NATURAL OCCURRENCE OF MYCOTOXINS IN BROAD BEAN (VICIA-FABA L) SEEDS AND THEIR EFFECT ON RHIZOBIUM-LEGUME SYMBIOSIS

Seeds of faba bean (Vicia faba L.) cultivar Giza 3 were screened for t he presence of mycotoxins. Eleven out of 100 samples were positive. Af latoxins B1 and B2 were found in 7 samples with a mean concentration o f 30 mug kg-1 seeds. Aflatoxins B1, B2, G1 and G2 and ochratoxin A wer e each detected twice in separate samples with a mean concentration of 25 and 20 mug kg-1, respectively. Mycotoxins at concentrations of 100 or 200 mug kg-1 soil significantly decreased nodule number, nodule fr esh weight and total nitrogenase activity. This was translated into re ductions in dry matter accumulation and nitrogen yield of the bean. My cotoxins also suppressed specific nitrogenase activity, NADH-dependent glutamate dehydrogenase (NADH-GDH) as well as glutamate synthase (NAD H-GOGAT) activities. In addition, mycotoxins inhibited synthesis of le ghaemoglobin, carbohydrate and protein in the nodule cytosol. Of the m ycotoxins tested, aflatoxin B1 was the most toxic. The decline in nito rgenase activity and total N concentration in the plants could be attr ibuted to mycotoxins interfering with normal nodule physiology and fun ction.