Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 21

amplification products revealed homologies between Helianthus genomes
and several other genera in the Helianthinae sub-tribe.
1.4.2 Plant Structure and Growth Habit
Sunflower is an annual crop distinguished from other cultivated crops by
its single stem and its large conspicuous inflorescence. When sunflower is
in full bloom, it is one of the most photogenic crops because of its large
inflorescence with showy yellow-orange ray flowers. Flowering sunflower
has served as an inspiration for artists, poets, and business promotions.
The inflorescence is a capitulum or head consisting of an outer whorl of
showy and generally yellow ray flowers and from 700 to 3,000 disk flowers
in oilseed hybrids up to 8,000 disk flowers in non-oilseed hybrids (Pustovoit
1975). The disk flowers are arranged in arcs radiating from the center of the
head and are perfect flowers that produce achenes. Involucral bracts or
phyllaries, which vary in form and size, surround the head.
The showy flowers on the outer whorl of the head have five elongated
petals united to form strap-like structures, which give them the name ray or
ligulate flowers. Ray flowers are usually golden yellow, but may be pale
yellow, orange-yellow, or reddish. Variation in petal color has been described
by Cockerell (1912, 1918) and Fick (1976). Ray flowers are normally sterile,
having a rudimentary pistil and vestigial style and stigma, but no anther.
Mutants occur producing more than the normal numbers of ray flowers, or
occasionally none.
The remaining flowers covering the large discoidal head are called disk
flowers. A single disk flower is often referred to as a floret. Each floret is
subtended by a sharp-pointed, chaffy bract, a basal ovary, and two pappus
scales (often considered being a modified sepal). The disk flower is perfect
(contains both a stamen and pistil).
At anthesis the outer whorl of disk flowers opens first, at about the time
that the ray flowers open from their folded position against the buds of the
disk flowers. Immediately after this stage is reached, the anther locules
dehisce, releasing pollen inside the anther tube. An elongation of the lower
portion of the style pushes the two-lobed pubescent stigma up the anther
tube. The stigma is not receptive at this stage because the two lobes are held
together covering the inner receptive surface. The stigma appears at about
the 1700 hour of the same day, and by the following morning it is fully
emerged, with receptive surfaces exposed. At this time, the staminal filaments
lose turgidity and the anther tube begins to recede into the corolla. The
beginning of flowering of disk flowers has been described as the R-5.1 stage
of sunflower development (Schneiter and Miller 1981). One to four rows of
disk flowers open successively daily for 5 to 10 days. The flowering period