PLANT‐INDUCED CHANGES IN THE RHIZOSPHERE OF RAPE (BRASSICA NAPUS VAR. EMERALD) SEEDLINGS

Abstract

SUMMARYChanges in the rhizosphere pH of rape plants (Brassica napus var. Emerald), grown at high root densities (> 90 cm cm−3) in a soil of low P status, were not associated with any detectable increase in the amount of extractable organic acids or their anions, or in the total amount of uronic acids in the soil. Microbial numbers in the rhizosphere soil and unplanted control soil were estimated by dilution plate counting, using an hydroxyapatite (HA) agar substrate, and those colonies capable of producing acid or of dissolving HA were identified. Neither the total number of colonies nor the number of acid‐producing colonies and P solubilizing colonies bore any obvious relationship to the pH of the soil from which they were isolated. However, analysis of the major cation and anion concentrations in the plant tissue showed that more cations than anions were taken up by the rape plants during the period when the rhizosphere pH decreased. The milliequivalents of H+ or OH− required to produce the observed pH changes were calculated from the soil's pH buffer curve and found to agree closely with the difference between the milliequivalents of cations and anions taken up by the plant. A steady decline in NO3− uptake and a small increase in Ca2+ uptake created the cation‐anion imbalance during the period of decreasing rhizosphere pH. H+ release from the roots must have occurred to maintain the charge balance across the root‐soil interface and was the most likely cause of decrease in rhizosphere pH.