Salinity Resistance in Rice ( Oryza sativa L.) And a Pyramiding Approach to Breeding Varieties for Saline Soils

Varieties and breeding lines of rice (Oryza sativa L.) differ in their resistance to NaCl salinity and in their Na uptake. These varieties (which are not single homozygous lines) also show large variation between individuals. The gross uptake of Na+ is, however, poorly correlated with growth and survival because net transport interacts with vigour and degree of dwarfing and because NaCl is compartmentalized within the plant confounding any simple relationship between tissue concentration, growth reduction and toxicity symptoms. It is argued that resistance is conferred by no single factor, but is the sum of a number of contributory traits which include variation in (1) NaCl entry via membrane leakage or apoplastic pathways; (2) preferential accumulation of Na+ in the older leaves; and (3) tolerance of NaCl within the leaf tissue which may reflect differences in apoplastic salt load. It is suggested that the salt resistance of rice can be increased by selecting separately for such physiological traits and then pyramiding them together.