Seedling diversity in Hydatellaceae: implications for the evolution of angiosperm cotyledons

BACKGROUND AND AIMS: Cotyledon number has long been a primary morphological feature distinguishing monocots from other angiosperms. Recent placement of Hydatellaceae near the early-divergent angiosperm order Nymphaeales, rather than in the monocot order Poales, has prompted reassessment of seedling morphology in this poorly known family. METHODS: Seedlings of six species representing all eco-geographical groups of Hydatellaceae are described using light and scanning electron microscopy. KEY RESULTS: Two seedling types were discovered. Material examined of Trithuria submersa, T. bibracteata, T. austinensis and T. filamentosa possess a transparent bilobed sheathing structure that surrounds the main axis below the first foliage leaf. The seed coat is attached to the sheathing structure. Seedlings of Trithuria lanterna and T. konkanensis lack a sheathing structure, and the seed coat is attached to a short, narrow lateral outgrowth on the main axis of the seedling. CONCLUSIONS: The sheathing structure that is present in seedlings of some Hydatellaceae could be homologized with the two united cotyledons of water lilies. It also resembles the single cotyledon of some monocots, and hence demonstrates a possible pathway of the origin of a monocot-like embryo, though no homology is implied. The sheathing structure is reduced in Trithuria lanterna and T. konkanensis, and the short, narrow outgrowth of its seedling could represent a single cotyledon. This synapomorphy suggests that the only Indian species of Hydatellaceae, T. konkanensis, is closer to the northern Australian T. lanterna than to the south-western Australian T. bibracteata.     

Development and plasticity of endangered shrub Lindera melissifolia (Lauraceae) seedlings under contrasting light regimes

Lindera melissifolia (Walt.) Blume seedlings were raised in a growth chamber to determine the effects of light availability on shoot growth pattern, and basic leaf and stem growth. Lindera melissifolia seedlings exhibited a sympodial shoot growth pattern for 3 months following emergence from the soil medium, but this pattern was characterized by a reduction in leaf blade area approximately 30 days after emergence, followed by increases in leaf blade area. Seedlings receiving low light were 76% taller than seedlings receiving high light. Seedlings receiving low light also had larger leaf blade dimensions, blade area, seedling leaf area, and greater mass. Seedlings raised in high light had a greater proportional distribution of biomass in the roots, suggesting possible water stress from greater vapor pressure deficits. Furthermore, these seedlings displayed sharp angles of blade inclination and blade folding – acclimation that reduces exposure to light and subsequent higher leaf temperatures in open environments. These differences in morphological response to light resulted in high phenotypic variability in L. melissifolia seedlings. Lindera melissifolia seedling development showed a brief period of phenotypic plasticity, followed by ontogenetic plasticity. The short period of phenotypic plasticity may, however, have profound ecological implications for the conservation and recovery of this federally endangered shrub. Further experimentation should take into account the development of ontogenetic standards for comparisons of plant traits in addition to temporal standards.     

不同波长蓝光LED对两个品种水稻秧苗生长的影响

传统的大田育秧方式容易受到自然条件的影响,如在连续的阴雨天容易导致烂秧,而采用人工可控的工厂化育秧则有助于解决此类问题。LED光源具有光质纯、光效高以及低能耗等优势,是一种有望在工厂化育秧中应用的人工光源。该研究以生长特性差异明显的水稻天优998(籼稻)和圣稻14(粳稻)为材料,在光强为(60±5)μmol·m~-2·s~-1、光周期为12 h·d~-1的条件下,以450、470和490 nm 3个波长的蓝光LED照射天优998和圣稻14秧苗,考察了不同波长蓝光对两个品种的三叶期和五叶期水稻秧苗生长的影响。结果表明:蓝光处理下两个品种、不同秧龄幼苗的壮苗指数、根系活力和根数增加,体内碳、氮代谢活性增强,抗氧化物酶活性也会增加。3个波长的蓝光对两个品种、不同秧龄水稻秧苗生长的影响存在差异,450 nm照射下水稻秧苗的各项生长指标较优,生产上育秧可根据秧龄选择450 nm或490 nm作为独立和组合光源。     

A unique set of 11,008 onion expressed sequence tags reveals expressed sequence and genomic differences between the monocot orders Asparagales and Poales

Enormous genomic resources have been developed for plants in the monocot order Poales; however, it is not clear how representative the Poales are for the monocots as a whole. The Asparagales are a monophyletic order sister to the lineage carrying the Poales and possess economically important plants such as asparagus, garlic, and onion. To assess the genomic differences between the Asparagales and Poales, we generated 11,008 unique ESTs from a normalized cDNA library of onion. Sequence analyses of these ESTs revealed microsatellite markers, single nucleotide polymorphisms, and homologs of transposable elements. Mean nucleotide similarity between rice and the Asparagales was 78% across coding regions. Expressed sequence and genomic comparisons revealed strong differences between the Asparagales and Poales for codon usage and mean GC content, GC distribution, and relative GC content at each codon position, indicating that genomic characteristics are not uniform across the monocots. The Asparagales were more similar to eudicots than to the Poales for these genomic characteristics.     

