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Studies of mycoheterotrophs, defined as plants that obtain carbon resources from associated mycorrhizal fungi, have fundamentally contributed to our understanding of the importance and complexity of symbiotic ecological interactions. However, to date, the reproductive ecology of these organisms remains empirically understudied, with existing literature presenting hypotheses about traits including a generalist pollination syndrome and autogamous self-pollination. To address this gap in our knowledge of the reproductive ecology of mycoheterotrophic plants, we comparatively analyzed three species of two monotropoid genera, Monotropa and Monotropsis. During three consecutive years of field observations and manipulations of four populations of Monotropa uniflora, seven of M. hypopitys (both red and yellow color forms), and two of Monotropsis odorata, we investigated flowering phenology, pollination ecology, breeding system, floral herbivory, and reproductive effort and output. Contrary to previous predictions, our results revealed that taxa are largely outcross-pollinated and specialized toward Bombus pollinators. Additionally, species differ in breeding system, timing and duration of reproductive development, fluctuations in reproductive effort and output, and fitness impacts of herbivory. This study is the first thorough investigation of the reproductive ecology of mycoheterotrophic species and provides insight into possible limitations in reproductive traits imposed by a mycoheterotrophic life history. Key words: breeding system; Ericaceae; Monotropa; Monotropsis; mycoheterotroph; phenology; pollination; reproductive ecology. doi: 10.3732/ajb.0800319

Poa is a complex genus taxonomically and genetically. As a result, relatively little information exists for this economically important genus with regards to reproductive mode and variability in chromosome number. We examined apomixis frequency and ploidy levels in 83 Poa accessions representing 33 species from the National Plant Germplasm System using flow cytometric techniques. In reproductive mode analysis, we analyzed at least three preparations of 50 seeds each from the accessions. In ploidy level analysis, at least three plants of each accession were analyzed. Sixty percent of the species had at least one apomictic or facultative apomictic accession; 40% were sexual. Thirty-three percent of the species had no sexual accessions. Autonomous apomixis was newly identified in P. bactriana and P. bulbosa in addition to it being previously reported in P. nervosa. Poa annua and P. trivialis were identified as obligate sexual, and P. nervosa as obligate apomictic. Two of four P. palustris accessions exhibited facultative apomixis. Ploidy levels among sexually reproducing Poa accessions ranged from 2n = 2x = 14 to 2n = 12x = 84, with most accessions between 2n = 3x = 21 to 2n = 8x = 56. Among apomictic accessions, ploidy levels ranged from 2n = 3x = 21 to 2n = 10x = 70, with most between 2n = 4x = 28 and 2n = 10x = 70. These results emphasize the wide variability in Poa, both among and within species.

Paspalum plicatulum Michx. is a wild forage grass species. The common races are tetraploid and apomictic, while sexual diploid representatives have been reported sporadically. Objectives of this study were to induce sexual 4x individuals from sexual diploids, determine the capacity for hybridization with other apomictic 4x species closely related with P. plicatulum, and thus create a tetraploid sexual material cross compatible with apomictic 4x species of the Plicatula group of Paspalum. Two induced tetraploid plants were recovered from germinating 2x seeds treated for 24 h with colchicine. Bivalent and quadrivalent chromosomes were the most common association at meiosis in these autotetraploids. Embryological analysis and progeny tests using molecular markers revealed that both induced tetraploids reproduced sexually. Single-seed screening by flow cytometry confirmed full sexual reproduction. These plants retained the high self-incompatibility system of the diploids, but set seed after reciprocal crosses and when crossed with pollen of apomictic 4x P. guenoarum Arechav. of the Plicatula group. Both sexual 4x plants constitute the foundational material for plant improvement through gene exchange and selection in apomictic 4x P. plicatulum and possibly in several apomictic species of the Plicatula group.

Development of theory has outpaced experimental tests for most maintenance of diversity mechanisms. Here we demonstrate how data from biodiversity--ecosystem functioning experiments can be used to determine the mechanisms that maintain plant species diversity. We hypothesized that grassland plant diversity is maintained by two classes of mechanisms: (1) equalizing mechanisms, which reduce asymmetric competition by reducing differences in monoculture biomass production among species in mixture, and (2) species interaction mechanisms, which increase overyielding by increasing niche partitioning and facilitation among species in mixture. Specifically, equalizing mechanisms reduce the coefficient of variation in monoculture biomass production among species in mixture. Species interaction mechanisms increase species overyielding in mixture, especially for low-biomass species. We tested these predictions with a seven-year data set from an experiment that varied grassland plant species evenness and richness. We used path analysis to model effects of these mechanisms on annual and multiyear changes in diversity. We found that diversity was frequently maintained by species interaction mechanisms and was infrequently maintained by equalizing mechanisms. Species interaction mechanisms maintained diversity by allowing the species that produced the least biomass in monoculture to benefit the most from species interactions in mixture. Equalizing mechanisms infrequently maintained diversity because asymmetric competition infrequently resulted in competitive exclusion. We propose that this mechanistic framework be used to better understand the specific processes that influence diversity. Key words: biodiversity; coexistence; complementarity; ecosystem function; net biodiversity effect; productivity.

