phytochrome on formation of light-grown turions. Formation of dark-grown turions was increased by about 240%by a single red light pulse and was reversed by an immediatelyapplied far-red light pulse. Consequently, under heterotrophicconditions phytochrome modulates the turion formation process. Spirodela polyrhiza L. Schleiden, duckweed, Lemnaceae, photomorphogenesis, phytochrome, turion     

菹草石芽大小和贮藏温度对萌发及幼苗生长的影响

通过萌发实验探讨了菹草石芽重量和贮藏温度对石芽萌发及幼苗生长的影响。结果表明：成熟的菹草石芽大小不一,按鲜重划分重量等级,各等级石芽数量占总数量的百分比差异很大,重量中等的石芽数量占到80%以上;重量对石芽最终萌发率没有影响,但重量小的石芽萌发时间较早,重量大的石芽虽然萌发较晚但是最终萌生的幼苗数目较多。石芽重量和萌发结束时幼苗数目之间呈显著的线性正相关(p＜0.05);连续去苗过程中,重量大的石芽萌发率和萌发幼苗数保持较高水平;经过贮藏的石芽与未经贮藏的石芽相比,萌发快且萌发整齐。经过15℃贮藏的石芽萌发最早,高温(25℃)和低温(4℃)贮藏均会使石芽最终萌发出的幼苗数目减少,3种温度下贮藏的石芽最终萌发率和幼苗长度无显著差异。     

Germination of Spirodela polyrhiza turions: the role of culture conditions during turion development

The rapidly germinating "old" turions of Spirodela polyrhizawere shown to derive mainly from the slowly germinating "young"turions. This modification to "old" turions could occur evenin isolated "young" turions, and was accelerated by sucrose.It is suggested that this modification is a form of turion senescenceand that turion initiation and maturation are strongly influencedby exogenous carbon and nitrogen sources. (Received November 19, 1979; )     

Cell wall polysaccharidase activities and growth processes: a possible relationship

The turions of Potamogeton crispus L. develop in early summer and function in propagation and dispersal. Under natural conditions during longday periods, an average minimum air temperature of more than 12°C was found to be important for turion formation. Experiments with controlled environments indicate that both temperature and photoperiod regulate turion formation. Turions can be induced at 13°C or above but not at 8 or 10°C. At a temperature range of 13–24°C turions form in both 12- and 16-h days, but not in 8-h days. By increasing temperature from 24 to 30 or 35°C turions can be induced under 8-h days. Light intensity was found to be important in the formation of turions.     

The binding of α-amylase to starch plays a decisive role in the initiation of storage starch degradation in turions of Spirodela polyrhiza

Degradation of reserve starch in turions, perennation organs of the duckweed Spirodela polyrhiza , is induced by continuous red light (cR). Irradiation of the turions with this light results in the autophosphorylation of starch-associated glucan water dikinase (GWD). The ensuing phosphorylation of the starch by this enzyme was proposed to result in the enhanced association of starch-degrading enzymes to the starch granules and in the initiation of starch breakdown. The present results confirm that the irradiation of dark-adapted turions with cR results in phosphorylation of the starch, accompanying changes in the capacity of the granule starch to bind turion endogenous α-amylase, as well as changes in the starch degradation level. All three effects show very similar dependence on the time of irradiation, suggesting that they may be linked. The α-amylase is a plausible candidate for effecting starch breakdown initiation. However, the increased binding capacity of the starch granules for this enzyme is insufficient to account for the initiation of the starch breakdown as this capacity is already high prior to the irradiation. The decisive effect of cR irradiation on starch degradation may lie in enabling α-amylase to gain access to otherwise sequestered starch granules or in activating α-amylase bound to the granules.     

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.     

Dormancy, Turion Formation, and Germination by Different Clones of Spirodela polyrrhiza

Some clones of Spirodela polyrrhiza form dormant bodies called turions which require several weeks of chilling treatment before they proceed to renew growth and develop into vegetative fronds. The individual fronds of Spirodela are less than 5 mm long and can be grown aseptically in liquid culture. Turion formation and germination can serve as a bioassay for the various compounds involved in dormancy development.

Turion formation can be induced by manipulation of light intensity during the day, photoperiod, night temperature, day temperature, and concentration of nitrate in the culture medium. Different clones of Spirodela from northeastern United States, Puerto Rico, and Argentina had different requirements for turion formation. The clones from Argentina and Puerto Rico did not form turions under any of the experimental conditions imposed. Turions of some clones required chilling treatments for renewed vegetative growth while others did not. Both gibberellic acid and long photoperiods were required to bypass the chilling requirements of some clones, but not others.

     

Environmental and hormonal control of turion formation in Myriophyllum verticillatum

The turions of Myriophyllum verticillatum, an aquatic vascularplant, develop in the fall and function in propagation and dispersalas well as in over-wintering. Experiments with controlled evnironmentsindicate that both temperature and photoperiod regulate turionformation. Turions can be induced at 15°C or lower, butnot at 20°C. At 15°C, turions form in both 8- and 12-hrdays, but not in 16-hr days. Plants collected in early springdo not form turions readily in response to short days unlesspreviously exposed to long days; thus, turion formation is along-day-short-day response. This combination of photoperiodand temperature requirements probably prevents turion developmentin early spring when the temperature and photoperiod are similarto those in the fall. Treatment of plants with ABA (10^–5M) enhances turion development under marginally inductive conditions(12-hr days at 15°C) but cannot induce it under long days.On the other hand, the cytokinin benzyladenine (10^–5 M)blocks turion formation. GA3 (10^–5 M) and AMO-1618 (10^–5M) exert only small qualitative effects on turion development,while IAA (10^–5 M) retards it. During turion development,the level of ABAlike activity and of one or two unidentifiedinhibitors increases. Cytokinin activity decreases at the startof turion formation, increases during development, then decreasesat abscission. Thus two lines of evidence suggest that a decreasein cytokinin activity and an increase in acidic inhibitor activityplay important roles in turion induction. ¹Present address: Biological Station, University of Michigan,Ann Arbor, Michigan 48109, U. S. A. (Received December 1, 1975; )