Agriculture Reference
In-Depth Information
action of low temperatures. Vernalization and light requirements largely determine
the response of cultivars to sowing date. Exploitation of genetic variations in day-
length sensitivity and vernalization requirement result in wider adaptability of wheat
crop. Allelic variations at the
VRN-1
locus are one of the main sources of genetic
differences in vernalization requirement in both diploid (
Triticum monococcum
)
and polyploid wheat (McIntosh et al. 2003). Common wheat is a hexaploid species
(2n = 42, genomes AABBDD) and carries three homologous copies of the
VRN-1
gene, one in each of the three genomes, which are designated
VRN-A1
,
VRN-B1
, and
VRN-D1
(McIntosh et al. 2003). A dominant (
Vrn
) allele at any one of these loci is
sufficient to confer a spring growth habit (Stelmakh 1998) and spring type flowers
without vernalization requirement in the presence of photoperiod-insensitive genes,
or once the long day requirement is met. Winter habit is conferred when the reces-
sive alleles are in the homozygous state across the
Vrn-1
locii. However, the differ-
ent dominant
Vrn
alleles have differential sensitivity to vernalization, with
Vrn-A1
being not only insensitive to vernalization but is also epistatic to
Vrn-B1
and
Vrn-D1
,
both of which have residual vernalization requirements (Pugsley 1971; Shindo and
Sasakuma 2002). Other vernalization-responsive genes like the
Vrn-2
series,
Vrn-B3
and
Vrn-D5
, have also been identified in wheat (Goncharov 2003; Kato et al. 2003;
Yan et al. 2006). The variation in response to vernalization due to the presence or
absence of different
Vrn
alleles causes differences in flowering time, where the cul-
tivar with
Vrn-1
allele flowers earliest and those with either
Vrn-D1
and
Vrn-D5
and
Vrn-B1
flower later under nonvernalizing conditions (Iqbal et al. 2011). The presence
of two dominant alleles results in early maturity and higher yield, whereas the domi-
nant allele at all the three loci results in earliest maturing but a low yielding cultivar.
In South Asia, most of the wheat are planted around mid-November, and the average
temperature in November hovers around 20 ± 5°C. Earlier popular wheat varieties
in India, such as HD2329, were of 135-140 days duration and therefore were able to
complete their life cycle under favorable crop growth conditions and rarely exposed
to terminal heat stress, if planted timely. A quantum jump in productivity of the
wheat cultivar HD2329 was realized with the development of Veery derivatives such
as PBW343, WH542, PBW373, and Raj3765 in India and Inqlab in Pakistan, which
are of 140-150 days growth duration. The presence of a rye segment in later culti-
vars buffers them against abiotic stresses; however, even the timely planted wheat
crop is often exposed to terminal heat and drought stresses because of longer dura-
tions. Farmers in the northwest of the Indo-Gangetic plains resorted to early sow-
ing of these varieties, such as to maximize wheat productivity. Besides the Veery
derivatives, a number of other varieties, such as HD2329, HD2894, and HD2851,
are also grown in the northwestern plains of Indo-Gangetic plains. If these cultivars
are seeded in October, many times they flower very early, in response to prevailing
high temperature owing to the presence of the inappropriate
Vrn-1
allecombina-
tions. This results in a drastic yield reduction of these cultivars (HD2329, HD2894,
and HD2851) under early sown condtions. However, variety like PBW343, because
of the
vrn-A1
and
Vrn-B1
alleles have partial vernalization requirement, does not
flower early even if planted early, as their vernalization requirement is met after
mid-December and early January. The
Vrn
allele with partial vernalization require-
ment therefore needs to be appropriately exploited to develop cultivars that can yield
