- 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.
- 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
- 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
- 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
- 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 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
- 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
- Probably the most important head/seed factors affecting yield are
the number of sound seeds per head, and sound seeds per unit area of
- 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
- 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
- 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
- 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.
- 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
- A colour rating system widely used in breeding or selection
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;
- 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.
- 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.