Effect Of Calcium On Water Uptake By Root Of Maize Seedling Under Water Stress And Rehydration Conditions

The effects of extra calcium (additions of 10 mmol L-1 CaCl2 into half-strength Hoagland solution) on maize (Zea mays L.) seedling root and individual cortical cells hydraulic parameters, root morphology and anatomy, leaf photosynthetic characteristics and leaf water potential (ψw) were investigated under water stress, which was simulated by PEG-6000 with osmotic potential (ψs) value of ?0.2 MPa for seven days, and subsequent two days rehydration.Whole root hydraulic conductivity (Lpr) and individual cortical cells hydraulic conductivity (Lp) of maize seeding were investigated using the pressure chamber and cell pressure probe. The Lpr and Lp of well watered plants was obviously decreased when extra Ca²⁺ supply into half-strength Hoagland solution. It could be seen that extra Ca²⁺ treated plants showed a less inhibitory effect of water stress than did those treated with regular Ca²⁺. In addition, the Lpr and Lp recovered to the level of the control after re-watering at high Ca²⁺ level, but the recovery of Lpr and Lp was only 59.06% and 64.51% at regular Ca²⁺ level. HgCl2 (50μmol L-1) treatment caused a sharp decline in Lpr, which was almost restored by treatment with 5 mmol L-1β-mercaptoethanol. The reduction of Lp by Hg2+ of well watered plants was 68.78% and 63.32% at regular and high calcium levels, respectively. And the reduction of Lp by Hg2+ of water stressed plants was 52.26% and 67.26% at regular and high calcium levels. The results suggest that Ca²⁺ decreased the passage of water through the cell membrane of roots by gating AQPs, but it increased the water permeability by increasing the activity of Hg-sensitive AQPs 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 photosynthetic parameters of maize seeding leaves were improved by extra Ca²⁺ under water stress and rehydration conditions. The improvement of Pn, Gs and Tr of extra Ca²⁺ treated plants was significantly especially at water stressed conditions, and which were recovered rapidly after re-watering.Leaf water potential (ψw) of maize seeding was determined by pressure chamber. There was no effect onψw when extra Ca²⁺ supply into half-strength Hoagland solution. The leaf water potential was declined significantly under water stress, particularly at regular calcium level with respect to the decrease of Lp. However,ψw recovered rapidly after 1 day 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 Ca²⁺ treated plants was recovered gradually with the prolonging of rehydration. Primary root cortex width and xylem vessel area of Ca²⁺ treated plants increased at well watered conditions, while those decreased at water stressed conditions. In addition, seedlings grown at water stressed and high Ca levels had the biggest mean V suggesting that water stress and calcium could decrease the cell V, respectively, and calcium could reverse that water stress induced decrease in cell volume.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.