This project aimed to identify factors that could substantially influence Bt efficacy in transgenic cotton. Cry1Ac protein concentration in cotton leaves was measured quantitatively using a commercial ELISA assay. Previous research suggested that Bt efficacy was compromised to some extent when environmental stresses were imposed on transgenic plants. Experiments were designed to investigate a range of factors that may affect Bt efficacy, including: crop nutrition, planting density, light intensity, water management, soil type, herbicides, temperature, soil fertility, growth regulators, and cotton cultivars.

Imposing very severe agronomic or environmental stresses on transgenic cotton had the potential to substantially reduce leaf Cry1Ac protein levels, although this did not always occur. Plant health/growth must be severely impaired before substantial reductions in leaf Cry1Ac protein levels occur.

Inadequate N nutrition reduced leaf Cry1Ac protein levels in the first year only, when N fertiliser application had a significant and positive effect on leaf Cry1Ac protein concentration. No effect was observed in the latter two years, despite there being significant responses to N fertiliser application. In some highly sodic commercial cotton fields, severe deficiencies of phosphorus and potassium were encountered that produced leaf senescence and a significant reduction in leaf Cry1Ac protein concentration. Early season zinc deficiency in other fields had no significant effect on leaf Cry1Ac protein concentration. Soil applied potassium fertilisers significantly reduced Cry1Ac protein in Bollgard II leaves.

Planting density had a small significant effect on leaf Cry1Ac protein concentration in the terminal leaves, with higher levels at higher plant density.

Herbicide application had no significant effect on leaf Cry1Ac protein concentrations. Similarly, the application of the plant growth regulator Pix® produced no significant effect on leaf Cry1Ac protein concentration.

Plants subjected to low light intensity (by shading) for one week contained slightly higher Cry1Ac protein concentrations in their leaves than plants subjected to normal light intensity.

Soil waterlogging produced no significant effect on Bt expression in two glasshouse experiments. However, in one experiment, Cry1Ac protein levels in Sicot 289i remained stable, whereas Cry1Ac protein levels in Siokra V-16i continued to decline as the soil dried out. Severe waterlogging of field-grown cotton produced a slight decline in leaf Cry1Ac protein concentration.

Imposing a period of water stress (drought) on Sicot 289RRi and Sicala V-3RRi significantly reduced the leaf Cry1Ac protein concentration.

Glasshouse experiments indicated considerable variability in Bt expression between individual plants of the same cultivar. Sicot 289i plants that expressed either high or low levels of Cry1Ac protein produced progeny with similarly high or low Cry1Ac protein levels.

The cotton industry sees transgenic Bt cotton as the basis for reducing the economic burden of Helicoverpa control and the environmental consequences of insecticide use. Identification of means of realising the potential of Bt cotton would assist the industry in economic terms and possibly help avoid problems of resistance to Cry1Ac genes.

This research has identified that agronomic factors have only small impacts on leaf Cry1Ac protein concentration assayed in cotton leaves. Continued research is required to assist cotton breeders to determine the efficacy of new cultivars.