Germination Shifts of C3 and C4 Species under Simulated Global Warming Scenario

Research efforts around the world have been increasingly devoted to investigating changes in C-3 and C-4 species' abundance or distribution with global warming, as they provide important insight into carbon fluxes and linked biogeochemical cycles. However, changes in the early life stage (e. g. germination) of C-3 and C-4 species in response to global warming, particularly with respect to asymmetric warming, have received less attention. We investigated germination percentage and rate of C-3 and C-4 species under asymmetric (+3/+6 degrees C at day/night) and symmetric warming (+5/+5 degrees C at day/night), simulated by alternating temperatures. A thermal time model was used to calculate germination base temperature and thermal time constant. Two additional alternating temperature regimes were used to test temperature metrics effect. The germination percentage and rate increased continuously for C-4 species, but increased and then decreased with temperature for C-3 species under both symmetric and asymmetric warming. Compared to asymmetric warming, symmetric warming significantly overestimated the speed of germination percentage change with temperature for C-4 species. Among the temperature metrics (minimum, maximum, diurnal temperature range and average temperature), maximum temperature was most correlated with germination of C-4 species. Our results indicate that global warming may favour germination of C-4 species, at least for the C-4 species studied in this work. The divergent effects of asymmetric and symmetric warming on plant germination also deserve more attention in future studies.