The number of ovules per flower varies over several orders of magnitude among angiosperms. Here we consider evidence that stochastic uncertainty in pollen receipt and ovule fertilization has been a selective factor in the evolution of ovule number per flower. We hypothesize that stochastic variation in floral mating success creates an advantage to producing many ovules per flower because a plant will often gain more fitness from occasional abundant seed production in randomly successful flowers than it loses in resource commitment to less successful flowers. Greater statistical dispersion in pollination and fertilization among flowers increases the frequency of windfall success, which should increase the strength of selection for greater ovule number per flower. We therefore looked for evidence of a positive relationship between ovule number per flower and the statistical dispersion of pollen receipt or seed number per flower in a comparative analysis involving 187 angiosperm species. We found strong evidence of such a relationship. Our results support the hypothesis that unpredictable variation in mating success at the floral level has been a factor in the evolution of ovule packaging in angiosperms. Key words: bet hedging; ovule; pollination; reproductive evolution; seed set; stigmatic pollen load. doi: 10.3732/ajb.0800183

Opuntia robusta has hermaphroditic, dioecious, and trioecious populations. To enhance our understanding of this breeding system diversity, we compared the reproductive output of males, females, and hermaphrodites in a trioecious population using field evaluations, controlled crosses, and progeny tests. Unisexuals were fully sterile in one sex function. Hermaphrodites were fully fertile for both functions. Consistent with the sex-allocation theory, unisexuality increased the quality and quantity (in males) of the gametes of the functional sex, relative to those of hermaphrodites, probably explained by maternal and paternal effects. The increase was higher in males than in females, suggesting a more expensive female function. Theoretically, this disproportional increase is required for unisexuals to invade a hermaphroditic population with prior selfing, negligible pollen discounting, and undetectable inbreeding depression, features found in O. robusta, therefore helping to explain dioecious populations. However, in the study population, the actual seed output of females was lower and had a higher variance than that of hermaphrodites, which also reproduce through pollen. Unisexuals are unlikely to be maintained by their actual reproductive output in this pollen-limited environment. Hermaphrodites may persist in this population by producing their seeds autonomously and by reducing interspecific fertilization by prior selfing and ovule discounting. Key words: cactus; dioecy; hermaphroditism; inbreeding depression; maternal effects; ovule discounting; pollen discounting; pollen limitation: pollen--ovule ratio; prior selfing; selfing; sex allocation. doi: 10.3732/ajb.0800301

Maturity classification is an important concept to provide the best allocation of resources for soybean [Glycine max (L.) Merr.] research and commercialization. A similar maturity group system used in North America is being used for some seed companies in Brazil and needs research to improve its use. This study evaluated the maturity stability of 48 midwestern and 40 southern Brazilian commercial cultivars ranging from North American maturity groups VI to VIII at 15 locations. Relative maturity groups were attributed to all cultivars. All trials were planted in the first half of November. The effect of location was very important in influencing the number of days to maturity, number of days to flowering and reproductive growth period (RGP). The genotype x environment interaction, although statistically significant, was much lower than the individual effects of environment and genotype for all traits and regions. Genotype x latitude and genotype x altitude, considering also years of evaluation, were generally low or nonsignificant. A recommended list was developed of the most stable genotypes and, consequently, of the most suitable check genotypes for each maturity group classification in the southern and midwestern regions. Results indicate that the use in Brazil of a maturity group system similar to that used in North America to classify soybean genotypes is an efficient method for describing relative maturity on a broad environmental basis.

Multiple field populations of two pairs of diploid sister taxa with contrasting mating systems in the genus Clarkia (Onagraceae) were surveyed to test predictions concerning the effects of resource status, estimated as plant size, on pollen and ovule production and on the pollen:ovule (P:O) ratio of flowers. Most theoretical models of size-dependent sex allocation predict that, in outcrossing populations, larger plants should allocate more resources to female function. Lower P:O ratios in larger plants compared to smaller plants have been interpreted as supporting this prediction. In contrast, we predicted that P:O ratio should not vary with plant size in predominantly selfing plants, in which each flower contributes to reproductive success equally through male and female function. We found that, in all four taxa, both ovule and pollen production per flower usually increased significantly with plant size and that the shape of this relationship was decelerating. However, ovule production either decelerated more rapidly than or at the same rate as pollen production with plant size. Consequently,the P:O ratio increased or had no relationship with plant size. This relationship was population-specifc (not taxon-specific) and independent of the mating system. Possible explanations for the increasing maleness with plant size are discussed. Key words: Clarkia: gametophyte production; mating system; Onagraceae; pollen to ovule ratio; size-dependent sex allocation.

