Predispersal seed predation in Bartsia alpina

Summary A northern Swedish population of Bartsia alpina, an arctic-alpine perennial herb, was found to suffer high levels of predispersal seed predation by larvae of two insect species, both specialists on rhinanthoid Scrophulariaceae hosts. The primary predator is Aethes deutschiana (Lepidoptera-Tortricidae), the host of which was previously unknown. The other predator is Gimnomera dorsata (Diptera-Scatophagidae), which is basically a Pedicularis specialist. Both predators are attacked by larvae of Scambus brevicorais (Hymenoptera-Parasitica-Ichneumonidae). Total predation pressure was more or less constant during 1985–1987, but in 1988 the level was doubled, the possible reasons of which are discussed. Large inflorescences of B. alpina suffer significantly higher predation pressures than small ones. It is shown that predation is most intense in the middle of the inflorescences. The same floral nodes are known to produce more selfed seeds than distal and basal nodes. Seed predation in B. alpina thus results in an increased proportion of outcrossed seeds entering the seed pool. Selection pressures on host plant and predator fauna are discussed.     

The genetics and ecology of seed size variation in a biennial plant,Hydrophyllum appendiculatum (Hydrophyllaceae)

The goal of this study was to elucidate the sources of seed size variation in Hydrophyllum appendiculatum, an outcrossing, biennial plant. The genetic basis of seed size variation was examined with a diallel breeding design. The analysis did not reveal any evidence for additive genetic variance, suggesting that seed size could not evolve in response to natural selection. A series of greenhouse experiments was conducted to determine the sensitivity of seed weight to a number of ecological variables. Seed weight was affected by inbreeding depression: seeds produced by self-pollinations were significantly lighter that outcrossed seeds. Maternal plants did not differentially provision seeds that were the result of crosses between subpopulations (separated by 300 m) or between populations (separated by 1.7 km). Mean seed size was independent of the number of outcrossed pollen donors (one vs. many) that sired seeds on an inflorescence; however, the variance was greater on inflorescences pollinated by multiple donors. Direct manipulations of the abiotic environment showed that seed size was greater on plants growing under full sunlight compared to shaded plants. Seed size was unaffected by soil type, fertilizer addition, or defoliation. Finally, I determined the effect of varying pollination intensity at the level of a single inflorescence, and at the whole plant level. Seed weight was greatest on plants that had only 1 and 5 inflorescences pollinated, and least on those that had 10 and 20 pollinated. At the inflorescence level, seed weights were greatest on those where all flowers were pollinated, compared to inflorescences where only half of the flowers were pollinated. Perhaps the greatest contributor to variance in seed size in this species was the temporal decline within plants through the flowering season. These results indicate that maternal plants are not capable of producing uniform seed crops. Rather, the final distribution of seed size produced by plants within a population will necessarily vary and be the result of pollination effects, heterogeneity in the abiotic environment, and developmental constraints.     

Variation in breeding system among populations of the common woodland herbAnemone nemorosa (Ranunculaceae)

Recent studies of germination in natural habitats, of genetic variation within populations and of the relative proportion of vegetative and sexual reproduction in the clonal plant speciesAnemone nemorosa suggest that sexual recruitment by seeds from outcrossed flowers is important for the maintenance of this species' populations. Because published reports on its breeding system are controversial, pollination experiments were performed in five natural populations ofA. nemorosa. Differences in ovule number per flower were recorded among populations, but they were not related to obvious habitat differences. Seed/ovule-ratios were significantly higher after open pollination and artificial crossing than after either artificial or spontaneous selfing. Populations had no effect on seed/ovuleratios. Different breeding indices indicated thatA. nemorosa is mainly self-incompatible. Nevertheless, some seed set also occurred after selfing, and both artificial and spontaneous selfing exhibited higher variation in seed/ovule-ratios than open pollination and artificial crossing. Continuous variation in seed/ovule-ratios after selfing suggested that the expression and effectiveness of the self-incompatibility system ofA. nemorosa is influenced by both genetic variation and phenotypic plasticity.     

