Abstract:

The effects of extra calcium (additions of 10 mmol L^-1 CaCl₂ into half-strength Hoagland solution) on maize (Zea mays L.) seedling root hydraulic conductivity (Lp_r), morphology and leaf water potential (ψ_w) were investigated under water stress, which was simulated by 10% PEG-6000 with osmotic potential (ψ_{s) value of -0.2 MPa for seven days, and subsequent two days rehydration. Whole root hydraulic conductivity (Lpr) and leaf water potential (ψw) were determined by pressure chamber. After these treatments, it could be seen that water stress reduced Lpr, which was restored when calcium was added to the solution for growing the water-stressed plants. In addition, the Lpr recovered to the level of the control after re-watering at high Ca²⁺ level, but the recovery of Lpr was only 59.06% at regular Ca²⁺ level. HgCl2 (50 μmol L^-1) treatment caused a sharp decline in Lpr, which was almost restored by treatment with 5 mmol L^-1 β-mercaptoethanol. The reduction of Lpr by Hg²⁺ was 53.20% and 74.55% at regular and high calcium levels, respectively under water stress condition, and 62.1% under normal condition. The results suggest that Ca²⁺ increased the passage of water through the cell membrane of roots by increasing the activity of Hg-sensitive AQP under water stress. The percentage of reduction by Hg²⁺ was decreased to the control level two days after re-watering in CaCl2 treated plants. The leaf water potential was declined significantly under water stress, particularly at regular calcium level with respect to the decrease of Lpr. However, ψw recovered rapidly onw day after re-watering. Furthermore, water stress had a detrimental effect on the root growth. The addition of calcium, especially at low water potential, increased the root surface area and primary root diameter, and it promoted primary root enlongation and lateral root development. Compared with the controls, the growth of Ca treated plants was recovered gradually with the prolonging of rehydration. Therefore, these results indicated that the extra calcium, with respect to root growth and water uptake, mitigates the negative effect of water stress and enhances the compensatory effects of rehydration.}

Key words: Water stress, Rehydration, Calcium, AQP, Root hydraulic conductivity