Photosynthetic and Physiological Responses to Combined Drought and Low–Temperature Stress in Poa annua Seedlings from Different Provenances

Combined drought and low–temperature stress is a crucial factor affecting turfgrass establishment and limiting the sustainability of the turfgrass industry in drought– and cold–prone regions. In this context, we evaluated the effects of regular watering (the soil water content was 80% of the maximum water–holding capacity of the field) at room temperature (25 °C) and combined drought (the soil water content was 30% of the maximum water–holding capacity of the field) and low–temperature (0 °C) stress on the morphology, photosynthesis, and physiology of wild Poa annua seedlings from different provenances (‘PA’, ‘WY’, ‘NX’ and ‘YC’). Results indicated that the combined drought and low–temperature stress changed the morphological and growth indicators of seedlings in four provenances to different extents. Moreover, combined drought and low–temperature stress reduced the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), water use efficiency (WUE), and chlorophyll content in seedlings from four provenances. However, intertemporal CO 2 concentration (Ci), relative electrical conductivity (REC), the contents of malondialdehyde (MDA), proline (Pro), soluble sugars (SS), the superoxide anion (O 2 •− ) production rate, the contents of hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (·OH), the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) were all increased. The increase in ‘PA’ was much greater than that in ‘NX.’ The comprehensive evaluation results showed that the order of combined drought and low–temperature resistance of seedlings from the four provenances was ‘PA’ > ‘YC’ > ‘WY’ > ‘NX’, which corresponded to the order of the morphological damage symptoms. In conclusion, ‘PA’ may maintain stronger combined drought and low–temperature resistance by improving the cellular water absorption and retention capacity, enhancing the function of the antioxidant defense system, and maintaining the integrity of the cell membrane, which is a crucial germplasm resource for breeding combined drought and low–temperature resistance in Poa annua .