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 the presence of mycotoxins. Eleven out of 100 samples were positive. Aflatoxins B1 and B2 were found in 7 samples with a mean concentration of 30 mug kg-1 seeds. Aflatoxins B1, B2, G1 and G2 and ochratoxin A were 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 fresh weight and total nitrogenase activity. This was translated into reductions in dry matter accumulation and nitrogen yield of the bean. Mycotoxins also suppressed specific nitrogenase activity, NADH-dependent glutamate dehydrogenase (NADH-GDH) as well as glutamate synthase (NADH-GOGAT) activities. In addition, mycotoxins inhibited synthesis of leghaemoglobin, carbohydrate and protein in the nodule cytosol. Of the mycotoxins tested, aflatoxin B1 was the most toxic. The decline in nitorgenase activity and total N concentration in the plants could be attributed to mycotoxins interfering with normal nodule physiology and function.