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Bt Research 2013, Vol.4 No.5 24-28
ISSN 1925-1939
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as the plant growth older. This mismatch leads to
weakining of foliage and reduced bolls setting in the
upper half of the plant. It is observed that the harvest
index in such hybrids is higher in the lower half and
lower in the upper half of the plant (Patil, 2010).
Many of seed companies converted their best non Bt
hybrids and released as many Bt hybrids, among them
some Bt hybrid versions performed well while some
failed to give high yield. These hybrids revealed
higher productivity than Bt hybrid leading to
overwhelming acceptance of today’s Bt cotton hybrids
in India. However, these observations also revealed
that the true potentiality of such hybrids is not release.
There is ample scope for manipulations leading to a
better balance between source and sink. All true
growth period of the crop among the evaluated hybrid
some have revealed higher productivity.
The effect of Bt gene
In Indian scenario bollworm has been a major cause
for square loss in non-Bt era. The plant after
sustaining square loss was conditioned to put forth
additional vegetative growth, but finally the total
vegetative growth was not translated into matching
reproductive growth (sink), as a result the harvest
index (HI) of plant was reduced thereby leading to the
decrease in yield (Figure 1). Hence, the genetic
potentiality of a genotype was not fully realized. In Bt
cotton, the Bt gene targets bollworms and by virtue of
this not only the cost of plant protection is reduced but
also the per se productivity of the genotype is
increased. By preventing the loss of squares, every
unit of vegetative growth is translated into matching
increase in reproductive growth. As a result the
harvest index improves and this contributes to
increase in yield. Integrating Bt gene as an integral
part of the genetic constitution has thus helped in
realizing the enhanced genetic potentiality in a cotton
ecosystem dominated by bollworms as a major pest.
The additional related impact of Bt gene is that plant
growth becomes determinate and the growth duration
is reduced leading to synchronous maturity of crop.
The impact of Bt gene in Harvest Index
It is important to know whether the genotype chosen
for incorporating Bt gene is capable of withstanding
the additional physiological load. Bt gene enhances
the sink with the same quantum of photosynthetic
output. In many genotypes the photosynthetic capacity
does not match the enhanced sink created by increase
in the sink attributed to the introduction of Bt gene. As
a result those genotypes which lack matching
photosynthetic potential (source) wither under the
pressure of enhanced sink. This impact is often
noticed in the latter half of the growth. As a result the
harvest index and the yield are reduced in upper half
of the plant. This impact is evidenced by some of the
popular hybrids of non- Bt era, which on developing
Bt version are clearly showing weaker upper half
since they cannot cope with this drain. Hence, the
symptoms of weakened leaves, more prone to
reddening, disease incidence and senescence are
noticed commonly.