A Comparative Study between Evergreen and Deciduous Daylily Species Reveals the Potential Contributions of Winter Shoot Growth and Leaf Freezing Tolerance to Foliar Habits

Evergreen ornamental plants can supply year-round aesthetic value and strong ecological benefits in comparison to deciduous species. However, less attention has been paid to the cause of foliar habits, especially in herbaceous perennials. A comparative study on evergreen and deciduous species with close relationships could help to uncover the factors that determine foliar habits when sibling mutants are absent. Three independent experiments were conducted in the present study. Experiment 1 was focused on the changes in the percentage of green leaves (PGL) between evergreen daylilies (Hemerocallis aurantiaca Baker) and the deciduous species (Hemerocallis fulva L.). The results showed that their foliar habits were consistent with those of previous reports but could be influenced dramatically by the air temperature. The leaf freezing tolerance (LFT) and relevant morphological, physiological and biochemical variations in leaf tissues in addition to the shoot growth status were determined during Experiments 2 and 3 when comparing natural cold acclimation to de-acclimation. Correlation analyses between the PGL and other indices indicated that the winter shoot growth probably maintained the green leaves of the evergreen daylilies, while a strong LFT might facilitate the defoliation of deciduous species. Furthermore, the dormant deciduous daylily is more freezing-tolerant than the ever-growing evergreen species. This study also possesses practical value for plants with variant green periods in landscaping.