Cellular integrity beyond xylem embolism underpins drought tolerance in zoysiagrasses

Drought tolerance is a critical component contributing to the overall drought resistance of grasses. Still, our understanding of tolerance mechanisms in this group remains limited. We sought to investigate the mechanisms contributing to drought tolerance in zoysiagrasses, an economically important group of turfgrass cultivated worldwide. Experiments were performed using four cultivars with contrasting tolerances. Declines in the integrity of leaf cells and photochemistry preceded the initiation of leaf embolism in drought-susceptible, but not in drought-tolerant, cultivars. Lobo, the most drought-tolerant cultivar, experienced cellular and photochemical damage only after extensive embolism. Empire, the most drought-susceptible cultivar, experienced considerable damage to cells and the photosynthetic apparatus before, or soon after, the initiation of embolism. The occurrence of cell damage independently of xylem embolism in some of these grasses contrasts with the typical pattern observed in woody species. Leaves of all zoysiagrasses were equally highly resistant to embolism, with large estimated stomatal safety margins. Although embolism resistance did not explain differences in tolerance among these cultivars, we cannot rule out the importance of such highly resistant xylem to their overall drought tolerance. These results describe critical tolerance mechanisms in grasses and have important implications for breeding programs improving drought resistance of cultivated grasses.