转Bt基因水稻根际土壤微生物多样性的磷脂脂肪酸(PLFAs)表征

以亲本水稻为对照,应用13C脉冲标记和磷脂脂肪酸技术,分析转Bt基因对水稻根际微生物多样性的影响.结果表明:转Bt基因水稻与亲本水稻根际均以饱和脂肪酸和支链脂肪酸为主,单不饱和脂肪酸次之,多不饱和脂肪酸最少.苗期、拔节期和抽穗期,转成基因水稻根际革兰氏阳性菌(G+)代表性磷脂脂肪酸含量显著低于亲本水稻;革兰氏阴性菌(G-)代表性磷脂脂肪酸含量显著高于亲本水稻.水稻各生育期,转Bt基因未对水稻根际土壤真菌、放线菌磷脂脂肪酸含量造成显著影响,且转Bt基因水稻与亲本水稻根际微生物磷脂脂肪酸13C含量无显著性差异.表明外源Bt基因插入仅对水稻根际微生物多样性造成短暂影响,不具有持续性.     

转crylAb基因克螟稻对根际可培养细菌类群的影响

田间栽培条件下,研究了转crylAb基因克螟稻1(KMD1)、克螟稻2(KMD2)及其亲本常规晚粳秀水11(XS11)不同生育期、不同根际部位(包括根际土壤、根表面和根内)可培养细菌类群数量、组成和动态变化.结果表明,各品种不同生育期根际平均细菌数量以拔节期最多,抽穗期最少;转Bt稻KMD1和KMD2根际平均细菌数量均显著高于XS11,且根表面和根内平均细菌数量也显著高于XS11,而根际土壤中平均细菌数量则低于XS11;试验共纯化菌株303个,经鉴定归于20个属,各品种根际出现的细菌类群大多为相同属,但数量有异,其中KMD1、KMD2根际的葡萄球菌属(Staphylococcus)、棒杆菌属(Corynebacterium)和壤霉菌属(Agromyces)均显著高于XS11,而XS11根际的束毛球菌属(Trichococcus)显著高于KMD1、KMD2;群落多样性分析表明,各水稻品种根际细菌群落的优势度指数C、多样性指数H′和均匀度指数J差异均不显著,表明转crylAb基因水稻对土壤细菌群落有一定影响,与水稻生育期和根际部位有关.     

转Bt水稻对土壤可溶性有机碳氮及微生物学性质的影响

研究了3种转Bt水稻［克螟稻（KMD）、华恢1号（HH1）和Bt汕优63（BtSY63）］及对应亲本在2年大田条件下对土壤可溶性有机碳（DOC）和可溶性有机氮（DON）及微生物学性质的影响.结果表明：测定指标均随采样时间发生显著变化.与对应亲本相比,转Bt水稻对土壤DOC、DON和微生物生物量氮（MBN）的影响不显著,而对土壤微生物生物量碳（MBC）、基础呼吸（BR）和微生物代谢熵（qCO₂）的影响在大田种植第1年某些时段达到显著水平（P<0.05）,但这种影响没有持续到第2年;3种亲本水稻土壤DOC、DON及微生物学性质差异均不显著,但相应转Bt水稻土壤MBC、BR、qCO₂差异显著,BtSY63土壤MBC和BR显著高于KMD及HH1,而qCO₂显著低于KMD及HH1.转Bt水稻2年的大田种植对土壤DOC和DON及微生物学性质的影响较小,但3种转Bt水稻之间微生物学性质的差异比亲本之间的差异大,表明长期监测可能有助于发现转Bt水稻对土壤生态系统结构和功能的影响.     

