Table 1 Factors reducing the Bt concentration in cotton

Salinity

Significantly decrease the concentration of Bt protein in cotton leaves and insecticidal activity against cotton bollworm hampered with enhancement of soil salinity (11.46 dS•m⁻¹). As for as soil salinity increases 9.1 dS•m^− 1, significant reduction of insecticidal protein in squares was observed.

Wang et al. 2018

Waterlogging

The Bt protein content greatly decreased in squares by waterlogging. It reduced the Bt protein content frome 38% to 50% in leaves from the 1^st to the 3^rd week under stress.

Luo et al. 2008

Drought

In water deficit stress the content of Bt protein in boll shells and associated insect resistance decreased. In moderate water-deficit conditions Bt concentrations decreased in leaves, flowers and bolls.

Zhang et al. 2017

Humidity

High humidity reduced the Bt toxins in leaf of cotton.

Yuan et al. 2012

Temperature

31–35 °C is the best range for maximum expression Cry1Ac toxin in transgenic cotton. High temperature reduced the leaf (37 °C) and square (above 38 °C) protein content, and notable reduction of insecticidal protein in boll shell was observed at 38 °C after 24 h.

Rana et al. 2015, Zhang et al. 2018

Genotype

Expression of Bt insecticidal protein varies among different genotypes of cotton.

Adamczyk and Sumerford 2001; Bakhsh et al. 2012; Khan et al. 2018b

Plant age

In Bt cotton, content of insecticidal protein varies with the age of plant, and after 110 days of planting, level of toxins falls below the threshold level.

Dong and Li 2007, Kranthi et al. 2009

Plant parts

Cry1Ac expression was the highest in leaves, followed by squares, bolls and flowers.

Chen et al. 2017a, Chen et al. 2018

Agronomic practices

Insect resistance and Bt expression were enhanced by nitrogen fertilizer. 14% increase of Bt toxins in leaf was observed by high nitrogen fertilizer whereas plant growth regulators enhance the Bt toxins in squares.

Chen et al. 2018, Chen et al. 2017a