Principles of Plant Genetics and Breeding

Climatic adaptation
MUTAGENESIS IN PLANT BREEDING 209
The main programs are based in the Nordic countries (Drudze 2004; Nybom 2004) and in Canada (Khanizadeh et al. 2004). The
objectives are to release winter-hardy cultivars adapted to a short and cool growing season. Specific local cultivars are used as
progenitors.
Programs are also being conducted in the Republic of South Africa and Brazil, releasing cultivars with low chilling requirements
adapted to the apple areas of these regions (Labuschagné 2004).
Tree habit
Almost all breeders concentrate their tree habit programs on compact trees. “Wijick”, a sport of “McIntosh” with columnar habit,
is often used but it is very susceptible to biennial bearing. At INRA, the aim for tree habit is to create weeping and not too vigorous
trees that give good and regular cropping and are easy to prune and harvest. Parents that transmit the trait “one fruit per cluster”
are also included (Laurens et al. 2000).
Selection process to release new scab-resistant apple cultivars
Compared to the main annual crops, breeding for fruit trees is particularly costly and long: the whole process to select a new
apple cultivar is likely to last between 15 and 20 years. Some specific characteristics of fruit trees explain this.
The selection process in all fruit species is linked to genetic, morphological, physiological, and agronomical constraints. The
length of the juvenile period is certainly the most restrictive. For the apple tree, it lasts from 3 to 6 years according to the genotype
and tree management. This period is characterized by a strong vegetative growth and no flowering. During these first years of
growing, no assessment can be performed on the fruits or on the tree, selection is limited to early resistance tests on seedlings.
Another big constraint for the breeder is the volume of each individual tree, which limits considerably the number of progenies
that can be studied in one site compared with annual crops.
Few genetic studies have been published on genetic parameters of agronomic traits in the apple; most of them have been dedicated
to resistance. In fact, work on fruit quality faces many problems: the length of the juvenile period, establishing objective criteria,
the close interaction between vegetative growth of the tree and fruit-quality traits, the complex genetic nature of the traits,
etc. More generally, the highly heterozygous status of the apple cultivars, due to self-incompatibility, makes the estimation of
genetic parameters very difficult as well as the breeding process.
Fortunately, the breeder can take advantage of the apple tree’s ability to be propagated by grafting. Once one initial tree is
found promising, it is easy to propagate it quickly and in large numbers to be tested in different sites.
Cycle of selection
All apple breeders roughly follow the same process of selection; only some practical points may change from one team to
another. As an illustration, this box will present the cycle of selection performed in France through a partnership started in 1996
between the INRA, a public research
institute, and NOVADI Ltd Co., which
employs 16 nurserymen involved in
the development of new, resistant
G
l
a
s
s
h
o
u
s
e
Nursery
Scab-resistant seedlings
5,000–6,000
Seedlings
15,000–20,000
INRA Angers
Scab-resistant and
mildew-tolerant
seedlings
Crosses
30–40
3,000
Grafting/M9
1,000 1,000 1,000
Six experimental sites
10 selections
Level 1
Level 2
National varietal tests
Figure 1 Cycle of selection in the collaborative INRA–NOVADI apple
breeding program.
N
O
V
A
D
I
apple cultivars. Work and responsibilities
are shared between INRA and
NOVADI as illustrated in Figure 1
(Laurens & Pitiot 2003).
Each year, cross-pollinations are
performed. Flowers are hermaphrodite,
so each genitor can be used as the
male or female parent. In general, the
earliest flowering parent is used as
the male. The collection of the pollen
and the different crossing techniques
are described by Janick et al. (1996).
The first step of selection occurs 1 year
later on young seedlings that are tested
in the greenhouse for scab resistance
(Figure 1). All the seedlings are sprayed
with a scab mixture, mainly race 1
(L. Parisi, personal communication),
obtained from scabbed leaves collected
in the INRA orchards. After two