Inbreeding alters a plant-predispersal seed predator interaction

The effect of inbreeding on genetic diversity is expected to decrease plant defences or vigour-related traits that, in turn, can modify the pattern of attack by herbivores. The selective damage caused by herbivores can produce variable fitness costs between inbred and outcrossed progenies influencing the evolution of a species’ plant mating system. By exposing inbred and outcrossed plants to natural conditions of seed predation, we assessed whether inbreeding increases weevil incidence and infestation, and how weevil seed predation affects the fitness of inbred and outcrossed progeny. To test if inbreeding affected the host’s plant quality, we weighed the biomass of weevils developed in inbred and outcrossed progenies. An additional experiment was carried out to examine whether weevils preferentially attack vigorous plants regardless from the level of inbreeding. The average value of leaf size was 21% larger in outcrossed plants than in inbred plants. Likewise, weevil incidence and infestation were 13 and 40%, respectively, higher on outcrossed plants relative to their inbred counterparts. However, the relative impact of seed predation was significantly lower in outcrossed progeny than in inbred progeny. In contrast, inbreeding did not alter host plant quality and weevils developed in inbred and outcrossed plants had a similar biomass. Variations in fruit number were consistently associated with the infestation level in both experiments, whereas leaf size only predicted the number of weevils in one experiment, suggesting that fruit number is the most influential vigour-related characteristic of a weevil attack. These findings indicate that the costs of inbreeding of the interaction D. stramonium-T. soror were higher for inbred plants than for outcrossed plants. The interaction between seed predation and inbreeding depression could prevent the fixation of selfing as a unique reproductive strategy in D. stramonium.     

Seedling performance in a trioecious cactus,Pachycereus pringlei: Effects of maternity and paternity

We studied seed germination and the growth and survivorship of seedlings of females and hermaphrodites ofPachycereus pringlei (cardon), a Mexican columnar cactus whose geographically variable breeding system includes trioecy and gynodioecy. Results of a two-year field experiment conducted near Bahia Kino, Sonora, Mexico and a ten-month laboratory experiment were similar and did not support the hypothesis that seedlings of females outperform those of hermaphrodites. In the field, percent seed germination and 2-yr seedling survivorship averaged 66% and 95%, respectively and did not differ among six treatment classes. Seedlings of hermaphrodites generally were larger than those of females at the end of both experiments. Selfed seedlings of hermaphrodites did not grow more slowly than outcrossed seedlings of hermaphrodites or females. Hermaphrodite seedlings performed best when pollinated with hermaphrodite pollen; female seedlings performed best with male pollen. We conclude that superior seedling performance cannot explain why females are able to coexist with hermaphrodites in populations of this cactus. Instead, we postulate that greater annual seed production, which averaged 1.6 times higher in females than in hermaphrodites in two years, may be sufficient to allow females to co-occur with hermaphrodites in this large, longlived plant, especially if sex determination involves cytoplasmic-nuclear inheritance.     

Different levels of inbreeding depression between outcrossing and selfing Serapias species

We quantified inbreeding depression for fruit production, embryo vitality and seed germination in three deceptive orchids, Serapias vomeracea, S. cordigera and S. parviflora, which do not provide any reward to their pollinators, and are predicted to experience high outcrossing. Of the three species examined only S. parviflora was autonomously selfing. Both S. vomeracea and S. cordigera showed highly significant differences in fitness between selfed and outcrossed progenies, resulting in high levels of inbreeding depression, which increased in magnitude from seed set to seed germination. Inbreeding depression may promote outcrossing in Serapias by acting as a post-pollination barrier to selfing. Cumulative inbreeding depression across three stages in S. parviflora was lower that in both outcrossing species. The large difference in germination between selfed and outcrossed seeds is an important issue in conservation biology.     

