菊科入侵植物三叶鬼针草的繁殖特征及其与入侵性的关系

三叶鬼针草(Bidens pilosa)是一种危害严重的菊科入侵植物.通过实验观察和人工控制套袋等方法,对其花序开花动态、花粉胚珠比(P/O)、自交亲和性、花粉活力、访花昆虫和种子(瘦果)的萌发率等与繁殖相关的特征进行了研究,探讨了这些繁殖特征与入侵性的关系.三叶鬼针草在10-11月开花,单个花序的花期约为5-6天.每小花的花柱基部有圆筒状的蜜腺环绕.单个花序内可自交亲和,自交结实率和花粉活力均较高,其P/O值为1754.12士29.87.主要访花者为灰蝶科(Lycaenidae)、粉蝶科(Pjerjdae)和茧蜂科(Braconidae)昆虫.三叶鬼针草所具有的灵活交配机制是其入侵成功的重要因素.此外,三叶鬼针草结实量大、种子产生迅速且适于传播,以及种子萌发范围广和短期快速萌发等特性也增强了其入侵性.     

菊科入侵植物三叶鬼针草的繁殖特征及其与入侵性的关系

三叶鬼针草(Bidens pilosa)是一种危害严重的菊科入侵植物。通过实验观察和人工控制套袋等方法, 对其花序开花动态、花粉胚珠比 (P/O)、自交亲和性、花粉活力、访花昆虫和种子 (瘦果)的萌发率等与繁殖相关的特征进行了研究, 探讨了这些繁殖特征与入侵性的关系。三叶鬼针草在10-11月开花, 单个花序的花期约为5-6天。每小花的花柱基部有圆筒状的蜜腺环绕。单个花序内可自交亲和, 自交结实率和花粉活力均较高, 其P/O值为1 7 54.1 2±29 .8 7。主要访花者为灰蝶科(Lycaenidae)、粉蝶科(Pieridae)和茧蜂科(Braconidae)昆虫。三叶鬼针草所具有的灵活交配机制是其入侵成功的重要因素。此外, 三叶鬼针草结实量大、种子产生迅速且适于传播, 以及种子萌发范围广和短期快速萌发等特性也增强了其入侵性。     

Performance and responses to competition in two congeneric annual species: does seed heteromorphism matter?

Variations in seed characteristics observed in heteromorphic species may affect various stages of their life cycles, e.g. seed dormancy, germination characteristics or even adult plant performance. Highly specialised seed morphs – described as colonisers and maintainers – exhibit a trade‐off between colonisation capacity and competitive traits. The performance of distinct seed morph progeny under competitive conditions, and especially in multi‐species arrangements, had previously not been given much attention. In this study, we compared performance and response to competition among distinct seed morph progenies in two congeneric, co‐occurring species: the invasive Bidens frondosa and the non‐invasive Bidens tripartita. We hypothesised that maintainer seed morphs of both species would perform better under increased plant densities and within inter‐morphic mixtures, while coloniser morphs would show stronger responses to increased densities and perform relatively poorly in inter‐morphic mixtures. We conducted a growth trial and a greenhouse experiment, which revealed that seed morph progeny differed significantly in plant height when grown without competition, while under competitive conditions such differences became less apparent. The observed pattern was more strongly pronounced in B. frondosa, which showed a general predominance in stature and biomass over its non‐invasive congener. Although seed morphs performed equally well under competitive conditions, increased plant height and more rapid germination can favour the maintainer seed morph on sites where vegetation is already present.     

多叶斑叶兰繁育系统与传粉生物学研究

为探讨多叶斑叶兰(Goodyera folisa)的繁育系统与传粉生物学特征,对其开花物候、花粉活力与柱头可授性、人工授粉、花的挥发性成分以及昆虫传粉行为进行了研究。结果表明,多叶斑叶兰的唇瓣黄色,萼片白色或白色带红褐色;单花花期为(9.4±0.8)d;花粉在开花后第1天具备活力,柱头在开花后第2天具备可授性,花粉活力和柱头可授性都在开花后第5天达到峰值。去雄套袋和不去雄套袋都不能结实,人工自花授粉、同株异花授粉和异株异花授粉的结实率分别为93.3%、95.0%和96.7%,自然结实率为43.3%。花朵的主要挥发性成分为1-辛烯-3-醇、3-辛醇和N,N-二甲基甲酰胺。多叶斑叶兰传粉者为中华蜜蜂(Apis cerana)。多叶斑叶兰具有自交亲和能力,但在自然界不具有主动自交现象,必须依赖中华蜜蜂传粉,花色及花香气味为吸引传粉者的主要因素。     