Several purported cases of plasticity in plant allocation patterns appear to be the effects of size and allometric growth ('apparent plasticity'). To ask whether there is true plasticity (i.e., a change in the allometric trajectory) in reproductive allocation in Senecio vulgaris L., we grew S. vulgaris plants at high and low levels of water, nutrients, and competition, and analyzed the relationship between vegetative and seed biomass. Plant size was the major determinant of reproductive output, accounting for 83% of the variation in log (seed mass). There were also significant effects of the treatments that were not due to size, accounting for an additional 9% of the variation. The treatments affected the allometric coefficient (intercept), not the allometric exponent (slope) of the relationship, reflecting a small but significant shift in the efficiency of conversion of total plant biomass into reproductive biomass. In a second experiment, we grew S. vulgaris plants at three nutrient levels and allowed all individuals to complete their life cycles. Again, nutrient level had a small but significant effect on the allometric coefficient. Plasticity in reproductive allocation exists, but is very limited. The primary effects of the environment on the reproductive output of S. vulgaris occur via plant size. Key words: allometry, common groundsel, plasticity, size effects. Resume : Plusieurs cas mis de 1'avant de plasticite dans les patrons d'allocation semblent resulter de la dimension et de la croissance allometrique (≪ plasicite apparente [much greater than]). Afin de verifier s'il existe s'il existe une vraie plasticite (i.e. un changement de la trajectoire allometrique) dans dalocation reproductive chez le Seneciao vulgaris L. les auteurs ont cultive cette plante a des degres eleves et faibles en eau, nutriments et competition et ils ont analyse la relation entre la biomasse vegetative et seminale. La dimension des plants constitue le determinant majeur du produit de la reproduction, expliquant 83% de la variation en log (masse seminale). On observe egalement des effets significatifs des traitements qui ne dependent pas de la dimension expliquant un 9 % additionnel de la variation. Les traitements affectent le coefficient allometrique (interception), et non 1'exposant allometrique (pente) de la relation, ce qui reflete un deplacement faible, mais significatif de l'efficacite de la conversion de la biomasse totale de la plante en biomasse reproductive. Dans une deuxieme experience, les auteurs ont cultive des plants de S. vulgaris en presence de trois teneurs en nutriments et ont permis a tons les individus de completer leurs cycles vitaux. Encore une fois la teneur en nutriments exerce un effet faible, mais significatif sur le coeffcient allometrique. Il existe une plasticite dans l'allocation reproductive, mais elle est tres limitee. Les effets primaires de l'environnement sur la productivite reproductive du S. vulgaris s'effectuent via la dimension de la plante. Mots-cles : allometrie, senecio vulgaire, plasticite, effets des dimensions., Introduction Growth and reproduction are two of the most fundamental processes for plants. Reproduction is the currency of natural selection, but plants must grow to build the machinery to reproduce. [...]

The size of acorns produced by several species of eastern North American oaks decreases with latitude. We investigated three hypotheses for this pattern in the bur oak (Quercus macrocarpa Michx.) using samples collected over 2 years throughout the species' range. We found strong support for the hypothesis that abiotic factors, including both temperature and rainfall, constrain acorn size. There was also a smaller but important difference in acorn size related to whether sites were glaciated or not, providing support for the hypothesis that some factor associated with the history of glaciation affects acorn mass. In contrast, although blue jays (Cyanocitta cristata L.) prefer smaller acorns and are an important dispersal agent, the latitudinal patterns of acorn size and variability were not consistent with predictions of the hypothesis that they are the product of blue jay dispersal during bur oak's postglacial Holocene expansion. Assuming that there is a lower limit to the size that acorns can be and still be successful, the strong role of environmental constraints on acorn size may be important in explaining the apparently contradictory interspecific pattern that North American oaks successfully colonizing areas further north during the Holocene are those that produce relatively larger acorns. Key words: blue jay, differential dispersal, life-history trade-offs, latitudinal gradient, seed dispersal, seed size. La dimension des glands produits par plusieurs especes de chenes nord-americains diminue avec la latitude. Les auteurs ont etudie trois hypotheses pour expliquer ce patron chez le chene a gros fruits (Quercus macrocarpa Michx.), en utilisant des echantillons recoltes sur deux annees dans l'ensemble de Faire de distribution de cette espece. Les auteurs ont trouve un support important pour l'hypothese basee sur des facteurs abiotiques restreignant la dimension des glands, incluant la temperature et les precipitations. On observe egalement une difference plus petite, mais significative dans la dimension des glands selon que les sites forment de la glace on non, supportant l'hypothese que certains facteurs associes a l'histoire de la glaciation affectent la dimension des glands. An contraire, bien que le geai bleu (Cyanocitta cristata L.) prefere les glands plus petits et en constitue un agent de dispersion important, la distribution latitudinale de la dimension des glands et de sa variability ne concorde pas avec les predictions de l'hypothese qu'elles proviendraient de la dispersion par les geais bleus an cours de l'expansion postglaciaire de l'Eocene, du chene a gros fruits. En assumant qu'il y a une limite a la dimension des glands pour demeurer fonctionnels, le role considerable des contraintes environnementales sur la dimension des glands pourrait etre important pour expliquer le patron interspecifique apparemment contradictoire que les chenes de l'Amerique du Nord colonisant avec succes les aires plus nordiques an cours de l'Holocene, sont ceux qui produisent des glands relativement plus gros. Mots-cles : geai bleu, dispersion differentielle, contre parties de l'histoire de vie, gradient latitudinal, dispersion des graines, dimension des graines. [Traduit par la Redaction], Introduction Offspring size is generally believed to be under strong selection to optimize fitness through balancing trade-offs between current and future reproduction, quantity and quality of offspring, and investment in [...]

