Effects of silicon on morphology, ultrastructure and exudates of rice root under heavy metal stress

Soil contamination with toxic heavy metals (such as Cd or Zn) is becoming a serious problem worldwide because of the rapid development of social economy. Silicon plays a substantial role in alleviating heavy metal toxicity in crop plants. In this study, two rice varieties, Feng-Hua-Zhan and Hua-Hang-Si-Miao, were chosen to determine the effects of Si application on root morphological traits, cell structure and exudates of rice roots under Cd and/or Zn stress. Single or combined applications of Cd and Zn resulted in significant reduction of total root length, root surface area, root volume, average root diameter and root activity. However, 1.5 mM Si addition reversed these negative effects. Transmission electron microscopy observations showed that rice root cortex cells were heavily damaged under Cd and/or Zn stress for both two varieties, whereas Si addition resulted in improved cell structure integrity. In addition, lower levels of oxalic, acetic, tartaric, maleic and fumaric acids in root exudates were observed for Feng-Hua-Zhan under Cd and/or Zn stress, but addition of Si increased the acid levels. For Hua-Hang-Si-Miao, heavy metal treatments significantly reduced oxalic and fumaric acid levels and increased acetic, tartaric and maleic acid levels, whereas Si treatment showed opposite results. The above results indicated that Si could ameliorate the toxicity of heavy metals (Cd and Zn) for rice which resulted in improving root traits, cell structure and influencing root exudates.