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     

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.     

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; )     

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。     

梁子湖苦草繁殖体的分布及其萌发初步研究

苦草鳞茎(冬芽)在梁子湖的垂直分布深度与其重量呈显著正相关,而重量也与鳞茎芽数显著相关 (P<0.05).埋藏深度、水分状况和鳞茎本身的大小都显著影响鳞茎的出土能力.去除第一位芽和切碎鳞茎后促进了其余芽的萌发;上年产生的未萌发的鳞茎在条件适宜的时候也可以很好的萌发,这些可能成为苦草在鳞茎被牧食后,维持种群数量的有效对策.沙质底比泥质底更有利于苦草种子的萌发,但在没有扰动的情况下,幼苗的定植能力在两种基质中没有显著差异.       

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.     

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.     

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.     

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.     

Clonal Differences in the Formation of Turions are Independent of the Specific Turion-inducing Signal in Spirodela polyrhiza (Great Duckweed)

Abstract: Turion (survival organ) formation in Spirodela polyrhiza includes a switch in the programming of the primordia from the formation of vegetative fronds toward resting turions. The specific turion yield (SY; number of turions formed by one frond) is used to evaluate the effect of three turion-inducing signals: low phosphate concentration (depleted due to frond growth), low temperature (15 °C) and exogenously applied abscisic acid (1 μM). The formation of turions was observed in the presence of any of the turion-inducing factors in all three clones of S. polyrhiza investigated (clones 9256 from Finland, SJ from Germany and SC from Cuba). The clone SC showed no specific induction by low temperature or phosphate limitation in one nutrient medium. Regardless of the specific signal applied, the SYs were highest in clone 9256 and lowest in clone SC, demonstrating signal-independent clonal differences. Clonal differences are therefore located in the developmental-specific common phase of the transduction chains leading to turion formation. We intend to use clonal differences in the molecular analysis of turion formation, e.g., by cDNA-based amplified fragment length polymorphism, to distinguish signal-specific and developmental-specific gene expression. In contrast, the total turion yield is useful in an ecological context to evaluate the number of turions available to support the survival of a population of plants but gives little information about the physiological process.     

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.     

Heterogeneity of dormancy in the turions of Spirodela polyrrhiza

Spirodela polyrrhiza exhibits at least two different types ofdormancy in turions induced by nitrogen deficiency, i.e., Y(young)- and O (old)-type turions produced at early and latestages in the formation process, respectively. On the otherhand, turions induced by relatively low temperature are mainlythe Y type. O-type turions germinate rapidly under darkness with the additionof NO₃-. However, under semi-anaerobic and anaerobic conditionsthey germinate only when exposed to light in the presence ofNO₃-. Y-type turions germinate in air after a definite dormant periodonly under light and in the presence of NO₃-. The dormancy isbroken by light regardless of the presence of NO₃-, althoughthe germination phase requires both. This dormancy is also brokenwhen a definite period of darkness is given in the absence ofNO₃-, and the germination proceeds in the presence of NO₃- withor without light. Thus, an alternative process exists for theY type in which the dormancy is released by either light ordarkness. (Received October 3, 1978; )     

The Physiological Role of Abscisic Acid in Eliciting Turion Morphogenesis

The exogenous application of hormones has led to their implication in a number of processes within the plant. However, proof of their function in vivo depends on quantitative data demonstrating that the exogenous concentration used to elicit a response leads to tissue hormone levels within the physiological range. Such proof is often lacking in many investigations. We are using abscisic acid (ABA)-induced turion formation in Spirodela polyrrhiza L. to investigate the mechanism by which a hormone can trigger a morphogenic switch. In this paper, we demonstrate that the exogenous concentration of ABA used to induce turions leads to tissue concentrations of ABA within the physiological range, as quantified by both enzyme-linked immunosorbent assay and high-performance liquid chromatography/gas chromatography-electron capture detection analysis. These results are consistent with ABA having a physiological role in turion formation, and they provide an estimate of the changes in endogenous ABA concentration required if environmental effectors of turion formation (e.g. nitrate deficiency, cold) act via an increased level of ABA. In addition, we show that the (+)- and (-)-enantiomers of ABA are equally effective in inducing turions. Moreover, comparison of the ABA; levels attained after treatment with (+)-, (-)-, and ([plus or minus])-ABA and their effect on turion induction and comparison of the effectiveness of ABA on turion induction under different pH regimes suggest that ABA most likely interacts with a plasmalemma-located receptor system to induce turion formation.     

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.     

