Introduction
India ranks third in global cotton production after the United States and China, and with 8-9 million hectares grown each year, India accounts for approximately 25% of the  total cotton area of the world and 16% of global cotton production. There are a number of benefits for adopting transgenic cotton, including environmentally safer pest control of major Lepidopterous insects, improved safety and health conditions for farmers, enhanced use of beneficial insects as biological control agents, and lower cost of production and higher yields (Edge et al., 2001).

Bt cotton is an increasingly important tool for farmers around the world. Statistics provided in the report indicates that 5.6 million small and marginal farmers in India during the year 2009 planted and benefited from 8.381 (~8.4) million hectares of Bt cotton, equivalent to 87% of the 9.636 (~9.6) million hectare national cotton crop. Large- and small-acreage farmers benefit from increased productivity, convenience, and time savings. The vast majority of farmers using Bt cotton globally are smallholder farmers who may reap economic, environmental, and social benefits from adoption of this important tool for agriculture. Adoption of this technology has led to positive implications for the farmers, their surrounding communities, and the future of agriculture (Purcell and Perlak, 2004).

The relationship between source and sink is different in the Bt cotton as compare to non Bt cotton, especially during the early stage of the growth. The Bt hybrids which are not able to put up sufficient vegetative growth during early stage of growth face with the problem mismatch source–sink relationship 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.

Figure 1 Comparison between Bt cotton and non Bt cotton

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

With Bt in genetic background, it is mandatory to enhanced the total photosynthetic output of the plant so that the plant can meet the demand of enhanced harvest index. Traits such as higher photosynthetic ability, higher leaf area, stay green nature, etc become very important in overcoming this Bt induced mismatch between source and sink. These traits first need to be incorporated in the parental genotypes so that they can be released as varieties and additionally they become good source for developing hybrids.

The objectives of the present study were: Bt gene produces poisonous protein to kill bollworm. Is it causing any other effect on the plant? Is the increased sink’s capacity realized? What is the impact of this on the Source? Is the plant phenology capable of sustaining enhanced sink?

This research work was aimed at seeking answer to these questions.

1 Results and Discussion
1.1 Productivity features of Bt cotton hybrids
The data on 15 hybrids developed through crossing were subjected to statistical analysis. The results revealed that among the entries tested, the hybrid 1 has registered the highest yield of 4726 kg/ha, this reflects that the harvest index of upper half and lower half same 0.32, 0.31 respectively, this hybrid came from a good crossing. The same result was found in hybrids 2,3,4,5,6,7,8,9,10 and 11. On the other hand, the hybrid 15 showed the lowest yield of 3106 ha/kg that means the harvest index in lower half (0.28) is more than upper half (0.21) and this hybrid came from a bad crossing. The same result was found in hybrids 12, 13 and 14 that recorded lower yield and differences between lower half and upper half that mean these hybrids came from bad crossing. From those data we can see that many of the less productive Bt hybrids revealed higher productivity in the lower half than the upper half, the harvest index was higher in the lower half as compared to upper half (Table 1). It can be inferred that Bt gene induces enhanced sink and in some of these less productive hybrids the vegetative growth can not match the demand of enhanced sink thereby leading to lower yield coupled with leaf reddening and early senescence.

Table 1 Productivity features of Bt cotton hybrids

1.2 The impact of enhanced sink
With same photosynthetic output (source) the quantum of sink has increased. Some genotypes lack matching photosynthetic potential (source) to meet extra demand of sink and they cannot cope with this drain.

Leaves are weakened, subsequent reddening, disease incidence and senescence (Figure 2). In some hybrids there is Mismatch between source and sink at a later stage of growth (upper half of the plant). By going to the table 2 we can see the physiological changes in cotton through introduction the Bt gene.

Figure 2 Leaf reddening

Table 2 Physiological changes in cotton through introduction the Bt gene

1.3 Overcoming the demand of enhanced sink induced due to Bt gene
There are two ways to overcome the demand of enhanced sink induced due to Bt gene.

1.3.1 Genetic intervention enhancing source to meet the demand of enhanced sink
Those Bt genotypes with enhanced photosynthetic ability (extra source) can match the demand of "enhanced sink (Bt gene induced)".Such of the genotypes reveal higher foliage, higher boll load in lower as well as upper halves and almost same harvest index values in both the halves. Stay green types known for green foliage even when bolls are open are likely to be more responsive to integration of Bt gene (Figure 3).

Figure 3 Higher photosynthetic ability, higher leaf area, stay green nature become very important in overcoming this mismatch

The breeder should have an insight into an ideal plant type which can maximize productivity based on optimum blending type which can maximize productivity based on optimum blending of desirable traits seen in different plant types. The robust plant type (tall growing) occupies a larger three- dimensional space but reveals a poor packing of bolls. In contrast with this a compact plant type occupies a smaller three- dimensional space but reveals better packing of bolls. It is possible to get robust and compact types, which are equally potential but it will be an ideal plant type that blends the desirable features seen in robust and compact plant types. Such ideal intermediate types may require a closer spacing and can maximize productivity per unit area. The features of ideal plant types again vary depending on the situation say like irrigated rainfed, machine picking, etc.

1.3.2 Chemical intervention (Ethrel spray)
Ethylene spray is expected to induce initial square dropping causing additional vegetative growth making the plant competent to bear enhanced sink (Figure 4).

Figure 4 Physiological Intervention

2 Materials and Methods
The experimental material used in the present study comprised of fifteen Bt cotton hybrids representing differences in Bt sources were evaluated at Dharwad, Karnataka (India) with two replication was laid out in Randomized Complete Block Design (RCBD) during 2009-2010. Each entry was sown in 2 row plots of 6 m length spaced at 90 cm with recommended dose of fertilizer and treatment of seeds with Imidochloprid, 2-3 seeds were dibbled per spot in each row and thinning was attended to retain one healthy plant per hill at 25 days after sowing. All the recommended package of practices were followed to rise healthy crop. Productivity of the hybrids was assessed in terms of seed cotton yield (kg/ha), mean boll weight (g), bolls number per plant, Harvest index and seed cotton yield of plants was determined as yield in upper half and lower half of the plant since the purpose of this evaluation was to understand reasons for the genotypic differences in potentiality of these Bt hybrids and know it’s implications in plant breeding , the identity of these hybrids is not disclosed and instead they are represented by code numbers (Table 1).

3 Conclusion
Often non Bt hybrid genotypes are selected based on potentiality and the parents are subjectedto transfer of Bt gene through back cross breeding. It is very important to know whether the genotype chosen for Bt gene has ability to withstand the additional reproductive load (sink). If the enhanced sink is not matched by the source (mismatch between source and sink ) this is reflected in the form of faster senescence reduced boll load especially in the upper half of the plant. These 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 seed cotton yield are reduced in upper half of the plant. Some hybrids of non –Bt era, which on developing Bt version are clearly showing weaker upper half since they cannot cope with this drain. The symptoms of weakened leaves, more prone to reddening, disease incidence and senescence are noticed commonly. This is leading to reduced boll load and lower harvest index in the upper half as compared to lower half.

References
Edge J.M., J. Benedict J.H., Carroll J.P., and Reding H.K., 2001, Bollgard cotton: an assessment of global economic, environmental, and social benefits, J. Cotton Sci., 5: 121-136

Patil S.S., 2010, Priorities of conventional breeding in the era of Bt cottons, In: Proc. National Symposium on Bt cotton: Opportunities and prospects., pp.46-52

Purcell J., and Perlak F., 2004, Global impact of insect– resistant Bt cotton, Ag. Bio. Forum, 7(1-2): 27-30