Abstract: Drought is one of the external forces that has promoted the evolution of the C₄ photosynthetic pathway. It is great of significance to explore the photosynthetic evolution of the C₄ pathway under drought stress for the protection and restoration of vegetation in areas threatened by desertification. Because the Salsola genus contains different photosynthetic types, it is an ideal material to study C₄ evolution driven by drought. Most previous studies on the photosynthetic evolution of plants have focused on changes at the anatomical level, rather than on the characteristics of growth adaptation. On the basis of the phylogenetic relationships of various species in the Salsola genus, we chose Salsola abrotanoides (C₃), Salsola laricifolia (C₃-C₄) and Salsola orientalis (C₄) as the experimental materials. We analyzed growth adaptation characteristics such as leaf morphology and anatomy, seed morphology, resistance physiological indicators, and ecological chemometric indexes. The results showed that the C₃-photosynthetic type in the Salsola genus is the original ancestor, and that the C₃-C₄ photosynthetic type and the C₄ photosynthetic type have evolved from the C₃ photosynthetic type. Compared with the C₃ species S. abrotanoides, the C₃-C₄ species S. laricifolia and the C₄ species S. orientalis have gradually adapted to drought by changing their external morphology and anatomical structure, and by adjusting their seed propagation strategies by forming differently sized seeds. The high photosynthetic capacity of C₄ plants provides materials (soluble sugars) involved in regulation, and with the evolution to C₄ photosynthesis, the drought resistance capacity of the Salsola genus has been enhanced. To increase the amount of carbon fixed by photosynthesis, S. laricifolia has increased its nitrogen and phosphorus use efficiencies, while S. orientalis has increased its nitrogen and phosphorus contents. Compared with S. abrotanoides, S. laricifolia and S. orientalis are more adapted to barren environments. These experimental data provide evidence of the physio-ecological adaptations that have improved C₄ photosynthesis, consistent with evolutionary theory.

Key words: Salsola genus, drought, C₄ evolution, physiological characteristics, ecological stoichiometry