Generative Reproduction in Allium Oleraceum (Alliaceae)
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Ann. Bot. Fennici 41: 1–14 ISSN 0003-3847 Helsinki 27 February 2004 © Finnish Zoological and Botanical Publishing Board 2004 Generative reproduction in Allium oleraceum (Alliaceae) Helena Åström & Carl-Adam Hæggström Department of Ecology and Systematics, P.O. Box 65, FIN-00014 University of Helsinki, Finland Received 31 Mar. 2003, revised version received 19 Aug. 2003, accepted 23 Sep. 2003 Åström, H. & Hæggström, C.-A. 2004: Generative reproduction in Allium oleraceum (Alliaceae). — Ann. Bot. Fennici 41: 1–14. The field garlic Allium oleraceum is reproducing by subterraneous bulbs, and above ground by bulbils and seed. Two chromosome numbers, tetraploid (2n = 32) and pen- taploid (2n = 40) have been found in Finland and Sweden. The seed production was studied in some populations of A. oleraceum mainly in south Finland. The fruits mature only if visited by insects and the seed ripen only in favourable, that is warm and not too dry, summers. Several insect species of the orders Hymenoptera, Diptera and Lepi- doptera were found to visit flowers of Allium oleraceum. The most frequent visitors were Bombus lapidarius (Apidae), Dolichovespula norwegica and D. saxonica (Vespi- dae), Eumenes spp. (Eumenidae), Volucella spp. and Syrphus spp. (Syrphidae), Pieris napi (Pieridae) and Autographa gamma (Noctuidae). Wasps of the family Vespidae are very regular visitors in the flowers of Allium oleraceum. Thus we suggest that Allium oleraceum is a species which has “wasp blossoms”. Both tetraploid and pentaploid plants are visited by insects and they both produce viable seed. The seed is described. In germination tests most of the seeds of both ploidy levels germinated well. Key words: Allium oleraceum, germination, pollination, seed, Vespidae, “wasp blos- som” Introduction umbel-like mass of young bulbils and flower buds develop enclosed by two spathe bracts The field garlic Allium oleraceum is a bulbous of unequal length with broad base and elon- geophyte with sympodial branching. The devel- gated tip. As the bulbils grow in size they burst opment of an individual plant begins in the between the bracts and occasionally also through autumn, when the offset bulb begins to develop their bases. This happens several weeks before roots, and lasts for almost one year. The plant the anthesis. The young flower buds develop either hibernates as a bulb below the ground or between the young bulbils and are protected by develops a short green leaf sheath above ground them at an early stage. Later on, the flower pedi- during warm and long autumns and stays over cels elongate and the flower buds begin to stand winter at this stage. The further development out in different directions between the bulbils. At of the plant starts in spring, in southern Finland this stage the green foliage begins to die back. In at the end of April or beginning of May, when SW Finland, the anthesis begins in late July or the scape and its foliage of three or four veg- early August and continues in an individual for etative leaves begins to elongate. The terminal about two weeks. The flowers are protandrous. 2 Åström & Hæggström • ANN. BOT. FENNICI Vol. 41 At anthesis the pedicels of the campanulate flow- ing Allium oleraceum specimens vary in size ers are curving downwards. from a few specimens to thousands, and the The number of flowers per plant is usually number in a certain population fluctuates from rather low. It varied between 0 and 41, with mean one year to another, quantitative studies are numbers between 2.0 and 9.3 in thirteen popula- extremely difficult to do in this species. Further, tion samples studied by us (C.-A. Hæggström & drought periods often put an end to fruit and H. Åström unpubl. data). However, a maximum seed development. number of 50 flowers was seen in a voucher specimen collected in the Botanical Garden of Helsinki (herbarium TMP). The number of bul- Material and methods bils normally exceeds the number of flowers; in our study it varied between 5 and 65 with means Insect visits in flowers of Allium oleraceum between 17.1 and 33.0 in nine population samples were studied in the island of Nåtö in 1999 in two (C.-A. Hæggström & H. Åström unpubl. data). populations: (1) in an open rock meadow and dry Four chromosome numbers are reported in and mesic meadow at Nåtö Biological Station Allium oleraceum: triploid (2n = 24; Vahtina (NBS) and (2) in a previously dense Fraxinus 1985), tetraploid (2n = 32; Levan 1931), pen- and Corylus stand at Själskatsudden belong- taploid (2n = 40; Geitler & Tschermak-Woess ing to the Nåtö-Jungfruskär Nature Reserve. 1946) and hexaploid (2n = 48; Pastor 1982). Occasional observations were also made at We have found both tetraploid and pentaploid Tvärminne Zoological Station on the south coast specimens in Finland and Sweden (H. Åström & of Finland in 1999, in southern Sweden 2000, in C.