Photosynthetic acclimation of an evergreen broadleaved shrub (Ammopiptanthus mongolicus) to seasonal climate extremes on the Alxa Plateau, a cold desert ecosystem

Key message: Survival of Ammopiptanthus mongolicus in a cold desert environment is facilitated by high photosynthesis rates in spring and summer, and efficient photoprotective strategies in winter cold. Woody evergreen plants inhabiting cold desert ecosystems must retain their foliage amidst chronically dry conditions and large seasonal temperature variations. To understand the strategies enabling survival of evergreens in these environments, we monitored seasonal changes in foliar gas exchange and photosynthetic traits of Ammopiptanthus mongolicus, an evergreen broadleaved shrub native to the cold desert of northwestern China. We found that photosynthesis was relatively higher in spring and summer and lower in fall and winter. Transitioning from spring to summer, A. mongolicus maintained high photosynthetic capacity (Amax). Transitioning into fall, the Amax and maximum stomatal conductance (gsmax) decreased, while the relative stomatal limitation to photosynthesis (Ls) increased. In winter, A. mongolicus decreased Amax, maximum quantum efficiency of photosystem II (Fv/Fm), maximum RuBisCo carboxylation rates (Vcmax), maximum RuBP regeneration rates (Jmax), and photosynthetic nitrogen-use efficiency (PNUEmax) relative to other seasons. Collectively, these results suggest that A. mongolicus adapts physiologically to maximize carbon assimilation during spring and summer, and to maximize foliar resistance to cold stress at the expense of photosynthesis in winter. Foliage was protected against photo-oxidative damage during temperature extremes in winter by dark-sustained thermal energy dissipation. Overall, our study reveals that multiple photosynthetic adjustments, varying among the seasons, enable the survival of cold desert evergreens.