DIURNAL RHYTHMICITY OF ROOT IRON REDUCTION IN SOYBEAN AS AFFECTED BY VARIOUS LIGHT REGIMES

Root iron (Fe) reduction is correlated with genotypic Fe efficiency in soybean (Glycine max L. Merr.) and is reportedly a reliable method for identifying chlorosis-resistant genotypes. If used on a large-scale, it would be advantageous to measure Fe reduction at various times of the day. Experiments were conducted to determine whether such a practice is possible without affecting the accuracy of the results. Despite extensive understanding of Fe-reducing activity in dicots, it is not known if root Fe reduction shows diurnal rhythmicity similar to phytosiderophore release, the Fe-deficiency stress response of grasses.. Five soybean cultivars showed a diurnally rhythmic pattern of root Fe reduction when grown in a normal 16-h light, 8-h dark cycle. Continuous illumination (no dark) slowed Fe reduction and eliminated the rhythmic pattern as well as any correlation between root Fe reduction and Fe efficiency. Preceding a continuous light period with two 16-h light, 8-h dark cycles did not improve the results. Satisfactory genotypic screening for Fe-efficiency in soybean cannot be accomplished in continuous light, but rather should be performed in normal light/dark cycles while limiting measurement to a 6- to 8-h period during illumination.