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。     

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; )     

Photophysiology of turion formation and germination inSpirodela polyrhiza

Standardized laboratory techniques for the vegetative growth of the duckweedSpirodela polyrhiza (Lemnaceaé), and for formation as well as germination of their turions were described. Increasing photon fluence rates of blue or red light increased the yield of turions. A specific stimulating effect of blue light was demonstrated under autotrophic but not under mixotrophic conditions. Therefore the spectral composition of light is not important in mixotrophic formation of turions whereas in autotrophic formation light sources with a higher portion of blue light are recommended. Dark-grown (etiolated) turions showed accelerated germination and higher germination percentage in comparison with light-grown turions after induction by a single red light pulse. This difference was overcome in continuous red light by speeding up the germination response of light-grown turions. Use of Petri dishes (8 cm³ nutrient solution) instead of Erlenmeyer flasks (50 cm³ nutrient solution) retarded germination response. Especially for long term experiments the use of Erlenmeyer flasks is recommended. Storage of turions for 72 h at 25 ‡C following at 5 ‡C in darkness after-ripening resulted in a decreased lag phase of the light-induced germination both after induction by a single light pulse and in continuous light. We thank Dr. Halina Gabrys, University of Crakow, Poland for critical discussion.     

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.     

水深、基质、光和去苗对菹草石芽萌发的影响

通过野调查,研究了水深对菹草石芽萌发率的影响,比较了梁子湖与湖北省其他四个不同水深的湖泊间菹草石芽明发率月动态;并通过萌发实验探讨了基质、光和去苗对菹草石芽萌发的影响。结果如下：无光环境下菹草石芽的萌发率较有光下的小,基质的有无及其类型对萌发率影响不大,去苗能使其萌发第二苗和第三苗的百分离分别从自然状态下的3．2％和1．0％提高至96．8％和64．0％（12月初）;五个湖泊的石芽均于7月初即开始萌发,相同月份不同湖泊石芽的平均萌发率基本与其平均水深成负相关关系,但12月初各湖泊的平均萌发率相近（＞95％）;同一湖泊水深越大,相同月份的萌发率越低,水深的增加能显著推迟萌发起始时间,但不改变其最终萌发率（12月初＞95％）。菹草石高萌发率的特征和极高的萌发第二、第三苗的潜力可能是其成为许多湖泊优势种的两个极其重要的维持机制。     

Overwintering temperatures affect freezing temperatures of turions of aquatic plants

The effect of different overwintering temperatures (2.5 ± 1 °C in a refrigerator or outdoor natural overwintering on wet topsoil with weak frosts) on the freezing temperature and survival rate of turions of 10 aquatic plant species with different ecological traits (free-floating habit or bottom rooting) was studied using mini thermocouples. Dormant, non-hardened turions of 9 species exhibited freezing within a narrow temperature range of ?7.0 to ?10.2 °C, while Hydrocharis morsus-ranae froze at ?3.6 °C. The survival rate of the turions after the measurements was, however, very low (0–38%). In several species, the freezing temperature of turions at the beginning of germination was not significantly different (at p < 0.05) from the dormant ones. The mean freezing temperature of outdoor hardened turions of 6 species was within a very narrow range of ?2.8 to ?3.3 °C and was thus significantly higher by 4–7 °C (p < 0.0002) than that for the non-hardened turions. It is assumed that the freezing temperatures indicate freezing of the extracellular water. The hardened turions of all 7 species were able to survive mild winter frosts under the topsoil conditions at a rate of 76–100%. These characteristics suggest that the turions of aquatic species can be hardened by weak frosts and that their frost hardiness is based on the shift from frost avoidance in non-hardened turions to frost tolerance.     

Effects of sediment anoxia and light on turion germination and early growth of Potamogeton crispus

