HOW CAN STOMATA CONTRIBUTE TO SALT TOLERANCE

Although some of the physiological mechanisms which contribute to salt tolerance in plants are known, there are still some major gaps in und erstanding and it remains impossible to provide a satisfactory integra ted picture for the plant as a whole. The operation of stomata in halo phytes has received little attention even though all of the salt prese nt in the shoot (apart from that taken in during submergence) is thoug ht to be carried in the transpiration stream. In non-halophytes, stoma tal function is damaged by sodium ions, and disruption of the normal r egulation of transpiration should be seen as a possible contributor to their inability to survive in salt-laden soils. The development of sa lt-tolerant cultivars of crops may require attention to the need for a ppropriate adaptations to the ionic relations of stomatal guard cells. Despite the small amount of evidence available, it is possible to ide ntify two alternative adaptations that occur in the stomata of halophy tes: (1) the guard cells can utilize Na+ instead of K+ to achieve thei r normal regulation of turgor; (2) the guard cells continue to use Kand are able to limit their intake of Na+. The second adaptation is wo rthy of further exploration because it may provide a means for 'top do wn' control of transpiration and, therefore, of the amount of salt del ivered to the shoot. This mechanism may be very important in some of t he glandless halophytes, and it could be of particular interest as a p otential contributor to the development of salt tolerance in crops. (C ) 1997 Annals of Botany Company.