Cytogenetic Effects of X-Rays and Thermal Neutrons on Dormant Tomato Seeds.

HE study reported in this paper concerns the effects of X-rays and thermal neuT trons on dormant seeds of the tomato, Lycopersicon esculentum, expressed as chromosomal abnormalities in root tip mitoses, seedling injury, and chlorophyll and morphological changes. XI and NI refer to the X-rayand thermal neutron-irradiated generations, respectively; Xz and Nz to the progenies of the XI and NI generations, respectively. A few studies have been reported concerning the cytogenetic effects of irradiation on the tomato. MARSHAK (1939) irradiated tomato root tips with fast neutrons and fixed them at various intervals after irradiation. He observed that the frequency of normal anaphase figures decreased to a minimum at from three to nine hours after irradiation, and increased after this time. A dose of 100 “n” units, which is equivalent to between 200 and 250r units (GILES 1943), resulted in 69.5 percent normal anaphase cells when the root tips were fixed three hours after irradiation. The effects of irradiation on the meiotic chromosomes were observed by GOTTSCHALK (1951) using X-rays on inflorescences and by BARTON (1954) using X-ray and ultraviolet radiations on pollen. LINDSTROM (1933) applied Monel-radium needles emitting gamma rays to tomato seeds, growing tips and developing fruit. He obtained the following recessive mutations which were heritable: ridged or rough leaves ( r i ) ; virescent-white ( v ) , a seedling chlorophyll mutation; yellow foliage (1-lutescent) ; yellow seedlings (ys) ; rolled cotyledons ( rc) ; and sterile plant (st), a mutation which produces a marked inhibition of growth. MACARTHUR (1934) irradiated dormant tomato seeds with various doses of X-rays, the maximum being approximately 20,000r units. Germination was not reduced in the X1 generation and, except for eight bright yellow-white I lethal seedlings from 1200 X-rayed seeds, the other XI plants appeared normal. In the Xp generation 43 mutations were found in 346 progenies (12.4 percent). Most of the mutations affected the chlorophyll system in the cotyledons and first leaves, and in three cases the stem color was green instead of the usual red. The cotyledons also showed changes in size, shape and number. Morphological changes included potato-leaf-shaped