Influence of the photoperiod duration on the biological properties of PGPR-bacteria of the soybean rhizosphere (Glycine max (L.) Merr.)

Abstract

Physiological and biochemical properties of the PGPR-bacteria isolated from the rhizosphere of the soybean (Glycine max (L.) Merr), grown under different photoperiod duration were investigated. A short-day soybean cultivar Clark was selected for the study; it was exposed to a long natural day (16 hours – control) and a short day (9 hours – test). The short day was created by darkening the plants with light-tight chambers for three weeks from 5 p.m. to 9 a.m. the next day. The field experiment was carried out at the experimental site of the Department of Physiology and Biochemistry of Plants and Microorganisms of V.N. Karazin Kharkiv National University, located in the Botanical garden of the University. Soil samples from the soybean rhizosphere were taken at the budding phase; the bacteria were isolated in a nitrogen-depleted medium. We studied cultural characteristics of bacteria (Gram stain, bacterial morphology, mobility, type of respiration, oxidase and catalase positivity) and their physiological and biochemical properties (the capability of proteolysis and nitrate reduction). A greater variety of bacteria in terms of cultural properties was isolated from the rhizosphere of soybean grown under long-day conditions. Probably, this is due to the higher intensity of root exudate excretion (chemoattagative factors) during the long day which is caused by different metabolic intensities at various photoperiod lengths. The highest number of isolates with proteolytic activity was observed in bacteria under long-day conditions. This property is considered to have a significant impact on the mineral nutrition of plants. Analysis of literature data showed that the higher amount of protein is accumulated in soybean leaves during the long day as a result of intensification of nitrogen nutrition provided by bacteria capable of proteolysis. Under conditions of a short day, a higher number of isolates capable of nitrate reduction was observed. A decrease in the number of isolates capable of nitrate reduction under a long-day condition can be explained by the possible repression of assimilative nitrate reductase by a large amount of ammonium formed during the ammonification of organic substances. Thus, the obtained results suggest that the photoperiod duration can determine physiological and biochemical properties of the rhizosphere microbiota of the plants sensitive to this factor.