Influence of two plant growth-promoting rhizobacteria on loblolly pine root respiration and IAA activity

Abstract

A clearer understanding of factors contributing to inconsistency in seedling root growth promotion among nurseries has become important to managers as the commercial use of rhizobacteria in forest nurseries increases. To address why some bacterial strains increase root growth response while others do not, two separate experiments were conducted with loblolly pine (Pinus taeda L.) seedlings. Experiment one examined growth media. Seed were sown in either a hand-mixed peat:vermiculite:perlite (2:1:1, v:v:v) potting medum or a commercial medum - Promix® (1:1 peat:vermiculite). Seed were inoculated at sowing with an equal amount of either Bacillus subtilis strain LS211, B. subtilis strain INR7, or sterile distilled water. Seedling biomass, root length, root surface area, average root diameter, and root volume were measured at 6 and 12 wk after sowing. Growth promotion was variable and dependent on media and weeks since sowing. In a second experiment, root respiration rate and total root indole-acetic acid (IAA)content were quantified at 6 and 12 wk for seedlings grown in Promix®. In experiment two, strain INR7 decreased whole root system respiration by 22% and increased root biomass and root length compared to controls at 6 wk. Bacterial strain LS211 had no effect on root respiration. Furthermore, bacterial strain INR7 produced 1.7 times the total root IAA concentration of controls at 6 wk, whereas LS211 had no effect. Although cause and effect could not be tested, these studies suggest that root growth promotion is influenced by growth media and that IAA concentrations and root respiration rates are two physiological mechanisms correlated with rhizobacterial activity and growth promotion.