温度和CO2浓度升高下转Bt水稻种植对土壤活性碳氮和线虫群落的短期影响

全球气候变化下转Bt水稻种植对土壤生态系统的影响仍是未知领域。利用野外开顶式气室(OTC)模拟气候变化,探讨了温度和CO2浓度([CO2])升高后种植转Bt(华恢1号)水稻对土壤活性碳氮和线虫群落的影响。结果表明:(1)温度和[CO2]升高条件下种植转Bt水稻显著影响土壤可溶性碳(DOC)、可溶性氮(DON)和硝态氮(NO-3-N)含量,同时DOC和DON受到升温和[CO2]升高与转Bt水稻的交互影响;在正常温度和[CO2]下,转Bt水稻显著降低了土壤微生物量碳氮含量(MBC、MBN),但在温度和[CO2]升高后转Bt水稻种植使土壤MBC和MBN含量显著高于亲本水稻。(2)在温度和[CO2]升高条件下,转Bt水稻土壤线虫总数呈高于亲本水稻的趋势,植食性线虫的比例随[CO2]和温度升高呈增大的趋势。此外,升温和转Bt水稻种植提高了土壤线虫群落的能流通道指数,而[CO2]升高和转Bt水稻种植则提高了土壤线虫群落的富集指数。总之,在模拟全球气候变化下,种植转Bt水稻在短期内对土壤活性碳氮和线虫群落均产生影响,但短期研究并未发现转Bt水稻种植对土壤生态系统的不利影响。     

转Bt基因玉米根际微生物和细菌生理群多样性

大田栽培条件下,以转Bt玉米Mon810及其亲本非转基因玉米为研究对象,在玉米的不同生育期测定根际土壤微生物的数量变化,并对细菌群落结构及多样性进行了分析。结果表明,各生育期内Bt玉米与对照相比根际土壤真菌无显著差异,但细菌在抽丝期,放线菌在苗期二者有显著差异。同种细菌功能群的数量变化在Bt玉米和对照各生育期内趋势一致,不同的菌群表现不同。亚硝化细菌、好气固氮菌、氨化细菌和钾细菌的数量变化波动较大,其中Bt玉米和对照相比好气固氮菌的数量在生育期内有5个生育期有显著差异,而氨化细菌、钾细菌和亚硝酸菌仅分别在乳熟期、喇叭口期及拔节期和抽雄期差异显著。好气纤维分解菌和硝化细菌在苗期、拔节期和喇叭口期差异显著,无机磷分解菌在乳熟期和完熟期数量差异显著。整个生育期内3种群落特征参数均保持基本稳定,除乳熟期和完熟期外Bt玉米根际微生物群落特征参数均高于对照。     

不同生育期转Bt基因棉种植对根际土壤微生物的影响

 在盆栽种植条件下,比较研究了两个转Bt基因棉(Gossypium hirsutum)与对照棉对根际土壤细菌、放线菌、真菌和主要功能类群及多样性的影 响差异。结果表明：两个转Bt基因棉根际土壤均可检测到Bt蛋白,且不同转Bt基因棉根系分泌Bt蛋白量以及Bt蛋白在根际土壤中的降解率不同 。与各自对照相比,转Bt基因棉对细菌和真菌生长繁殖有促进作用,对放线菌、好气固氮菌和钾细菌数量没有显著影响。苗期和花期转Bt基因 棉均可显著提高氨化细菌、显著降低无机溶磷菌数量,花期均可显著提高好气纤维分解菌、显著降低有机溶磷菌数量,‘Bt冀668’苗期也可显 著提高好气纤维分解菌数量。转Bt基因棉根际土壤好气纤维分解菌、有机和无机溶磷菌多度发生了变化。尽管功能类群总数转Bt基因棉高于各 自对照常规棉,但群落多样性和均匀度都有所下降,优势集中性表现明显,且花期转Bt基因棉多样性参数值以及功能类群数量的变化幅度大于 苗期。     

