Silicon nutrition promotes root growth and tissue mechanical strength in symbiotic cowpea

Application of metasilicic acid (0, 0.04, 0.08, 0.20, 0.40 or 0.80 g L^-1) to hydroponically-grown, Bradyrhizobium-infected cowpea [Vigna unguiculata (L.) Walp.] plants showed an increased assimilation of silicon into roots and shoots, which triggered a significant (P<0.05) promotion of root growth, but not shoot growth. Root : shoot ratio therefore, increased markedly (P<0.05) with higher metasilicate application. Mechanical strength of stems and peduncles also increased significantly (P<0.05) with silicon nutrition compared with control plants receiving no metasilicate. But the mechanical strength of roots was not affected. Radioimmunoassay of lateral roots, free of nodules, from plants fed metasilicate revealed markedly (P<0.05) increased concentrations of endogenous ABA, a hormonal signal that stimulates root growth. In contrast, a decreasing concentration of the cytokinin zeatin ribose was obtained with increasing metasilicate supply. These data show that silicon nutrition in symbiotic cowpea promotes an increase in mechanical strength of stems, which bear the entire weight of shoots, and peduncles, which, in turn, support weighty reproductive structures including developing pods and seeds. The increased concentration of ABA in roots as a result of improved silicon nutrition suggests that this element might be an elicitor of ABA biosynthesis and / or its accumulation, which then affected lateral root growth in this study.