Does Leaf Rolling Serve as a Phenotype Index for Drought Tolerance in Grasses? A Review

Leaf rolling is a distinct morphological response to drought stress observed in grasses, including major cereal crops such as rice, wheat, and maize. While parameters associated with leaf rolling are often proposed as potential criteria for screening and selecting drought-tolerant genotypes, their efficacy and reliability as definitive indicators of drought tolerance remain subjects of ongoing debate. In this review, we first summarize common patterns of leaf rolling and provide a critical overview of quantification methods, ranging from subjective visual scoring and simple indices like the leaf rolling index to objective morpho-geometric measurements, advanced image analysis techniques, and integrated or remote sensing approaches. We also summarize the significant inter- and intraspecific variations observed. Subsequently, we delve into the physiological mechanisms linking leaf rolling to both avoidance and tolerance. Recent evidence suggests a tight temporal correlation between the initiation of visible rolling and critical physiological thresholds, such as the bulk leaf turgor loss point and substantial stomatal closure. This supports the hypothesis that bulliform cells might act not only as the effectors executing the rolling movement but also as highly sensitive physiological sensors monitoring internal water status. Furthermore, the initiation phase of rolling—potentially governed by the sensory function of bulliform cells—may provide novel insights into physiological tolerance, even if the extent of rolling remains an equivocal trait for breeding selection. Finally, potential future research directions stemming from this analysis are also discussed.