Individuals in large plant populations are expected to benefit from increased reproductive success relative to those in small populations because of the facilitative effects of large aggregations on pollination. As populations become small, the inability to attract sufficient numbers of pollinators can reduce reproduction via pollen limitation. This study experimentally tested whether such trends occur for the herbaceous biennial Sabatia angularis (L.) Pursh (Gentianaceae). We created artificial populations of varying size consisting of potted S. angularis plants in two field sites to determine whether population size affected mean fruit and seed set. We also examined whether population size affected the degree of pollen limitation using a supplemental pollination design in one of the sites. Our results showed that, on average, seed set was lower in large populations, not small populations, of S. angularis and that this result may be due to increased pollen limitation in large populations. We suggest that in certain contexts, small populations may enjoy reproductive advantages over large populations by escaping intraspecific competition for pollinators. Key words: competition, facilitation, pollen limitation, population size, reproduction, seed set. On s'attend a ce que les individus de grandes populations beneficient d'un succes reproductif relatif plus grand que ceux des petites populations du aux effets encourageants d'une forte agregation de pollen. A mesure que les populations deviennent petites, l'incapacite d'attirer des nombres suffisants de pollinisateurs pourrait reduire la reproduction par une limitation du pollen. Dans cette etude experimentale, les auteurs ont verifie si de telles tendances existent chez (espece herbacee Sabatia angulares (L.) Pursh (Gentianaceae). Ils ont cree des populations artificielles de diverses dimensions constituees de plantes en pot du S. angulares dans deux sites de prairie, afin de determiner si la dimension des populations affecte les moyennes de fruits produits et de graines formees. Ils ont egalement verifie si la dimension des populations affecte le degre de limitation par le pollen en utilisant une pollinisation supplementaire sur un des sites. Les resultats montrent qu'en moyenne la formation des graines est plus faible dans les grandes populations, et non dans les petites, du S. angulares et que ce resultat pourrait provenir d'une augmentation de la limitation par les pollens dans les grandes populations. Les auteurs suggerent que dans certains contextes, les petites populations possedent des avantages reproductifs par rapport aux grandes populations en echappant a la competition infra specifique pour les pollinisateurs. competition, facilitation, limitation par le pollen, dimension des populations, reproduction, formation des graines. [Traduit par la Redaction], Introduction Reduced reproduction in small plant populations is becoming a widely recognized phenomenon (Leimu et al. 2006), and has primarily been attributed to reductions in the quantity and quality of [...]

In dioecious plants, females often incur greater reproductive costs than males, owing to the production of seed. This has led to evolution of cost-compensatory mechanisms in females. In trees, however, reproductive cost and compensatory mechanisms are not always detectable, probably because costs and compensation traits change with time, plant size, and modular level (i.e., individual shoots and whole plants). Herein we investigated sex-specific reproductive allocation, growth, and carbon acquisition mechanisms at different hierarchical levels in a dioecious tree, Salix integra Thunb. At both shoot and whole-plant levels, females invested more resources into reproduction than males, but without an associated reduction in vegetative growth, suggesting compensatory mechanisms in females. Females had greater biomass allocation to photosynthetic organs and higher photosynthetic rates than males. Although photosynthetic rates decreased with age, higher shoot turnover in females maintained higher productivity by consistently locating new leaves in favourable light. Females had larger areas of leafy bracts beneath peduncles, suggesting a reduction in the translocation distance of the assimilation to mature seeds. Females meet greater reproductive costs not by reducing growth, but by increasing the carbon uptake ability of different modular units in well-illuminated microenvironments. Key words: dioecy, module, photosynthetic rate, reproductive allocation, reproductive size, willow. Chez les plantes dioiques, les femelles supportent souvent des couts de reproduction plus eleves que les males, compte tenu de la production des grannes. Ceci a favorise l'evolution de mecanismes compensatoires des couts Chez les femelles. Cependant, Chez les arbres, les couts de la reproduction et les mecanismes compensatoires ne sont pas toujours detectables, probablement parte que les caracteres des couts et de la compensation changent avec le temps, la dimension des plants et les echelles modulaires (i.e., tiges individuelles et plante entiere). Les auteurs ont examine l'allocation specifique aux activites sexuelles, incluant les mecanismes de la croissance et de l'acquisition du carbone, a differentes echelles hierarchiques chez un arbre dioique, le Salix integra Thunb. A l'echelle des tiges aussi bien qu'a cene de la plante entiere, les femelles investissent plus dans la reproduction que les males, sans pour autant voir leur croissance vegetative reduite, ce qui suggere l'existence de mecanismes compensatoires chez les femelles. Les femelles allouent une plus grande biomasse aux organes photosynthetiques avec de plus forts taux de photosynthese que les males. Bien que les taux photosynthetiques diminuent avec l' age, un cyclage plus rapide des tiges maintient une plus forte productivite chez les femelles en exposant constamment de nouvelles feuilles a des illuminations favorables. Les bractees foliaires des femelles occupent des surfaces plus grandes sous les pedoncules, ce qui suggere une reduction de la distance de translocation des assimilats vers les grannes matures. Les femelles rencontrent les couts plus eleves de la reproduction non pas en reduisant leur croissance, mais en augmentant leur capacite a absorber le carbone de differentes unites modulaires, en profitant de microenvironnement favorablement illumines. Mots-cles : dioecie, module, taux photosynthetique, allocation a la reproduction, dimensions reproductrices, saule. [Traduit par la Redaction], Introduction In dioecious plants, reproductive allocation (RA) in females usually exceeds that of males, owing to the production of seeds (Lloyd and Webb 1977; Delph 1999; Obeso 2002; Pickering and [...]

