Approaches to identifying genes for salinity tolerance and the importance of timescale.

Soil salinity reduces the ability of plants to take up water, and this quickly causes reductions in the rate of cell expansion in growing tissues. The slower formation of photosynthetic leaf area in turn reduces the flow of assimilates to the meristematic and growing tissues of the plant. Later, salt may exert an additional effect on growth. If excessive amounts of Na(+) or Cl(-) enter the plant it may rise to toxic levels in the older transpiring leaves. This injury, added to an already reduced leaf area, will then further limit the flow of carbon compounds to meristems and growing zones in leaves. This chapter analyses the various plant responses over time, to provide a conceptual framework on which the different approaches to gene discovery can be based. Knowledge of the physiological processes that are important in the tolerance response, and the time frame in which they act, will enable further progress in understanding of the molecular regulation of salt tolerance.