Reproductive traits associated with invasiveness in Conyza sumatrensis

Conyza sumatrensis (Retz.) E. Walker, a member of Asteraceae, is a highly invasive species. However, its reproduction biology remains poorly known. To understand the role of reproductive traits in successful invasion of the species, we studied several traits of its reproductive system: the miniature capitulum and gynomonoecious sexual system, the biology and phenology of capitula and florets, pollen/ovule ratio, the mating system (selfcompatibility), flower visitors, physical traits and dispersal potential of achenes, germination potential of achenes from manually pollinated capitula, and the association of these traits with invasiveness. Our study showed that the reproductive traits of autonomous seed production, versatile mating system of self- and cross-pollination, and generalized pollination system might contribute to the species' successful invasive capability. The invasiveness was further enhanced by the high and rapid production of achenes, as well as the high percentage, rapid germination rate and high dispersal capability of achenes. It was concluded that in annual or winter-annual weeds, autonomous seed production contributed significantly to the invasiveness of the species.     

一年生短命植物疏齿千里光果实异形性的生态学意义

疏齿千里光(Senecio subdentatus)是分布在新疆北部古尔班通古特沙漠中的一种具异形瘦果的菊科一年生短命植物。将野外观测与室内实验相结合, 对该物种异形瘦果的形态、扩散和萌发特性, 以及异形瘦果产量与植株大小的关系进行了研究, 并对其生态学意义进行了探讨。结果表明: 疏齿千里光果序中的外围果和中央果均为柱形, 但前者为淡黄色, 后者为褐色, 且二者在大小、冠毛长度及果皮微形态等方面均存在明显差异。两种瘦果均以单个果实为扩散单元, 且在静止空气中的降落速度和在1 m·s^-1与2 m·s^-1风速下的扩散距离无显著差异, 说明虽然果实大小和冠毛长度对瘦果扩散具有不同的影响, 但对其整体扩散能力无明显影响。在各温变周期(5/2、15/2、20/10、25/15和30/15 ℃)处理中, 淡黄色外围果的萌发率均高于褐色中央果的, 且不同温度间两种瘦果的萌发率均存在显著差异, 但光照条件对其无显著影响。果序中的中央瘦果数明显多于外围果的, 且植株中外围果所占比例与植株大小间呈显著负相关关系, 而中央果所占比例与植株大小间呈显著正相关关系。这些特点说明, 该物种的小植株倾向于产生较多较易萌发的外围果, 大植株倾向于产生较多不易萌发的中央果。在古尔班通古特沙漠不可预测的极端环境中, 疏齿千里光可通过异形瘦果间的萌发差异及调节其不同大小植株中异形瘦果的比例, 来减少同胞后代之间的竞争, 增加其对不同微环境条件的生态适应性。     

