Effets de l'irradiation γ directe de plantules de tabac sur leur morphologie générale et sur les chloroplastes et la teneur nucléaire en ADN de leur tissu chlorenchymateux

Abstract

Plantlets of Nicotiana tabacum var. xanthi and of its Dulieu chlorophyll mutant showed a reduced growth and became yellowish when treated with different γ-ray doses (100–1000 Gy). In bom varieties, the size of the cotyledons and of the first leaves was increased and their aspect was modified after irradiation at 500 and, especially, 100 Gy. Furthermore, in the cotyledon green tissue where chloroplast replication is inhibited by irradiation, a marked dose-dependent increase in chloroplast size and starch storage was observed: in the Dulieu variety, at 500 and 1000 Gy, the chloroplast mean area was respectively 74 and 121 times larger than the control area, whereas the starch granule mean area was respectively 28 and 39 times superior to that of the control. Direct irradition of plantlets induced a nucleus endopolyploidization process directly related to the nucleus size in the green tissue of the cotyledons and first leaves of these two tobacco varieties; the endopolyploidization level was very high at 100 Gy (8C, 16C, and > 16C), high at 500 Gy (8C → 16C), and lower but still significant at 1000 Gy (8C). In this tissue, the G2 → M transition point was more sensitive to strong γ irradiation (100–1000 Gy) than the G1 → S transition point. In conclusion, the plantlets of the two tobacco varieties did not show any significant differences in response to strong γ irradiation. Key words: plant morphology, γ irradiation, chloroplasts, nuclear DNA, cytophotometry.