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Future of Bt Cotton    

Muhammad Faisal Bilal and Dr. Muhammad Farrukh Saleem
Department of Agronomy, University of Agriculture, Faisalabad.


Every new technology has its benefits and risks; the benefits associated with the use of transgenic crops include: a dramatic decrease in the use of conventional and broad-spectrum insecticides, target pest specificity, improved yield, lower production costs and compatibility with other biological control agents while some risks are also associated with transgenic crops include out-crossing by the transfer of pollen to non-transgenic plants, food safety concerns, development of resistance in target pests and effects on non-target organisms and biodiversity. Global adoption of Bt cotton has risen dramatically from 0.76 million hectares in its introductory period in 1996 to 37.67 million hectares in 2010. Bollworms are serious pests of cotton causing 30-40% yield reduction in Pakistan, and 20-66 % potential crop losses in India. The major advances shown in this review are.

The growing of transgenically modified cotton that expresses insecticidal protein derived from Bacillus thuringiensis (Bt) is revolutionizing cotton production on a global scale. The first Bt transgenic cotton variety (called Bt cotton), expressing the same gene construct of Cry IAc, was commercially released in Austriala (INGARD) and the United Stated (BOLLGARD) in 1996. Agriculture plays a pivotal role in the economy of Pakistan. It contributes about 24% to national GDP and employs 44% of the total labor force; Cotton (Gossypium hirsutum L.) is the main cash crop and is known as “White Gold”. Pakistan is the fourth largest producer of cotton after China, USA and India. During year 2010, Pakistan Agricultural Research Council (PARC) imported almost 950 kg seed of five different types of Bt cotton from China under special permission for conducting trails directly on farmers' fields without following the rules and regulations designed by NBC, PCCC, FSC & RD.

Transgenic cotton cultivars showed possible role to control three main pests i.e., Helicoverpa armigera, Diparopsis castanea and Earias biplaga. Farmers are using US$ 300 million worth of pesticides annually, out of which more than 75% is used on cotton crops to control pests, especially bollworms. However due to introduction of Bt cotton, the number of spray operations per crop (cotton) has been reduced. Field studies in China have shown that by adopting Bt cotton, farmers have reduced pesticide and labour costs, and there is less exposure to toxic insecticides.

Bt transgenic cotton line increased leaf amino acid content, but more nutrients were utilized for stem and branch growth. High nitrogen and high vegetative growth produced significantly less lint in Bt cotton cultivar. An increase of leaf NR activity and NO3-N enhanced boll shedding for Bt cotton cultivars. Total nitrogen reduced sharply in the bolls of the Bt cotton cultivars; the reduction of total nitrogen caused a decrease in nitrogen metabolism and limited boll development; there was significantly positive correlation between GA3 (Gibbleric Acid) content at flowering and boll size in Bt cotton cultivars. This result also suggests that reduction of GA3 may induce the decline of nitrogen absorption and metabolism, thus affecting boll development. In Pakistan cotton growers use a desirable amount of N (125 kg ha-1) but use of K is negligible (0.7 kg ha-1). The less use of K fertilizer in Bt cotton may have serious consequences including depressed growth and development in the form of reduced plant height, leaf area index, leaf and stem weights, decreased photosynthesis and stomatal conductance, increased mesophyll resistance, low chlorophyl content, poor chloroplast ultrastructure and ultimately the total plant however, the termination of reproductive growth and natural cutout occurs earlier than those who received full potassium fertilizatoin. Excessively hot weather also compounds K-deficit problems in cotton.

Factors affecting toxicity of Bt cotton Cry1Ac protein content in Bt cotton was significantly reduced by high temperature, NaCl stress, and nitrogen deficiency, whereas high dose of N fertilizer, or foliar applications of the plant growth regulator Chaperone, greatly improved the Cry1Ac protein levels, resulting in increased mortality of bollworms feeding on the treated plants. The field observations of Australian Bt cotton showed that plants expressing the Cry1Ac protein are less toxic to first-instar H. armigera when the leaves are from fruiting versus presquare plants. Terpenoids fluctuate temporally and levels of condensed tannins generally increase with plant age, so both may play some part in changing the efficacy of Cry1Ac protein. Tannins can alter the efficacy of Bt toxins against target species. Many researchers reported increased mortality in lepidopteran species when hydrolysable tannin compounds were combined with various Bt toxins in bioassays. Removal of fruiting forms leads to great morphological and physiological changes, including lint yield variation ranging from small increase to large decrease. Nitrogen metabolism affected Cry1Ac protein content, and removal of early fruiting forms could change the nitrogen metabolism, we also hypothesized that removal of early fruiting branches may increase the Cry1Ac protein content in Bt cotton plants. Due to industrial revolution level of atmospheric CO2 concentration has risen from 280-360 ppm which is anticipated to double by the end of this century. This increase may have a variety of direct and indirect effects on relationships between host plant, their herbivores and the herbivores natural enemies. Elevated CO2 tends to increase photosynthetic rates, growth, yield and C:N ratio in most C3 plants. Only limited research has been reported on the effect of elevated CO2 on transgenic Bt cotton or the effects on bollworms fed Bt cotton grown in elevated CO2. An elevation of CO2 level from 330-660 ppm led to 95 % yield increase in cotton. Increases in solube sugar, starch, total non-structural carbohydrates (TNC), TNC:Nitrogen ratio, condensed tannin, gossypol and decreases in water content, nitrogen and Bt toxin protein were observed in the young bolls from cotton plants grown under elevated CO2 conditions compared with those in ambient CO2–grown cotton for both Bt and non-Bt cotton.


 

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