Relationships between leaf physiognomy and sensitivity of photosynthetic processes to freezing for subtropical evergreen woody plants

Subtropical and tropical species in high altitude suffer from low temperature more frequently than those from temperate regions. Chlorophyll fluorescence analysis can measure the primary photochemical processes of photosystem II (PSII) and help evaluate the sensitivity of evergreen woody plants to low temperature. Coupled with leaf physiognomy, it has allowed to examine the potential thermal regulation of evergreens in response to extreme coldness. The leaf physiognomy (length, width, thickness and ratio of length/width) and chlorophyll a fluorescence (Fv/Fm, maximum potential photochemical efficiency of PSII; NPQ, non-photochemical quenching of chlorophyll fluorescence; and Y(II), effective photochemical quantum yield of photosystem II) under natural freezing and recovery conditions of nine evergreen woody trees were measured to analyze their relationships. Results showed that the changes of Fv/Fm under freezing versus recovery had a positive relationship with leaf length and width, while a negative relationship with leaf thickness. Similar to leaf size, leaf shape also influenced the photoinhibition levels of evergreens by regulating the leaf boundary layer thickness. Leaves with an oval-like shape suffered less from freezing than leaves with a lanceolate-like shape. A relatively weaker relationship between NPQ and Y(II) was found at freezing than after recovery for species with larger and lanceolate-like leaves. Our findings are helpful to understand the adaptation strategy of evergreen woody species to extreme low temperature in subtropical areas and to provide guidance for the management of evergreen plants introduced in botanical gardens.