Genetic analysis of osmotic adjustment in crop plants

Phenomenon of osmotic adjustment Land plants have been coping with water stress for some Plant water deficit is a component of several different 400 million years, ever since they first left the seas and stresses, including drought, salinity and low tempercolonized dry land (Thomas, 1997). Because land plants ature, which severely limit plant growth and crop experience constant fluctuations in the availability of productivity. Genetic modification of plants to allow water, they have evolved adaptive features to search for growth and yield under unfavourable conditions is an and absorb water through their root systems, to prevent important component of the solution to problems of excessive transpirational water loss using cuticles and environmental stress. While disagreement and even stomata, and to adjust their physiology and metabolism confusion may characterize some of the discussions for continued growth and survival (Zhu et al., 1997; on what constitutes a significant and an effective Zhang et al., 1996). mechanism of drought resistance in crop plants, Although plants are exposed to many types of environosmotic adjustment (OA) is receiving increasing mental stresses, osmotic stress, whether by drought, recognition as a major mechanism. This paper starts salinity or low temperature, constitutes the most serious with the review of OA functions, genetic variation and limitation to plant growth, productivity and distribution inheritance, and theories and principles involved in (Boyer, 1982). Plants respond to these stresses at morphocommonly used protocols for quantifying OA. logical, anatomical, cellular, and molecular levels. Emphasis is placed on a summary of current molecuParticular morphological adaptations may be vital in lar strategies and advances in the improvement of specific plant species, but are not commonly used in all plant stress resistance through manipulating OA. plants. Basic cellular responses to water deficit, however, They include a genetic engineering approach and a appear conserved among all plants (Zhu et al., 1997). QTL mapping approach. Future promising strategies Osmotic adjustment (OA), for example, is common even for improving drought resistance lie in molecular in cellular organisms. technology that allows genes or QTLs controlling OA While disagreement and even confusion may characterto be tagged and isolated, these genes to be ize some of the discussions on what constitutes a significexpressed in transgenic plants, and efficiency of ant and an effective mechanism of drought resistance in breeding via marker-assisted selection to be crop plants, OA is receiving increasing recognition as a improved. Aspects of QTL utilization in plant genetics, major mechanism. In earlier days OA was defined as the breeding and physiology and future research direcosmotic potential difference between plants and the tions are discussed. medium in which they grew (Bernstein, 1961). This definition did not separate the net accumulation of solutes from the solution concentration effect due to osmotic stress. As water is being removed from the cell, osmotic