Identification of Sri Lankan rice varieties having osmotic and ionic stress tolerance during the first phase of salinity stress

Salt tolerance of plants during the first phase of salt stress requires a combination of physiological traits conferring tolerance to both osmotic stress and ionic stress. Accordingly, the objectives of the present study were: (a) to screen a substantial number of varieties (102) from the Sri Lankan rice (Oryza sativa) germplasm and thereby identify varieties with osmotic and ionic tolerance during Phase I of salt stress development; and (b) to identify the physiological mechanisms that are responsible for osmotic and ionic tolerance. The 102 varieties tested included 51 traditional, 4 old-improved and 47 new-improved varieties, which were screened in nutrient solutions at 100 mM NaCl. Whole plant salt tolerance was quantified using relative leaf area (RL) and relative total dry weight (RW), i.e. leaf area and total dry weight at 100 mM Na+ as ratios of their respective values at 1 mM Na+, the unstressed control. Cluster analysis based on RL and RW enabled classification of Phase I salt tolerance of the screened varieties into five groups as, ‘highly tolerant’ (6 varieties), ‘tolerant’ (25), ‘moderately tolerant’ (32), ‘susceptible’ (30) and ‘highly susceptible’ (9). Capacity to maintain relatively high tissue water content, capacity for Na+ exclusion at the root surface and root xylem and tissue tolerance to excess shoot Na+ were identified as tolerance mechanisms of rice for salinity at Phase I. High shoot tolerance to Na+ contributed to tolerance in 40 out of the 63 varieties identified as Phase I-tolerant to varying degrees. In comparison, high or medium capacity to maintain either whole plant or shoot relative water content contributed to tolerance in 48 varieties, while Na+ exclusion at either of the two control points contributed to tolerance in 33 varieties. Based on the varietal phenotyping of the present study, suitable varieties can be selected: (a) for direct recommendation to areas of mild to transient salinity; (b) as parents in conventional breeding programmes to develop salt tolerant rice varieties; (c) for molecular biological studies to identify genes responsible for Phase I salt tolerance.

DOI: http://dx.doi.org/10.4038/jnsfsr.v40i3.4699

J.Natn.Sci.Foundation Sri Lanka 2012 40 (3):251-280