NaCl-induced Senescence in Leaves of Rice (Oryza sativaL.) Cultivars Differing in Salinity Resistance

The influence of NaCl on senescence(definition)-related parameters (protein and chlorophyll concentrations, membrane permeability and chlorophyll fluorescence) was investigated in young and old leaves of five rice cultivars differing in salt resistance. NaCl hastened the naturally-occurring senescence of rice leaves which normally appears during leaf ontogeny: it decreased chlorophyll and protein concentrations and increased membrane permeability and malondialdehyde synthesis. Such an acceleration of deteriorative processes affected all leaves in salt-sensitive cultivars while it was more marked in oldest than in youngest leaves of salt-resistant genotypes. NaCl-induced senescence also involved specific modifications, such as an increase in basal non-variable chlorophyll fluorescence ( F0 ) recorded in all cultivars or a transient increase in soluble protein concentration recorded in salt-resistant genotypes only. Alteration of membrane permeability appeared as one of the first symptoms of senescence in rice leaves and allowed discrimination among cultivars after only 7 d of stress. In contrast, Fv / Fm ratio (variable fluorescence/maximal fluorescence) was the same for all cultivars during the first 18 d of stress and thus could not be used for identifying salt-resistant rice exposed to normal light conditions. Relationships between parameters involved in leaf senescence are discussed in relation to salinity resistance of rice cultivars.