Abstract:

With global warming， global temperatures continue to rise， extreme drought hydrological events are frequent， and they have a significant impact on ecosystems in global （especially arid and dry） regions. Haloxylon ammodendron has extremely strong tolerance to drought and high temperatures， and it is widely planted in the arid desert regions of northwest China. Exploring the adaptive characteristics of photosynthesis and water physiology in different age H. ammodendron （5， 10， 20， 30， 40 a） under extreme high temperature conditions can help predict the survival of different age H. ammodendron in the future under global. warming （extreme hightem perature）. conditions. The temperature photosynthesis response curve of H. ammodendron was measured using the LI-6800F photosynthesis-fluorescence automatic measurement system. In order to simulate the warming caused by the continuous global warming， an experimental temperature gradient of 25，27.5， 30， 32.5， 35， 37.5， 40 °C was set. The gas exchange differences between different age H. ammodendron were analyzed， and the Water potential of assimilating branches of different age H. ammodendron was measured using the 1515D pressure chamber instrument. The results Showed that： （1） There were significant differences in assimilating branch water potential of different age H. ammodendron. The water potential of 5-30 a H. ammodendron increased with increasing age， while that of 40adecreased slightly more than that of 20a H. ammodendron. （2） There are obvious differences in photosynthesis characteristics of different age H. ammodendron as temperature rises. The net photosynthesis rate （P_n） of 5-year-old H. ammodendron is the smallest throughout the entire experimental temperature range due to the most severe water stress. The P_nof 10-year-old H. ammodendron increases linearly and does not reach a peak within the experimental temperature range. The P_nof 20， 30， and 40-year-old H. ammodendron is higher and fluctuates less with increasing temperature. The P_nof different age H. ammodendron continues to increase （10-year-old）， remains stable （20- and 30-year-old）， or decreases （5- and 40-year-old） under extreme high temperature （35-40 °C）. （3） There are also significant differences in water physiological characteristics among different age H. ammodendron， especially the differences at extreme high temperature， which are closely related to the survival and development of plants. Young H. ammodendron （5- and 10-year-old） takes a more conservative water utilization strategy， The physiological characteristics of stomatal conductance （G_s）， water use efficiency （WUE） and transpiration rate （Tr） fluctuate little at extreme high temperature and maintaining a high P_n， which makes them have strong resistance to extreme high temperature environment； Old H. ammodendron （30- and 40-year-old） takes a more aggressive water utilization strategy， continuously increasing Tr at extreme high temperature， resulting in a continuous decrease in WUE， although they can maintain a high P_n. However， this may also limit tree growth in the long run and eventually lead to tree death. The gas exchange traits of H. ammodendron at different ages vary significantly under extreme high temperatures， and diverse adaptation tactics to extreme high temperatures are employed. Notably， the adaptation tactics of H. ammodendron aged （30， 40 a） are more intense. In the future， it is imperative to focus on monitoring the population of H. ammodendron aged （30， 40 a） in the light of global changes （particularly under extreme high temperatures）.

Key words: global warming, Haloxylon ammodendron, photosynthesis, water use efficiency, transpiration rate