Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 22

is prolonged if heads are larger, or if the weather is cool and cloudy. The
stigmas remain receptive for up to 4 or 5 days.
The achene, or nut-like fruit of the sunflower, consists of a seed, often
called the kernel, and the adhering pericarp, usually called the hull. Achenes
mature from the periphery of the whorl to the center. As achenes mature, the
withered calyx, corolla tube, anther, stigma, and style drop off at the point of
their attachment. The achenes usually are largest on the periphery of the
head and smallest at the center. Disk flowers in the center of the head in
some breeding lines fail to produce filled achenes. The lack of achene filling
in the center of some heads is influenced both by genotype and environment.
The dimensions of achenes range from 7 to 25 mm long and from 4 to 13
mm wide. The achenes of oilseed sunflower are usually black. Achenes are
much smaller in the wild species where they are from 2 to 7 mm long and
generally from 1 to 2 mm wide. The weight of 100 achenes of cultivated
sunflower ranges from 4 to 20 g. The weight of an individual achene ranges
from 40 to 400 mg. Lengthwise, the achenes may be linear, ovoid, or almost
round, and in cross-section they may be flat to almost round. Large achenes
usually have thick hulls and relatively small kernels. Small achenes, in
contrast, usually have thin hulls tightly fitting around the kernel. Thinner-
hulled achenes usually have higher oil content than the thicker-hulled
achenes.
Cultivated hybrid sunflower plants are annual with non-branched stems,
while the wild species have dominant branching. Branched types that
possess recessive genes for branching are frequently used as parental male
breeding lines or pollinator lines in commercial seed production. Stem
dimension and development are influenced by the environment and by
plant population. Branching can also be influenced by the environment,
especially when the terminal buds are injured early in phenological
development.
Many degrees of branching occur in sunflower, ranging from a single
stem with a large solitary inflorescence in cultivated types to multiple
branching from axils of most leaves on the main stem in the wild species.
Branch length varies from a few centimeters to a distance longer than the
main stem. Branching may be concentrated at the base or top of the stem, or
spread over the entire plant. Generally, heads on branches are smaller than
heads on the main stem. Occasionally, some first-order branches have a
terminal head almost as large as the main head. In most wild species, the
head on the main stem blooms first, but generally is no larger than those on
the branches.
Stem (plant) height of commercial sunflower cultivars ranges from 50 to
over 500 cm, and stem diameter from 1 to 10 cm. Rare plant types 12 m tall
were reported by Dodonaeus (cited by Cockerell 1915). Cultivars which are
used for forage are usually tall and late flowering. Long-season oilseed