Agriculture Reference
In-Depth Information
many parts of the world, most notably in certain states of the USA. This work has
had a huge impact through the development of F
1
hybrid cultivars (see Fig. 3.5),
based on the discovery and exploitation of male sterility genes by Henry Jones and
his colleagues at the University of California at Davis (Jones and Clarke, 1947). In
the breeding of new varieties in state-funded institutions the systematic exchange
and exploitation of genetic material on a global basis has been scientifically
documented (e.g. Corgan, 1988). Goldman
et al.
(2000) give the history of
publicly funded onion breeding programmes in the USA, including the pedigrees
of the resulting cultivars. In recent decades public funding, coordinated by the
International Board for Plant Genetic Resources (IBPGR), has been directed
toward collecting, characterizing, documenting, conserving and distributing the
world's diverse heritage of onion varieties (Astley, 1990). This in turn is
facilitating the worldwide exchange and utilization of genetic material in both
publicly and privately funded onion breeding.
The successive stages of cultivar development are well illustrated by the
progress of the early-maturing, sweet onion crop of the extreme south of the
USA. In the late 19th and early 20th centuries, flat-shaped, sweet, short-
storing onions, known as Bermuda types, began to be grown in southern
Texas. These were of Italian origin (Magruder
et al.
, 1941). In 1925, seed of a
short-day, early-maturing, sweet, short-storing Babosa type onion, grown as
an overwintered crop on the Mediterranean coast of Spain, was imported from
the Valencia region into the USA. This was selected at the New Mexico
Agricultural Experiment Station to produce a round top-shaped, thin-skinned,
mild-flavoured, sweet variety which was named 'Early Grano'. This cultivar
was higher yielding than the Bermuda types and began to supplant them after
its introduction.
From this variety the cv. 'Texas Early Grano' was bred at the Texas
Agricultural Experiment Station and released in 1944. Resistance to the soil-
borne disease pink root (see Chapter 5) was transferred from resistant
Bermuda types by crossing and selection. A whole series of Texas Grano-type
onions, differing slightly in maturity date, has since been developed at the
Texas A&M University, so that a succession of maturity dates between March
and May can be achieved in the Rio Grande Valley. These were developed from
a cross of cv. 'Texas Grano' with the later-maturing Israeli cv. 'Ben Shemen',
itself derived from the Californian cv. 'Sweet Spanish' (Pike
et al.
, 1988).
Similarly, in other parts of the world with mild winters, such as Israel and
the Mediterranean coast of Spain, different seasonal cultivar groups mature
successively from early spring through the summer to maintain fresh bulb
supplies over a long season. In the USA in the mid-1950s H.A. Jones developed
the hybrids of the Granex series, starting with cv. 'Yellow Granex', by selection
from 'Texas Grano' and incorporation of cytoplasmic male sterility (see Fig. 3.5).
Yellow, red and white Granex hybrids are now available and they are among the
most widely grown cultivars, since they produce high yields at tropical latitudes.
However, as a consequence of their ancestry and their original purpose as an