A remarkably diverse Lower Cretaceous (Valanginian-Hauterivian) flora at Apple Bay, Vancouver Island, preserves seed plants at an important time of floristic evolutionary transition, about the same time as the earliest flowering plant megafossils. The fossils are permineralized in carbonate concretions and include tetrahedral seeds within cupule- or carpel-like structures. These enclosing structures, composed of elongate sclerenchyma cells with spiral thickenings that grade externally to a few layers of parenchyma, are vascularized by one collateral vascular bundle and lack trichomes. They apparently broke open to release the tightly enclosed seeds by valves. Seeds are similar to those of the Triassic seed fern Petriellaea, but are about 100 million years younger and differ in size, vascularization, integumentary anatomy, seed attachment, and number of seeds/cupule. These new seeds are described as Doylea tetrahedrasperma gen. et sp. nov., tentatively assigned to Corystospermales. Inverted cupules are reminiscent of an outer angiosperm integument rather than a carpel. Like fruits, cupules opened to release seeds at maturity, thereby foretelling several aspects of angiospermy. They show that nearly total ovule enclosure, a level of organization approaching angiospermy, was achieved by advanced seed ferns during the Mesozoic. Key words: angiospermy; corystosperms; Cretaceous; cupule; Petriellaea; seed ferns.

Timing of pollen arrival may affect the level of seeds fertilized by pollen from outside seed orchards, especially in seed orchards of Scots pine (Pinus sylvestris L.) established in southern Finland with stock from northern populations. We performed sequential pollination experiments with Scots pine pollen collected from seed orchard clones originating from southern and northern Finland and recipient strobili in clones originating from southern Finland. When southern pollen was introduced first, seeds were sired equally by northern and southern genotypes. When northern pollen was introduced first, northern genotypes were found in 75% of the resulting seeds. When both pollen types were introduced at the same time, the resulting seed crop was split almost equally between northern and southern genotypes. These results do not unconditionally support the idea that the first pollen grain in the pollen chamber always fertilizes the ovum; instead, they suggest a more complex way of competition between pollen grains. Le moment ou le pollen arrive peut affecter la quantite de graines fertilisees par le pollen provenant de l'exterieur des vergers a graines, particulierement dans les vergers a graines de pin sylvestre (Pinus sylvestris L.) etablis dans le sud de la Finlande avec du materiel provenant de populations nordiques. Nous avons realise des experiences de pollinisation sequentielle avec du pollen de pin sylvestre collecte dans des vergers a graines de clones originaires du sud et du nord de la Finlande et des strobiles recepteurs chez des clones originaires du sud de la Finlande. Lorsque le pollen provenant du nord etait introduit en premier, 75 % des graines produites correspondaient aux genotypes du nord. Lorsque les deux types de pollen etaient introduits en meme temps, les graines etaient reparties presque egalement entre les genotypes du nord et du sud. Ces resultats ne supportent pas inconditionnellement l'idee que le premier grain de pollen dans la chambre pollinique fertilise toujours l'ovule. Au contraire, ils laissent entrevoir un mecanisme plus complexe de competition entre les grains de pollen. [Traduit par la Redaction], Introduction While pollen competition that leads to nonrandom mating is well studied in angiosperms (Skogsmyr and Lankinen 2002; Bernasconi 2003), few experiments have focused on coniferous species such as Scots [...]

The coexistence between genetically modified (GM) and non-GM maize (Zea mays L.) fields is subjected to regulation in several countries. A strategy to reduce cross-pollination from GM to non-GM fields was evaluated, on the basis of reducing the flowering coincidence by sowing at separate times. The trial included narrow plots in which transgenic maize was sown before, simultaneously, and after adjacent non-GM maize. A three-week delay between GM and non-GM sowing dates resulted in flowering delays below 5 d for early sowings, but up to 10 to 15 d for late sowings. Cross-pollination was monitored by xenia effect and validated by real-time PCR. The combined effect of flowering delay and distance from the pollen source was evaluated. Cross-pollination dramatically decreased with distance between pollen source and receptor. Flowering delay interacted with distance to the pollen donor source and further reduced GM pollen flow in all cases. Thus, it acted as a regulatory factor. A flowering delay of 92.9 growing degree units (GDU) (7 d in our experimental conditions) was enough to reduce the adventitious presence of GM kernels in non-GM maize plots down to residual values. This study illustrates the possibility of improving coexistence through temporal separation of flowering dates on the basis of appropriate delays in sowing dates.

Spikelet number per panicle is a major target trait for improving rice (Oryza sativa L.) yield in upland fields and is strongly affected by water stress. The rice panicle consists of rachis branches and spikelets, in which preflowering spikelet abortion often occurs. We investigated the effect of timing and intensity of water stress at the early reproductive stage on the morphology of rice panicles in field and pot experiments. In the field experiment, water stress caused high rates of preflowering spikelet abortion, and spikelet number was reduced by 48% on average. Pot experiments revealed the effects of water stress on panicle morphology were dependent on the developmental stage, and water stress at the meiosis stage (10-20 d before heading) induced the highest frequency of preflowering spikelet abortion. At the meiosis stage, a mild water stress (no visible effect and less than 20% reduction in biomass production) was sufficient to cause a considerable increase in secondary rachis branch abortion (70%) and resulted in a 40 to 45% reduction in spikelet number per panicle. Reduction in plant water status under drought conditions was related to preflowering spikelet abortion, suggesting that the drought avoidance mechanism is effective for maintaining spikelet number in upland rice.

