USE OF HYDROGEN RELEASE OR A COMBINATION OF HYDROGEN RELEASE AND IRONREDUCTION 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 solutio n. In addition to field chlorosis rating studies, several methods have been developed to screen cultivars. One of the most promising is quan tifying individual plant response mechanisms induced during Fe-deficie ncy 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 we re grown in solutions of low (0.05 mg L-1) and no (0 mg L-1) Fe, respe ctively. Beginning at day 2 after imposition of low Fe treatments, pla nt 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 so lutions (no plants grown in solution, but air bubbled through it for o ne hr). The results of six consecutive days were summed and correlated with field chlorosis scores. In dry bean, there was a statistically s ignificant positive relationship between H+ release and field chlorosi s scores, which is opposite of the theoretical relationship, i.e., dry bean cultivars with high chlorosis scores released the largest quanti ty of H+ ion and vice versa. Soybean exhibited a statistically signifi cant negative relationship between quantity of H+ release and field ch lorosis scores, but Fe reduction measurements provided better screenin g ability than H+ ion release. Adjusting the measurements for fresh ro ot weight did not alter these relationships; thus, eliminating the nee d to weigh roots in a breeding nursery. Combining the effects of H+ re lease and Fe reduction using standardized scores did not improve the s election of Fe efficient cultivars over Fe reduction alone in either d ry bean or soybean. Hydrogen ion quantification was time consuming, te dious, and gave conflicting results in the two species studied. Thus, it is not recommended as a screening technique in either dry bean or s oybean.