Studies on the effect of chronic gamma rays on Ricinus—I : Development of pollen, ovule and female gametophyte

Abstract

Young seedlings of Ricinus commuais cv. ‘early spineless’ (Syn. Anjou, Mautner) at the three-leaf-stage of growth were put in the gamma greenhouse at approximate dose rates of 500, 400, 300, 200, 100, 50 and 25 R per day (exposure period of 22 hr/day) for varying periods of time. Plants at 500 R/day did not produce any inflorescences due to growth inhibition. At 400 R/day the inflorescences formed were extremely reduced and non-viable. At 300 R/day and lower doses the flowers in the inflorescence were usually unisexual-male at the base, intergrades in the centre and female at the top. At chronic doses of 100 and 200 R/day terminal hermaphrodite flowers were produced occasionally and in about 5–10 per cent of the plants at 300 and 400 /Rday. Morphogenetic abnormalities described are dose related, but we do not have enough data to construct a dose-response curve for each organ or stage of development. The gynoecium under irradiation may show a variable number of carpels, open ovary chamber, the formation of accessory gynoecia and transformation of carpellary tissue into pollen sacs. The development, structure and fate of the anther wall were similar under unirradiated and irradiated conditions, but variations were observed in microsporogenesis, in microspores and in mature pollen, probably due to failure of cytokinesis, excessive vacuolation and failure of one or both meiotic divisions. The orientation or organization and development of the ovules and of the embryo sac in two-thirds of the plants were abnormal at 300 and 400 R/day, and to some degree also at 200 and 100 R/day. These are the most radiosensitive stages of flower development. Isolated tracheids in the nucellus were observed under irradiated conditions only. The development of the female gametophyte in a majority of cases followed the monosporic (Polygonum) type, but a tendency in irradiated plants toward the bisporic and tetrasporic type was also reflected. Ionizing radiations affect all developmental stages of pollen, ovule and female gametophyte except ( a) the mechanism responsible for the movement of the two first division nuclei toward the poles of the embryo sac and ( b) the position of vacuole and nucleus in the synergid and egg nuclei, both of which seem to be radioresistant.