Inoculation of pea (Pisum sativum L.) by Rhizobium leguminosarum bv. viceae preincubated with naringenin and hesperetin or application of naringenin and hesperetin directly into soil increased pea nodulation under short season conditions

In short season areas, low soil temperature is the major limiting factor for symbiotic nitrogen fixation of legume. One greenhouse and four field experiments were conducted in 1999 to determine whether the pre-incubation of Rhizobium leguminosarum bv. viceae with hesperetin and naringenin or application of these compounds onto the seed surface or into the seed furrow at the time of planting can increase pea nodulation and final grain yield. The results from these experiments clearly indicated that application of naringenin and hesperetin by either pre-incubating R. leguminosarum bv. viceae prior to inoculation of plant or directly applying onto the seed surface or into seed furrow at the time of planting can increase pea nodulation, and plant pod numbers. Interactions existed between symbiotic signal compounds and pea cultivars or R. leguminosarum bv. viceae strains. However, there was no impact on the final grain yield by the treatments from the field experiments. The effects of these treatments on the final grain yield have to be farther tested.     

Effects of inoculation methods on the incidence of ear-cockle and ‘tundu’ on wheat under field conditions

Wheat,Triticum aestivum L. cv. Mexipak was artificially inoculated withAnguina tritici (Steinb.) under field conditions. Nematode inocula: 0, 0.1, 0.2 and 0.3 g of seed galls/50 seeds or, germinated seeds of wheat (sown in a 1-m long row) were either added as intact seed galls or as a suspension of 2nd stage juveniles. Inoculation of ungerminated seeds with juveniles produced the highest incidence of ear-cockle disease. Whereas, intact seed galls inocula produced the highest incidence of tundu disease. Ear-cockle incidence was always greater than that of tundu at all treatments. The highest reduction in grain yield was associated with the treatments that caused the highest incidence of tundu disease.     

凋落物物理阻隔对格氏栲种子萌发及胚根生长的影响

为了探讨凋落物物理阻隔对格氏栲天然林自然更新状况的影响,通过模拟野外凋落物覆盖,设置格氏栲种子上层覆盖厚0 cm(CK)、2 cm(D2)、4 cm(D4)、6 cm(D6)、8 cm(D8)及种子下层铺垫厚2 cm(U2)、4 cm(U4)凋落物等7个处理,分析凋落物覆盖方式及厚度对格氏栲种子萌发及胚根生长的影响。结果表明:(1)凋落物覆盖方式及厚度对种子萌发进程存在显著影响。CK萌发持续时间最长,上层覆盖处理(D)次之,下层铺垫处理(U)的种子起始萌发时间显著滞后。(2)CK种子萌发率最高,其次D6处理发芽速度较快,发芽整齐;U处理较D处理的发芽率及发芽势均显著降低,且萌发抑制率显著增加。(3)D处理的种子胚根生长速度快,胚根长度大于CK;U处理的种子胚根生长速度呈先慢后快趋势。可见,凋落物是影响格氏栲种子萌发及胚根生长的重要因素,主要通过阻碍种子与土壤接触而抑制萌发,影响格氏栲林更新。     

Importance of fragmentation-tolerant species as seed dispersers in disturbed landscapes

Forest fragmentation can negatively affect plants if animal seed-dispersers become locally extinct in fragments. We conducted a 2-year experiment to evaluate the importance of tree squirrels (Sciurus) as seed dispersers for Quercus, Carya, and Juglans, and to assess dispersal consequences in patches where fragmentation-sensitive eastern gray squirrels (Sciurus carolinensis) are absent. We accounted for fates of ∼15,700 seeds from five tree species in four exclosure treatments at 18 fragments during a high (2003–2004) and low seed (2004–2005) year. Two treatments excluded Sciurus to mimic disperser loss. We sampled nut-tree seedling density at 259 sites across eight watersheds, half of which were too fragmented to support S. carolinensis, but supported fragmentation-tolerant fox squirrels (Sciurus niger). Autumn-to-spring seed survival was low (∼1%) for all species during low seed production. During high seed production, survival was higher for Juglans nigra (20%) and Carya ovata (16%) than for three Quercus species (∼4% for Quercus palustris and Quercus rubra in two exclosure types; ∼1% for Quercus alba in all treatments). Survival of J. nigra, C. ovata, and Q. rubra was ≥2.1–7.7 times higher for seeds in exclosures that Sciurus could access. Seed displacement distance was higher in the low seed than the seed-rich year, but the proportion of seeds surviving to greater distances was higher in seed-rich years for all seed types except Q. rubra. This affirms the importance of masting to seed survival and dispersal, but also suggests an advantage to trees of producing seed in non-mast years. Seedling densities were comparable in watersheds with and without S. carolinensis. These results demonstrate the importance of tree squirrels as dispersers of nut-bearing trees, but suggest that fragmentation may not disrupt dispersal of certain species if losing S. carolinensis from disturbed landscapes is compensated for by fragmentation-tolerant fox squirrels (S. niger).     