茄子嫁接苗与自根苗光合特性比较

对茄子嫁接苗与自根苗的光合生理特性进行了比较研究。结果表明,不同时期嫁接苗功能叶片的净光合速率（Pn）均显著高于自根苗;茄子嫁接苗与自根苗功能叶片的Pn日变化均呈“双峰”曲线,但嫁接苗的“午休”程度较自根苗轻;嫁接苗比自根苗有较低的光补偿点（LCP）和CO2补偿点（CCP）,较高的光饱和点（LSP）和CO2饱和点（CSP）;有较低的光合冷限温度和较高的光合热限温度,光合最适温度两者之间没有明显的差别;有较高的光饱和、CO2饱和及最适温度时的Pn;嫁接苗Pn高是因为嫁接苗胞间的光合反应的底物浓度大（Ci高）,表观量子效率（AQY）、羧化效率（CE）和光合能力（A350）高的缘故。     

光对热带雨林冠层树种绒毛番龙眼种子萌发及其幼苗早期建立的影响

绒毛番龙眼(Pometia tomentosa)为西双版纳热带季节雨林标志树种,被列为中国珍稀濒危植物。对其开展种子及幼苗生态学研究可为珍稀濒危树种的保护及当地森林恢复提供科学依据。试验采用遮荫试验和野外试验相结合的方法,研究光对绒毛番龙眼种子萌发及幼苗早期建立的影响。结果表明:在遮荫试验30%(林窗中心光)、10%(林窗边缘光)和3.5%(林下光)3个光处理及森林3种生境(林窗中心、林窗边缘和林下)条件下,种子萌发率差异无显著性差异,均达到95%以上,且平均萌发周期小于6 d。3个光处理下的幼苗生长、生物量分配模式及气体交换参数差异显著。30%光下绒毛番龙眼幼苗的根重比(RMR)和茎重比(SMR)最高,10%和3.5%光下幼苗的叶重比(LMR)最高,3.5%光下的叶面积比(LAR)显著高于30%光下。30%光下绒毛番龙眼幼苗的最大净光合速率(P_nmax)、暗呼吸速率(R_d)和光饱和点(I_sat)在3个光处理中均最大,光补偿点(I_comp)则无显著性差异。绒毛番龙眼幼苗具耐荫性,能够在低光环境下长期存活且能缓慢生长;同时具有适应林窗光环境生长的能力,如高的质量相对生长速率(RGR_M)和高度相对生长速率(RGR_H)。林窗的出现是绒毛番龙眼进行成功更新的必要条件,水分可能是限制其幼苗生境选择的另一环境因子。     

香果树实生苗的光合特性及其与环境因子的关系

为确定香果树实生苗的适生环境并为其自然更新提出有针对性策略,研究了不同生境(冠下、冠缘、林窗和林缘)中2年生香果树实生苗的净光合速率、水分利用效率、叶绿素含量、苗高、基径、生物量等的变化及其与生态因子之间的关系.结果表明: 4种生境中的光合有效辐射最大值为50～1380 μmol·m^-2·s^-1,冠下和冠缘中香果树实生苗的净光合速率日变化呈单峰型,而林窗和林缘实生苗的净光合速率日变化呈双峰型;香果树实生苗为耐阴植物,但耐阴能力较弱,其功能叶的光饱和点、补偿点和暗呼吸在4种生境中大小顺序为: 林缘>林窗>冠缘>冠下,表观量子效率的变化规律与之相反;林窗和冠缘2种生境中香果树实生苗的适应能力较强,叶片的蒸腾速率、气孔导度、水分利用效率和净光合速率较高;林窗中香果树实生苗叶片的叶绿素含量较低,但实生苗的生长速度最快,生物量最大;香果树实生苗的净光合速率与光合有效辐射和气孔导度呈显著正相关.对于冠下生境,需降低林冠层密度,增加透光率,以利于香果树实生苗的光合作用;对于林缘生境,则需要增加植被盖度,降低光照强度,以利于其快速生长.     