种植转Bt基因棉土壤中Bt蛋白的分布

采用盆栽试验结合酶联免疫法(ELISA)测定了转Bt基因抗虫棉及常规棉花不同生育期根际及非根际土壤中的Bt蛋白含量.结果表明:红壤、黄棕壤及黄褐土种植转Bt基因棉花后根际土中Bt蛋白含量显著高于非根际土,而种植常规棉花的根际土与非根际土中Bt蛋白含量差异不显著.在初蕾期转Bt基因棉根际土中的Bt蛋白含量为黄褐土＞黄棕壤＞红壤,分别为常规棉根际土的144%、121%和238%;而在盛花期为黄棕壤＞黄褐土＞红壤,分别为常规棉根际土的156%、116%和197%.无论种植转Bt基因棉还是常规棉,供试土壤的根际和非根际土中Bt蛋白含量都随棉花生育期的推进先增加后减少,在盛花期达最大值.整个生育期内,转Bt基因棉根际土中的Bt蛋白含量大于其非根际土;种植转Bt基因棉土壤中Bt蛋白含量高于常规棉,说明转Bt基因棉花的Bt蛋白可释放到根际土中.     

转Bt cry1Ab基因水稻对稻田弹尾虫种群数量的影响

以转Bt cry1Ab基因水稻克螟稻1号(KMD1)和克螟稻2号(KMD2)及其亲本非转基因水稻秀水11(XS11)为材料,于2003年9月和翌春4月,调查了KMD1和KMD2对水稻灌浆期和收割后休田期稻田土表落叶层中弹尾虫种群数量的影响.结果表明,KMD植株表达的cry1Ab杀虫蛋白可在稻田环境中残留160 d以上;在水稻灌浆期采用吸虫器法在稻田落叶层中采集到灰橄榄长角跳虫(Entomobrya griseoolivata)和钩圆跳虫(Bourletiella christianseni)等2种弹尾虫,其中灰橄榄长角跳虫在KMD1和KMD2稻田中的种群密度显著高于XS11稻田;在水稻收割后休田期采用网袋法采集到灰橄榄长角跳虫、钩圆跳虫、球角跳虫(Hypogastrura matura)和等节跳虫(Isotoma monochaeta)等4种弹尾虫,其中转Bt基因水稻稻田中的灰橄榄长角跳虫和球角跳虫的种群密度显著高于XS11稻田,且其植株组织残体生物量损失率显著高于XS11.     

不同有机类肥料对小麦和水稻根际土壤线虫的影响

国内外对土壤线虫的研究绝大部分都是基于旱地的非根际土壤,对水田土壤,尤其是水旱轮作下根际土壤线虫对有机肥的响应研究非常地有限。因此,探讨在稻麦轮作体系中以猪粪堆肥和秸秆为原料的不同有机类肥料,对水稻和小麦根际土壤线虫的影响。田间试验处理为:CK(不施肥)、F(100%化肥)、PF(猪粪堆肥+50%化肥)、SF(秸秆全量还田+100%化肥)、PSF(猪粪堆肥+秸秆全量还田+50%化肥)、PMF(猪粪商品有机无机复合肥)。结果表明:小麦根际主要线虫优势属为头叶属(Cephalobus)、拟丽突属(Acrobeloides)和丝尾垫刃属(Filenchus);水稻根际则为杆咽属(Rhabdolaimus)、潜根属(Hirschmanniella)和丝尾垫刃属(Filenchus)。小麦根际土壤线虫总数显著高于水稻根际;SF和PSF对两季土壤线虫总数的提高显著高于PF和PMF,F对线虫总数没有显著影响。线虫营养类群表现为小麦根际食真菌线虫相对丰度显著高于水稻根际,植食性线虫和杂食/捕食性线虫丰度显著低于水稻根际。有机肥对稻季线虫群落的影响远大于麦季,秸秆配施有机肥显著提高麦季瓦斯乐斯卡指数(WI);施肥均显著提高稻季的多样性指数(H'),降低优势度指数(λ)。土壤线虫对肥料的响应取决于作物及肥料的质量和养分含量;在稻麦轮作系统下,同时添加秸秆和猪粪对提高土壤线虫数量和食微线虫丰度更有益。     

