Studies were conducted over several years at field sites in south east Queensland to evaluate the response of diverse soybean genotypes to saturated soil culture (SSC), relative to that in conventional irrigation (CI). In an initial study at Dalby, 56 accessions from 12 countries were tested, and all exhibited the ability to acclimate to, and grow in SSC. The chance occurrence of residual picloram herbicide on the site confounded interpretation of differential genotypic responses, but highlighted the potential vulnerability of the SSC system to chemical residues in the surface soil. In more detailed studies with representative subsets of lines at Lawes, there was large genotypic variation in relative responsiveness (RR, defined as {[Response in SSC-Response in CI]/Response in CI}) of yield to SSC, with seed yields reduced by up to 52%, enhanced by up to 37%, or unchanged, depending on genotype and agronomic management. Genotypic variation in RR was generally consistent across seasons, and was variously associated with genotypic differences in phenology when grown under CI. In general, the most positively responsive genotypes were those that started flowering late enough for the plants to have already acclimated to SSC, and that then flowered for a longer duration. Negatively responsive genotypes were those in which the pre-flowering period was sufficiently short for flowering to have commenced before acclimation was complete, i.e. where the acclimation phase intruded into reproductive ontogeny. Very late genotypes were also less responsive, perhaps because of greater lodging under the very favourable growth conditions provided by SSC. The studies indicated the potential for predicting responsiveness to SSC from knowledge of genotypic responses under conventional irrigation.
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