Calcium regulates the cell-to-cell water flow pathway in maize roots during variable water conditions

Soil water shortages can decrease root hydraulic conductivity and affect Ca uptake and movement through the plant. In this study, the effects of extra Ca2+ applied in nutrient solution on the hydraulic properties of the whole roots (Lpr) and cortical cells (Lpcell) of maize (Zea mays L.) subjected to variable water conditions were investigated. Under well-watered conditions, extra Ca2+ significantly increased the root Ca content, total root length, and lateral root number; however, it reduced the root cortical cell volume, Lpr, and Lpcell. Hg2+ inhibition experiments suggested that extra Ca2+ could reduce the contribution of the cell-to-cell water flow pathway. Osmotic stress (10% PEG6000) significantly decreased the cortical cell volume, Lpr, and Lpcell in the control plants, but smaller decreases were observed in the extra Ca2+ plants. The Hg2+ treatment reduced the Lpr larger in the extra Ca2+ plants (74.6%) than in the control plants (53.2%), suggesting a higher contribution of the cell-to-cell pathway. The larger Hg2+ inhibition of the Lpcell in the extra Ca2+ roots (67.2%) when compared to the controls (56.4%) indicated that extra Ca2+ can mitigate the inhibition of aquaporin expression and/or activity levels via osmotic stress. After 2 d of rehydration, the extra Ca2+ helped the Lpr and Lpcell to recover almost completely, but these properties only partially recovered in the control plants. In conclusion, extra Ca2+ may adjust the contribution of cell-to-cell pathway by regulating the expression and/or activity levels of AQPs according to water availability; this regulation may weaken negative effects and optimize water use.