How Does Silicon Mediate Plant Water Uptake and Loss Under Water Deficiency?

Abstract

In plants, water deficiency can result from a deficit of water from the soil, an obstacle to the uptake of water or the excess water loss; in these cases, the similar consequence is the limitation of plant growth and crop yield. Silicon (Si) has been widely reported to alleviate the plant water status and water balance under variant stress conditions in both monocot and dicot plants, especially under drought and salt stresses. However, the underlying mechanism is unclear. In addition to the regulation of leaf transpiration, recently, Si application was found to be involved in the adjustment of root hydraulic conductance by up-regulating aquaporin gene expression and concentrating K in the xylem sap. Therefore, this review discusses the potential effects of Si on both leaf transpiration and root water absorption, especially focusing on how Si modulates the root hydraulic conductance. A growing number of studies support the conclusion that Si application improves plant water status by increasing root water uptake, rather than by decreasing their water loss under conditions of water deficiency. The enhancement of plant water uptake by Si is achievable through the activation of osmotic adjustment, improving aquaporin activity and increasing the root/shoot ratio. The underlying mechanisms of the Si on improving plant water uptake under water deficiency conditions are discussed.

Keywords: drought; salt stress; silicon; transpiration; water balance; water status; water uptake.

FIGURE 1

Possible mechanisms for silicon (Si) mediated water balance of plants experiencing water deficiency. (1) Si enhances the aquaporin activity by up-regulating the expression of Plasma membrane Intrinsic Protein (PIP) aquaporin genes and alleviating the ROS (reactive oxygen species)-induced aquaporin activity inhibition. (2) Si enhances the accumulation of soluble sugars and/or amino acids in the xylem sap by osmorugulation; Si activates the K⁺ translocation to xylem sap by the activation the expression of SKOR (Stelar K⁺ Outward Rectifer) gene. The osmolyte accumulations in the xylem sap increase the osmotic driving force. (3) Si might adjust the root growth and increase root/shoot ratio, which together with enhancement of aquaporin activity and osmotic driving force contribute to the improvement of root hydraulic conductance. The higher root hydraulic conductance results in increased uptake and transport of water, which helps to maintain a higher photosynthetic rate and improve plant resistance to water deficiency.