The cultivated sunflower is a tall, erect, unbranched, coarse annual, with a distinctive large, golden head, the seeds of which are often eaten, and are commonly crushed for their oil. Since it is often necessary to define the development stage of a crop in the field, a series of general and easily observable changes in plant growth forms a reasonable basis.

Plant

• Annual with rather stout, erect, herbaceous stem, 2.5 to 7.5 cm in diameter and 1.5 to 6.0 metres in height, rough, hairy or hispid, usually without branches except near the top, but occasionally branching freely throughout; leaves alternate except near the base of the stem leaves with rather stout petioles, 5 to 25 cm long and about two thirds as wide, rough on both surfaces coarsely and irregularly toothed on the margins, pointed at the apex , heads, one to six or more in the small flowered branching forms, terminating the stem or branches, 10 to 50 cm in diameter with 40 to 80 yellow rays and numerous brown or nearly black discs.

Stem

• The stem is robust, circular in section, usually 3-6 cm in diameter but occasionally reaching 10 cm, bearing rough hairs, and may have slight longitudinal ridges.
• The woody exterior is filled with a stiff white pith, and frequently becomes hollow with age.
• The main constituent of the stem of sunflower were 53% cellulose, 17% lignin, 17%pentosan, 3 per cent crude protein and 8 per cent ash.
• The stem is usually green or greenish yellow, occasionally mottled, or with a bluish tinge.
• The plant grows rapidly, the stem varying in height from 1 to 3 m when full grown, frequently taller, and individual plants of giant varieties have been recorded as reaching 5 m.
• Saline soil or irrigation water can reduce both the stem diameter and height.
• Branching was common but undesirable in commercial crops grown for seed, since it adversely affects ripening, head size and seed-oil content.
• Branches are usually small, occur towards the top of the stem, and may vary in number from one to a dozen or more, but branching is uncommon in present commercial cultivars.

Leaves

Leaves are usually alternate, occasionally opposite on the lower stem and alternate above, large, ovate, cordate, frequently heavy and carried on long petioles.

• Their colour is usually dark green, but may sometimes have a bluish or reddish tinge.
• Some twenty to forty are produced per plant, their rate of development and number being a varietal characteristic.
• Within a variety final leaf number per plant is mainly an effect of environment, i.e. plant population, soil moiture or temperature.
• Irrigated plants for instance, can produce two to three times the leaf area than when rainfed.
• Leaves are highly heliotrophic prior to anthesis, but this diminishes with plant maturity.
• Leaf production and stem elongation continue until the inflorescence opens and flower begins, when the number of active leaves decline.
• Artificial defoliation to stimulate severe insect or hail damage, has shown that some 50 per cent of leaf area must be lost before there is any significant depression in seed yield or seed-oil content.
• The most critical period for defoliation to occur is between bud formation and flowering.

Head

The disc-shaped head, capitulum, is borne terminally on the main stem and branches where these occur. Head size varies between cultivars, seasons, soil type, etc., and is commonly 10-30 cm in diameter, occasionally much larger, and the greatest diameter has the most significant effect on seed yield, but there is usually an optimum diameter for maximum seed production in the field.

• Probably the most important head/seed factors affecting yield are the number of sound seeds per head, and sound seeds per unit area of the head.
• Within these parameters, selection for a long, thin-hulled seed would appear theoretically desirable, but in practice such seed is difficult to sow in commercial plot and air seeders.

Flowers of the capitulum are of two types

• An outer row of brightly coloured, sterile, ligulate flowers, which are usually yellow but can vary from deep yellow to red, and the brown or purplish disc flowers.
• There can be 1,000-4,000 individual florets per head, which are arranged in spinal whorls originating at the centre of the inflorescence, and mature progressively from periphay to the centre of the disc.
• Opening of all florets on a single head is usually completed in 5-10 days, but if individual florets are not quickly pollinated, they can remain receptive for up to 14 days, with a greatly reduced possibility of being fertilized.
• Flowering within a crop of hybrid sunflower is remarkably uniform, with 80-90 per cent of heads opening within 3-4 days of the first to do so.
• Introduction of the reproductive phase is related to day length, and although the relationship appears complex, it is considered that short days favour floral initiation.
• As a field guide, floral initiation occurs about eight-leaf stage of development.Flowers are protandrous and normally cross-fertilized, pollination being affected by insects walking on the surface, wind pollination being rare.
• It would appear that honey-bees are the main, sometimes almost exclusive pollinators, and a major cause of poor seed set in commercial sunflower plantings was inadequate pollination.
• A second factor reducing number of sound seeds on the heads is that centre florets produce less pollen than ray florets, and are also less likely to be visited by insects.
• Growth regulators not only affect plant growth, they can also affect seed characterstics.
• For instance, when TIBA was sprayed on the flower heads as ray florets opened, seed yield and 1,000seed weight increased and the number of pops were reduced.

Seed

The fruit commonly known as the sunflower seed is more correctly an achene, and commercially grown varieties range in colour from black through to white, but brown, striped or mottled seed can also occur. A colour rating system widely used in breeding or selection programmes is

a) black;

b) black with grey stripe;

c) black with white/light-grey stripe;

d) dark grey with white/light-grey stripe;

e) grey with white stripe;

f) grey with grey stripe;

g) white.

• Many growers still equate dark seed with the oil market and light or striped seed with birdseed or confectionary sectors.
• Although there is often a relationship between a dark hull and high average seed-oil content, an increasing number of hybrids have light-coloured seed with high oil content.

Roots

The root system is substantial but often shallow, and although tap root going upto 3 meter depth with a larger lateral spread of feeder roots it rapidly reduces in diameter from the soil surface and so offers but increase support to the mature plant. In the more arid tropics it is often partially destroyed by termites prior to harvest, so increasing the incidence of lodging. In unrestricted conditions, the tap-root grows at approximately the same rate as the stem for the first 5-6 weeks, then more slowly.

• Root growth is generally prolific and since the majority remain near the surface, too deep inter-row at 60 cm row spacing, and many roots will be severed by ill-adjusted weeders.
• Mature plants of the giant types can have roots that penetrate to a depth of 2.5m, although the bulk generally remain in the 0-0.5m horizon.
• Root distribution and penetration is most important for rainfed crops, for late vegetative growth.

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