Photophysiology of turion germination inSpirodela polyrhiza (L.)Schleiden. The cause of germination inhibition by overcrowding

Red-light-induced (via phytochrome) germination decreased with increasing numbers of turions per germination flask (overcrowding). Three hypotheses concerning the mechanism of this germination inhibition were tested, related to abscisic acid, ethylene, and oxygen deficiency: (i) Although abscisic acid is a powerful inhibitor of turion germination it had to be excluded as a cause, because abscisic acid was not secreted from turions into the nutrient solution, (ii) Ethylene (ethrel) strongly inhibited growth of newly formed sprouts, but germination response itself was not inhibited, (iii) Germination inhibition did not appear if short light pulses were substituted by continuous irradiation. It reappeared in the presence of the photosynthesis inhibitor 3-(3, 4-dichlorophenyl)-l, 1-dimethylurea, but it was not observed in aerated nutrient solutions, or when Petri dishes instead of Erlenmeyer flasks were used. Decreased oxygen concentrations in the nutrient solution were produced by turion respiration. Consequently, anaerobiosis within the nutrient solution caused by turion respiration was the reason for germination inhibition by overcrowding.     

Ecophysiological Characterization of Dormancy States in Turions of the Aquatic Carnivorous Plant Aldrovanda vesiculosa

Two main dormancy states, innate and imposed dormancy, were characterized in turions (winter buds) of the aquatic carnivorous plant Aldrovanda vesiculosa L. (Droseraceae) kept at 3 ± 1 °C in a refrigerator over the winter. As a result of the breaking of imposed dormancy by a temperature increase (at 15 – 20 °C), some of the turions rose to the water surface within 1 – 3 d and germinated. Turion leaves contained large lacunae with a slimy reticulum and were filled by water over winter. As a result of breaking imposed dormancy, the proportion of gas volume in inner turion leaves rose from 10 – 20 % to 100 % of leaf lacunae volume. The aerobic dark respiration rate of the turions [0.74 – 1.5 μmol O₂) kg⁻¹(FM) s⁻¹] slightly increased during innate dormancy after 1 – 2 d at 20 °C, while it was almost constant during the breaking of imposed dormancy. The anaerobic fermentation rate of the turions was only 1.5 – 7 % of the oxygen respiration rate and also was constant during the breaking of imposed dormancy. In turions, the content of glucose, fructose, and sucrose was the same for the two states of dormancy, but starch content was greatly reduced for the imposed dormancy (10 – 11 vs. 32 % DM). It may be suggested that a temperature increase causes an increase of fermentation or respiration which is responsible for the evolution of gas in turion lacunae and, thus, for turion rising.     

Stay dormant or escape sprouting? Turion buoyancy and sprouting abilities of the submerged macrophyte Potamogeton crispus L.

The ability of asexual propagules to disperse is an important ecological determinant of the spread and establishment of many aquatic species. However, few previous studies have addressed the relationship between the asexual propagule buoyancy and sprouting abilities in submerged macrophytes. For this reason, turions of Potamogeton crispus samples were collected from Lake Liangzi, and an incubator sprouting experiment was conducted. Our results revealed that the floating turions showed higher sprouting rates than that of sinking turions, indicating the former ones are possibly with high levels of primary metabolites. The higher N and P concentrations in the floating turions caused lower C:N, C:P, and N:P ratios in these turions compared with sinking turions, which confirmed the activation of floating turions. The free amino acid and soluble carbohydrate concentrations were also higher in floating turions than those in sinking turions. Our results also revealed that turion leaf porosity rather than starch concentration may determine the density of P. crispus turions. This study makes a contribution to our understanding of how the internal characteristics of turions can (at least partly) determine dispersal outcomes and offers new insights into the dispersal and sprouting of asexual propagules of submerged macrophytes.     

On the germination of turions and the life cycle of Potamogeton trichoides Cham. et Schld.

The life cycle of P. trichoides Cham. et Schld. has been studied in a pond near Nijmegen (The Netherlands); the germination of its turions has been studied in relation to temperature, cold induction (stratification) and light dependency. The species shows a summer-green annual life cycle, surviving the winter period by means of turions. Timing of turion germination appears to be well regulated by the water temperature, but light was also effective.     

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.     

Developmental Control of Apiogalacturonan Biosynthesis and UDP-Apiose Production in a Duckweed

Vegetative fronds of Spirodela polyrrhiza were induced to form dormant turions by the addition of 1 micromolar abscisic acid or by shading. The cell wall polymers of fronds contained a high proportion of the branched-chain pentose, d-apiose (about 20% of total noncellulosic wall sugar residues), whereas turion cell walls contained only trace amounts (about 0.2%). When the fronds were fed d-[³H]glucuronic acid for 30 minutes, the accumulated UDP-[³H]apiose pool accounted for about 27% of the total phosphorylated [³H]pentose derivatives; in turions, the UDP-[³H]apiose pool accounted for only about 4% of the total phosphorylated [³H]pentose derivatives. We conclude that the developmentally regulated decrease in the biosynthesis of a wall polysaccharide during turion formation involves a reduction in the supply of the relevant sugar nucleotide. One controlling enzyme activity is suggested to be UDP-apiose/UDP-xylose synthase. However, since there was a 100-fold decrease in the rate of polysaccharide synthesis and only a 9-fold decrease in UDP-apiose accumulation, there is probably also control of the activity of the relevant polysaccharide synthase.