COMBINED USE OF COLORIMETRIC AND MICROELECTRODE METHODS FOR EVALUATING RHIZOSPHERE PH

Plant control of rhizosphere pH is important for nutrient mobilization and uptake, and also affects microbial activity and pathogens in the vicinity of the root. Limited information is available on the ability of plant species and genotypes within a species to induce pH changes i n the rhizosphere. A growth chamber study was conducted to characteriz e patterns of pH change within the rhizosphere of selected genotypes i n an alkaline environment with a balanced nutrient supply. After germi nation in incubators, seedlings of 32 genotypes of maize (Zea mays L.) , soybean (Glycine max. L.), sorghum (Sorghum bicolor L.), sordan [sor ghum (Sorghum bicolor L.), sudangrass (Sorghum sudanese L.) hybrid], w heat (Triticum aestivum L.), oats (Avena sativa L.), and barley (Horde um vulgare L.) were transferred into aseptic agar medium (pH 7.6) with bromocresol purple indicator. Ability of the embedded roots to induce rhizosphere pH change was followed by photographing the color change of the bromocresol purple indicator. The pH for selected genotypes at different root zones (maturation, elongation, meristematic) was also m onitored by a microelectrode at l-, 2-, 3- and 4-mm distances from the root surface. Rhizosphere acidification for selected genotypes within a species were in the order: soybean, Hawkeye > PI-54169; maize, Pion eer-3737 > Pioneer-3732 > CM-37; sordan, S-757 > S-333; sorghum, SC-33 -8-9EY congruent-to SC-1 18-15E; barley, Bowman > Primus 11; oats, Hyt est > SD-84104. The pH patterns within the root system varied from spe cies to species. The highest amount of acidification was found at the elongation and meristematic zones for soybean, while the highest amoun t of acidification was found at the maturation zone for barley under t he same experimental conditions. The agar method allowed the determina tion of a genotype's capability to induce rhizosphere pH changes while the microelectrode method is necessary for quantifying the spatial va riation of specific root developmental zones with high resolution.