珍稀濒危植物距瓣尾囊草(Urophysa rochkii)的开花物候和繁育系统特性

通过野外观察,运用套袋授粉和联苯胺-过氧化氢法等实验方法对距瓣尾囊草开花物候以及种群的繁育系统特性进行研究。研究结果:1)距瓣尾囊草为两性花,聚伞状花序,花期较长,从头年12月持续到次年4月,单花花期持续8—15 d,种群花期可持续4个月左右。2)距瓣尾囊草在花朵未开放前花粉没有活力,花药开裂当天花粉活力达到94.8%,第2天为90.3%,第3天为81.6%,第4天为62.9%,此后其花粉活力明显减弱;柱头可授性于散粉后第3天开始出现,散粉后4—5天最强,散粉后第8天柱头失去可授性;柱头可授性和花粉活力有5 d左右的重叠期。3)其花粉胚珠比P/O为938.5±250,按照Cruden标准划分,其有性繁育系统为兼性异交。4)按照Dafni的标准,距瓣尾囊草的OCI=4,即繁育系统为异交、部分自交亲和、需要传粉者。5)人工异花授粉结实率达80%以上,略高于自然结实率78.94%;直接套袋结实率为5.71%;去雄套袋和去雌套袋均不结实。以上结果表明,距瓣尾囊草繁育系统表现为异交为主、部分自交亲和并且不存在无融合生殖现象,其开花物候、花部特征和繁育系统为适应特殊的环境提供了一定的生殖保障,本次研究结果为距瓣尾囊草的迁地保护和种群的繁衍复壮提供理论依据。     

Ecology of achene dimorphism in Leontodon saxatilis

BACKGROUND [corrected] AND AIMS: Leontodon saxatilis produces two morphologically distinct achenes (morphs) in a single capitulum: one row of dark brown achenes without a pappus lies at the edge ('peripheral achenes'; 0.74 +/- 0.18 mg) while the inner ones are light brown with a pappus ('central achenes'; 0.38 +/- 0.07 mg). The hypothesis that achene heteromorphism in L. saxatilis widens its ecological amplitude was tested. KEY RESULTS: Achenes of both morphs germinated over the same range of temperatures (6-33 degrees C) but the central achenes showed significantly higher germination percentages, and the two also differed significantly in their annual dormancy cycle, with the peripheral achenes showing greater dormancy for part of the year. Seedlings from the two morphs did not differ significantly in total biomass after 2 and 4 weeks of growth, neither did they differ significantly in root and shoot weight and root:shoot ratio. Plants from both morphs growing at different regimes of soil moisture, nutrients and competition did not differ significantly in their number of achenes per capitulum. While the number of central achenes varied, that of peripheral achenes remained constant at approx. 13. Drier soil led to an increase in the number of central achenes in plants from both morphs. CONCLUSIONS: The peripheral achenes can replace the mother plant in the following growing season, whereas the central achenes are well adapted for wind dispersal and thus for colonization of new sites. However, the extent of variation in germination characteristics was similar to that found in seed populations of homomorphic plants, which suggests that germination percentage and other growth characteristics do not contribute to widening the ecological amplitude. The habitat of most heteromorphic species, the morphs of which differ greatly in germination and/or growth characteristic, are deserts or highly disturbed areas where such differences are highly advantageous, but in the moderate habitat of L. saxatilis the differences may prove a disadvantage.     

野生荞麦细柄野荞麦的繁殖生物学特性研究

该研究通过野外观察和人工控制实验相结合的方法,从开花动态、花部基本特征、繁育系统、传粉生物学及种子性状等方面对荞麦属(Fagopyrum Mill.)植物细柄野荞麦(Fagopyrum gracilipes)的繁殖生物学特性进行了探究,并分析了各性状对其繁殖的贡献。结果表明:在贵州威宁,细柄野荞麦的花果期常为每年的6—10月,单花序和单花的花期分别为13～21 d和1～3 d。花较小,直径为(3.99±0.12) mm,花柱和花药高分别为1.30和1.65 mm,花直径与花被片长和花被片宽呈显著正相关,花柱高与花药高呈极显著正相关。细柄野荞麦花粉胚珠比为371±16.40,杂交指数为2,套袋实验显示其自交、异交亲和,表明其繁育系统为兼性自交,部分异交亲和。细柄野荞麦的访花昆虫较少,主要为膜翅目(Hymenoptera)、双翅目(Diptera)和鞘翅目(Coleoptera) 7个科的9种昆虫,食蚜蝇科(Syrphidae)昆虫是其主要传粉昆虫。细柄野荞麦果实存在有翅和无翅两种类型,有利于其适应不同的传播方式,种子较小,千粒重为(1.05±0.04) g,萌发率较低,播种后30 d的累积萌发率为(19.60±2.14)%,但萌发整齐,主要集中在前5 d。综上所述,细柄野荞麦灵活的繁育系统为其产生大量种子提供了保障,多样的果实传播方式和整齐的种子萌发特性为其占据更广阔的生境成为群落优势种创造了基础。     

