Determination of appropriate irrigation time based on rewatering water-use efficiency

Photosynthesis, water potential (Ψ) and carbonic anhydrase (CA) activity of leaves of Orychophragmus violaceus were examined in the course of drought and rewatering. Leaves were exposed to drought stress induced by poly-ethylene glycol (PEG) 6000 at 5 levels (1, 2, 4, 8% and 0 as control) for 4 h, which was regarded as drought phase. Then the leaves which were soaked in 1, 2, 4, 8% were transferred into control, 1, 2, 4%, respectively, which was regarded as rewatering phase and lasted for 2 h. In drought phase, CA activity of O. violaceus was activated under 2% and became the highest in 4%. CA of leaves translated HCO₃^- into H₂O, CO₂ and provided water for O. violaceus. And values of Ψ could remain stable between levels ranging from 1 to 4% during drought and rewatering phases, but the decrease of Ψ did not cause decline in net photosynthetic rate (P_N) or stomatal conductance (g_s) in 8%. Water regulation caused by enzymes, such as CA in plants, changed the water status; reduced the accuracy of diagnosis of water deficit using Ψ, P_N or g_s. Therefore, rewatering water-use efficiency (WUE_R) which meant increment of P_N per increment of water content (WC) in leaves of O. violaceus from drought to rewatering phase was defined. Higher WUE_R meant better rewatering effect on plants when they suffered from drought stress. Value of WUE_R3^2 in leaves were the highest. It was better to provide water for leaf of O. violaceus under 2% (-0.22 MPa osmotic stress or 17.5% water content in loam soil). Soil water potential after rewatering was better to be -0.14 MPa (1% PEG level or 17.9% water content in loam soil). The results provided a general method to determine appropriate irrigation times for crops.