Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 23

hybrids of the southern hemisphere are taller than hybrids of the northern
hemisphere. A generally accepted ideotype of productive sunflower is a
medium plant height of 160 to 180 cm (Skoric 1988). Commercial hybrids
with a height of 120 to 150 cm have been developed and are referred to as
semi-dwarf hybrids, while dwarf sunflower are 80 to 120 cm tall (Schneiter
1992). Stem length is determined by the number and length of internodes.
Both tall and short plants with many internodes will have thick stems
because of the positive association between numbers of internodes and stem
thickness. Stems that are short because of fewer internodes will be thinner
(Knowles 1978).
Information on development of sunflower roots is limited. Early studies
by Weaver (1926) on the cultivar “Russian” at Lincoln, Nebraska, found
that a strong central taproot penetrates to a depth of 150 to 270 cm. Pustovoit
(1967) reported root depths of 4 to 5 m in cultivars with a growth period of
100 to 110 days. Root depths of 2 m or more have been frequently reported
for sunflower (Jones 1984; Gimenez and Fereres 1986; Sadras et al. 1989;
Angadi and Entz 2002a, b). The rooting system of sunflower can be
considered “explorative”, i.e., a large volume of soil is explored with a
combination of thick and thin roots, low specific root length, and low root-
length density, as opposed to “exploitative”, root systems characterized by
predominantly fine roots, high specific root length, and large root-length
density (Boot 1990). Adventitious roots may be complementary to primary
root systems and may function in plant anchorage and in water absorption
and conductance.
The leaf consists of the blade (lamina) and a stalk-like part, the petiole,
which connects it to the stem. Leaves in sunflower are rarely sessile (without
a petiole), except in some wild species. The leaf is highly variable in both
structure and function. In sunflower, as seedlings emerge from the soil,
cotyledons unfold and reveal the first pair of true leaves at the top of the
shoot. Leaves are produced in opposite alternating pairs, and after five
opposite pairs appear a shorter form of alternate phyllotaxy develops
(Palmer and Phillips 1963). The number of leaves on single-stemmed plants
may vary from as few as eight to as many as 70 (Knowles 1978). There
appears to be some association between the number of leaves and time to
maturity for plants with numerous leaves (Knowles 1978). Plants with
numerous leaves are also usually late maturing. The number of leaves on a
plant and their expansion, size, shape, and duration can be greatly
influenced by environmental factors. There is considerable variation in leaf
size, shape of the entire leaf, shape of the tip, base, margin, surface
pubescence, and petiole characteristics. The length and width of leaves
vary with the height of the stem. The ratio of length:width may be a useful
criterion for cultivar description.