BIOLOGY OF SEED GERMINATION IN SOME ONION SPECIES (ALLIUM L.)

The possibility of seed reproduction is one of the factors of successful cultivation of resource plants in specific environmental conditions. The aim of the work was to study the germination behavior, quality and longevity of seeds in 15 onions (Allium L.) from the collection of the Central Siberian Botanical Garden SB RAS, Novosibirsk. The research was carried out in 1996–2019. Laboratory germination of seeds was determined according to generally accepted methods. Seeds were tested 3–7 months after harvesting at 17–23 °C in the light. The number of seeds in each Petri dish – 25–50; double replication. In the case of hindered germination, a two-month chilling at 4 °C was applied. In the future, laboratory germination was determined after 3, 5 and 7 years of room storage. We have established that onion seeds have three types of germinating. Seeds of type I – eight species, one variety from the subgenus Rhizirideum – usually lack dormancy. Germination is fast and simultaneous, and average germination percentages are 77.1–92.2 %. Seeds of type II – A. leucocephalum and A. microdictyon from the subgenus Rhizirideum, also A. caeruleum and A. flavum from the subgenus Allium – are characterized by stretched germination period due to the shallow dormancy, and their germination percentages vary from 32.6 % to 81.1 %. Seeds of type III – A. obliquum from the subgenus Rhizirideum and two species of the subgenus Melanocrommyum – do not germinate in room conditions, or germinate with low germination percentages, whereas chilling increases them up to 47.0–67.0 %. The hindered germination of these seeds is due to the deep organic dormancy. The economic longevity of onion seeds is 3–5 years, and the biological longevity varies at interspecific level within 5–8 years. Thus, the species that produce plump and good germinating seeds suitable for long-term storage – A. altaicum, A. bidentatum, A. flavum, A. microdictyon, A. nutans, A. ramosum, A. rubens, A. schoenoprasum, A. senescens, A. senescens var. glaucum, and A. strictum – have the highest reproductive potential in culture.

The Synaptonemal Complex and the Achiasmatic Condition

The onion species Allium amplectans includes both a triploid and a tetraploid variety. By light microscopy both varieties appear to have normal synapsis during pachytene of meiosis. However, the triploid does not form chiasmata and exhibits almost total asynapsis following pachytene. The tetraploid forms at least one chiasma per homologue and retains pairing through metaphase I. Electron-microscopic examination of pachytene nuclei in these 2 varieties reveals apparently identical synaptonemal complexes. Three-dimensional reconstructions of chromosome arrangements in triploid pachytene nuclei confirm that synapsis is as complete as could be expected in an autotriploid. These observations give firm support to the hypothesis that the presence of apparently structurally normal synaptonemal complexes is not a sufficient prerequisite to ensure chiasma formation. It is suggested that a faulty or missing endonuclease which is normally involved in crossing over is responsible for the achiasmatic condition in triploid A. amplectans.