Effects of extreme soil water stress on photosynthetic efficiency and water consumption characteristics of Tamarix chinensis in China's Yellow River Delta

Soil moisture is a major limiting factor for plant growth on shell ridge islands in the Yellow River Delta. However, it is difficult to carry out situ experiment to study dominant plant photosynthesis physiological on the shell ridge islands under extreme soil water stress. To evaluate the adaptability of plants to light and moisture variations under extreme soil moisture conditions present on these islands, we measured photosynthetic gas exchange process, chlorophyll fluorescence, and stem sap flow variables for 3-year-old trees of Tamarix chinensis Lour, a restoration species on these islands, subjected to three types of soil water levels: waterlogging stress (WS), alternating dry-wet (WD), and severe drought stress (SS) to inform decisions on its planting and management on shell ridge islands. Gas exchange, chlorophyll fluorescence, and stem sap flow in T. chinensis were then measured. Net photosynthetic rate (P-N), transpiration rate (E), and water use efficiency (W-UE) were similar under WS and alternating dry-wet conditions, but their mean E and W-UE differed significantly (P < 0.05). Under SS, the P-N, E and W-UE of T. chinensis leaves varied slightly, and mean P-N, E and W-UE were all low. Apparent quantum efficiency (A(QY)), light compensation point (L-CP), light saturation point (L-SP), and maximum net photosynthetic rate (P-Nmax) of leaves were not significantly different (P > 0.05) under WS and dry-wet conditions; however, under extreme drought stress, compared with the dry-wet conditions, L-CP was higher, L-SP was lower, and A(QY) and P-Nmax were both at the lowest level. Therefore, drought stress weakened light adaptability of leaves, and the efficiency of light transformation was poorer. (3) Maximum photochemical efficiency (F-v/F-m) and the actual photochemical efficiency (Phi(PSII)) were similar under waterlogged stress and dry-wet conditions, indicating a similar healthy photosynthetic apparatus and photosynthetic reaction center activity, respectively. Under SS, F-v/F-m was 0.631, and the coefficient of non-photochemical quenching (N-PQ) was 0.814, which indicated that while the photosynthetic mechanism was damaged, the absorbed light energy was mainly dissipated in the form of heat, and the potential photosynthetic productivity was significantly reduced. The daily cumulants of sap flow of T. chinensis under dry-wet alternation and severe drought stress were 22.25 and 63.97% higher, respectively, than under waterlogging stress. Daily changes in sap flow velocity for T. chinensis differed under the three soil water levels. Stem sap flow was weak at night under severe drought stress. Under dry-wet alternation, daytime average stem sap flow velocity was the highest, and night stem flow accounted for 10.26% of the day cumulants, while under waterlogged stress, the average nightly stem flow velocity was the highest, accounting for 31.82% of the day cumulants. These results provide important information for regional vegetation restoration and ecological reconstruction.