Introduction

Many early studies reported the molybdenum (Mo) requirements of bacteria (Bortels, 1930; Horner et al., 1942), fungi (Steinberg, 1936, 1937), green algae (Arnon et al., 1955), and higher plants (Arnon and Stout, 1939) long before recognition of its various biochemical and physiological functions. Because of subsequent advances in biochemical and physiological research techniques and instrumentation, Mo has been found to be an essential constituent of several important enzymes, besides having a range of nonspecific roles in plant metabolism. This chapter reviews the information available on the biochemical and physiological roles of Mo in crop plants.

Chemical Basis for the Roles of Molybdenum in Biochemical Reactions

It is worthwhile to examine the chemistry of Mo to emphasize what makes it suitable for catalyzing many unique biochemical reactions. Molybdenum is a transition element with an atomic number of 42 and electronic configurations of Is2, 2s22p6, 3s23p63d10, 4s24p64d5, and 5s1. The outermost 4d electrons can be easily removed to yield several oxidation states of Mo, ranging from zero to VI. However, the oxidation states IV, V, and VI are more common and involve only low potentials. Higher oxidation states of Mo have a tendency to form oxocomplexes. As Mo(VI) has no electron in its d orbital, it accepts two e– from an O2– anion to form a sigma (σ) bond. As the Mo—O bond length is short, the d orbital of Mo suffers sidewise weak overlap with the p orbitals of an O atom, to yield a slightly double-bond (Mo=O°) character.