Sorghum (Sorghum bicolor L. Moench) grown in semiarid regions is often exposed to short periods of high-temperature (HT) stress during reproductive development. Objectives of this research were (i) to quantify the effects of short episodes of HT stress during reproductive development on physiological, growth, and yield processes of grain sorghum and (ii) to identify the stage(s) most sensitive during the reproductive development phase to HT stress. Plants of hybrid DK-28 E were grown in growth chambers at daytime maximum/nighttime minimum optimum temperature (OT) of 32/22[degrees]C until 29 d after sowing. Thereafter, plants were exposed to OT or HT (40/30[degrees]C) or were reciprocally transferred at 10-d intervals (10 d before flowering, 0, 10, 20, and 30 d after flowering [DAF]) from OT to HT and vice versa. Transferred plants remained in the new temperature regime for 10 d before being returned to their original temperature regime. Continuous HT stress delayed panicle emergence and decreased plant height, seed set, seed numbers, seed yield, seed size, and harvest indices but did not influence leaf photosynthesis. Exposure to short (10-d) periods of HT stress at flowering and 10 d before flowering caused maximum decreases in seed set and seed yield, and HT stress during postflowering stages (10, 20, and 30 DAF) decreased seed yield, with a larger reduction at early stages of seed development.

A well-known, but largely untested, prediction in plant reproductive ecology is that dioecious taxa should produce larger, more, higher-quality, or better-defended seeds than cosexual taxa. Using a data set composed of 972 species in 104 families, representing the flora of the Tambopata Wildlife Reserve (Madre de Dios, Peru), we evaluated the first component of this prediction, examining ecological and evolutionary relationships between breeding system and mean seed size with two kinds of tests. First, we conducted cross-species analyses to determine whether species with different breeding systems differed significantly with respect to mean individual seed size. Second, we used a hypothesized phylogeny to identify pairs of the most closely related taxa or clades within the Tambopata community that differed with respect to breeding system. Comparing pair members allowed us to determine whether evolutionary divergence in breeding system (between taxa with unisexual vs. cosexual individuals) was consistently associated with evolutionary change in seed size. In both analyses, we controlled for potentially confounding effects of growth form by examining these relationships within woody and nonwoody taxa. Cross-species analyses revealed that dioecious species produced larger seeds than cosexual species among woody species, shrubs, lianas (each growth form analyzed separately), and all species pooled, but not among trees. Phylogenetically independent contrasts upheld the significant association between breeding system and seed size among woody taxa, lianas, and all taxa pooled, but not among shrubs. We discuss the implications of our findings for evolutionary hypotheses regarding associations between dioecy and seed size. Key words: angiosperms, breeding system, dioecy; monoecy; plant growth forms; rain forest; seed size.

The late-nineteenth/early-twentieth century debate over homologous versus antithetic alternation of generations is reviewed. Supporters of both theories, at first, used Coleochaete as a model for the origin of land-plant life cycles. The early debate focused on the morphological interpretation of the sporophyte and on whether vascular cryptogams had bryophyte-like ancestors. The terms of the debate shifted after the discovery that the alternation of morphological generations was accompanied by an alternation of chromosome number. Supporters of homologous alternation now promoted a model in which land plants had been derived from an algal ancestor with an isomorphic alternation of haploid and diploid generations whereas supporters of antithetic alternation favored a model in which land plants were derived from a haploid algal ancestor with zygotic meiosis. Modern evidence that embryophytes are derived from charophycean green algae is more compatible with an updated version of the antithetic theory. Keywords Alternation of Generations * Apogamy * Apospory * Coleochaete * Dictyota * Embryophytes * Gametophyte * Sporophyte, Introduction For more than a century, theories of the 'antithetic' origin of sporophytes have been juxtaposed with theories of their 'homologous' origin. During this same period, there have been profound [...]

Age and size at the first reproduction and the reproductive allocation of plants are linked to different life history strategies. Aleppo pine only reproduces through seed, and, as such, early female reproduction confers high fitness in its infertile highly fireprone habitats along the Mediterranean coast because life expectancy is short. We investigated the extent of ecotypic differentiation in female reproductive allocation and examined the relation between early female reproduction and vegetative growth. In a common-garden experiment, the threshold age and size at first female reproduction and female reproductive allocation at age seven differed significantly among Aleppo pine provenances of ecologically distinct origin. Significant correlations among reproductive features of the provenances and the ecological traits of origin were found using different analytical tools. In nonlinear models of cone counts vs. stem volume, medium-sized trees (not the largest trees) produced the highest cone yield, confirming that, at the individual level, early female reproduction is incompatible with fast vegetative growth. The contribution of founder effects and adaptation to contrasting fire regimes may be confounding factors. But considering all traits analyzed, the geographical patterns of resource allocation by Aleppo pine suggest ecotypic specialization for either resource-poor (favoring early reproduction) or resource-rich (favoring vegetative growth) habitats. Key words: age at first reproduction; cones; Mediterranean; Pinaceae; Pinus halepensis; reproductive allocation; reproductive effort; threshold size.

