DIFFERENTIAL GENOTYPIC AND ROOT TYPE PENETRATION OF COMPACTED SOIL LAYERS

Total number of root penetrations of compacted subsoil layers have bee n commonly used to assess plant tolerance to subsoil compaction. Root- type specific responses to compacted subsoil layers have not been inve stigated despite the documentation that plant root types are physiolog ically and genetically different, A set of maize (Zea mays L.) and soy bean (Glycine max L. Merr.) cultivars were grown in growth chambers in containers with compacted middle layers (1.2 and 1.65 Mg m(-3)) of Ho ward soil (loamy skeletal, mixed, mesic Glossoboric Hapludalf) with 17 % clay content. The containers mere exposed to a neutron beam to deter mine the relative roof lengths (RRL) of taproot (RTRL) and basal root (RBRL), the numbers of lateral roots in the compacted lager, and lengt hs of lateral roots above the compacted laver. The resulting neutron r adiographic images revealed that maize 'SA-3' and soybean 'PI 416937' taproots penetrated the compacted layer and maintained >90% RTRL in th e subsoil, but their basal roots hardly penetrated the compacted lager .In contrast, maize 'La Posts' and soybean 'Weber' taproots mere sever ely restricted by the compacted layer, but their basal roots penetrate d the leger and had >70% RBRL in the subsoil. Maize 'TS-6' taproots we re also restricted by the compacted layer, but TS-6 basal roots had a mode rate penetration that produced a 38% RBRL in the subsoil, On the other hand, soybean 'Perry' had a moderate penetration and RRLs in the subsoil with both tap and basal roots. This variation in root type re sponse to subsoil compaction among cultivars stresses the importance o f considering different roof types as distinct entities in studies on plant tolerance to compacted subsoil conditions.