Changes in the germinability of seeds of dicotyledonous herbs from anthropogenic and wild habitats during two initial years in a seed bank

The germinability of buried seeds changes with time, and the direction and periodicity of these changes differ among plant species. In 116 abundant dicotyledoneous herb species, we investigated the changes in seed germinability that occurred during the 2-year period following burial in the soil. We aimed to establish differences between seeds collected in “anthropogenic” (ruderal, arable land) and “wild” (grassland, forest) habitats. The seeds were buried in a field one month after collection, exhumed at regular intervals and germinated at 25 °C. During the two-year study period, four categories of species-specific patterns of germinability changes were found: seeds demonstrating seasonal dormancy/non-dormancy cycles (31 species), seeds germinating only in the first season after burial (16 species), seeds germinating steadily (38 species) and seeds whose germinability changed gradually, with increasing (7 species) or decreasing (18 species) germinability. The seeds of 6 species did not germinate at all. We found no significant difference in the frequency of these categories between species typical for anthropogenic and wild habitats. The cause for this result may be dramatic human influences (changes of agricultural practices), the pressure of which impedes the development of floras specific for certain habitats, as distinguished by the frequency of species with particular patterns of seed germinability. These frequencies varied among taxa with the growth form, seed mass and flowering phenology of species.

Innovative RNAi Strategies and Tactics to Tackle Plum Pox Virus (PPV) Genome in Prunus domestica-Plum

We developed an innovative RNAi concept based on two gene constructs built from the capsid gene (CP) cistron of the Plum pox virus (PPV) genome. First, designated as amiCPRNA, a potential molecule interfering with PPV genome translation and the second one is the ami-siCPRNA to target viral genome translation and PPV RNA replication. Following the previous engineering of these constructs in an experimental herbaceous host, they were introduced into Prunus domestica (plum tree) genome. Previously propagated onto a susceptible rootstock, these clones were graft-inoculated with PPV. After four dormancy cycles, and consistent with our experience of PPV infection, some clones showed a common phenomenon of silencing that can differ between the detailed plant phenotypes. Three different phenotypes were developed by the amisiCPRNA clones. First, the high resistance character shown by the amisiCPRNA plum-7 that was similar to the resistance expressed by HoneySweet plum. Secondly, a recovery reaction was developed by the two other amisiCPRNA plum-3 and plum-4 that differed from the rest, characterized as susceptible clones, among these were the amiCPRNA plums. Having assessed the behavior of these plums versus the herbaceous host accumulating the similar form of RNAi: ami-, si-, and ami-siRNA, challenging assays in perennials consistently reflect the natural context of viral genome targeting.

Germination ecology of the perennial herb Xyris longiscapa: inter-annual variation in seed germination and seasonal dormancy cycles

Germination ecology was investigated in a natural population of Xyris longiscapa, a perennial herbaceous species endemic to the Brazilian campo rupestre. Seeds were collected over four consecutive years (2014 to 2017) to evaluate germination responses to a range of temperatures (from 15 to 30°C). The light requirement was evaluated in seeds collected in 2014. Seeds collected in 2014 were also buried in soil in the natural habitat of the species to evaluate changes in germinability at different temperatures over the time. Seeds showed an absolute light requirement for germination. Seed germination was affected by temperature, collection year and the interaction between these two factors. Seeds collected in 2014 showed a narrower temperature range for germination (15–20°C), compared with the seeds collected in 2015, 2016 and 2017 that germinated in a temperature range of 15–25°C. Buried seeds remained viable in soil for at least 14 months and exhibited seasonal dormancy cycling. Secondary dormancy was induced during the rainy season and alleviated during the dry season, following a conditional dormancy/dormancy cycle. The degree of primary dormancy appeared to be influenced by the environmental conditions experienced by seeds during maturation. Primary dormancy (when present), seed persistence in soil and seasonal dormancy cycles are strategies of X. longiscapa to enhance regeneration success in the harsh environment of the Brazilian campo rupestre.

Will the sleeping beauties wake up? Seasonal dormancy cycles in seeds of the holoparasiteCuscuta epithymum

Seed banks are often crucial for the survival of plant species, especially short-lived ones. Nevertheless, empirical studies about the fate of seeds under natural conditions are scarce, particularly for parasitic plants. Therefore, fresh seeds of the holoparasiticCuscuta epithymumwere buried in the field under natural conditions in Belgium or kept at room temperature for up to 31 months, and germination was tested periodically by incubating seeds in light at 23°C. BuriedC. epithymumseeds had cyclic changes in their germinability, while those stored dry at room temperature never germinated without scarification. Buried seeds exhibited a seasonal cycle of physiological dormancy, the first to be reported for a species with combinational (i.e. physical+physiological) dormancy. The physiological dormancy of the embryo was broken during winter ( ≤ 5°C) but induced during exposure to late spring temperatures (>10°C). Therefore, germination ofC. epithymumseeds is fine tuned to seasonal temperature changes. Each year only a portion of the viable seeds could germinate. The portion of seeds buried in the soil that remained available for delayed germination, i.e. in the seed bank, decreased rather quickly, with only 8.5% of them being viable after 31 months of burial; all seeds stored in the laboratory remained viable. Nonetheless, we concluded that someC. epithymumseeds are capable of surviving for several years in the soil. These data indicate that the parasite is well adapted to survive in a dynamic heathland landscape where conditions for survival change constantly, but it is still sensitive to local extinction.

Seasonal dormancy cycles in the biennial Torilis japonica (Apiaceae), a species with morphophysiological dormancy

Torilis japonica (Apiaceae) has a widespread distribution, extending from western Europe to eastern Asia. In Europe, it usually behaves as a spring-germinating biennial species. Ripe seeds of T. japonica have an underdeveloped embryo and can persist in the soil for several years. The aim of this research was to reveal the mechanisms regulating the seasonal emergence pattern of seedlings. Experiments in a natural environment were performed to study phenology of seedling emergence and embryo growth. Seasonal changes in the dormancy status of T. japonica seeds were examined by regularly exhuming buried seeds and incubating them in controlled conditions. The action of temperature and light in regulating dormancy, embryo growth and germination was studied in the laboratory. Results showed that seeds of T. japonica have non-deep, simple morphophysiological dormancy (MPD), whereby physiological dormancy is broken by moist chilling (5°C). Once MPD was broken, embryo growth and subsequent germination started in spring, when appropriate temperature and light conditions were present. Seeds buried at a depth where light could not reach them showed cyclic changes in their dormancy status; embryo growth in these seeds could not be initiated because of the lack of a light stimulus. As far as we know, this is the first extensive study of seasonal dormancy cycles in a spring-germinating species of the Apiaceae. T. japonica occurs in a temperate climate with cool winters and warm, moist summers. In this climate, spring is often the most favourable season for seedling establishment.