How can stomata contribute to salt tolerance?

Abstract

Although some of the physiological mechanisms which contribute to salt tolerance in plants are known, there are still some major gaps in understanding and it remains impossible to provide a satisfactory integrated picture for the plant as a whole. The operation of stomata in halophytes has received little attention even though all of the salt present in the shoot (apart from that taken in during submergence) is thought to be carried in the transpiration stream. In non-halophytes, stomatal function is damaged by sodium ions and disruption of the normal regulation of transpiration should be seen as a possible contributor to their inability to survive in salt-laden soils. The development of salt-tolerant cultivars of crops may require attention to the need for appropriate adaptations to the ionic relations of stomatal guard cells. Despite the small amount of evidence available, it is possible to identify two alternative adaptations that occur in the stomata of halophytes: (1) the guard cells can utilize Na+ instead of K+ to achieve their normal regulation of turgor; (2) the guard cells continue to use K+ and are able to limit their intake of Na+. The second adaptation is worthy of further exploration because it may provide a means for 'top down' control of transpiration and, therefore, of the amount of salt delivered to the shoot. This mechanism may be very important in some of the glandless halophytes, and it could be of particular interest as a potential contributor to the development of salt tolerance in crops.