LAUSR

The leaves of spring wheat (Triticum aestivum L.) and winter rye (Secale cerealе L.) were used to examine the dynamics of peroxidase activity and the content of hydrogen peroxide, proline, flavonoids, and organic acids (oxalic, succinic, malic, and citric) at early stages of alkaline (pH 10) and acidic (pH 3) stresses imposed by the pH change in the root medium. The results are discussed on the basis of oxidative stress theory and the mechanisms of intracellular pH homeostasis. The dynamics of hydrogen peroxide content and the increased peroxidase activity provided evidence for the enhancement of redox processes. The accumulation of low-molecular-weight polyfunctional compounds, such as proline and flavonoids, indicated the mobilization of plant defense systems. At the same time, stress-related adaptive changes differed qualitatively in different treatments: the leaves of wheat plants transferred to acidic media accumulated proline, while alkaline-treated plants accumulated predominantly flavonoids. The peroxidase activation and proline accumulation after pH changes in the root medium were more strongly pronounced in rye than in wheat plants. A comparatively stable trend to the decrease in the total amount of malic, citric, and succinic acids was observed within 24 h after the onset of stress treatment; in rye leaves, this trend was apparent both with acidic and alkaline root media, while it was evident in wheat leaves under acidic pH of the solution. The observed changes in the content of organic acids that are involved in the Krebs cycle are regarded as a manifestation of the pH-stat mechanism.