Ontogenetic Changes in the Photosynthetic Apparatus and Effects of Cytokinins (Review)

Data on structural and functional changes in the photosynthetic apparatus of agricultural plants are presented. Results of experiments with the regulatory effects of cytokinin-like phytohormones on photosynthesis are discussed. Cytokinins are involved in the structural formation and maintenance of the photosynthetic apparatus, stomatal function, supply of CO₂ to carboxylation sites through the leaf mesophyll, synthesis of pigments and enzyme systems, and regulation of photoreduction and carbon metabolism. Possible mechanisms mediating the regulatory effects of cytokinins are discussed.     

The Activity and Content of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Wheat Plants as Affected by Water Stress and Kartolin-4

The carboxylating activity and content of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO, EC 4.1.1.39), and other soluble proteins in young seedlings and mature leaves of Lutescens-758, a drought-sensitive cultivar of soft spring wheat Triticum aestivum L., were studied under the conditions of drought and subsequent rehydration. Seedlings and mature plants preliminarily treated with the cytokinin-like compound kartolin-4 were compared to untreated plants. Drought-induced decrease in RuBPCO activity should be attributed not only to proteolytic decomposition of the enzyme protein itself but also to a partial inhibition of its catalytic activity. The decrease in RuBPCO activity was larger than that in RuBPCO content. Water stress induced a marked decrease in the soluble protein content. Kartolin-4 increased the resistance to drought.     

Effects of Cytokinin Preparations on the Stability of the Photosynthetic Apparatus of Two Wheat Cultivars Experiencing Water Deficiency

Carboxylase activity of the key enzyme of carbon metabolism, ribulose-bisphosphate carboxylase/oxygenase (RuBisCO; EC 4.1.1.39), and phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), as well as the intensity of carbon dioxide photosynthetic assimilation in young seedlings and adult leaves of the wheat Triticum aestivum L. cultivars Mironovskaya 808 (a more tolerant) and Lyutestsens758 (a less tolerant), were compared under conditions of progressive water deficiency. The water stress had more pronounced negative effects on all the studied characteristics of the photosynthetic apparatus of the cultivar Lyutestsens758. Its seedlings were more sensitive to water stress. Compounds with a cytokinin activity (6-benzylaminopurine, thidiazuron, kartolin 2, and kartolin 4) played a protective role, increasing the stability of the photosynthetic apparatus under conditions of water deficiency. Preparations of kartolins displayed the maximum protective effect.