Abstract: Thamnolia subuliformis (Ehrh.) Culb. is a sterile lichen-forming ascomycete with no known sexual or vegetative reproductive structures except fragments of thallus branches. The open tundra in northern Manitoba contains [...]

A strong vernalization requirement can increase the amount of time needed per breeding cycle for cool season grasses, and the presence of awns can be an eye irritant to grazing livestock. Our objective was to identify awnless tetraploid (2n = 4x = 28) Elymus accessions capable of flowering quickly and consistently in the greenhouse. A total of 244 accessions, encompassing 27 Elymus species and subspecies, were evaluated during three different time periods in the greenhouse at Ardmore, OK. Heading date of accessions ranged from 53 to 90 d after planting, with 175 accessions representing 20 species or subspecies, flowering within 90 d of planting in the greenhouse. Eighty-eight accessions, representing 11 species or subspecies, were awnless or awnletted (i.e., awns

Secondary pollen presentation is the relocation and presentation of pollen in floral structures (termed pollen presenters) other than the anthers. These pollen presenters are often found close to the stigma and have been hypothesized to increase the accuracy of pollen transfer, although no experimental studies have been done. We examined the function of the pollen presenter and its efficiency in pollen dispersal, female fitness, and the degree of interference created by self-pollen in the shrublet Polygala vayredae, an insect-pollinated species with secondary pollen presentation. Herkogamy, a mechanism generally involved in the reduction of self-interference, was also evaluated. Significant pollen was lost (49% of total pollen) during the secondary relocation in the pollen presenter. However, pollen was exported from the pollen presenter, and subsequent pollen losses were similar to those in species with primary pollen presentation. Despite the presence of a self-incompatibility system, the numbers of developed pollen tubes as well as fruit and seed production were significantly reduced by the self-pollen interference created at the stigmatic papillae level. The extent of herkogamy correlated positively with female fitness. The secondary pollen presentation mechanism may in fact be an accurate system for pollen transport, but it may also have its costs. Further comparative studies involving species with primary and secondary pollen presentation are needed to fully understand the advantages and disadvantages of secondary pollen presentation. Key words: female fitness; herkogamy; pollen dispersal; pollen presenter; pollen relocation; Polygalaceae; self-incompatibility; self-interference.

Abstract: We describe and illustrate conidiogenesis in 21 species of Strophariaceae sensu Singer in culture, including first reports of conidiogenesis from nine species. Two modes of thallic conidiogenesis were revealed. [...]

Rosa rugosa, a vigorous ornamental shrub introduced from Asia in the 19th century, is now naturalized in coastal northeastern North America, where it occasionally grows in sympatry with the native R. blanda. To document hybridization between these species, evaluate its extent across the area of sympatry, and examine the use of morphology as a field monitoring tool, we sampled 179 individuals of parental species and putative hybrids in 13 pure and 11 mixed populations. We developed allele-specific primers to assay single nucleotide polymorphisms (SNPs) markers from one chloroplast region and four low-copy nuclear introns. Our results revealed frequent bidirectional hybridization and infrequent introgression in sympatric populations of these species. The recurrent presence of [F.sub.1] hybrids in mixed populations indicated the weakness of early-acting reproductive barriers. Morphological data were concordant with molecular data and provided additional evidence for the presence of a few backcrosses. Morphological analyses yielded diagnostic characters for identifying hybrids and monitoring the hybrid zone. Such hybridization could ultimately lead to the genetic assimilation of R. blanda in mixed populations and to the formation of invasive hybrid genotypes, a phenomenon that is of economic and ecological concern because of the increasing number of exotic species worldwide. Key words: allele-specific primers; eastern North America; hybridization; introduced species; introgression; Rosa blanda; Rosa rugosa; Rosaceae.

Intraspecific variation in resistance to herbivory among genders and life-history phases of primary producers can significantly alter the ecological and evolutionary consequences of plant herbivore interactions. Seaweeds (macroalgae) with complex life histories have multiple distinct phases with associated variation in traits that can potentially lead to differences in resistance to consumers and provide a unique system in which to simultaneously test the effects of sex and life-history stage on herbivory. We tested the susceptibility to grazing of the three life-history stages and separate sexes of the chemically defended red alga Asparagopsis armata against the sea hare Aplysia parvula, and we related this to the plant quality traits of different stages and genders. Differences in nutrient content and halogenated secondary metabolites between life-history phases were highly sex dependent. Male gametophytes had a low concentration of secondary metabolites and the highest nutrient content. The highest secondary metabolite content was found within the female gametophyte, in the wall of the reproductive structures (cystocarps) that contain the microscopic carposporophyte phase. Feeding choices by A. parvula were consistent with differences in algal quality and defense and resulted in the haploid male gametophytes being the most preferred food type. The diploid carposporophyte found inside the chemically rich cystocarps was the least consumed life-history stage. Selective herbivory of male gametophytes by A. parvula is consistent with an observed shift in gametophyte sex ratio in the field from unity at the beginning of the reproductive season to female bias at the end. The variation in susceptibility to herbivory found between sex and life-history stages of A. armata represents the first example of sex-biased consumption in seaweeds and may contribute to the maintenance of complex life histories such as those found in red algae. Key words: algae; Aplysia parvula; Asparagopsis armata; chemical defense; dioecy; herbivory; heteromorphic life cycle; life history; plant-herbivore interaction.

