82
1989 ; Calderini et al. 1995 ; Bodega and Andrade 1996 ), usually without a reduction
in size (Ledent and Stoy 1988 ; Hay 1995 ).
A further increase in harvest index is diffi cult to achieve without a deterioration
in adaptability, so in the future higher yields are likely to be the result of an increase
in biomass or an improvement in water and nutrient use effi ciency (White and
Wilson 2006 ; Parry and Hawkesford 2010 ).
3.2 Traditional Wheat Breeding Methods
The breeding of a wheat cultivar can be divided into several stages. The fi rst step is
to develop populations with very broad genetic variability. After crossing, the
diverse properties of the parents recombine in the progeny, producing genetically
variable populations from which stable, homozygous lines can be selected over a
period of years. The third stage of breeding is testing. After several years of experi-
ments in an increasing number of environments it is possible to identify lines that
have signifi cantly better productivity than earlier cultivars, combined with a satis-
factory level of yield stability and with quality that meets the demands of users.
The most important method applied to develop populations with wide genetic
variability is the crossing of parental cultivars with diverse genetic backgrounds.
This may be a single cross (A×B), a top cross [(A×B) × C], a double cross [(A×B)
× (C×D)] or an even more complicated multiple cross. The choice of crossing
scheme depends greatly on the properties of the parental cultivars.
If the aim is to introduce into the breeding material a new trait that is only to be
found in initial stocks which are not adapted to local environmental and technologi-
cal conditions, it is advisable to use a crossing scheme where the donor genotype
contributes to a smaller extent to the development of the progeny generation. A
special crossing method designed for this purpose is the backcross [(A×B) × A] ×
A, where the aim is to improve a recipient or recurrent parent (an elite cultivar) by
introducing one or a few favourable traits (e.g., resistance, chemical quality, etc.)
from a donor (B) carrying alien genes. After a few backcrosses, followed by con-
tinuous selection for the desired trait, the result is practically the same as the initial
recipient genotype (A), except for the improvement in resistance or some other trait.
When applying this method it is important to remember that a new genotype devel-
oped in this way is essentially a derived version of the recurrent cultivar, so if the
initial material is protected by plant breeder’s rights, this protection extends to the
modifi ed/improved form of the cultivar.
The success of crossing programmes is greatly infl uenced by the properties of
the initial breeding material. A survey that involved breeders from 52 countries
demonstrated that in the majority of cases breeders use their own advanced lines
(39.7 %) or registered cultivars (17.4 %) for crossing, while 11 % of the crossing
partners are foreign lines and 17.4 % registered cultivars. Landraces are used in
3–4.5 % of cases. A considerable proportion of the parental partners (18.6 %) are
made up of genotypes included in international nurseries organised by CIMMYT
Z. Bedő and L. Láng