Subcellular organization of N2-fixing nodules of cowpea (Vigna unguiculata) supplied with silicon

Provision of silicon (0, 0.048, 0.096, 0.24, 0.48, and 0.96 g/l) in the form of silicic acid (H4SiO4) to nodulated cowpea plants (Vignia unguiculata [L.] Walp.) grown in liquid culture resulted in considerable changes in the internal organization of nodule structure. Compared to the control plants which received no added silicate, bacteroid numbers increased significantly (P < or = 0.05) at silicate concentrations of both 0.096 and 0.48 g/l. The number of symbiosomes also increased by 3.2-fold at the silicate concentration of 0.96 g/l compared to the control. In contrast, the size of bacteroids and symbiosomes decreased significantly (P < 0.05) inside nodules of silicate-treated plants. The peribacteroid space was also decreased considerably (P < 0.05) with the application of 0.096 and 0.96 g of silicate per liter to plants. However, the size of intercellular spaces adjacent to infected and uninfected interstitial cells within the nodule medulla increased significantly (P < or = 0.05) at 0.096 g of silicate per liter followed by a sharply marked (P < or = 0.05) decrease with each subsequent increase in silicate application. The result was a large decrease (P < 0.05) in the area of bacteria-infected tissue occupied by intercellular space at the highest silicate concentration, which was caused by a significant (P < or = 0.05) increase in cell wall thickness. Our findings show that the positive effects of silicon on N2 fixation might actually be due to an increased number of bacteroids and symbiosomes.