Keywords: abiotic stress, NADPH oxidase, photosynthesis, potassium deficiency, reactive oxygen species, stress tolerance.
Plants exposed to environmental stress factors, such as drought, chilling, high light intensity, heat, and nutrient limitations, suffer from oxidative damage catalysed by reactive oxygen species (ROS), e.g., superoxide radical (O °2–, hydrogen peroxide (H2O2) and hydroxyl radical (OH °). Reactive O2 species are known to be primarily responsible for impairment of cellular function and growth depression under stress conditions. In plants, ROS are predominantly produced during the photosynthetic electron transport and activation of membrane-bound NAD(P)H oxidases. Increasing evidence suggests that improvement of the potassium (K) nutritional status of plants can greatly lower the ROS production by reducing activity of NAD(P)H oxidases and maintaining photosynthetic electron transport. Potassium deficiency causes severe reduction in photosynthetic CO2 fixation and impairment in partitioning and utilisation of photosynthates. Such disturbances result in excess of photosynthetically produced electrons and thus stimulation of ROS production by intensified transfer of electrons to O2. Recently, it was shown that there is an impressive increase in capacity of bean root cells to oxidise NADPH when exposed to K deficiency. An increase in NADPH oxidation was up to 8-fold higher in plants with low K supply than in K-sufficient plants. Accordingly, K deficiency also caused an increase in NADPH-dependent O °2– generation in root cells. The results indicate that increases in ROS production during both photosynthetic electron transport and NADPH-oxidising enzyme reactions may be involved in membrane damage and chlorophyll degradation in K-deficient plants. In good agreement with this suggestion, increases in severity of K deficiency were associated with enhanced activity of enzymes involved in detoxification of H2O2 (ascorbate peroxidase) and utilisation of H2O2 in oxidative processes (guaiacol peroxidase). Moreover, K-deficient plants are highly light-sensitive and very rapidly become chlorotic and necrotic when exposed to high light intensity. In view of the fact that ROS production by photosynthetic electron transport and NADPH oxidases is especially high when plants are exposed to environmental stress conditions, it seems reasonable to suggest that the improvement of K-nutritional status of plants might be of great importance for the survival of crop plants under environmental stress conditions, such as drought, chilling, and high light intensity. Several examples are presented here emphasising the roles of K in alleviating adverse effects of different abiotic stress factors on crop production.
Ismail Cakmak, Sabanc? University, Faculty of Engineering and Natural Sciences, 34956 Istanbul, Turkey.
20 pages, 6 figures, 3 tables, 81 references.