Effects of burial in sand and water supply regime on seedling emergence of six species

BACKGROUND AND AIMS: Air seeding has long been regarded as a quick and successful measure for vegetation rehabilitation in China. However, seedling emergence of often-used species including Agriophyllum squarrosum, Artemisia sphaerocephala, Artemisia ordosica, Hedysarum fruticosum, Caragana korshinskii and Medicago sativa is low. Experiments were conducted under controlled conditions to study the effects of sowing depth and water supply on seedling emergence, in order to understand the requirements for increasing seedling emergence. METHODS: Seeds were exposed to different environments of burial and water supply regimes in PVC pots (7 cm in diameter and 11 cm in height) under the same light intensity and alternating temperature regimes in a growth chamber. KEY RESULTS: Seedlings of three species (Agriophyllum squarrosum, Artemisia sphaerocephala, Artemisia ordosica) with relatively light seeds emerged well at a 0.5 cm sowing depth under a 7.5 and 10 mm water supply regime. However, few seedlings of these species emerged when the sowing depth was over 1 cm or when water supply was 5 mm. Seedlings of Caragana korshinskii, Hedysarum fruticosum and Medicago sativa emerged from sowing depths of 0.5-4 cm, 0.5-3 cm, and 0.5-4 cm, respectively, under both 7.5 and 10 mm water supply regimes. Under a 5 mm water supply regime, seedlings of these species also emerged at over 1 cm sowing depth. Seeds of all six species sown on the surface of sand did not germinate, and seedlings did not emerge when they were sown at depths greater than 6 cm. CONCLUSIONS: Based on these experiments, a 0.5 cm sowing depth resulted in the highest seedling emergence and it is concluded that this is the optimal sowing depth for seedling emergence of all six species.     

南亚热带森林24种乔木的种子萌发和幼苗生长

以膨胀珍珠岩为基质,在光和暗的条件下,对２４种南亚热带森林乔木的种子萌发和幼苗生长进行了研究。种子的形态和重量与种的演替阶段有关,种子的重量也与不同的种有关。不同种的萌发率差异较大,种子较大的种,萌发率较高。肉质果实的种子,开始萌发的时间较长。光和暗条件对萌发率和萌发速度无明显影响。幼苗高度和种子重量呈正相关,这一相关在光条件下比暗条件下更明显。暗条件明显地引起群落演替早期的树种的茎徒长,限制根的生长,而对演替后期种无显著的影响。幼苗地上部分和根系生物量的分配与种的演替阶段有关,也与不同的科有关。幼苗地上部分和根系生物量的分配也受光暗条件的影响,而不受种子重量的影响。根和叶的生物量分配和种在其群落演替阶段有关。幼苗的生物量与种子重量有显著的正相关。幼苗的相对生长率相差较大,在没有营养供应的生长基质中,以演替中间阶段的种的相对生长率较大。幼苗从种子的物质利用效率与种子重量呈负相关,而与种的演替阶段无明显的相关。     

Seedling establishment of Acacia tortilis and Hyphaene compressa in the Turkwel riverine forest, Kenya

This study reports on the spatial and temporal patterns of seedling establishment in the Turkwel riverine forest, Kenya. Seedlings of the dominant tree species Acacia tortilis and Hyphaene compressa were mapped and monitored to assess the underlying causes of seedling recruitment and mortality. The broad‐scale distribution of A. tortilis was not correlated with any environmental variables, while H. compressa seedlings were confined to flood‐exposed sites in the arid downstream section of the floodplain. One year of monitoring showed that seedling recruitment of A. tortilis was evoked by prolonged rainfall, while seedling mortality was caused by desiccation, browsing and trampling. In contrast, seedling recruitment and mortality of H. compressa was largely unaffected by rainfall and livestock, probably due to the high moisture requirements of seeds and the tolerance of seedlings to disturbance. There were no effects of soils, light, or seedling density on the establishment of A. tortilis and H. compressa seedlings. This study demonstrates the importance of parallel mapping and monitoring of riverine seedlings in order to understand patterns and processes of forest regeneration in arid and semi‐arid floodplains.     