转Bt基因抗虫棉根际微生物区系和细菌生理群多样性的变化

在大田栽培条件下 ,以转 Bt基因抗虫棉 GK-12和常规棉花泗棉 3号作为材料 ,在棉花不同发育时期 ,于 2 0 0 1和 2 0 0 2连续两年测定棉花根际土壤细菌、放线菌和真菌数量的变化 ,并在 2 0 0 2年棉花的花铃期和吐絮期对根际细菌生理群的数量和多样性进行了分析 ,结果表明 :虽然不同年份和生育期棉花根际微生物数量存在差异 ,但是 ,年度间和相同的发育时期棉花根际微生物的数量变化趋势一致。在棉花的苗期和吐絮期 ,转 Bt基因抗虫棉根际微生物的数量与对照差异不显著 ;在棉花的花铃期 ,转 Bt基因抗虫棉根际细菌的数量比对照增加 ,放线菌的数量差异不显著 ,而真菌的数量变化没有规律。在棉花发育的花铃期和吐絮期 ,Bt棉根际细菌生理群的总数量比常规棉增加 ,但是根际细菌生理群的 Simpson指数、Shannon-Wiener指数和细菌生理群分布的均匀度下降     

Bt toxin is not taken up from soil or hydroponic culture by corn,carrot, radish,or turnip

The culture of transgenic Bt corn (Zea mays L.) has resulted in concern about the uptake of the Cry1Ab protein toxin by crops subsequently grown in soils in which Bt corn has been grown. The toxin released to soil in root exudates of Bt corn, from the degradation of the biomass of Bt corn, or as purified toxin, was not taken up from soil, where the toxin is bound on surface-active particles (e.g. clays and humic substances), or from hydroponic culture, where the toxin is not bound on particles, by non-Bt corn, carrot (Daucus carota L.), radish (Raphanus sativus L.), and turnip (Brassica rapa L.). The persistence of the toxin in soil for 90 days after its addition in purified form or for 120–180 days after its release in exudates or from biomass, the longest times evaluated, confirmed that the toxin was bound on surface-active particles in soil, which protected the toxin from biodegradation. The greater toxicity of the toxin in soil amended with 9% montmorillonite or kaolinite than in soil amended with 3% of these clay minerals indicated that the binding and persistence of the toxin increased as the clay concentration was increased.     

Microsites for seedling establishment of subalpine conifers in a forest with moss-type undergrowth on Mt. Fuji, central Honshu, Japan

We examined microsites for the seedling establishment of Tsuga diversifolia, Abies veitchii and Abies mariesii in a subalpine coniferous forest with moss-type undergrowth dominated by Hylocomium splendens and Pleurozium schreberi on Mt. Fuji, a less-snowy mountain located on the Pacific Ocean side of the central part of Honshu Island, Japan, and compared these microsites with cases in other regions of Japan with different undergrowth vegetation (herb type, dwarf-bamboo type) under snowier conditions. L-shaped size structures with abundant seedlings and small trees were identified for all three species. Established seedlings of T. diversifolia showed remarkable preference for elevated microsites such as fallen logs and convex ground (i.e. ground raised around trunk bases), while A. veitchii and A. mariesii indicated no significant bias toward such elevated microsites in the large seedling stage. However, the relative frequency of T. diversifolia seedlings established on flat ground reached high values of 54–77%, since the relative occupation area of the flat ground was large (85–89%). Such a large share suggests that flat ground should be regarded as the most important microsite for the seedling establishment of T. diversifolia in forests with moss-type undergrowth. This preference trait for T. diversifolia observed with moss-type undergrowth has not been reported in previous studies examining forests with herb- and dwarf bamboo-type undergrowth, where T. diversifolia seedlings are scarce on the ground and restricted to elevated microsites. We conclude that the major microsite for T. diversifolia seedling establishment changes according to the undergrowth vegetation type: a large ground share for moss-type undergrowth and a small ground share for herb- and dwarf bamboo-type undergrowth.     