Variation in floral morphology and plant reproductive success in four Ipomoea species (Convolvulaceae) with contrasting breeding systems

• This study tested the hypothesis that self‐compatibility would be associated with floral traits that facilitate autonomous self‐pollination to ensure reproduction under low pollinator visitation. In a comparison of two pairs of Ipomoea species with contrasting breeding systems, we predicted that self‐compatible (SC) species would have smaller, less variable flowers, reduced herkogamy, lower pollinator visitation and higher reproductive success than their self‐incompatible (SI) congeners.
• We studied sympatric species pairs, I. hederacea (SC)– I. mitchellae (SI) and I. purpurea (SC)–I. indica (SI), in Mexico, over two years. We quantified variation in floral traits and nectar production, documented pollinator visitation, and determined natural fruit and seed set. Hand‐pollination and bagging experiments were conducted to determine potential for autonomous self‐pollination and apomixis.
• Self‐compatible Ipomoea species had smaller flowers and lower nectar production than SI species; however, floral variation and integration did not vary according to breeding system. Bees were primary pollinators of all species, but visitation rates were seven times lower in SC than SI species. SC species had a high capacity for autonomous self‐pollination due to reduced herkogamy at the highest anther levels. Self‐compatible species had two to six times higher fruit set than SI species.
• Results generally support the hypothesis that self‐compatibility and autonomous self‐pollination ensure reproduction under low pollinator visitation. However, high variation in morphological traits of SC Ipomoea species suggests they maintain variation through outcrossing. Furthermore, reduced herkogamy was associated with high potential for autonomous self‐pollination, providing a reproductive advantage that possibly underlies transitions to self‐compatibility in Ipomoea.

     

Breeding system of the annual Cruciferae, <Emphasis Type="Italic">Arabidopsis kamchatica</Emphasis> subsp. <Emphasis Type="Italic">kawasakiana</Emphasis>

The breeding system of an annual Cruciferae, Arabidopsis kamchatica subsp. kawasakiana, was studied in three natural populations. We applied four experimental treatments, open pollination, bagging, emasculation + bagging, and emasculation + hand-pollination + bagging. None of the emasculated flowers with bags produced fruits but we observed high fruit sets in the other three treatments. The results confirmed that A. kamchatica subsp. kawasakiana is a self-compatible, non-apomictic species that can produce seeds through auto-pollination. Considering the life cycle as an annual, increased reproductive assurance through auto-pollination should be critical for the maintenance of populations of A. kamchatica subsp. kawasakiana.     

Reproductive features of Celmisia species (Asteraceae) in relation to altitude and geographical range in New Zealand

A range of seed and flower characters was examined in 37 species of the New Zealand genus Celmisia (Asteraceae) to determine whether there were any relationships between reproductive features and the geographic or altitudinal ranges of the species. Using published sources and herbarium material, flower attractiveness to pollinators was measured in terms of capitulum size and ray/disc length ratio. Dispersibility was measured in terms of seed dry weight, seed terminal velocity, pappus/seed length ratio, and scape/leaf length ratio. Altitude (minimum and maximum) and geographic ranges (number of 1^o latitude x 1^o longitude squares occupied) were obtained from published sources. A multivariate principal components analysis using 12 morphological characters revealed the presence of five groups of species differing in capitulum size, seed weight, pappus length, number of pappus bristles, leaf length and scape length. Linear regressions showed that altitude was significantly and negatively related to seed dry weight and capitulum size, and positively to scape/leaf length ratio. Geographic range is negatively related to capitulum size, and positively to ray/disc ratio and altitudinal range. Amongst the seed features measured, seed weight was the best predictor of terminal velocity (an inverse measure of dispersability), but lighter seeds also had a higher pappus/seed length ratio. The frequency distribution of the geographic ranges of the species is highly skewed, with many local species (86% occurring in 15 or fewer one‐degree squares) and very few common ones. Two species (C. gracilenta and C. graminifolia) are notably more common than all the others. A comparison of these species with the others shows that, on average, they have lighter seeds with more bristles, taller scapes and higher ray/disc ratios, but have smaller capitula. The low geographic abundance of the rare species could not readily be attributed to any specific reproductive feature, possibly because of the multiple types of rarity represented within the genus.     