Pollination ecology of Penstemon roseus (Plantaginaceae), an endemic perennial shifted toward hummingbird specialization?

Summary Pink-flowered tubular Penstemon roseus (Plantaginaceae), which has shifted partially to hummingbird pollination, blooms on high-elevation slopes in the mountains in Tlaxcala, Mexico. We studied the interactions between pollinator visitation rates to flowers, pollen removal and deposition, flower size, and nectar removal frequency on seed production in P. roseus. We combine observational and experimental studies in two contrasting natural populations. Our manual pollinations revealed that P. roseus is fully self-compatible. Autonomous self- and manual self-pollinated flowers matured as many seeds as when outcrossed, but outcrossing seems to become better than selfing as the flowering season progressed. Early in the season flowers that were bagged and hand-selfed, hand-outcrossed, or autonomously selfed, or unbagged and naturally pollinated had equal seed set in all four treatments. But later in the season, outcross pollen gave approximately twice as much seed set as the two self-treatments. Low levels of pollen receipt and pollen removal were consistent with the long time elapsed for a given plant to be visited by hummingbirds, which suggests pollen shortage in both sites. Despite differences in pollinator visitation rates to flowers, probability of flower visitation, removal and deposition of pollen, and nectar production rates between populations, we found that total nectar production had no effect on seed production at either site. The daily nectar secretion rate of 0.3–0.65 mg sugar per flower per 1–3 days was low relative to other hummingbird-adapted Penstemon species (typical range: 1.5–5 mg sugar per flower), and it might be intermediate between hummingbird- and bee-adapted Penstemon flowers. Our results support the hypothesis about a shift toward hummingbird pollination, and provide an example of a ‘despecialized’ Penstemon species, which attracts high-energy pollinators (hummingbirds) and profits from outcrossing, but retains bee-syndrome floral traits and low sugar production rates.     

Influence of litter removal and mineral soil disturbance on the spread of an invasive grass in a Central Hardwood forest

Soil and litter disturbances within Central Hardwood forests may facilitate exotic plant species invasion of interior forest areas. Microstegium vimineum is an annual exotic grass that has become common throughout the Southeastern United States. Three replicates of three different mineral soil and litter disturbance treatments, plus a control with no disturbance, were established on the leading edge of M. vimineum patches prior to seed fall. All patches were located in areas with similar forest canopy structure and slope in three Central Hardwood forest stands prior to seed fall. At the beginning of the following growing season, each individual M. vimineum seedling was mapped within the treatment plots. The mean number of M. vimineum individuals that established within each treatment did not differ significantly from the control. The distance at which 90% of the individuals had spread, and the overall mean distance spread were significantly farther for the litter removal treatment than the control. The farthest individual seedling from the boundary of existing patches in both the litter removal and the mineral soil disturbance and litter removal treatments were significantly farther than the control. The individuals that spread the farthest are of most concern due to the large number of viable seed that a single M. vimineum plant can produce. These results suggest that disturbance of the litter layer may increase the spread rate of M. vimineum and facilitate its invasion of new habitats, and that leaving litter layers intact may slow the spread of M. vimineum.     

Distyly and incompatibility inVillarsia congestiflora (Menyanthaceae), with comparative remarks onV. capitata

The Western Australian annual of restricted distribution,Villarsia congestiflora, has distylous flowers and pollen-size heteromorphism. Results of a crossing program indicate that this species is strongly self-incompatible, that crosses among individuals of the same morph produce little or no seed, and that intermorph crosses produce copious seed. The species shares a number of morphological and ecological traits withV. capitata, another distylous, self-incompatible annual species of Western Australia with a greater area of distribution. Artificial hybrids between the two, however, showed reduced pollen stainability. Both species combine a number of specialized morphological features with a breeding system that is primitive for the familyMenyanthaceae.     