Decomposition of Bt transgenic rice residues and response of soil microbial community in rapeseed–rice cropping system

In this study, Bt transgenic rice (KMD rice) residue decomposition and the associated microbial community in a rapeseed–rice cropping system were assessed in comparison with its parental non-Bt rice variety (XiuShui 11). Decomposition was measured as mass loss by straw and root decay in litterbags over two consecutive years. Bacterial and fungal community compositions associated with residue decomposition were detected by terminal restriction fragment length polymorphism (T-RFLP) and the additive main effects multiplicative interaction (AMMI) analysis model. Decomposition dynamics and bacterial and fungal communities associated with decomposition were strongly affected by surface and incorporated placements, and by temporal factors. However, no significant differences were observed between Bt and non-Bt rice variety in either decomposition dynamics or in the soil microbial communities associated with residue decay. Our field study indicated that the insertion of the cry1Ab gene into Xiushui 11 rice genome had no significant effect on the residual decay and decomposition-associated microbial community compositions in the rapeseed-rice cropping system.     

Difference in microhabitat-related regeneration patterns between two subalpine conifers, Tsuga diversifolia and Abies mariesii, on Mount Hayachine, northern Honshu, Japan

Seedling and tree-establishment microhabitats of Tsuga diversifolia and Abies mariesii were examined on the herb- and dwarf bamboo-dominated forest floor on Mount Hayachine, a mountain under intermediate snow conditions in northern Honshu, Japan. The four microsite types were fallen logs, buttresses, rocks and ground. The ground substratum was further divided into four subtypes by dominant undergrowth species: Lycopodium, Pteridophyllum, Carex and dwarf bamboo. The establishment of T. diversifolia seedlings on the ground was scarce, and depended mostly on non-ground microsites (i.e. fallen logs, buttresses and rocks). The seedling establishment of A. mariesii was not dependent on specific substrata, although on the ground, establishment sites were limited to the Lycopodium subtype situated on convex sites. Among the microhabitats for seedling establishment, larger trees of A. mariesii rarely occurred on higher portions of the non-ground microsites. In contrast, T. diversifolia could grow up to the height of canopy-layer trees in such microsites. Thus, non-ground microsites seem to be unsuitable for tree establishment in A. mariesii, and are probably useful microhabitats for regenerating T. diversifolia to avoid competition with A. mariesii. We also compared seedling-establishment microhabitats for the two conifers between Mount Hayachine and two other mountain regions under different undergrowth conditions (moss-dominated and dense dwarf bamboo-dominated). Our findings suggested that seedling recruitment in non-ground microsites was primarily determined by undergrowth conditions; T. diversifolia preferred such microsites where the moss-covered area was low, and A. mariesii preferred where dwarf bamboo-covered area was high.     

Bt抗虫棉秸秆还田对土壤养分特征的影响

【目的】研究转基因作物秸秆或残茬还田可能对土壤养分特性造成的影响。【方法】以不同抗虫水平Bt棉花和常规棉花(泗棉3号)为研究材料,分别在经过一、二个生长周期后将秸秆机械粉碎后原位还田,40 d后测定分析土壤中Bt蛋白含量及肥力相关的养分含量变化。【结果】Bt棉秸秆还田后,所有品种棉花土壤中Bt蛋白含量与还田前无显著增加,且转Bt基因棉与非转基因棉还田对土壤Bt蛋白含量的影响并无显著差异。同时,棉秸秆还田可显著提高土壤有机质、速效磷、碱解氮、速效钾、全氮、全磷和全钾含量,提升土壤pH值;增加幅度在不同抗虫水平Bt棉花间及与非转基因常规棉花品种间皆无显著性差异。【结论】秸秆还田对土壤肥力的提升与Bt棉的抗虫水平无关。“转Bt基因”不成为Bt棉秸秆还田提高土壤肥力的限制性因素,其秸秆还田不会对土壤肥力质量产生负面影响,可使土壤养分含量增加,有效提升土壤肥力。秸秆原位还田简单、无害又提升肥力,有条件作为转Bt基因植物秸秆无害化处理的理想方式。     

