The alpine homoploid hybrid Pinus densata has greater cold photosynthesis tolerance than its progenitors

The diploid hybrid tree species Pinus densata inhabits high-altitude regions, usually at higher altitudes than its parental species, P. tabulaeformis and P. yunnanensis. Its success in alpine habitat has been hypothetically attributed to enhanced cold tolerance. We tested this hypothesis by subjecting all three species to cold stress for 33 days (12/5 °C, day/night) and measuring changes in their photosynthetic function. Significant decreases in net photosynthetic rate (Pn), stomatal conductance (gs) and transpiration rate (E) were observed in all three species in response to cold stress. The depression of Pn was primarily attributable to stomatal closure in P. densata, but to both stomatal and non-stomatal limitations in its progenitors, as the parental species had increased intercellular CO2 concentrations (Ci) under the stress. Significant decreases in chlorophyll fluorescence parameters were also observed in both parental species, but they varied little throughout the experiment in P. densata. Furthermore, the chlorophyll and carotenoid concentrations decreased significantly under the cold stress, while anthocyanin concentrations increased, and the anthocyanin concentration in P. densata was significantly higher than in either parental species at the end of the exposure. Finally, P. densata recovered more quickly than its parental species when the cold treatment ended. These results show that P. densata maintains a high photosynthetic rate and exhibits elevated PSII activity and anthocyanin levels under cold stress. These adaptations suggest that P. densata is more cold-tolerant than its parental species and may have been important for its colonization of high alpine habitats.

► Pinus densata is an infrequently diploid hybrid tree species in the alpine region.
► We test whether P. densata enhances cold tolerance compare to its progenitors.
► P. densata showed high photosynthetic rate and elevated PSII activity.
► P. densata also exhibited high anthocyanin levels during cold stress.