Abstract: Stem photosynthesis, as a way of plant carbon sequestration, does exist but has long been neglected, which can help terrestrial plants maintain their most basic physiological activities under extreme environments. In this study, the stem photosynthesis of Populus tomentosa and Caragana microphylla were monitored through assimilation chamber combined with CO₂/H₂O infrared gas analyzer, while leaf photosynthesis was monitored by LI-6400. Combining with their stem economic and optical functional traits, the regulation of stem photosynthesis with its influencing factors was examined. The mean of total stem photosynthetic rates of branches ofP. tomentosa and C. microphylla were 0.39±0.08 and 0.20±0.12 μmol·m^-2·s^-1, as 2.31% and 2.37% of leaf photosynthetic rates, respectively. Stem photosynthesis was determined by both light interception and utilization. Light interception was affected by stem epidermal color and thickness, which determines the ability of branches in their reflectance, absorption and transmission of visible light. Light utilization was affected by stem chlorophyll a/b, C/N, and water content. In conclusion, although leaf photosynthesis is the main way to obtain plant carbon and the contribution of stem photosynthesis is only below 2.5%, stem photosynthesis can maintain stable under stressful environment, which is of great significance for the resistance and resilience of plants to extreme drought events.

Key words: stem photosynthesis, stem respiration, chlorophyll, CO₂ flux, cortical thickness, photochemical trait.