Bt rice expressing Cry1Ab does not stimulate an outbreak of its non-target herbivore, Nilaparvata lugens

In this study, the non-target effects of Bt rice “KMD2” expressing a Cry1Ab protein on the performance of the brown planthopper (BPH), Nilaparvata lugens, over multiple generations were evaluated under laboratory and field conditions. In the laboratory, BPH was reared to observe the impact of the Bt rice as compared to its parental non-Bt cultivar Xiushui 11, while the population dynamics and oviposition performance of BPH were investigated in the field. The survival of BPH nymphs fed Bt and non-Bt rice did not differ significantly. The nymph developmental duration of BPH was significantly delayed by the Bt rice by comparison with the non-Bt rice for the 1st and 2nd but not the 4th generation. Most importantly, the fecundity of BPH on the Bt rice was significantly decreased in every generation when compared with the non-Bt rice. In the field investigations, the population density of BPH nymphs was significantly lower in the Bt rice field. However, the temporal pattern of population dynamics of BPH adults was similar between the Bt and non-Bt rice, presumably due to migratory interference of the adults. In the Bt rice field, the percentage of tillers with eggs and the number of eggs per tiller were also significantly lower from tillering to mature stage. Additionally, Cry1Ab protein could not be detected in guts from single BPH adults. In general, our results suggest that the Bt rice “KMD2” could not stimulate an outbreak of BPH.     

Root proliferation characteristics of seven perennial arid-land grasses in nutrient-enriched microsites

We compared root proliferation in fertilized microsites among seven cultivars of five commonly planted cool-desert perennial grass species that differ in productivity and competitive ability. In a greenhouse experiment on nutrient-limited plants, one soil microsite in each pot received distilled water (control) and a second microsite received a rich, complete nutrient solution (fertilized). Roots in and adjacent to the microsites were mapped on Mylar windows for 22 days after the injections to determine the magnitude and timing of response in root length relative growth rates (RGRs). Because we provided adequate water, used a high level of fertilization in the treatment microsites, and conducted the experiments during rapid vegetative growth, the results provide a measure of the relative capacities and maximal rates of the grasses responses to enriched microsites. Root samples were harvested from control and fertilized microsites at the end of the experiment to determine the morphological basis of the proliferation response. In all seven grasses fine roots proliferated in the fertilized microsites faster than in the control microsites. The grasses did not differ in the timing of their response which showed a peak 7–8 days after injection. Although one species, Pseudoroegneria spicata cv. Goldar, had higher maximum root length RGR and higher RGR ratio (RGR in fertilized to RGR in control microsites) 7–8 days after injection, the seven grasses did not differ significantly in the magnitude of root length RGR response to fertilizer integrated over the 22 day experiment. The grasses also did not differ significantly in root morphological changes in fertilized mocrosites. Compared to roots in control microsites, roots in fertilized microsites had greater specific root length, length of secondary roots per length of main axis, number of lateral and sublateral roots per length of main axis, and mean lateral root length. Root proliferation was mainly the result of increased lateral branching and lateral root growth in all seven grasses. The consistency of root proliferation responses among these seven cultivars suggests that differences in the capacity for, maximum rate, or morphological basis of root proliferation are not directly related to ecological characteristics such as productivity and competitive ability. Other aspects of root response to nutrient enrichment, such as differential responses as a function of microsite nutrient concentration, plant phenology, plant nutrient status, or specific nutrient element(s), may still be important, but further experiments are required to determine whether different responses to enriched soil microsites among species correspond with know species differences in ecological characteristics.     

