Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 75

trials are expensive to carry out, and it has been noted frequently that the
genotypes with the highest ratios were not generally those with the highest
yields under either drought or watered conditions. Breeders have searched
for some simple characters that could be measured on inbred lines. An
example was reported by Serieys (1989) who showed that the desert species
Helianthus argophyllus had less permeable leaves than cultivated sunflower.
He selected two populations which differed in their leaf transpiration rates
and rapidity of wilting. Baldini et al. (1996) proposed the use of characters
such as carbon exchange rate, transpiration rate and leaf relative water
content in breeding for drought resistance. However, the cost of such
measurements have meant that a few programs with intentional selection
of drought resistance have been carried out, generally varieties which adapt
well to dry conditions have been identified in yield trials in these conditions.
2.4.7 Resistance to High and Low Temperatures
The lowest soil temperature for germination and plant growth of sunflower
is about 6°C, but seedlings at the cotyledon stage can resist slight frosts,
whereas older plants may lose leaves or become branched because of partial
destruction of the terminal bud. For sunflowers to be grown as a winter crop
in Mediterranean type climates, resistance to slight frosts would be useful.
Restarting of growth at low temperatures would also be useful, not only for
autumn sown crops but also for very early spring sowings which could
allow the crop to escape summer droughts. Some direct breeding work has
been done (Gosset and Vear 1995), but large-scale studies have not been
made on the variability available or the genetics of such characters.
Resistance to high temperatures often concerns sunflower crops in the
same regions, in this case when they are sown in mid- or late-spring. Again
most work has involved practical breeding rather than genetics, with
elimination of genotypes which show symptoms of burnt dry capitular bracts.
2.4.8 Lodging Resistance and Reduced Height
Lodging may be at stem base or mid-stem, but it is always catastrophic. As
a factor of resistance, reduced height may be selected, but often the two
characters are independent. Breeding consists in eliminating the genotypes
which show lodging. The behavior of inbred lines and their hybrids are
generally significantly correlated.
Independently of lodging resistance, reduced height is of interest: to
make possible increases in plant population, to reduce the proportion of
dry matter in the stem and to facilitate surveillance and harvest of the crop.
Inheritance of plant height is generally quantitative, with broad sense
heritability of 6% and narrow sense of 20 to 40% (Fick 1978). In the 1980s,