Ecology of Aglaia mackiana (Meliaceae) Seedlings in a New Guinea Rain Forest1

The large seeds of Aglaia mackiana (Meliaceae) germinate and produce vigorous seedlings under closed canopies or in large gaps. To assess seedling ecology after germination, we measured growth, herbivore damage, and survivorship of seedlings over one year. The sample included shaded seedlings from dispersed seeds, undispersed seeds under parent trees, and seedlings transplanted to gaps. We quantified the light environment using hemispherical canopy photographs taken above seedlings at the beginning and end of the one–year study. Seedlings transplanted to gaps grew faster and had more leaves, larger total leaf surface area, longer secondary roots, and greater root mass than shaded seedlings. Seedlings in gaps did not differ from shaded seedlings in survivorship or amount of herbivore– and pathogen–caused leaf damage. The canopy photographs taken one year apart suggest there is a rough equilibrium in closed canopies with slight changes occurring around an average light level. Sites with < 0.06 ISF (a unitless, relative measure of canopy openness or reflected sunlight) tended to remain the same with minor fluctuations toward brighter or darker. Sites with canopy openness > 0.06 ISF tended to close; few gaps grew larger. Seedlings under parenr trees and seedlings away from parent trees had similar amounts of leaf damage and virtually identical survivorship after 18 months, but seedlings under parent trees had slower growth rates and smaller total leaf surface areas. Dispersal did not strongly benefit seeds via escaping high levels of mortality or comperition around the parent.     

Seed dispersal, seedling establishment and gap partitioning among tropical pioneer trees

1 We examined the abundance and distribution patterns of pioneer seeds in the soil seed bank, and of pioneer seedlings in 53 recently formed gaps, in a 50‐ha forest dynamics plot on Barro Colorado Island (BCI), Panama. The aim was to assess the importance of dispersal limitation (failure of seeds to arrive at all sites suitable for their germination) and establishment limitation (failure of seeds having reached a site to germinate successfully and establish as seedlings) in determining patterns of gap occupancy.
2 The abundance of seeds in the soil seed bank was strongly negatively correlated with seed size, but was not correlated with the abundance of reproductive‐sized adult trees in the plot. In contrast, the abundance of pioneer seedlings > 10 cm height in natural gaps was strongly correlated with adult abundance, but was not correlated with seed size.
3 Seedlings were non‐randomly distributed among gaps, but seedling abundance was not directly related to gap size, and there was no evidence of partitioning of the light environment of gaps by small seedlings. Large differences in growth and mortality rates among species were observed after 1 year, and this may result in the gap size partitioning previously found in saplings of the same species.
4 Seedlings of most species, particularly those with large seeds, were relatively more abundant than expected in gaps close to their conspecific adults. Proximity to reproductives, and by inference dispersal limitation, therefore exerts some effect on seedling distribution. None the less, large differences between seed and seedling abundances for some species, and low seedling occupancy rates in some gaps close to adult conspecifics, suggest that seedling emergence probabilities and species‐specific establishment requirements may also be important determinants of local abundance.     

Palatability of silver birch seedlings to root voles Microtus oeconomus

We used laboratory feeding trials to test the palatability of silver birch Betula pendula seedlings to root voles Microius oeconomus Seedlings of two B pendula families (A and B) were grown in growth chambers on three soil types and under two light intensities. Seedlings from family A grew taller and had more resin droplets on their bark than seedlings from family B The more light or nutrients the seedlings received the taller they grew and the more resm droplets they had on their bark We offered 10 root voles both basal and top 5–10 cm segments of winter-dormant seedlings one treatment at a time (no-choice arrangement) in a random order The seedlings grown on low-fertility soil were eaten more than those grown on non-fertilized or fertilized peat The top segments of seedlings grown in low light were slightly more eaten than those grown in high light Birch family did not have any effect on the consumption Consumption correlated negatively with the amount of resin droplets on the bark when the seedling material was compared over different treatments These results indicate that root voles would prefer young birch seedlings that have low amount of resin droplets on the bark and that grow in poor soils or under low light conditions     

The influence of light on the growth of watercress (<Emphasis Type="Italic">Nasturtium officinale</Emphasis> R. Br.)

In stream ecosystems, the growth of aquatic primary producers is affected by spatial and temporal variations in the riparian canopy, which can influence the availability of light resources. Aquatic plants can acclimate to low light environments by employing a suite of morphological or physiological mechanisms to increase light capture or photosynthetic efficiency. Some species may also use alternate types of propagules to colonize environments with heterogeneous light environments. In a greenhouse experiment we examined the morphological and physiological response of watercress (Nasturtium officinale R. Br.) to a gradient of increasing light levels, which ranged from 7% ambient light to full sunlight. We also determined if watercress seedlings and vegetative fragments differed in their growth response to increasing light levels. Total biomass and root biomass of seedlings and vegetative fragments decreased with decreasing light levels. The difference in plant biomass across treatments was due to morphological changes in total canopy area and leaf area, both of which increased with decreasing light levels. Seedlings and vegetative fragments did not differ in their response to light availability, but vegetative fragments had higher final biomass as a result of higher initial biomass. Physiological acclimation to low light levels appears to be of secondary importance for watercress as the concentrations of total chlorophyll, chlorophyll a, chlorophyll b, and chlorophyll a:b did not differ among light levels or between seedlings and vegetative fragments. Seedlings and vegetative fragments grown under high light levels had a greater percentage of carbon and a lower percentage of nitrogen than plants grown under low light conditions. The results of this study indicate that watercress displays considerable morphological plasticity and acclimates to low light conditions primarily by increasing leaf area and canopy surface area. There is no evidence that the type of watercress propagule (seedling vs. vegetative fragment) imparts any growth advantage in low light environments and watercress grown from either type of propagule showed no differences in their morphological or physiological responses to varying light regimes. Handling editor: S. M. Thomaz     

