An analogy can be drawn from disease resistance breeding. In breeding for disease resistance
horizontal resistance (which can be defined as resistance to a number of physiological races of a dis-
ease) can be achieved by pyramiding different genes specifically resistant to individual physiological
races. This contributes to the stability of a genotype across years in disease-prone environments. The
same concept could also be applied to the genetic improvement of salinity tolerance, whereby pyra-
miding of genes that regulate various specific physiological traits into a single genotype or variety
could provide that genotype with the necessary genetic means to respond to different types and levels
of salinity stress that it is likely to experience at different locations and sites and over years. This would
contribute to its stability of production as well as widening its adaptability to a greater range of saline
environments.
872 SUBBARAO AND JOHANSENFigure 4 Continued.