Stronger drought tolerance in C4 compared with C3 grass crops is achieved via both avoidance and resistance strategies

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Grasses comprise many of the world's major cereal crops, yet the physiological mechanisms underlying their resilience to environmental stress remain unclear. While C4 grasses tend to display higher water use efficiency than C3 grasses, little is known about the underlying physiological and structural dynamics during mild and severe dehydration. We tested the sequence of decline with dehydration in leaf, root and whole plant hydraulic functions, and in leaf gas exchange and photochemistry (Fv/Fm) across three C3 and three C4 grass crop species, along with their rehydration capacity post-dehydration. We observed distinct strategies between the C3 and C4 species in response to water stress. The three C4 species experienced their peak intrinsic water-use efficiency earlier during dehydration, displayed greater hydraulic supply relative to demand, and had higher turgor loss points. The three C4 species also showed greater resistance to photochemical damage and embolism under severe dehydration. The three C4 species had higher leaf capacitance, and experienced greater leaf shrinkage than the three C3 species. Our findings reveal that variation among photosynthetic types can be associated with contrasts in both drought avoidance and resistance traits, contributing to a broader understanding of how different photosynthetic pathways influence plant performance under stress.

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