Being a biannual crop onion has two crop cycle, Seed cycle is the important one where we studied floral biology and phenology. The majority grow as biannual, producing an underground storage bulb at the end of the first growing season, which flowers in the next.
Onion plants started flowering in January that is exactly 60-67 days after bulb sowing (Table.1). Onion flowers were produced on single elongated inflorescence stalk called scape which is hollow from inside. Each Onion plant produced 6.72±0.01 scapes which bears exactly same number of inflorescences. Onion flowers born on inflorescence called umbel. All the plants belong to genus Allium sps produce umbelliferous inflorescence (Fig.1). These comprises average of 479±19.06 numbers of small flowers called florets which were bounded in a membranous 2-3 white coloured spathe. Due to the pressure created by growing flower buds the spathe splits open. Each floret lives for about 6 days. Number of florets per umbel differ from plant species to species and depends on plant genotype, planting time, size and storage conditions of mother bulb (Sunita Devi et al., 2015).
The florets were of white in colour with green stripes. Colour of flowers also varies from species to species. Viz., Allium afatunense flower tepals are initially violet and then turn to purple (Beata Zuraw et al., 2009) and Allium giganteum produce purple red cluster (Beata Zuraw et al., 2010). Sunitha Devi et al., reviewed that flowers are white to bluish in colour which is contradict to our result.
Onion flowers possess very strong odour. This odour was characteristic and was created by chemical alteration of a sequence of volatile secondary metabolites like sulphur containing compounds (Jones et al., 2004).
Onion plants took almost 67 days to produce first open flower and it required 91 days to produce maximum number of open flowers. Only a few flowers open on the umbel at the beginning of flowering which gradually increases until more florets open in one day at full bloom. Umbels were in bloom for a period of 30 days as the florets continue to open over a 2?3 week period. Flower initiation in onion is sensitive to temperature and depends on photoperiod, number of leaves and bulb development. The reproductive response of onion plants was affected by photo-thermal conditions (Branca and Ruggeri. 1994).
Anthesis take place on early morning hours, usually 06-07 am. The anthesis commences from outer layer of florets in umbel and goes to central in succession. Pollen fertility was found to be highest on the day of anthesis. Stigma receptivity was also high on the day of anthesis. Both perianth and stamens were six in number arranged in two whorls. Stamen was found to be 5.33 mm in length and consists of bilocular anthers which splits longitudinally to release pollen. Anther dehiscence occurred between 07 am to 05 pm. Anthers produced pollen grains of size 0.04 x 0.02 mm in length. Each anther produced on an average of 1609.58 anthers which is sum up to 9657.41 pollen grains per flowers. Florets produced elongated pistil having length of 4.56 and superior ovary (Fig.2).
Mutualistic inter-species relationships are very common in all kingdoms of living organisms since practically every species is involved in one or the other such associations. (Bronstein et al., 2006). Plants are attached to the ground and have restricted chance of movements, the benefits they receive in mutualistic interactions with animals, and especially insects i.e the ability to cover long distances and play a vital role in pollen transfer in the plant and collect reward for themselves. (Schoonhoven et al., 2005; Rico-Gray and Oliveira, 2007). The ecological importance of pollen and nectar as a food-reward, offered by animal-pollinated plants to their pollen vectors, has long been recognized (Simpson and Neff 1981).
In pollination ecology, the flowers of Allium are classified in morphological terms as bowl shaped flowers with hidden nectarines like under part of ovary. Onion flowers were found to be good source of floral nectar and pollen. The nectar produced from the nectaries was found collected in three cups between the lower ovaries wall and inner whorl of stamens. Undisturbed nectar on florets was easily visible like a small sparkling bubble in the sunlight (Fig.2). Onion flowers produced nectar volume of 1.80micro liter/ floret with average total soluble solids 63.80. LCMS sugar analysis data showed that onion floral nectar was rich in fructose (20.339 mg/g), glucose (10.99 mg/g) and sucrose (0.151 mg/g) sugars (Table.2).. Our results was in accordance with Silva and Dean (2004) showed that individual onion flowers produced the highest amount of nectar and Onion nectar was predominatly hexose rich with very low sucrose (Silva and Dean, 2000).
Onion being a cross pollinated and entamophilic crop, enhancement of our understanding on its flower morphology, floral biology, phenology and pollination syndromes helps in appreciate natural pollinator complex and plan seed production programs. Even though lot of variation exists in plant behaviour and phenology among different cultivars, in different region, at different agro climatic zones, effort has been made to understand the fundamental aspects of floral biology of the crop.