Population Dynamics of Rhizobium in the Rhizosphere of Pea

Abstract

Pea growers in western Canada have a greater selection of inoculant formulation than ever before, but lack information on their efficacy to deliver viable rhizobia to the rhizosphere. Peat and peat-based granules have been the carriers of choice by most commercial manufacturers of inoculants. While they remain as excellent carriers of rhizobia, several manufacturers have successfully developed liquid carriers for rhizobia. In field studies trials conducted at numerous sites in Saskatchewan and Alberta, the yield of pea following application of liquid inoculants were as high or higher than peat inoculants (Hynes et al. 1995). However, poorer yields have been observed with liquid inoculants in the Peace River region of northern Alberta. To determine the reason for this, we are studying the population dynamics of Rhizobium leguminosarum in the rhizosphere of pea (Pisum sativum) following inoculation. A spontaneous antibioticresistant mutant of Rhizobium leguminosarium (128C56G strr) was formulated in liquid, peat and peat-based granules and applied to pea (cv. Express) in field trials at Saskatoon, SK and Beaverlodge, AB in 1996 and 1997. Treatments were replicated 4 times in a randomized complete block design. The population of R. leguminosarium 128C56G strr was enumerated on TY (Beringer 1974) amended with 500 μg streptomycin/mL and 0.03% Rose Bengal (TY+). At frequent intervals over the growing season, 10 seeds or seedlings were removed from each plot, shaken in peptone-phosphate buffer and 128C56G strr was enumerated on TY+ by dilution plate count. Nodule occupancy was determined by surface sterilization of 16 nodules per plot and squeezing them onto TY+. At Saskatoon in 1996, the population of 128C56G strr from seed-applied formulations decreased 1–2 log units during the initial 24 h, but increased to approximately 106/g dry root over the next 20 days, a value similar to that from granular inoculant. Formulation did not significantly affect nodule number; nodule occupancies were 64, 69 and 79% for the liquid, peat and granular formulations, respectively. At Beaverlodge in 1996, the population of 128C56G strr from peat and granular formulations was 105/plant at 21 days after planting, but only 102/plant from the liquid formulation. Nodule number per plant for peat, granular and liquid formulations were 25, 27 and 7, respectively, while nodule occupancies were 86, 89 and 24%, respectively. Field studies are continuing to understand the difference in performance of the formulations at these two sites.