The role of the storage carbon of cotyledons in the establishment of seedlings of Hymenaea courbaril under different light conditions

BACKGROUND AND AIMS: Hymenaea courbaril (Leguminosae-Caesalpinioideae) is a tree species with wide distribution through all of the Neotropics. It has large seeds (approx. 5 g) with non-photosynthetic storage cotyledons rich (40 %) in a cell wall polysaccharide (xyloglucan) as a carbon reserve. Because it is found in the understorey of tropical forests, it has been considered as a shade-tolerant, late-secondary species. However, the physiological mechanisms involved in seedling establishment, especially regarding the interplay between storage and light intensity, are not understood. In this work, the ecophysiological role of this carbon cotyledon reserve (xyloglucan) is characterized, emphasizing its effects on seedling growth and development during the transition from heterotrophy to autotrophy under different light conditions. METHODS: Seedlings of H. courbaril were grown in environments with different light intensities, and with or without cotyledons detached before xyloglucan mobilization. Development, growth, photosynthesis and carbon partitioning (dry mass and [14C]sucrose) were analysed in each treatment. KEY RESULTS: The detachment of cotyledons was not important for seedling survival, but resulted in a strong restriction (50 % less) of shoot growth, which was the main sink for the cotyledon carbon reserves. Carbon restriction promoted an early maturation of the photosynthetic apparatus without changes in the net CO2 fixation per unit area. The reduced surface area of the first leaves in seedlings without cotyledons was evidence of limited growth and development of seedlings in low light conditions (22 micromol m(-2) s(-1) photon flux). CONCLUSIONS: There is an increase in the importance of storage xyloglucan in cotyledons for H. courbaril seedling development as light intensity decreases, confirming that this polymer plays a key role in the adaptation of this species to establish successfully in the shadowed understorey of the forest.     

不同光强下几种南亚热带森林乔木的种子萌发和幼苗生长观察

对泡桐,春椿,木蝴蝶,枳Ｊｕ扁斗青冈和罗浮柿等亚热带森林乔木树种在不同强条件下的种子萌发及其幼苗生长进行观察,泡桐和枳Ｊｕ种子在光（１００％）条件下的萌发率最高,香椿和罗浮肺种子在轻阴（６２．２％）条件下的萌发率最高,木蝴蝶和扁斗青冈种子在中阴（３４．４％）条件下萌发率最高,阳性树种泡桐,香椿,木蝴蝶和枳Ｊｕ幼苗生长速度较快,属速生型树种,其幼苗在自然光条件下的总生物量最大;扁斗青冈和罗浮柿幼苗生     

The response of Spartium junceum roots to slope: anchorage and gene factors

BACKGROUND AND AIMS: Plant anchorage is governed by complex, finely regulated mechanisms that occur at a morphological, architectural and anatomical level. Spanish broom (Spartium junceum) is a woody plant frequently found on slopes--a condition that affects plant anchorage. This plant grows throughout the Mediterranean area where it plays an important role in preventing landslides. Spanish broom seedlings respond promptly to slope by altering stem and root morphology. The aim of this study was to investigate the mechanisms whereby the root system of Spanish broom seedlings adapts to ensure anchorage to the ground. METHODS: Seedlings were grown in tilted and untilted pots under controlled conditions. The root apparatus was removed at different times of growth and subjected to morphological, biomechanical and molecular analyses. KEY RESULTS: In slope-grown seedlings, changes in root system morphology, pulling strength and chemical lignin content, all features related to plant anchorage in the soil, were related to seedling age. cDNA-AFLP analysis revealed changes in the expression of several genes in root systems of slope-grown plants. BLAST analysis showed that some differentially expressed genes are homologues of genes induced by environmental stresses in other plant species, and/or are involved in the production of strengthening materials. CONCLUSION: Plants use various mechanisms/strategies to respond to slope depending on their developmental stage.