The sunflower, Helianthus annuus L., belongs to the family compositae. The genus name Helianthus is derived from Greek helios ‘sun’ and anthos ‘flower’, Locally it is know as surajmukhi. Sixty-seven species of this genus are recognized. Besides the cultivated species, many are perennials, ornamentals, and weeds. The sunflower has a shallow root system. The taproot grows as deep as 3 m, but the bulk of the root system generally remains in the 0-0.5 m horizon. The stem is circular, 3-6 cm in diameter, and strong, with rough hairs and slight longitudinal ridges.
Sunflowers usually grow 1-3 m tall; however cultivars as tall as 5 m have been reported. The number of branches varies from 1 to 12; they are fairly small and are found towards the top of the plant. However, branching is undesirable in sunflower, and the present commercial cultivars are single-stemmed and single-headed. The alternate leaves are usually large, hairy, and ovate, attached with long petioles. The number of leaves varies from 20 to 40, depending on cultivar and growing conditions.
Before anthesis, the leaves are highly heliotropic. Generally up to 50% defoliation does not affect yield or oil content. Sunflower produces a disc-shaped head called the capitulum on top of the stem. The head commonly varies from 10 to 30 cm in diameter, and heads as big as 76 cm in diameter have occasionally been produced. In each head there are 1000-4000 flowers. The flowers are of two types: 1) the outer row of brightly-coloured, sterile ligulate flowers, and 2) the inner, brown or purplish, fertile disc flowers. The flowers are arranged in spiral whorls originating at the centre of the inflorenscene. All the florets on a head open in 5-10 days.
Although floral initiation is a complex phenomenon, it is favoured by short days, the begins at about the eight-leaf stage of development. The flowers are normally cross-pollinated, mainly by honey-bees. The introduction of self-compatible hybrids is reducing the necessity of honeybees for pollination. The head is heliotropic until most of the flowers are fertilized, after which it generally remains fixed facing the east. The heliotropic movement is related to the level of auxins and their distribution in actively growing parts of the plant (Weiss 1983`410). Seeds start to mature at the periphery and progress towards the centre of the disc. Seed colour varies from black to white, with striped or mottled appearance.
The seeds are usually 10-25 mm long, 8-15 mm wide, and 3-7.5 mm thick. One thousand seeds weigh 50-100 g and in many cases much more. The size, weight, and viability of the seeds decreases from the periphery to the centre of the head. The oil content of the seeds varies from 25 to 48%. Cultivars with larger white or striped seed coats and low oil content are grown for confectionery and birds. The discovery of cytoplasmic male sterility and fertility restoration has made it possible to produce sunflower hybrids with high oil content. Sunflower hybrids are now commercially available for cultivation (Weiss 1983:415).