Changes in plant response to NaCl during development of rice (Oryza sativa L) varieties differing in salinity resistance

The effects of NaCl stress on growth and development of rice (Oryza sativa L.) were studied and compared in varieties of various origins. During the vegetative stage, tall indica landraces (Nona Bokra, Buhra Rata, Panwell, and Pokkali) appeared to be resistant throughout while in japonica varieties (I Kong Pao (IKP) and Tainung 67) and elite breeding lines (IR 4630, in 2153 and in IR 31785), resistance fluctuated. Panwell, which was the only indica variety evaluated during the reproductive stage, also expressed salt resistance during booting, heading and grain maturation while varieties with the greatest variability in salt stress response during the vegetative phase (IR 4630, in 31785 and IKP) also showed the greatest variability during reproductive development. Thus, varietal levels of resistance to salinity at different growth stages are not necessarily interdependent characteristics. Variability in salt resistance of different genotypes during the vegetative and reproductive phases of development was not correlated to their mean level of relative resistance. There was an ontogenic evolution of salt resistance and the young seedling stage appeared to be the most sensitive to NaCl during vegetative growth. Nevertheless, short- and middle term effects of stress have to be distinguished for each genotype since some varieties showed better growth during the second week of stress than during the first, whilst others showed an opposite trend. Moreover, even at specific stages of development, plant responses to NaCl were quite variable according to the criterion used to quantify salinity resistance. The identification of genotypes resistant to NaCl at specific developmental stages is essential to improve the understanding of the effects of salt stress upon phenology and to elaborate further breeding programmes.