Mating system and seed variation of <Emphasis Type="Italic">Acacia</Emphasis> hybrid (<Emphasis Type="Italic">A. mangium</Emphasis> × <Emphasis Type="Italic">A. auriculiformis</Emphasis>)

The mating system and seed variation of Acacia hybrid (A. mangium × A. auriculiformis) were studied using allozymes and random amplified polymorphic DNA (RAPD) markers, respectively. Multi-locus outcrossing rate estimations indicated that the hybrid was predominantly outcrossed (mean±s.e. t _m = 0.86±0.01). Seed variation was investigated using 35 polymorphic RAPD fragments. An analysis of molecular variance (AMOVA) revealed the highest genetic variation among seeds within a pod (66%–70%), followed by among pods within inflorescence (29%–37%), and the least variation among inflorescences within tree (<1%). In addition, two to four RAPD profiles could be detected among seeds within pod. Therefore, the results suggest that a maximum of four seeds per pod could be sampled for the establishment of a mapping population for further studies.     

Effect of <Emphasis Type="Italic">Trichoderma</Emphasis> spp. isolates for biological control of tan spot of wheat caused by <Emphasis Type="Italic">Pyrenophora tritici-repentis</Emphasis> under field conditions in Argentina

Trichoderma spp. have been used as biocontrol agents to protect plants against foliar diseases in several crops, but information from field assays is scarce. In the present work, experiments were carried out to determine the effect of six isolates of Trichoderma harzianum and one isolate of T. koningii on the incidence and severity of tan spot, caused by Pyrenophora tritici-repentis (anamorph: Drechslera tritici-repentis) under field conditions. Significant differences between years, wheat cultivars and treatments were found. In 2003, two of the isolates assayed (T5, T7) showed the best performance against the disease applied as seed treatments or sprayed onto wheat leaves at different stages. The application of six of the treatments on wheat plants significantly reduced disease severity by 16 to 35% in comparison with the control. Disease control provided by isolate T7 was similar to that provided by the fungicide treatment (56% reduction). This is the first report on the efficacy of Trichoderma spp. against tan spot under field conditions in Argentina.     

Biological control of maize seed pathogenic fungi by use of actinomycetes

The effectiveness of twoStreptomyces spp. strains to controlpathogenic fungi was studied in stored maizegrain. The treatments included seeddisinfection and inoculation withStreptomyces spp. strains previously isolatedfrom maize rhizosphere. Actinomycete inoculumconsisted of filtered suspension and totalsuspension of fermentor-producedStreptomyces spp. strains biomass. Treatmentswith Streptomyces spp. strains aloneeffectively suppressed the development ofAspergillus spp., Curvularia lunata, andDrechslera maydis and significantly(p < 0,05) reduced the incidence ofFusarium subglutinans and Cephalosporiumacremonium. Among the inoculation treatments,nondisinfested seed inoculated with filteredsuspension was the only treatment that did notsuppress the development of Penicilliumspp. Maize seed inoculation with totalsuspension of strains was the most effectivetreatment to control the incidence of seedpathogenic fungi. The development of theDiplodia maydis was only suppressed by thecombination of seed disinfection andinoculation with total suspension of strains.Although, the strain DAUFPE 11470 showed thegreatest effectiveness for controlling thefungi pathogenic to seed, root and shootdevelopment was reduced by treatment with thisstrain.The results indicate thatStreptomyces spp. strains reduce the incidenceof seed pathogenic fungi and have potential asa biological control agent. However, an efficient methodof seed treatment with the biological controlagent must be developed before it can become anagricultural practice.     

Selfing inNarthecium ossifragum (Melanthiaceae)

Hagerup's (1950) observations of rain pollination inNarthecium ossifragum are commonly admitted. Nevertheless,Daumann (1970) stated that pollen and stigmas suffer considerable damage from rain impact. Experimental pollinations and pollen observations were conducted at a single site in the Upper Ardennes, Belgium, during three years. Rain water had a deleterious effect on both fruit and seed set and on pollen germination. Fruit and seed set differed among years mainly due to rainy weather. Fruit set also varied among experimental treatments. Pollination trials showed thatNarthecium ossifragum is self-compatible and autofertile. Emasculation had a deleterious effect and caging had no effect on fertility.     

