A test of potting methods used to control soil water was applied to study the physiological characteristics and drought resistance of five plants used for highway shoulder greening and ecological restoration in northern China. Leaf water potential, photosynthesis parameters and fluorescence parameters of five plants were measured under four levels of water stress. The drought resistance of the plants was evaluated to study the average rate of change in various parameters.
The results show: (1) as water stress increased, leaf water potential (Ψ_L), net photosynthesis (P_n), transpiration (T_r), and stomatal conductance (G_S) of the five plants gradually decreased; as water stress increased, water use efficiency (WUE) of the five plants first increased and then decreased but the internal CO₂ (C_i) of the five plants first decreased and then increased. There were some differences in the changing amplitudes and inflection points of the physiological indices. Sedum spectabile and Iris lactea var. chinensis had relatively small average rates of change, while Opisthopappus taihangensis and Saponaria officinalis had relatively large average rates of change. (2) The C_i of S. spectabile and I. lactea var. chinensis reached a minimum value under moderate water stress (W₂), while the C_i of Parthenocissus quinquefolia, O. taihangensis and S. officinalis reached a minimum value under mild water stress (W₁). Changes in Gs and Ci under water stress were reflected in changes in P_n; the decrease in P_n observed in S. spectabile and I. lactea var. chinensis were mainly caused by stomatal limitation resulting from the change from a normal water supply (CK) to moderate water stress (W₂), while non-stomatal factors caused the changes observed under severe water stress (W₃). The decrease in Pn in P. quinquefolia, O. taihangensis and S. officinalis was mainly caused by stomatal limitation under normal water supply conditions (CK) and mild water stress (W₁), and by non-stomatal factors under moderate water stress (W₂) and severe water stress (W₃). WUE of S. spectabile and I. lactea var. chinensis had the highest under moderate water stress. WUE of P. quinquefolia, O. taihangensis and S. officinalis had the highest under mild water stress. (3) As water stress increased, initial inflorescence (F_v), maximum inflorescence (F_m), the ratio of variable to maximum (F_v/F_m), or initial inflorescence (F_v/F_o) of the five plants decreased gradually and F_o of the five plants increased gradually. This indicated the energy of PSII was transformed into a reduction in photochemical energy and additional energy was lost as heat and through fluorescence, which led to reduced photosynthesis. (4) The drought resistance of the five plants was evaluated by principal component analysis based on the average rate of change of each plant under water stress. The comprehensive drought evaluation indices of S. spectabile and S. officinalis/O. taihangensis were the highest (0.898) and lowest (0.125), respectively. The drought resistance order follows: S. spectabile > I. lactea var. chinensis > P. quinquefolia > O. taihangensis > S. officinalis. The results of cluster analysis shows S. spectabile and I. lactea var. chinensis were the most drought resistant plants, P. quinquefolia exhibited a medium level of drought resistance, and O. taihangensis and S. officinalis had poor drought resistance.