| Gamma irradiated morphogenesis in bread wheat (Triticum
aestivum L.) A. S. LARIK Department of Botany and Plant Breeding, Sind University Agriculture College Campus, Tandojam, Pakistan Morphogenetic variations in crop plants are not uncommon after exposure to ionizing radiation. The normal expressions of leaves, roots, stems, spikes and flowers are often altered by radiation. During the course of investigations to produce beneficial mutants in M2, interesting morphological changes were noticed, a preliminary report of which is given here. Seeds of four varieties of Triticum aestivum L. em. THELL namely, Wisconsin Supermo, No. 43, Kenya Plume and Keyna Hunter were exposed to acute gamma radiation with dose ranges from 25 to 45 kR from Co60 source. Flag leaves were deformed in few populations especially in case of 35 kR, and 45 kR. During blade formation the flag leaves were very redumentary in some plants of No. 43 and Kenya Hunter population. Curling of primary leaves in 35 kR and 45 kR populations were observed. The coleoptile in these cases failed to open in time, resulting into forced emergence of primary leaves from midway of coleoptile forming loops. Chimerical transformations were observed in the region of head. This observation is quite interesting and specific examples like one twin ear chimera in dosage 25 kR of No. 43. Kenya Hunter had greater variation with respect to ear types showing from lax to compact types and from large to short ones. Wisconsin Supremo was least susceptible to chimera though it possessed a few such plants. Three plants in 35 kR and two in 45 kR of No. 43 had axillary spikes on nodes (branches) other than those found at usual terminal position. The spikes were completely sterile, for they bore no seeds. Twisted spike internodes and spike arising from sheathless flag leaves were observed in Kenya Hunter, a few in No. 43 in all the three treatments especially in 35 and 45 kR. Two plants (with three and two tillers) in 35 kR and one (two tiller) in 45 kR of No. 43 had solid upper-culm internode chimera. Supernumerary spikelets were of frequent occurrence in Kenya Plume, many in Kenya Hunter and few in Wisconsin Supremo and No. 43. One plant in Keyna Hunter was recorded with only three fully awned tillers while the rest were awnless as per varietal characteristics. No awnless mutant could be observed in awned variety. Occurrence of chimeras in nearly all the populations corroborate the established and known facts that various chimeric forms are quite frequently met with during second mutation generation raised from seeds treated with ionizing radiations. Different mechanisms are involved for their production and mode of inheritance. Most of the head pecularities recorded were chimerical arising most probably from a disturbed sub-dermato-gen sectors of the head primordia. These views have been expressed by notable workers like SEARS (1956), MAC KEY (1960), TAVCAR (1962), GAUL (1963), GUSTAFSSON (1963) and SWAMINATHAN (1964). The head chimeras obtained by the author might be useful in unravelling the problem of zygote differentiation. There exists a strong indication of existence of more than one bud in the seed embryo which are most probably in different stages of cell differentiation. The size of the sectors may reveal the size of the radiation flux which inflicted the damage, if the number of cells is known. This can be estimated from the number of heads bearing the heteromorphic characters. DAMATO et al (1962) and GAUL (1965) have concluded that the size of the spike sector depends on the irradiations dose. GAUL explained that with no irradiation or with low doses a single spike (generative tissue) is presumably formed by about four, and possibly by exactly four embryo cells. With increasing dose the number of embryo cells per spike primordia decreases. This results in an increased size of mutated spike sectors. The increasing sector size with increasing dose appears to be a results of the increased killing among the initial cells of the spike primordia. |
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