Effects of the interaction between soil water content and nitrogen fertilisation (W×N) on (a) leaf net photosyn-thetic rates (Pn), (b) SPAD-502 leaf chlorophyll meter reading (SPAD), and (c) stable carbon isotope signatures
Our objective was to identify combinations of irrigation and fertiliser application that promoted the photosynthetic productivity and water use efficiency (WUE) of Catalpa bungei. Using response surface methodology based on a rotatable central composite design, we conducted a pot experiment with C. Bungei clone 004-1 seedlings. The experiment quantified the effects of interactions between soil water content (W), nitrogen application rate (N), and phosphorus application rate (P) on leaf net photosynthetic rates (Pn), SPAD values (SPAD-502 meter readings of leaf chlorophyll), and carbon isotope composition (δ13C). N had significantly positive effects on the all three leaf parameters, while W had significantly negative effects on leaf δ13C signatures but positive effects on leaf Pn and SPAD values. The magnitude of the N effect exceeded that of the W effect. P had no significant effects on any of the three leaf parameters. The W× N interaction had significant positive effects on Pn and SPAD, but significant negative effects on leaf δ13C. Thus, irrigation and nitrogen application had different effects on seedling growth and WUE, and the two effects were interactive. Catalpa bungei growth was promoted by appropriately increasing nitrogen application and irrigation, but WUE was improved by increasing N and decreasing irrigation. Therefore, strategies for the irrigation and fertilisation of C. bungei plantations should be designed to appropriately balance plant growth and WUE.
soil water and fertilizer, net photosynthetic rate, SPAD, δ13C, Catalpa bungei