Use of hydrogen release or a combination of hydrogen release and iron reduction for selecting iron-efficient dry bean and soybean cultivars

Iron (Fe) chlorosis continues to be a problem in the world where crops are grown in calcareous soils. Although it is possible to remedy this problem with fertilizer application, selection of cultivars resistant to Fe-deficiency chlorosis is a more economical and practical solution. In addition to field chlorosis rating studies, several methods have been developed to screen cultivars. One of the most promising is quantifying individual plant response mechanisms induced during Fe-deficiency stress, such as Fe reduction or hydrogen ion (H+) extrusion by the roots. Eight dry bean (Phaseolus vulgaris L.) and Il soybean (Glycine max L.) cultivars of varying resistance to Fe-deficiency chlorosis were grown in solutions of low (0.05 mg L-1) and no (0 mg L-1) Fe, respectively. Beginning at day 2 after imposition of low Fe treatments, plant roots were incubated for one hour in a solution to collect H+ ions. This solution was then titrated with NaOH to the pH of the control solutions (no plants grown in solution, but air bubbled through it for one hr). The results of six consecutive days were summed and correlated with field chlorosis scores. In dry bean, there was a statistically significant positive relationship between H+ release and field chlorosis scores, which is opposite of the theoretical relationship, i.e., dry bean cultivars with high chlorosis scores released the largest quantity of H+ ion and vice versa. Soybean exhibited a statistically significant negative relationship between quantity of H+ release and field chlorosis scores, but Fe reduction measurements provided better screening ability than H+ ion release. Adjusting the measurements for fresh root weight did not alter these relationships; thus, eliminating the need to weigh roots in a breeding nursery. Combining the effects of H+ release and Fe reduction using standardized scores did not improve the selection of Fe efficient cultivars over Fe reduction alone in either dry bean or soybean. Hydrogen ion quantification was time consuming, tedious, and gave conflicting results in the two species studied. Thus, it is not recommended as a screening technique in either dry bean or soybean.