Page 11 - Molecular Plant Breeding

Molecular Plant Breeding 2013, Vol.5, No.9, 47-63
http://mpb.biopublisher.ca
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also reported that salinity might directly or indirectly
inhibit cell division, which will result in leaves and
stems of the affected plants become stunted. Besides
some of the RILs in the experiments had higher plant
height under saline conditions than both parents
(43.93 cm for RILs vs 15.0 cm for IR29 and 28.0
cm for Hasawi) (Table 1). Of the 300 RILs evaluated,
86 had an increase of up to 15 % in root length over
Hasawi; the donor and tolerant parent, whilst the
effect of salinity on root length was 20% relative to
the control trials.
There was a retardation of root growth under saline
stress compared to the non-saline experiments. Some
RILs were less affected by saline stress and had lower
root length reduction (15% compared to 37% in the
sensitive RILs and 29% in Hasawi, the tolerant check).
The result indicates that root length decreased under
salinity stress; Maiti et al. (2006) identified wide
genetic variation under saline condition at the seedling
stage using similar parameters as per this study. It has
also been reported that root length and number of
roots /plant are reduced with an increasing effect of
salinity (Roy et al., 2002; Rodrigues et al., 2002)
Wide variation in seedling dry weight relative to the
none saline control were observed in the experiments;
for instance, some RILs were highly affected by
salinity stress (over 90%), whilst some lines even had
higher shot weight (26% increase). This finding add
credence to the earlier one made by other researchers
(Al-Amin et al., 2013, Maiti et al., 2006) that wide
genetic variation exist in rice under saline condition
at the seedling stage using parameters such as root
length, shoot and root dry weight. Islam et al. (2007)
reported that total chlorophyll content is reduced when
salinity level is increased resulting in decreasing in
shot dry weight. Hossain (2004), Roy et al. (2002)
and Khan et al. (1997) also found that seedling height
and dry weight of different rice genotypes showed a
declining trend with the increase of salinity level.
The significant correlations between plant height and
or shoot dry weight and all other parameters suggest
that salt-tolerant RILs had a high seedling height and
shoot dry weight (Table 3). An increase in seedling
height seems to be responsible for increasing total
shoot dry weight and have been associated with high
grain yield potential in rice (Peng et al., 1999; Zhang
et al., 2004). The extent by which the studied
parameters affects biomass was also reflected in their
correlations with fresh weight under salt stress,
especially with SES score (r=0.58), seedling height
(r=0.49), root length (r=0.40) and dry weight (r=0.23).
This suggests the importance of these growths
attributes and therefore yield loss under salt stress.
Reductions in survival and growth of rice seedlings
under salt stress are the major causes of stand failure
and biomass reduction leading to yield loss in
salt-affected areas, as has been documented in
numerous studies (Pearson and Bernstein, 1959;
Kaddah, 1963; Flowers and Yeo, 1981; Heenan et al.,
1988; Lutts et al., 1995; Zeng and Shannon, 2000a;
Zeng and Shannon, 2000b; Zeng et al., 2001). Low
levels of salinity such as EC of 1.9 dS/m has been
reported to cause considerably reduction in shoot
biomass; besides seedling survival can substantially
be reduced at an EC of 3.4 dS/m, reflecting the high
sensitivity of rice to salt stress at early stages (Zeng
et al., 2001).
Considerable variation in plant vigor and growth
parameters was observed between the tolerant and
sensitive RILs reflects in the strong interaction
between genotype and all the treatment in the trials
under saline stress which is mainly due to greater
variation among genotypes under salt stress (Table 2).
The variation between the tolerant and sensitive RILs
was clearly reflected by the averaged reduction in
seedling height under salt stress, where it decreased
by about 5.5% in the tolerant RILs but to a much
greater extent (24%) in the sensitive RILs. Based on
SES score under stress and also other important traits
studied under both control and saline stress conditions,
31 RILs have been identified for advancement in our
breeding program at AfricaRice center (Appendix 1).
2.2 QTLs identified in relation to already identified
ones
It has been suggested that for an efficient QTL study,
using a 384- plex SNP set, a suitable polymorphism
rate of more than 200 markers is preferred (Thomson
et al., 2010b; 2012), but in this study, only 194
markers representing about 50% were polymorphic.
This might be due the fact that the donor parent,
Hasawi even though SNP-facilitated allelic diversity
has revealed it to be an Aus cultivar suggesting a high