Basic HTML Version
Cotton Genomics and Genetics
61
while the physiological basis of heterosis for lint yield
in cotton is probably attributed to the enhanced
photosynthetic capacity, increased dry weight
accumulation and more partitioning of assimilates to
reproductive sinks (Whisler et al., 1986; Wells et al.,
1988; Li and Jiang, 1992). The photosynthetic rates of
cotton leaves under a given environmental conditions
is a function of the various biophysical and
biochemical processes involved during the diffusion
of CO
2
from atmosphere to chloroplast and the
subsequent enzymatic reactions. The leaf transpiration
and stomatal resistance are directly related to number
of stomatal present per unit leaf area (Van de Roovart
and Fuller, 1935). It has been suggested that a reduced
stomatal frequency would be expected to reduce
stomatal conductance (Penman and Schofield, 1951)
which in turn reduce rate of water loss and increase
the ratio of photosynthesis to transpiration in wheat
(Jones, 1977). Similary Austin (1977) reported that
low stomatal frequency increased stomatal resistance
and decreased the transpiration in barley which inturn
increased the yield due to increased water use
efficiency under rainfed conditions.
Shimshi and Ephrat (1975) were of the opinion that
wheat cultivars with a wide stomatal aperture produce
higher yields without consuming more water.
However, they also stated that permeability was
significantly correlated with short term transpiration,
short term photosynthesis and yield.
Wong et al. (1979) reported that stomatal conductance
was correlated with photosynthetic rate and stomatal
aperture is determined by the capacity of mesophyll to
fix carbon. Further, Hutmacher and Kreig (1983)
noticed that photosynthetic rate of leaves had a
curvilinear relationship with leaf conductance.
Gopinath and Madalageri (1985) reported heterosis
for stomatal frequency and leaf area over mid parent
values in F
1
hybrids of egg plant. In a similar
observation Hazra et al. (1989) noticed marked
heterosis for length, breadth and number of stomata on
upper and lower surface in F1s from ten genotypes of
vigna unguiculata.
Dhopte et al. (1988) reported that boll number and
transpiration rate had direct positive effect on yield,
while stomatal conductance had a direct negative
effect.
Heterosis is defined as the increased vigour of the F
1
generation over the mean of the parents or over the
better parent (Hayes et al., 1955). Shull (1914) first
coined the term heterosis. Heterosis has been observed
for yield and other characters in cotton by many
workers. Commercial exploitation of hybrid vigour in
cotton has been successful in India with release of
Hybrid 4 in 1969.
Heterosis produced by the joint effects of all the loci
as the sum of their separate contributions can be
represented by the formula (Falconer, 1981).
HF1 = dy
²
Where,
d = Magnitude of dominance
y = Allelic frequency differences at a locus in the
parental populations
The genetic causes involved in the expression of
heterosis are dominance and non-allelic interactions
(Hayes and Foster, 1976). The magnitude of heterosis
can be maximized if the parents are genetically
diverse from each other. Parents should differ for
maximum number of yield influencing loci so that F
1
exhibits the dominance effect at as many of the yield
influencing loci as possible.
Heterosis works as a basic tool for improvement of
crops in form of F
1
and F
2
populations, and economic
heterosis (over standard cultivar). It also contributes to
choose genotypes with desired genetic variance, vigor
and maternal effects. Therefore, it is essential to have
detailed information about desirable parental
combiners in any breeding program, which can reflect
a high degree heterotic response. In intra- and
inter-specific heterosis, yield increase over better
parent or greater than best commercial cultivar (useful
heterosis) has been documented (Baloch et al., 1993b;
Galanopoulou- Sendouca and Roupakias, 1999; Wei et
al., 2002; Yuan et al., 2001 & 2002; Khan et al., 2007;
Khan, 2011). Both positive and negative heterotic
values have been detected, demonstrating potential of
hybrid combinations for traits improvement in
breeding programs (Hassan et al., 1999; Khan et al.,
2009). F
1
hybrids with high heterosis were also
associated with higher inbreeding depression;
therefore, moderate type of heterosis has some
stability in segregating populations (Tang et al., 1993;
Cotton Genomics and Genetics