EFFECTS OF DROUGHT ON SYMBIOTIC NITROGEN FIXATION, PHOTOSYNTHESIS, AND PRODUCTIVITY OF RED CLOVER (TRIFOLIUM PRATENSE L.) UNDER CO-INOCULATION WITH RHIZOBIA AND PLANT GROWTH-PROMOTING RHIZOBACTERIA

Abstract

Background. Red clover is an important forage crop that enhances soil fertility. Pre-sowing inoculation of legume seeds with nitrogen-fixing microorganisms is an effective way to increase their productivity. It is known that co-inoculation with rhizobia and plant growth–promoting bacteria helps protect plants from abiotic stresses, particularly drought. The aim of this study was to investigate the effects of drought on nitrogen-fixing activity, net CO₂ assimilation rate, and productivity of clover plants inoculated with microbial complexes based on nodule bacteria and free-living soil microorganisms.
Materials and Methods. The study was conducted on symbiotic systems involving clover plants (Trifolium pratense L.) of the Tina variety and nodule bacteria Rhizobium leguminosarum bv. trifolii 348а strain, as well as in their combinations with Azotobacter chroococcum T79, Pseudomonas fluorescens 33, and Neorhizobium galegae MC-1 strains. Research methods – microbiological, gasometric, gravimetric, statistical analysis.
Results. Drought suppressed both the nitrogen-fixing activity (NFA) of root nodules and the net CO₂ assimilation rate (A_N) in clover leaves, with a stronger effect on NFA than on A_N. A significant increase in leaf photorespiration (R_light) was observed in drought-exposed plants, and R_light/A_N ratio remained elevated compared to control plants even after drought termination. Drought was also found to weaken the correlation between NFA and A_N, which was strong under optimal water conditions. A stronger correlation was observed between A_N and whole-plant dry weight (r = 0.84) than between NFA and dry weight (r = 0.71). The data clearly show that neither NFA nor A_N returned to well-watered control levels within seven days of drought termination. Seed treatment with the combination of Rhizobium leguminosarum bv. trifolii 348a + N. galegae MC‑1 stimulated NFA earlier in plant development than other treatments, maintaining peak activity from the eight-leaf stage onwards, whereas treatments with R. leguminosarum bv. trifolii 348a alone and in combination with A. chroococcum 79 reached comparable NFA values only at the budding stage.
Conclusions. The results indicate that inoculation of clover plants with different combinations of nitrogen-fixing microorganisms increased their productivity and improved their performance under drought conditions, although to varying extents. The combination R. leguminosarum bv. trifolii 348a + N. galegae MC1 was the most effective in stimulating clover productivity under both optimal and stress conditions. The R. leguminosarum bv. trifolii 348a + P. fluorescens 33 treatment was the worst-performing inoculant combination.