Abstract: Aims The increasing frequency and intensity of extreme climate events such as drought and heatwave,. has heightened the need to understand how plants with different photosynthetic pathways (C3 vs. C4) respond physiologically to combined heat and drought stress. However, a through understanding of their photosynthetic responses under such compound stresses, as well as the differences in their recovery mechanisms once the stress is alleviated, remains limited. Methods Using a pot-based experiment, we measured responses in photosynthetic physiology, carbon assimilation, and allocation in dominant C3 and C4 grass species from the Songnen Grassland to individual and combined extreme drought and heatwave treatments, followed by a recovery period. Important findings Under the heatwave conditions, the studied C4 grasses maintained higher leaf temperatures yet sustained high water use efficiency, whereas the C3 grasses employed transpirational cooling, at the expense of reduced water use efficiency. The imposed stress treatments promoted the accumulation of soluble sugars in the leaves of both photosynthetic types, notably, the C4 grasses exhibited the greatest increase under the combined treatment and tended to allocate more sugars to stems. Morphological and biomass allocation responses were more pronounced in the C3 grasses, while the C4 grasses displayed a relatively conservative strategy. During the recovery, the C4 grasses demonstrated significantly greater photosynthetic capacity and biomass recovery than the C3 grasses. These findings indicate that C4 grasses possess stronger adaptability and resilience under extreme climate events such as heatwaves and droughts, which can be attributed to their more efficient photosynthetic apparatus, superior water use efficiency, and more flexible resource allocation patterns.

Key words: C3/C4 grasses, heatwave, drought, photosynthetic physiology, assimilates, resilience