Floral display and habitat fragmentation: Effects on the reproductive success of the threatened mass‐flowering Conospermum undulatum (Proteaceae)

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二型花柱植株金荞麦繁殖特征

开花物候及繁殖分配是植物适应环境的重要因素。对金荞麦开花物候、繁殖分配及策略进行了研究。结果如下:金荞麦的花果期为每年的8—11月,9月集中开花,其集中开花模式有助于吸引昆虫传粉,提高繁殖成功率;金荞麦单花开花持续时间为1—2 d,种群花期均为85d。L型花序花期为15—26d,S型花序花期为14—27d,两者没有显著差异;L型单花序开花数为26—131朵,S型单花序开花数为36—147朵,两者没有显著差异。L型和S型花序开花动态呈现单峰曲线,在花序开花后第11天L型和S型都达到最大值,分别为7.30%和7.20%,且两种花型具有较高的开花同步性,这有助于其繁殖适应性的提高。同一个花型中,雌蕊长、雄蕊长之间存在极显著负相关,但雌雄总长不存在显著差异,表明雌蕊长、雄蕊长可能存在权衡关系;金荞麦的繁殖器官和营养器官生物量在L型和S型间不存在显著差异,但其花生物量与植株生物量表现出极显著正相关关系。金荞麦L型花生物量分配极显著大于S型,而总生物量不存在显著差异,说明金荞麦植株的营养生长与有性繁殖间存在权衡关系。     

Reproductive versatility in legumes: the case of amphicarpy in Trifolium polymorphum

Amphicarpy is a fascinating reproductive strategy, defined as fruit produced both below the soil surface and as aerial fruit on the same plant. Trifolium polymorphum is a grassland species subject to herbivory that combines amphicarpy with vegetative reproduction through stolons. Underground flowers have been described as obligate autogamous and aerial ones as self‐compatible allogamous, with aerial floral traits favouring cross‐pollination. In the present work we performed different pollination treatments on aerial flowers to analyse rates of pollen tube development and offspring fitness, measured as fruit set, seed production and germination percentage. This last variable was compared to that of seeds produced underground. No significant differences were found between fruit set in self‐ and cross‐pollinations. Seed production was higher in self‐pollinations, which is consistent with the higher rate of pollen tube development observed in self‐crosses. Spontaneous self‐pollination is limited in aerial flowers; thus pollen transfer by means of a vector is required even within the same flower. Germination tests showed that aerial seeds produced after self‐ and cross‐pollination did not differ in fitness, but underground seeds had higher germination percentage than aerial ones. Thus, we conclude that T. polymorphum has a mixed mating system. In grasslands with heavy grazing pressure, clonal propagation and underground seed production ensure persistence in the field. An intermediate level of selfing in aerial flowers ensures offspring, but morphological (herkogamy) and functional (dicogamy) floral traits maintain a window to incorporate genetic variability, allowing the species to tolerate temporal and spatial pressures.     

Divergence on floral traits and vertebrate pollinators of two endemic Encholirium bromeliads

Shifts in pollen vectors favour diversification of floral traits, and differences in pollination strategies between congeneric sympatric species can contribute to reproductive isolation. Divergence in flowering phenology and selfing could also reduce interspecific crossing between self‐compatible species. We investigated floral traits and visitation rates of pollinators of two sympatric Encholirium species on rocky outcrops to evaluate whether prior knowledge of floral characters could indicate actual pollinators. Data on flowering phenology, visitation rates and breeding system were used to evaluate reproductive isolation. Flowering phenology overlapped between species, but there were differences in floral characters, nectar volume and concentration. Several hummingbird species visited flowers of both Encholirium spp., but the endemic bat Lonchophylla bokermanni and an unidentified sphingid only visited E. vogelii. Pollination treatments demonstrated that E. heloisae and E. vogelii were partially self‐compatible, with weak pollen limitation to seed set. Herbivores feeding on inflorescences decreased reproductive output of both species, but for E. vogelii the damage was higher. Our results indicate that actual pollinators can be known beforehand through floral traits, in agreement with pollination syndromes stating that a set of floral traits can be associated with the attraction of specific groups of pollinators. Divergence on floral traits and pollinator assemblage indicate that shifts in pollination strategies contribute to reproductive isolation between these Encholirium species, not divergence on flowering phenology or selfing. We suggest that hummingbird pollination might be the ancestral condition in Encholirium and that evolution of bat pollination made a substantial contribution to the diversification of this clade.     

