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 fo r 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 practi ce is possible without affecting the accuracy of the results. Despite extensive understanding of Fe-reducing activity in dicots, it is not k nown if root Fe reduction shows diurnal rhythmicity similar to phytosi derophore release, the Fe-deficiency stress response of grasses.. Five soybean cultivars showed a diurnally rhythmic pattern of root Fe redu ction when grown in a normal 16-h light, 8-h dark cycle. Continuous il lumination (no dark) slowed Fe reduction and eliminated the rhythmic p attern as well as any correlation between root Fe reduction and Fe eff iciency. Preceding a continuous light period with two 16-h light, 8-h dark cycles did not improve the results. Satisfactory genotypic screen ing for Fe-efficiency in soybean cannot be accomplished in continuous light, but rather should be performed in normal light/dark cycles whil e limiting measurement to a 6- to 8-h period during illumination.