In animal-pollinated plants, autonomous selfing provides reproductive assurance under conditions of infrequent pollinator visits or a lack of mates, but few data are available for wind-pollinated species or species with combined insect and wind-pollination, for which it is often assumed that pollen availability does not limit reproduction. In this study, the capacity of autonomous selfing was investigated in the temperate forest herb Paris quadrifolia, and an emasculation experiment was performed under natural field conditions to investigate the contribution of autonomous selfing to total seed set across a continuous gradient of densities of flowering conspecifics. In the absence of wind or pollinators, autonomous selfing was observed through anthers approaching stigmas at the end of flowering and the capacity for autonomous pollination was about 0.34. Under natural conditions, considerable outcross pollination was observed, but the proportion of ovules successfully fertilized signifcantly decreased with decreasing density of conspecifics when flowers were emasculated, but not when flowers were left intact. These results indicate that autonomous selfing resulted in reproductive assurance (RA = 0.16) and thus support the hypothesis that autonomous selfing can also provide reproductive assurance in wind-pollinated species. Key words: autogamy; delayed selfing; mixed mating; Paris quadrifolia.

Abstract: To examine the function of morphologically hermaphroditic, but functionally sterile, florets in female inflorescences of Petasites japonicus (Sieb. & Zucc.) Maxim., we conducted experiments in which all those florets [...]

Abstract: We studied the effects of inbreeding depression and the level of self-compatibility on overall fitness parameters in the invasive species Senecio inaequidens DC. and the native Senecio malacitanus Huter, [...]

Apomixis--asexual reproduction through seed--orovides a convenient means to faithfully propagate even heterozygous genotypes and hence expIoit heterosis, in several naturally apomictic, warm-season forage grasses. Inheritance of apomixis has been shown to be monogenic dominant in at least four economically important panacoid grasses Previously proposed breeding schemes for apomicts do not provide a means to accumulate genes contributing to nonadditive, heterotic effects over cycles of selection and recombination. Following the development of successful brachiariagrass [Brachiaria (Trin.) Griseb] cultivars by ecotype selection, artificial hybridization of brachiariagrasses began in the late 1980s with the development of a sexual tetraploidized biotype of the natural diploid, sexual ruzigrass (Brachiaria ruziziensis Germain and Evrard). A breeding scheme--recurrent selection for specific combining ability designed to accumulate nonadditive effects, originally proposed for sexual maize (Zea mays L.), is suggested as an appropriate scheme for improvement of apomictic tropical grasses, Recurrent selection on specific combining ability or interpopulation selection schemes such as reciprocal recurrent selection should be appropriate for other asexually propagated crops.

Cassava (Manihot esculenta Crantz) is a vitally important food source for many people in developing tropical countries. There are significant opportunities for improving the compositional qualities and pest resistance of cassava, and modern biotechnology is expected to play an important role in these improvements. The testing and development of genetically modified cassava will of course be subject to regulatory review, and experimental field trials must be performed in a fashion that prevents gene flow from the regulated plants. Methods to ensure reproductive isolation will be derived from a fundamental understanding of the biology of the crop. A current and comprehensive document on cassava reproductive biology is not yet available but is essential to guide regulators and scientists in planning and evaluating measures for reproductive isolation of confined field trials. This paper compiles a current view of the reproductive biology of cassava for use in experimental design and regulation of confined field trials. With the current state of knowledge on gene flow and seed dormancy in cassava, three methods for reproductive isolation of regulated experimental plots may currently be recommended: (i) removal of flower buds before flowering, (ii) destruction of plants before flowering, and (iii) floral bagging to contain pollen and seed. Areas for further research in cassava biology and biosafety are suggested.

Although most living organisms reproduce sexually, some have developed a uniparental reproduction where the embryo usually derives from the female parent. A unique case of paternal apomixis in plants has been recently reported in Cupressus dupreziana, an endangered Mediterranean conifer. This species produces unreduced pollen that develop into all-paternal embryos within the seed tissues. We analyzed seedlings produced by open-pollinated C. dupreziana seed trees using morphological descriptors, ploidy levels assessed through flow cytometry, and AFLP genetic diversity. In situ C. dupreziana seed trees (from Algeria) produced only diploid C. dupreziana progeny. In contrast, only one-third of the progeny produced by ex situ C. dupreziana seed trees planted in French collections were similar to C. dupreziana seedlings; the other progeny were haploid or diploid C. sempervirens seedlings. These results demonstrate that C. dupreziana ovules allow for the development of all-paternal embryos from pollen produced by another species, C. sempervirens. Thus, the in planta androgenesis is achieved through the combination of the embryogenic behavior of pollen grains and the ability of seed tree ovules to act as a surrogate mother. This phenomenon offers a unique opportunity to produce, by natural means, highly valuable material for genetic studies and selection of sterile cultivars.