Exogenous Calcium Induces Different Hydraulic Strategies in Response to Osmotic Stress in Maize Seedlings

Recent discoveries regarding the signal molecules involved in abiotic stresses require integration into the field of plant hydraulic property research. Although calcium (Ca) is an important second messenger involved in numerous complex, abiotic stress-induced signaling pathways, it remains unclear how exogenous calcium mediates cellular signaling to promote plant drought resistance. We investigated the effects of calcium on the water balance and hydraulic properties in maize seedlings (Zea mays L.) under osmotic stress simulated by 10% (m/v) PEG-6000 in a hydroponic culture. The osmotic stress dramatically decreased the photosynthetic rate, transpiration rate, stomatal conductance, leaf water content, and root water absorption. However, the short-term (2 h) and long-term (10 d) exogenous Ca²⁺ (CaCl₂: 10 mM) treatments had different effects on the maize gas exchange parameters and leaf water status. The short-term treatment improved the leaf transpiration by inhibiting the abscisic acid (ABA) synthesis and accumulation in the leaves, generating a stronger transpiration pull and enhancing the root water absorption and axial flow path water transport by increasing the root hydraulic conductance to relieve the osmotic stress-induced inhibition. The long-term treatment induced the ABA and H₂O₂ accumulation in the roots and leaves. Under osmotic stress, the accumulation of ABA, H₂O₂, and Ca²⁺ rapidly repressed the transpiration and enhanced the radial flow path water transport, decreasing the water loss and improving the stress tolerance. These insights suggest a role for a judicious use of Ca fertilizer in reducing the adverse effects of drought on agricultural production.