Intensity of photosynthesis and transport of assimilates in Solanum tuberosum under the action of 24-epibrassinolide

Brassinosteroids are a unique class of steroid hormones. They have a wide functional activity, combining the properties of growth stimulants and inducers of protective reactions that reduce the damaging effect of stressors on the plant organism. They regulate the processes of ethiolation, the synthesis of other groups of phytohormones. There is information about the participation of brassinosteroids in the expression of light-regulated photosynthetic genes and regulation of the functioning of the photosynthetic apparatus. Nevertheless, information regarding the content of pigments is conflicting. Activation of the enzymes of the Calvin cycle is shown. Some researchers noted stimulation of CO2 uptake and a positive effect on the yield of various crops. However, there are practically no data on the effect of brassinosteroids on the transport of assimilates to the attracting centers of plants. In this work, we established the effect of brassinosteroids on the rate of photosynthesis of potato plants and for the first time revealed their participation in the regulation of the transport of assimilates to tubers through changes in the content of ABA in the basal zone of the stem and cytokinins in tubers. Our work aimed to study the effect of 24-epibrassinolide on the intensity of the photosynthesis process, the content of assimilates in different zones of the stem, and to reveal the participation of ABA and cytokinins in the outflow of assimilates into tubers in potato plants. The Solanum tuberosum L. cv. Skoroplodny plants were grown in a growing house in soil culture. At flowering phase, the plants were sprayed with a 1.47½10-8 M solution of 24-epibrassinolil (Institute of Bioorganic Chemistry of the National Academy of Sciences of Belarus); the control plants were sprayed with water. The intensity of photosynthesis was assessed using the 14C radioactive isotope generated in a gasholder from a mixture of radioactive and non-radioactive sodium bicarbonate. At the end of flowering, a clothespin chamber was attached to an intact leaf of the eighth layer. In the chamber, the leaf was exposed to 14СО2 atmosphere (10 ml, 0.6% 14СО2, 0.0334 mBq/nM) for 10 minutes. To determine the content of 14C-assimilates, sections of the stem zones were fixed 48 hours after exposure to the 14CO2 atmosphere. The radioactivity was measured (a T-25-BFL end counter, Isotope, Russia). The sucrose content was measured refractometrically (an RPL-3 refractometer, OAO Kyiv plant “AnalytPribor”, Ukraine). The concentration of abscisic acid (AA) in the zones of the stem and cytokinins in the tubers were determined by enzyme-linked immunosorbent assay (ELISA) method. AA and zeatin (Serva, Germany) served as standard solutions of phytohormones. The content of chlorophylls a, b and carotenoids was determined in 80% acetone extracts (a KFK-3-01 photometer, AO ZOMZ, Russia). The thickness of the phellema was measured on intravital cross sections in the middle part of the tuber using an eyepiece micrometer (a Biolam microscope, LOMO, Russia). After the end of flowering, epibrassinolide increased the intensity of 14СО2 assimilation by 23 % (р £ 0.05). The treated leaves exposed to 14СО2 contained more sucrose as compared to the control leaves. An increase in the content of chlorophylls a, b and carotenoids occurred. The concentration of sucrose and 14C-assimilates differed between various stem zones. In the basal zone, the concentrations were lower than in the middle part. Epibrassinolide increased the gradient of 14C- assimilates and sucrose between the zones of the stem, which may indicate an increase in their outflow into tubers. Simultaneously, the level of endogenous abscisic acid in the basal zone increased, which facilitates unloading of phloem endings. Under the influence of epibrassinolide, the AA gradient between the zones was 41 % vs. 26 % (р £ 0.05) in the control. In tubers, due to the exogenous epibrassinolide, the level of cytokinins which exhibit an attracting effect was higher compared to the control. The brassinosteroid increased the productivity of potato plants by 25 % (р £ 0.05) and stimulated phellema formation in the tubers. The research data obtained suggest that epibrassinolide regulates the intensity of the photosynthesis process, the outflow of assimilates into the forming tubers through the participation of AA in the creation of a gradient of assimilates in the stem zones and an increase in cytokinins in tubers, which attract assimilates. This ultimately affects the productivity of potato plants.