Comparative growth and propagule production byHydrilla verticillata grown from axillary turions or subterranean turions

Hydrilla verticillata (L.f.) Royle produces two types of vegetative propagules, subterranean turions and axillary turions. After 8 or 12 weeks growth under similar conditions, plants grown from subterranean turions weighed 1.7 to 2 times as much as plants grown from axillary turions. Subterranean turion-derived plants produced more propagules (by weight and number) per plant than plants from axillary turions. Characteristics of weight frequency distributions (median, minimum, maximum, and coefficient of variation) for new subterranean turions were also influenced by the type of propagule from which the parent plant was derived. The number of root crowns per plant which reflects the plant's ability to expand horizontally was significantly greater for subterranean turion-derived plants (by 2 times) than for plants from axillary turions. These results support the hypothesis that following colonization of an area the impact ofHydrilla on resident species changes over time.     

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.     

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.     

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     

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; )     

Serum ferritin and serum iron changes after cross-country and roller ski endurance races

We have studied the variations induced in iron status parameters by four endurance races of different lengths. A comprehensive group of 48 healthy, non-iron deficient, endurance athletes were evaluated before and after four different cross-country and roller ski races: I = Skirollonga, roller ski race for individuals (n = 10), mean duration (MD) = 1 h 48 min; II = Marcialonga, cross-country ski race for individuals (n = 9) MD = 3 h 10 min; III = 12-h of Caldonazzo (Trento-Italy) roller ski relay race (n = 13) MD = 12 h; IV = 24-h of Pinzolo (Trento-Italy) cross-country ski relay race (n = 16) MD = 24 h. In the relays the MD includes both exercise and recovery times. Blood samples were taken before and after every race for the determination of the following haematological parameters: red blood count, haemoglobin, and packed cell volume, serum iron concentration [SI], serum ferritin concentration [FERR] and total iron binding capacity (TIBC). The results showed a constant significant increase of [FERR] after the races (+44.9% in I, +50.5% in II, +51.2% in III and +36.5% in IV, P less than 0.01) while [SI] increased only in the first two races (+28.2% in I and +19.7% in II, P less than 0.01) and showed a remarkable decrease in the longer races (-46.1% in III and -39% in IV, P less than 0.01). The TIBC increased in all the races (except II) to the same extent (range 10%-12%).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Endogenous Gibberellins in Floating Plants and Turions of Wolffiella floridana

Endogenous gibberellins from floating plants and in vitro induced turions of Wolffiella floridana (Lemnaceae) were extracted and partially purified. Gibberellin-like activity was detected in two zones of the chromatogram corresponding to Rf 0-0.1 and Rf 0.4-0.5 by dwarf pea bioassay. The active compounds in the two zones have been referred to in this paper as factor I and factor II respectively. There were quantitative differences in the gibberellin-like substances of floating plants and turions. The floating plants contained more of factor I but less of factor II as compared to the turions. The chromatographic behavior of factor I was similar to either gibberellin A₁ and A₃ while the identity of factor II was uncertain.     

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.     

Propagule type influences competition between two submersed aquatic macrophytes

Summary Glasshouse competition experiments with Hydrilla verticillata (L.f.) Royle indicate that plants grown from turions are weaker competitors than those grown from tubers, when compared to the widely distributed macrophyte, Potamogeton pectinatus L. These results support an earlier hypothesis about the importance of propagule size for predicting the outcome of plant competition (Grace 1985; Schaffer and Gadgil 1980). Results of outdoor growth experiments indicate that even though Hydrilla plants from turions are relatively weaker competitors, they are able to grow succesfully in an existing macrophyte bed composed of either, P. pectinatus or P. gramineus. During the early stages of Hydrilla invasion into an area of existing macrophytes, native plants may coexist with Hydrilla. However, once the abundance of Hydrilla tubers in the sediment increases, Hydrilla may displace existing plants.     

Influence of photoperiod on growth,pigment composition and vegetative propagule formation for Potamogeton nodosus Poir. and Potamogeton pectinatus L.

Photoperiods of 10 or 12 h enhanced vegetative propagule production by Potamogeton nodosus Poir. and P. pectinatus L. The proportion of plants producing vegetative propagules and the number and weight of propagules per plant increased over time, and were greatest after 8 weeks. Maximum vegetative propagule production was 12±6 tubers per plant (mean±S.D.; n=7) for P. pectinatus and 5±3 for P. nodosus. Tuber or winter-bud dry weight accounted for 38 or 27% of total plant dry weight, respectively, for plants grown for 56 days under a 10-h photoperiod. Vegetative propagules appeared to be produced instead of new shoot tissue under short-day conditions. Chlorophyll a and carotenoid content were lower for P. nodosus at short photoperiods, but not for P. pectinatus. The changes in chlorophyll a:b and carotenoid:ratio chlorophyll a suggest that synthesis/degradation of these pigments is tightly coupled and that short photoperiods may initiate senescence in P. nodosus.     

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.       

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。     

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.     

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.     

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.     

Climate change can reduce the risk of biological invasion by reducing propagule size

Climate change has been conclusively linked to species extinctions, and to expansion and contractions and shifts of species ranges. Climate change is exerting similarly profound pressures on the individual stages of biological invasion which can significantly impact the biodiversity and ecology of invaded areas. Propagule pressure is perhaps the single most important determinant of invasion success, but the effects of climate change on propagule pressure are still largely uncertain because we have few observations of introduction events (or their size) that can be analyzed together with climate records. The common surrogate variables for propagule pressure do not logically respond to climate. Here I use a process-based simulation model to examine the potential effects of climate change (specifically temperature) on propagule size of a common invading insect species by estimating in-transit survivorship rate of propagules using historical and future (projected) temperatures and two common trade routes between a donor and a recipient location (Yokohama, Japan and Sydney, Australia). Propagule size (=the number of individuals in an introduction event) was lower under climate change temperatures than under historical temperatures in both routes. The route had significant effects on propagule size through its influence on the duration (and also the timing) of exposure to temperature conditions that are of time-sensitive importance to the development of the invasive species. Under historical temperatures propagule size was higher and less variable in the direct than the indirect route in 20 independent iterations. Under the future temperatures propagule size was also higher in the direct route but it was more variable than in the indirect route. Increased trade is increasing the opportunities for introductions, but the results reported here suggest that climate change will have inconsistent effects on biological invasion because of the complex relationship between temperature and insect ontogeny.     

Influence of photoperiod on tuber production in various races of Hydrilla (Hydrilla verticillata)

The potential for asexual reproduction throughproduction of tubers was evaluated in various races ofan International Hydrilla [Hydrilla verticillata (L.f.)Royle] germplasm collection. Interracecomparisons were made under both shortday and longdayphotoperiods in environmental growth chambers. Allthe races evaluated produced rates varying from91-7182 tubers per sq m of production under shortdayconditions. Tuber production occurred under longdayphotoperiods in several races. Under propertemperature conditions, all the monoecious racescurrently established in the U.S. appear capable oftuber production throughout the year.