Biology Reference
In-Depth Information
hybrids are from the S 2 or S 3 generations. Selection of useful inbred lines is
most commonly based on measurements of general combining ability with
one or more tester lines. For new male (restorer parents) known inbred CMS
lines (or unrestored single cross hybrids) can be used. For new female
(sterility maintainers) lines, in the first generations, when no CMS version
is available, it is more complicated although there are several possibilities.
Unrelated CMS lines: however, if the line belongs to the same group,
heterosis may not represent that with restorer lines. In addition, the hybrid
would be male sterile and Vear (1984) showed that such hybrids
overestimated the value of the new line, especially for oil content.
A restorer line sterilized with gibberellin: very often the quantity of seed
obtained is too small.
A restorer line with a cytoplasm different from PET1, so that it is male
sterile. The validity of this depends on whether the hybrid produced is male
fertile or male sterile.
A restorer line with genic male sterility: in this case the restorer acts as
female parent and the hybrid is male fertile. Branches may be removed from
the restorer line to increase the main capitulum and seed size.
To obtain an estimate of general combining abilities, several tester lines
are used in the course of breeding. Then the best female (CMS) lines are
crossed with the best male (restorer) lines and further yield trials are
necessary to determine the best specific combining abilities. All these trials
are an expensive part of breeding programs.
Recently, research has been carried out on possible methods of predicting
hybrid yield from characteristics of inbred lines. Debaeke et al. (2004) and
Triboi et al. (2004) observed that the maintenance after flowering, of a large
area of actively photosynthesizing leaves is correlated with seed yield in
hybrids. This character is measured by total leaf area at flowering, duration
of leaf area after flowering and nitrogen contents of leaves at and after
flowering (which represent enzyme activity). These characters were shown
to be heritable between lines and their hybrids, so it would be possible to
select between early generation plants or families for “residual leaf area”,
for example, and only make hybrids with the best material. However, the
measurement of leaf area by hand (length
0.71, for sunflower)
requires a lot of time and some automation will be necessary to use it on a
large scale.
In spite of the complications in judging seed yield, this character has
been significantly improved over the last 40 years. In most European
countries, hybrids have to show improved yields compared with the most
widely grown varieties (103%) to be registered on the European and National
Catalogs. In France, for a crop of 600-800 000 ha, 20-30 new varieties are
registered each year. In 2000-2001, the French sunflower industry as a whole
carried out trials with the most widely grown varieties from 1970 to 2000 to
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