The effect of galls induced by the gall fly Procecidochares utilis on vegetative growth and reproductive potential of crofton weed, Ageratina adenophora

Reproductive potential and vegetative growth of Ageratina adenophora (Spreng.) K. & R. (crofton weed) were measured under ambient conditions to evaluate the efficacy of the gall fly, Procecidochares utilis Stone as a biocontrol agent for this weed. Galls induced by the fly resulted in the production of underdeveloped capitula and a reduction in the number of capitula and filled achenes per capitulum. Galling had no effect on subsequent germination of the seed produced. Where vegetative growth was concerned the effect of galling ranged from a temporary cessation of growth to death of stems above the gall. Although galling promoted production of side shoots, the dry mass of above-ground material was markedly reduced.     

Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.     

Reproductive biology of Aechmea bracteata (Sw.) Griseb. (Bromelioideae: Bromeliaceae)

• Individuals of Aechmea bracteata show inflorescences with red scape bracts and odourless, yellow, tubular diurnal flowers, with closely arranged sexual organs, producing a large amount of fruits.
• In order to investigate the reproductive system of this species, a suite of characters was assessed: phenology, floral morphology and biology, nectar production dynamics, and fruit and seed production and germination, as a result of controlled pollination crosses. The study was conducted during two flowering seasons in wild populations in Yucatán, Mexico.
• Results suggest an annual flowering pattern with one flowering peak; flowers were diurnal, showing partial dichogamy (protandry)‐herkogamy, anthers and stigma become mature before floral aperture, which could lead to self‐pollination, nectar is produced during anthesis, varying in volume and total sugar concentration during the day; fruits and seeds were produced in all experimental crosses (cross‐pollination, obligated cross‐pollination, assisted and unassisted selfing, geitonogamy and apomixis), as well as high percentage seed germination.
• Several species of Aechmea are reportedly self‐compatible and autogamous, as suggested by results of selfing and non‐assisted selfing crosses, but these results are negated by the presence of apomixis, indicating that the species is apomictic. This is the first report of this breeding system for subgenus Aechmea and the sixth for Bromeliaceae. Polyembryony is here suggested for the first time in this genus and family based on the fact that more seeds were recorded that expected based on ovule numbers. Finally, when performing experimental crosses, estimating reproductive success based on number of seeds is a better approach than number of fruits, due to the effect of pseudogamy.

     

Effect of source–sink ratio on seed set and filling in sunflower (Helianthus annuus L.)

Poor seed development in sunflower may result from insufficient assimilate supply (source limitation). To test this hypothesis, the effects of changed source–sink ratio on seed set (measured as percentage of empty achenes) and seed filling (measured as dry mass per filled achene) in individual plants were investigated. Source–sink ratio, defined as leaf area per floret (LAF), was experimentally altered using invasive (floret removal, defoliation) and non‐invasive (pulse of chilling, short days or shading during leaf or floret initiation) treatments. Shading at floret initiation proved the most effective non‐invasive method. Generally, an increase, or decrease, in LAF improved, or impaired, both seed set and filling. Increasing LAF by 2.0 cm²[95% confidence interval (1.5, 2.5)] decreased the percentage of empty achenes by 36.9%‐points (?41.9, ?30.9) and increased dry mass per filled achene by 20.1 mg (13.6, 26.7) in the capitulum centre. The effect of source–sink ratio on seed set was always strongest in the centre, whereas peripheral whorls were not affected. Achene mass was affected in all parts of the capitulum. It is concluded that source limitation is a major cause for empty achenes in sunflower plants grown under non‐stress conditions.