Potamogeton crispus is a cosmopolitan aquatic species and is widely used as a pioneer species for vegetation restoration of eutrophic lakes. However, many restoration projects applying P. crispus turions have not been successful. Earlier studies focused on effects of light and temperature on turion germination. The purpose of this study was to determine whether sediment anoxia and light interactively affected the turion germination and early growth of P. crispus. Anoxic conditions in the experiment were produced by adding sucrose to the sediment. The germination rate of the turions was 68–73% lower in the highly anoxic condition treatment than in the control. Medium light intensity (10% of natural light at the water surface) was more favorable for germination under slightly anoxic conditions than either low or high light intensity. The growth of newly-formed sprouts was also significantly inhibited by sediment anoxia. Photosynthesis and shoot biomass were reduced under sediment anoxia, whereas total chlorophyll content, root biomass, and soluble protein content were highest in the low anoxic condition treatment. Medium light improved net photosynthesis and biomass production of the sprouts. We conclude that turion germination and sprout growth can be significantly inhibited by sediment anoxia. Medium light intensity may alleviate this inhibition by anoxia, but light has little effect when sediment anoxia is severe. For the purposes of vegetation restoration, more attention should be paid to the role of sediment anoxia, and it is necessary to improve sediment and light conditions for turion germination and early growth of P. crispus in eutrophic lakes. These results will contribute to a more complete understanding of turion germination dynamics of P. crispus and will be useful for future restoration programs. Handling editor: S. M. Thomaz     

Life cycle and growth ofPotamogeton crispus L. in a shallow pond,ojaga-ike

The life cycle and growth ofPotamogeton crispus L. were studied in a shallow pond, Ojaga-ike. With respect to the shoot elongation and seed and turion formations, the life cycle of this plant in the pond could be divided into following five stages: germination, inactive growth, active growth, reproductive and dormant stages. It was suggested that the plant showed these successive stages depending mainly upon water temperature. The turions germinated on the bottom in autumn when the water temperature fell below ca. 20 C. The plant showed hardly any growth during winter (December—early March) when the temperature was below 10 C. In the spring when the bottom water temperature rose to above 10 C (mid-March), the plant started to grow again and the shoot elongated rapidly at the rate of 4.2 cm day⁻¹ until the shoot apex reached the pond surface in late April. Both the increment of node number and the internodal elongation were associated with this rapid shoot growth. On 10 May (last sampling date), the mean values of shoot length, internodal length and the number of nodes estimated for 10 predominant plants were 238.2±5.6 cm, 7.1±0.8 cm and 34.9±4.0 cm, respectively. The turion formation and flowering occurred during the period from mid-April to mid-May when the surface water temperature ranged 19 and 22 C. The dry weight of a plant reached the maximum mean value of 1180 mg on 10 May. At its peak biomass, an individual plant produced 1–10 turions (5.5 on average) of which the mean individual turion dry weight was 53.2 mg. The turion dry weight accounted for ca. 42% of the total plant biomass m⁻² at that time.     

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.     

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.

     

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.     

The causes of sinking and floating in turions of Myriophyllum verticillatum

The sinking and floating (density) of the special winter buds (turions) produced by many aquatic plants play important roles in their life cycles, yet there has been little experimental study of these phenomena. However, it has been postulated that starch determines the density of turions. In order to test this hypothesis, turions of Myriophyllum verticillatum L. were collected from October through April and analyzed for density, dry-to-fresh-weight ratio and starch content. Density did not correlate closely (R² = 0.37) with the dry-to-fresh-weight ratios. Density strongly correlated (R² = 0.99) with starch only in the January collections, but not at several other times, so starch content alone is not always the major determinant of turion density. Intracellular lacunae (potentially gas spaces) also occur within the axes of these turions, and the lacunal volumes increased ca. 20-fold as they resumed growth, at which time the turions started to float. Significantly, turions, which were near the density of water, could be sunk or floated by changing the ambient pressure (which would alter their internal gas volume). Thus, starch has a role in determining the density of M. verticillatum turions, but the amount of intercellular gas space may be more important, especially when they resume growth in the spring.     

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.     

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.     

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.       

Dormancy and germination in axillary turions ofHydrilla verticillata

Effects of environmental conditions and growth regulators on release from dormoncy of axillary turions inHydrilla verticillata were investigated. Coll treatment at 2 C for 33 days produced the most complete release from dormancy. One week of 2 C treatment was sufficient for the germination; however, longer cold periods produced more rapid growth in shoot or root lengths as well as a shorter lag time for germination. Dormancy in turions could be broken by a photoperiod of 16 hr but not by on of 8 or 12 hr, nor by continuous lighting. When a cold treatment was applied turions grew out in response to all of the photoperiodic conditions. Red and far-red irradiation during the incubation after a cold treatment promoted gremination; blue and green light markedly inhibited the germination. At 10⁻⁴ and 10⁻⁵ M, gibberellic acid broke dormancy of non-cold treated turions, but was toxic at 10⁻⁴ M to the development after germination. Gibberellic acid promoted growth of cold treated turions even at 10⁻⁶ M. Indoleacetic acid at 10⁻⁴, 10⁻⁵ and 10⁻⁶ M induced outgrowth of both non-cold treated and cold treated turions. Apparently normal growth and development was observed in a high concentration of indoleacetic acid.     