Performance of carrot and onion seed primed with beneficial microorganisms in glasshouse and field trials

Beneficial microorganisms (Clonostachys rosea IK726, Pseudomonas chlororaphis MA342, Pseudomonas fluorescens CHA0, Trichoderma harzianum T22 and Trichoderma viride S17a) were successfully applied to carrot and onion seed during a commercial drum priming process. Applied microorganisms were recovered above the target of at least 1 × 10⁵ cfu g⁻¹ seed following subsequent application of pesticides to the seed according to standard commercial practices of film-coating carrot and pelletting onion seed. Two glasshouse experiments consistently showed that priming improved emergence of carrot seed and that C. rosea IK726 further improved emergence time. Priming improved emergence of onion seed in one glasshouse experiment, but had an unexpected negative effect on emergence in the second experiment, possibly due to the proliferation of an unidentified indigenous microorganism during priming, becoming deleterious in high numbers. In this experiment, the application of beneficial microorganisms during priming negated this effect and significantly improved emergence. For each crop, a series of field trials was also carried out over three years, at two different sites each year. Although some positive effects of different seed treatments were seen on emergence or yield in individual field trials, no consistent effects were found for primed or microorganism-treated seed across all sites and years. However, a combined analysis of data for all years and sites indicated that pesticide application did consistently improve emergence and yield for both carrot and onion. This is the first comprehensive study assessing glasshouse and field performance of carrot and onion seed primed with beneficial microorganisms during a commercial process of drum priming in the UK.     

Fruit-set and seed weight variation inAnthyllis vulneraria subsp.vulgaris (Fabaceae)

Fruit-set and seed weight variation was studied in a population ofAnthyllis vulneraria subsp.vulgaris (Fabaceae) in northwestern Spain. The plants produce several shoots, each bearing two to four inflorescences that open one at a time from bottom to top. Fruit-set, seed-pod weight and seed weight were found to be significantly higher in proximal inflorescences than in distal inflorescences of the same shoot; mean seed weight was up to three times higher in proximal than in distal inflorescences. By contrast, none of the three variables varied significantly among plants or among shoots of the same plant. Similarly, none of the three variables differed significantly between early- and late-flowering plants, or between plants monitored in 1993 and in 1994. These results are compatible with the view that shoots function as semiautonomous units as regards resource allocation, and that within the shoot resources are preferentially allocated to proximal (= early-opening) inflorescences. In the plants studied, the ratio of seed-pod weight to seed weight was fairly constant, suggesting that the pod is important for seed success.     

干旱胁迫下白刺花种子大小与萌发对策

种子大小与种子萌发及其与环境因子的关系是植物种子萌发对策研究中的重要科学问题之一。采用PEG模拟干旱法研究不同干旱胁迫强度(0,5%,10%,15%,20%)下,白刺花(Sophora davidii)种子萌发进程、种子大小与种子萌发及种子命运的关系。结果表明:不同干旱胁迫下,白刺花种子具有相似的萌发进程,但中度干旱处理(10%PEG)萌发率显著高于零干旱(0%PEG)和重度干旱处理(P0.05),重度干旱处理(20%PEG)种子萌发开始时间晚于零干旱和中度干旱处理;种子大小与种子萌发开始时间的关系表现为零干旱处理下呈极显著负线性关系,中度干旱处理(5%PEG,10%PEG)下无相关关系,重度干旱处理(15%PEG,20%PEG)下呈负二次曲线关系;种子大小对种子命运的影响表现为零干旱处理有利于大、小种子萌发和小种子休眠,中度干旱处理(10%PEG)增加中等种子萌发、大种子休眠和小种子死亡风险,重度干旱处理(15%PEG,20%PEG)增加大种子死亡风险、中等种子和小种子休眠。综合分析表明,白刺花种子大小与萌发行为及种子命运的关系具有较强的环境依赖性,即种子萌发行为表现为顺境下种子越大萌发越快,逆境下小种子和大种子较中等种子萌发更快;种子命运表现为顺境增加种子死亡的风险,中度干扰有利于种子萌发,逆境则有利于种子休眠。