Abstract

Chemical mutagens, such as sodium azide, have attracted the interest of plant breeders. Azide creates DNA point mutations and affects plant growth and development, disturbs metabolic activity and inhibits protein and DNA replication, whereas mannitol is used to simulate drought stresses in tissue culture. To identify biochemical markers for stress tolerance, maize seeds were germinated under mannitol and sodium azide induced stress in controlled conditions for 7 days. Then levels of chlorophyll, carotenoids, phenolics and aldehydes produced were subsequently determined. Germination percentage was not affected by either mannitol or sodium azide and was always above 85%. However, total fresh weight decreased by 50% with the application of 153.4 mM mannitol and 0.26 mM azide in combination. This treatment significantly reduced plantlet growth from 0.94 g in the control to 0.53 g in the treated materials. Root weight reduced by 68.1%, cotyledons by 14.3%, stems by 65.0% and leaves by 70.0% in treated samples. The level of carotenoids in roots was the clearest biochemical indicator of stress produced by the mannitol and sodium azide treatment. Carotenoids increased from 0.01 mu g g(-1) fresh weight in the control to 9.03 mu g g(-1) fresh weight in the treated materials. A large-scale seed treatment with mannitol and sodium azide was carried out. 2296 seeds were placed in magenta containers with 153.4 mM mannitol and 0.26 mM NaN3. At 7 days of germination, the heaviest seedlings (450) (450/2296=20%) were transferred to soil environment. Forty-two plants (42/450=9.3%) were off-type phenotypes at 45 days. Genetic variants may have been obtained following the novel procedure described here which combines chronic treatment with sodium azide and selection pressure with mannitol to simulate drought conditions.