Contribution to the ecology and seed germination ofCucumis callosus

Ecology and germination behaviour of the seeds of two categories ofCucumis callosus Cogn. has been studied here. The seeds were found to be rigid in their requirement of light and temperature for germination. Red and far-red lights were found to affect germination and a phytochrome system appeared to be present. Temperature was found to have a significant effect on seed germination.     

The appearance of glutamine synthetase in turions of Spirodela polyrhiza (L.) Schleiden as regulated by blueand red light, nitrate and ammonium

Control by light and nitrogen (nitrate and ammonium) of theappearance of glutamine synthetase (GS; EC 6.3.1.2 [EC] ) in turionsof Spirodela polyrhiza (L.) Schleiden, strain SJ, was investigatedduring the pregermination period, i.e. up to 48 h after onsetof light. Immediately after transfer from after-ripening conditions(5C, darkness, D) to germination conditions (25C), GS activitydid not respond to light or nitrate. After 72 h in D (25C)activity increased in continuous light. Therefore, the regulatoryrole of light, nitrate and ammonium in the process of appearanceof GS was mainly studied between 72 and 120 h after transferfrom after-ripening to germination conditions (phase II of thepre-germination process). The inducing effect of red light ismediated by the photoreceptor phytochrome: the effect of long-termcontinuous red light (6 or 24 h) can be reversed, at least inpart, by a subsequent far-red light pulse (‘end of day’Irradiation). Blue light is more effective than red light ininducing the appearance of GS. Therefore, a specific blue lighteffect has to be assumed. This represents a novel mode of lightaction in regulating the level of the ammonium assimilatingenzyme in an angiosperm system. lmmunoblots showed that (i)increase in the enzymatic activity is caused by de novo synthesisof the enzyme protein, (ii) two different subunits (38 and 42kDa) contribute to the total activity which must be attributedto two different isofornis. In accordance with results fromother higher plants, the 38 kDa subunit (presumably relatedto the cytosolic isoform) did not increase in the presence oflight, whereas the 42 kDa subunit (presumably related to theplastidic isoform) was induced. The maximal enzyme level wasreached only in the presence of both light (blue light) andnitrate. Light induction was also observed in the presence ofammonium; however, GS activity was decreased, when comparedto nitrate-treated turions. Comparison of these results withprevious observations suggest that the influence of light andnitrate on the germination response and regulation of the nitrate/ammoniumassimilation pathway in turions appear to be unrelated phenomena. Key words: blue light, germination, glutamine synthetase, phytochrome, Spirodela polyrhiza, turion     

Bottom-up Assembly of the Phytochrome Network

Plants have developed sophisticated systems to monitor and rapidly acclimate to environmental fluctuations. Light is an essential source of environmental information throughout the plant’s life cycle. The model plant Arabidopsis thaliana possesses five phytochromes (phyA-phyE) with important roles in germination, seedling establishment, shade avoidance, and flowering. However, our understanding of the phytochrome signaling network is incomplete, and little is known about the individual roles of phytochromes and how they function cooperatively to mediate light responses. Here, we used a bottom-up approach to study the phytochrome network. We added each of the five phytochromes to a phytochrome-less background to study their individual roles and then added the phytochromes by pairs to study their interactions. By analyzing the 16 resulting genotypes, we revealed unique roles for each phytochrome and identified novel phytochrome interactions that regulate germination and the onset of flowering. Furthermore, we found that ambient temperature has both phytochrome-dependent and -independent effects, suggesting that multiple pathways integrate temperature and light signaling. Surprisingly, none of the phytochromes alone conferred a photoperiodic response. Although phyE and phyB were the strongest repressors of flowering, both phyB and phyC were needed to confer a flowering response to photoperiod. Thus, a specific combination of phytochromes is required to detect changes in photoperiod, whereas single phytochromes are sufficient to respond to light quality, indicating how phytochromes signal different light cues.