Thermonastic leaf movements in Rhododendron during freeze-thaw events: Patterns, functional significances, and causes

- Thermonastic leaf rolling and drooping are considered in relation to cold hardiness.
- The magnitude of leaf rolling is positively correlated with plant cold hardiness.
- Thermonastic leaf movements minimize photo-damage during freezing winter conditions.
- Rhododendron species without leaf movements have enhanced photo-protection.
- Opposing longitudinal and transverse forces together cause thermonastic leaf rolling.

Adaptations to freezing air temperature in temperate understory species of Rhododendron include physiological processes, anatomical changes, and thermonastic leaf movements. Leaves roll transversely and leaf-lamina angle decreases in relation to horizontal as temperatures decrease below a critical freezing temperature. Within the genus Rhododendron, tolerance of cold conditions is greater in species with thermonastic leaf movements than species without. The leaf movements protect critical physiological processes such as photosynthesis from damage due to the synergistic effects of cold temperatures, and high light common to winter conditions in temperate forests. In particular, the absence of these leaf movements increases photoinhibition, and species that lack these adaptations exhibit distinctly different physiological and anatomical mechanisms of photo-protection during cold conditions. The biomechanical or physiological causes for thermonastic leaf movements have been difficult to resolve because of the lack of distinctive anatomical and morphological features associated with these leaf movements. Nevertheless, it is firmly established that the lower the petiole turgor potential the lower the leaf-lamina angle in relation to horizontal. However, the cause of leaf rolling is unclear. In this study, experiments on sectioned leaves implicate both longitudinal and lateral thermonastic rolling forces, likely driven by water redistribution between apoplast and symplast, and regulated by aquaporins. This should result in abaxial-adaxial differential turgor pressures that vary markedly along the mediolateral direction. We postulate that the combined effect of the leaf morphology, anisotropy in rolling forces and the geometrical constraints due to the relatively stiff midrib causes leaf rolling, consistent with the mechanics of a thin plate with anisotropic spontaneous curvature. We expect a correlation between the rolling forces and veination microstructure that remains to be explored for a complete understanding of leaf adaptations to freezing in understory evergreen Rhododendron species, as well as more general thermonastic responses.