Relationship among rice root aerechyma, root radial oxygen loss and rhizosphere nitrification

A soil culture experiment with rhizobox was carried out to study the difference of rice root growth, aerenchyma development, root radial oxygen loss (ROL) and root surface and rhizosphere nitrification activities. Two Indica varieties with different yield potentials, Zhonghan 22 (ZH, high yield) and Hesheng 10 (HS, low yield) were used and the samples were collected at 40, 50 and 60 d after sowing. The results showed that root diameter, root number and root dry weight of ZH were always significantly greater than those of HS, expect for the root number and root dry weight at 40 d after sowing. The radial-arrangement aerenchyma tissues were examined at 20 mm behind root tips. The parenchyma cells of ZH entirely collapsed and the radial cell wall aggregated together to form a gas-filled space connected the stele and exodermis, while the parenchyma cells of HS partially collapsed and a gas-filled space connected the stele and exodermis was formed with the residual parenchyma cells and aggregated cell walls. The volume of porosity of root (POR) of ZH was significantly higher than that of HS, indicating better developed aerenchyma for ZH. The cortical prothenchyma cell (ROL barrier) size of ZH was relatively small and closely arranged, and the thickening effect for the cell wall was obvious, whereas that of HS was big and loosely arranged, suggesting that a strong ROL barrier effect is important to provide sufficient oxygen for rice root growth. Except for the difference of ROL per root between ZH and HS at 40 d after sowing, the ROL per plant, ROL per dry weight and ROL per root measured at 40, 50 and 60 d after sowing for ZH were always significantly higher than those for HS (P < 0.01). Rhizosphere nitrification activities of both varieties were 3 to 6 times higher than those measured in the root surface soil. Although there had no significant difference of root surface soil nitrification activities between these varieties, the rhizosphere nitrification activities for ZH were generally twice as high as those for HS, and appeared to be significantly correlated to rice ROL per plant, POR, root diameter, root number and root dry weight (r = 0.803, 0.808, 0.637, 0.646 and 0.696, respectively, P < 0.01). Because the nitrification prefers aerobic environment, the rice variety with well-developed root system and aerenchyma as well as correspondingly high ROL allows relatively high oxygen entering rhizosphere soil, which might improve rhizosphere nitrification activit