Photosynthetic response of Bothriochloa ischaemum (L.) Keng to drought and short-term rewatering when intercropped with Lespedeza davurica (Laxm.) Schindl. in a pot experiment

To clarify photosynthetic characteristics and their relationship with soil water conditions of Bothriochloa ischaemum (L.) Keng intercropped with Lespedeza davurica (Laxm.) Schindl. in the loess hilly-gully region, a pot experiment was conducted in the weather enclosure of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau. B. ischaemum (B) was intercropped with L. davurica (D) in six mixture sowing ratios of 2︰10, 4︰8, 6︰6, 8︰4, 10︰2, 12︰0), and these combinations were marked as B2D10, B4D6, B6D6, B8D4, B10D2 and B12D0, respectively. Three soil water conditions were set in the experiment, which were high water (80% field capacity, HW), middle water (60% field capacity, MW) and low water (40% field capacity, LW). Additionally, during the elongation stage of B. ischaemum, soil water content was improved separately from MW to HW, LW to HW and MW, respectively, i.e. middle water to high water (MHW), low water to high water (LHW) and middle water (LMW). Altogether, there were 36 treatments in the experiment, and each was replicated three times. Photosynthetic characteristics, net photosynthetic rate (P_n), transpirationrate (T_r), stomatal conductance (G_s), water use efficiency (WUE) of B. ischaemum were measured after soil was rewatered. Results showed that under relative stable water conditions (i.e. HW, MW, and LW), photosynthetic parameters of B. ischaemum were significantly highest under HW treatment, and significantly lowest under LW treatment for all mixture sowing ratios, indicating water was the key factor affecting B. ischaemum photosynthesis. After re-watering, the largest and smallest increases of photosynthetic parameters of B. ischaemum appeared under LHW and LMW respectively. For all mixture sowing ratios, photosynthetic activity of B. ischaemum under MHW recovered to stable level in the shortest time, suggesting that higher water content in early stage and higher rewatering level benefited B. ischaemum photosynthesis improvement. B. ischaemum presented not only higherP_n, lower T_r and higher WUE under B10D2 and B8D4 treatment, but also greater increase in P_n and WUE and smaller increase in T_r after rewatered, implying that the two mixture sowing ratios were beneficial for improving the photosynthesis efficiency of B. ischaemum. The research results suggested that photo-physiological activities of B. ischaemum was recovered after re-watering from short-term drought stress, and the recovery speed and level were correlated with mixture sowing ratio with L. davurica and soil water content before re-watering.