Effects of aflatoxin B₁ on in vitro cultures of Nicotiana tabacum var. Samsun - 1: Callus growth and differentiation

Calli of Nicotiana tabacum (tobacco) were treated with two dose ranges of aflatoxin B₁ (0.1-2.0 μg ml^-1 - low does; 5-25 μg ml^-1 aflatoxin B₁). The ability of calli to recover following 3 weeks of toxin exposure was also investigated. The I₅₀ (50% inhibition) value for fresh mass accumulation was approximately 2 μg ml^-1 AFB₁. Fresh mass accumulation was significantly lower than the control value from 0.5 μg ml^-1 AFB₁. Following 3 weeks growth without a toxin source, the growth of calli up to and including 10 μg ml^-1 AFB₁, was significantly greater than control calli, indicating reversibility of the toxic effects. With increasing toxin concentration, chlorophyll content of callus was inhibited from 0.5 μg ml^-1. Transfer to a toxin-free medium resulted in a degree of recovery (up to 0.5 μg ml^-1). In the dose range 5-25 μg ml^-1, the levels of chlorophyll were drastically reduced, with no recovery following AFB₁ removal. Electron microscopy revealed a disruption of chloroplast structure as an early deteriorative event in AFB₁ exposure of callus cells. Protein levels were less sensitive, with inhibition manifested only in the high dose range. Shoot development occurred at all concentrations, but was significantly inhibited from 5 μg ml^-1 AFB₁. Recovery following toxin removal was minimal at these higher AFB₁ concentrations. The number of necrotic calli increased progressively from 5 μg ml^-1 as toxin levels increased.