Development of a model system to assess the impact of genetically modified corn and aubergine plants on arbuscular mycorrhizal fungi

We developed an experimental model system to monitor the impact of generically modified (GM) plants on arbuscular mycorrhizal (AM) fungi, a group of non-target soil microorganisms, fundamental for soil fertility and plant nutrition. The system allowed us to study the effects of root exudates of both commercial Bt corn and aubergine plants expressing Dm-AMP1 defensin on different stages of the life cycle of the AM fungal species G. mosseae. Root exudates of Bt 176 corn significantly reduced pre-symbiotic hyphal growth, compared to Bt 11 and non-transgenic plants. No differences were found in mycelial growth in the presence of Dm-AMP1 and control plant root exudates. Differential hyphal morphogenesis occurred irrespective of the plant line, suggesting that both exuded Bt toxin and defensin do not interfere with fungal host recognition mechanisms. Bt 176 affected the regular development of appressoria, 36% of which failed to produce viable infection pegs. Our experimental model system represents an easy assay for testing the impact of GM plants on non-target soil-borne AM fungi.     

Gap formation, microsite variation and the conifer seedling occurrence in a subalpine old-growth forest, central Japan

To evaluate the effects of canopy gaps and forest floor microsites (soil, fallen logs, root-mounds, buttresses and stumps) on regeneration of subalpine forests, the gap regeneration and seedling occurrence of conifers (Abies mariesii, Abies veitchii, Picea jezoensis var. hondoensis and Tsuga diversifolia) were studied in two stands of a subalpine old-growth forest, central Japan. The percentage of gap area to total surveyed area was 11.2–11.3% in the stands. Gap regeneration was not common for P. jezoensis var. hondoensis and T. diversifolia. In contrast, gap regeneration by advanced regeneration was relatively common for Abies. Seedling occurrence of P. jezoensis var. hondoensis and T. diversifolia was restricted on elevated surfaces such as stumps and root-mounds, while Abies seedlings could occur on soil as well as on elevated surfaces. Rotten stumps were the most favorable microsites for conifer seedling occurrence, which covered small area in the forest floor. Although canopy gaps were not always favorable for seedling occurrence, all conifer seedlings were larger under canopy gaps than under closed canopy. Canopy gaps and forest floor microsites clearly affected seedling occurrence and growth of conifers. This suggests that regeneration of conifers is related to the difference of growth advantage under canopy gaps and favorable microsites for seedling occurrence.     

Roles of cottony hairs in directed seed dispersal in riparian willows

Willows usually establish on wet substrates with fine sediments at sites that are created by large disturbances, but suitable microsites are spatially and temporally limited. Thus, we hypothesized that willow seeds are selectively dispersed to suitable microsites, such as those with a wet substrate, rather than unsuitable microsites, such as those with a dry substrate, with seedling establishment mediated by the cottony hairs attached to seeds (directed dispersal). To test our hypothesis, we compared several recruitment-related traits, including buoyancy, germination, and trapping at favorable microsites, in seeds of the riparian willows Salix sachalinensis and S. integra with and without cottony hairs in laboratory and field experiments. In both field and laboratory experiments, more seeds with cottony hairs were trapped in water and wet sand than in dry sand, in which no seeds of either species germinated. These results indicate that cottony hairs facilitate the recruitment of seeds to microsites favorable for seed germination and help seeds avoid unfavorable microsites. On the water surface, 17.6% of S. sachalinensis seeds and 68.0% S. integra seeds with cottony hairs floated for more than 6 days, whereas all seeds without cottony hairs sank immediately after being placed on the water surface. These results suggest that cottony hairs facilitate long-distance dispersal via flowing water and also help avoid germination under water, where willow seedlings fail to establish. Seeds of the two willow species were released from the cottony hairs and germinated immediately after the seeds were placed on wet sand, but not after placement on water or dry sand. These results suggest that the seeds are released from the cottony hairs when the hairs become wet and the seeds are striking to a suitable microsite for seedling establishment, such as wet sand. In riparian willows, the cottony hairs promote directed dispersal by moving seeds to discrete and predictable microsites where the seedling establishment is disproportionately high.