|Sunflower (Helianthus annuus) belongs to the genus Helianthus composed of 19 species and 37 perennial species. The only cultivated sunflower, H. annuus var. macrocarpus (DC) Ckil. is said to originate from North America.|
Currently it is cultivated all over the world with the largest areas being in the USA, Argentina, the Ukraine and Volga regions. Tropical type of sunflower is found in Australia and parts of Africa and Argentina. This germplasm is characterized by thick hulled seed and tall plants (2.5-3.0 m).
Sunflower belongs to the Compositae family. The sunflower plant is an erect annual herb with height ranging from 0.5 m to 3.5 m tall. The plant has a tap root which can extend down to 3 m supported by a large lateral spread of surface roots. The stem of sunflower plant can be branched or unbranched with an angular shape and a hollow to soft pitch. Leaves of the plant are oppositely positioned at the base and alternate towards the top of the stem. The shape of the leaves are variable from being cordate to ovate.
The floral part of the plant, the capitula or the head, is terminal and has a diameter ranging from 10 cm to 40 cm. Larger heads can be obtained under low plant population and higher fertilizer application. The head can be drooping or erect. The florets are hermaphrodite and aggregate into variable head sizes.
The head is surrounded by free connate bracts. On the immediate inside of the bracts are ray florets which are bright coloured, yellow to orange. In the inner part of the ray florets are disc florets which are hermaphrodite. Occasion