The influence of magnetic pulsation on the genetic variability of tomato Ursul S . V . , Ursul N . A . Institute of Vegetable Breeding and Seed Production (VNIISSOK), p/o Lesnoy Gorodok, Odintsov Region, Moscow District, 143080, RUSSIA . There is much data in the scientific literature on the effective influence of magnetic fields on the germination of plant seeds (Saktheeswari, Hussain, 1995; Aksenov et al . , 1996), plant growth (Namba, 1996; Yano et al . , 1997), drought resistance (Suven et al . , 1992), and plant yields (Pietruszewski, 1993; Gabrielian, 1996), but the question of the influence of magnetic pulsation on the genetic variability of plants is also very important .   The tomato F1 hybrids between Mo628 and Likurich were exposed to a low - frequency magnetic field ("CEF" device) using patented methods (RU 2083074 C1, 1997), where Mo628 ( L . esculentum ) is a multimarker mutant line homozygous for recessive linked markers on chromosomes 4 ( ful,e / ful,e ), and 11 ( hl,a / hl,a ) . Marker ful (4, 24) is associated with yellow leaf colour at the growing points; e (4, 66) - serrated leaves with curved central vein; hl (11, 48) - hairless plant; a (11, 68) - lack of anthocyanin in the hypocotyl, stem and leaves of seedling (Tanksley, Mutschler, 1989) . Likurich is a commercial tomato variety . Three time exposures (1, 4 and 8 hours) using wetted seeds, pre - meiotic buds (1st cluster), and also their combined action were studied . Ten F1 plants of each variant were examined . The collecting of F2 seeds for assessment of the crossing - over frequency was carried out separately on each of the plants and fruits of the first cluster . Recombination frequency (rf) was estimated by maximum likelihood method for each fruit of a plant and on the variants in whole (Fisher, 1958; Bailey, 1961) . Statistical analysis was performed on an IBM PC compatible computer using the software package "BIOSTAT" (Preygel, 1986) . The F1 plants were grown in the greenhouse . During the growth of the vegetation we observed that 7% of the F1 had a very interesting phenomenon - mottled regions that were probably a consequence of mitotic crossing over which took place in the somatic tissue of the tomato (Photo 1 - 2) . The stem of the plant usually has hairs and anthocyanin but here there appeared a region without hairs or anthocyanin . This is a consequence of the transition of recessive mutant genes to the homozygous state as a result of mitotic crossing over . Quite possibly the cause of the phenomenon was the magnetic field as usually this is a very infrequent event .   The analysis of recombination frequencies has shown the disposition towards an increase of crossing over frequency in both segments simultaneously with increasing magnetic field exposure . But only at 8 hours treatment (seeds + buds) was the difference statistically significant at the 5%