Effects of cold stress on leaf physiological characteristics in Forsythia suspensa (Thunb.) Vahl seedlings

Forsythia suspensa (Thunb.) Vahl is a common early spring flower and bulk cultivated medicinal crop. Low temperature is the main stress factor for this species in spring. To date, however, no physiological research has explored the effects of low temperature on F. suspensa growth. Here, we used the leaves of F. suspensa seedlings and detected changes in relative conductivity, malondialdehyde (MDA), proline content (Pro), soluble sugar (SS), soluble protein (SP), chlorophyll content, superoxide dismutase (SOD), peroxidase (POD) activity, and chlorophyll fluorescence parameters under four temperature conditions. We also detected chlorophyll fluorescence parameters after recovery. Results showed that low temperature stress caused an increase in relative electrical conductivity and MDA content in the leaves, which affected growth to some extent. The resistance of F. suspensa leaves to low temperature was achieved by multiple pathways. SP, SS, and SOD played major roles at the early stage of cold stress, while Pro and POD played important roles when stress intensity increased. The JIP-test showed that low temperature had a significant effect on photosynthesis, mainly reflected in the inhibition of electron transfer and damage to the receptor side. Treatment damage at 0℃ was the most serious. Low temperature stress affected the growth of F. suspensa seedlings. Low temperature stress above 4℃ affected the seedlings, but growth recovered after the stress was removed. Damage caused by low temperature stress at 0℃ was irreversible. Thus, F. suspensa should be cultivated in areas where low temperatures are less common in late spring, or cold-tolerant varieties should be selected for cultivation in areas where low temperatures are more frequent in late spring.