-A. Hæggström unpubl. data). The tetraploid the Botanical Garden in Helsinki in 2001 and at and pentaploid specimens are morphologically NBS 2001–2003. Some of the flower visitors discernible, especially during anthesis. This was were collected for later identification. not known to us before August 2000. To elucidate the role of insects visiting the In late July and early August 1997, the flowers of Allium oleraceum, well developed author C.-A.H. observed that flowers of Allium flower and bulbil heads were enclosed in mid- oleraceum were regularly visited by wasps of July 1998 and 1999 before anthesis in pergamine the family Vespidae at Nåtö Biological Station and tulle bags (Fig. 1). In 1998, 10 pergamine in the Åland Islands, SW Finland. Later it was and 8 tulle bags were used at NBS. In 1999, 10 observed that some flowers were seemingly pergamine bags and 3 tulle bags were used at developing into fruits; they had risen more or NBS and 3 pergamine and 5 tulle bags at Själ- less straight up, the campanulate flowers were skatsudden. The bagged plants were collected constricted and the pedicel seemed to be thicker and inspected for possible fruits in late August. than in flowers still curving downwards. In late The pollen viability was studied on penta- August and early September, some of the flowers ploid plants from the Åland Islands. Fresh pollen had developed capsules with one to three seeds. grains were dyed with methylgreen-phloxin More than 350 seeds could be gathered from the (according to Gurr 1965), which stains the A. oleraceum population comprising over one cytoplasm of the pollen grains red and the walls thousand specimens in the meadow area next to green. In the pollen of Allium oleraceum, the Nåtö Biological Station. cell walls were only faintly stained. Wholly red Qualitative studies on insects visiting flow- coloured pollen grains were regarded as viable, ers of A. oleraceum and the development of fruit faintly and unevenly coloured lacking a homog- and seed were continued in 1998–2001 by both enous cell content were regarded as non-viable. authors. Pollen grains were also tested for pollen tube The aim of this study was to investigate development on agar with added nutrients (10% which insects visit the flowers of Allium oler- sucrose, 0.03% CaCl2 and 0.01% boric acid in aceum and if these visits lead to fruiting. Fur- 1% agar). ther, our aim was to study the seeds, especially Fruiting plants were searched for in numer- their germination. As the populations of flower- ous localities in the Åland Islands, in the south- ANN. BOT. FENNICI Vol. 41 • Generative reproduction in Allium oleraceum 3 Fig. 1. Unbagged and bagged Allium oleraceum specimens at Nåtö Bio- logical Station. A few taller specimens of Allium scoro- doprasum are seen, too. Photo: C.-A.H., 6.VIII.1998. ern part of mainland Finland and in southern and Percival (1961), who studied the nectar compo- mid Sweden. Seeds were collected whenever sition of Allium fistulosum, A. schoenoprasum, found. In order to get ripe seed from localities A. triquetrum and A. ursinum, reported that that could not be revisited, plants with unripe the nectar of these species was of the dominant fruits were cut, placed in vessels with tap water fructose and glucose type also containing some and kept either out-of-doors or in the laboratory. sucrose. In the two first mentioned species mal- In such a way many more seeds were gath- tose was found, too. No study on the sugars of ered. Two batches of seeds, one from Germany the nectar of A. oleraceum is known to us. and one from Italy, were received through the Insects of the orders Hymenoptera, Diptera exchange of the Botanical Garden, University and Lepidoptera were observed to visit flow- of Helsinki. ers of Allium oleraceum (Table 1). 30 and 31 Seeds were germinated on moist soft paper July 1999, were sunny and warm (25 °C) days in petri dishes, if not otherwise stated. In some in Nåtö. Some species of bumblebees (Bombus germination experiments air dry seeds were pre- lapidarius, B. pascuorum, Psithyrus bohemicus treated by storing them in a refrigerator at 7 °C. (Apidae) visited flowers of Campanula rotundi- Germination was carried out at two temperatures: folia, Centaurea jacea, Lathyrus pratensis and (1) in dark (the petri dishes were covered by alu- Trifolium pratense at NBS. Only one of these, minium foil) and cool (7 °C) in a refrigerator and namely queens and workers of Bombus lapidar- (2) at 21–23 °C in the laboratory in dark. ius, frequently visited flowers of Allium oler- aceum. One other bumblebee species was once seen in an Allium oleraceum flower; it could not Results be caught for identification. Bombus lapidarius visited several (2–10) Insects visiting Allium oleraceum flowers in each Allium plant.