Tree peony is a famous ornamental plant in the world. However, little is known about the leaf color mutants in tree peony. The present study monitored the physiological and photosynthetic properties of a yellow leaf mutant (yl1) in tree peony. The results showed that the yl1 mutant had lower pigment contents, but increased chlorophyll (Chl) a/b and carotenoids to Chl ratio. Microstructure and ultrastructure analysis showed that the yl1 mutant had smaller chloroplasts, few thylakoid stacks, and a few stroma thylakoid membranes remained along with clusters of osmiophilic granules, which might result from inhibition at the reactions from coproporphyrinogen III to protoporphyrin IX. The yl1 mutant had lower leaf net photosynthetic rate, stomatal conductance, transpiration rate, and stomata limitation value, but higher intercellular CO2 concentration. Analysis of simple sequence repeat markers indicated that four pairs of primers could obtain different bands in the genome of the yl1 mutant compared to the wild type.

In order to explore the adaptive strategies of tree peony yl1 mutant in response to photooxidative stress, the changes of biochemical parameters, chlorophyll fluorescence, and the modulated 820-nm reflection of yl1 mutant and wildtype plants were compared. We found the activities of superoxide dismutase and catalase, the contents of soluble protein, proline, superoxide radicals, hydrogen peroxide, malondialdehyde, and relative electrical conductivity were significantly higher than those of the wild type. The photochemical efficiency of PSII significantly decreased, while the proportion of heat dissipation of yl1 mutant increased greatly. The donor and acceptor side and electron transfer of PSII were greatly inhibited. Meanwhile, the performance of PSI and coordination of two photosystems decreased markedly in yl1 mutant. The yl1 mutant could reduce the damage caused by photooxidative stress by increasing the activity of antioxidant enzymes, the content of some osmoregulatory substances, and the proportion of heat dissipation.studying the mechanism of photosynthesis, the structure and function of the photosystems, and Chl biosynthesis, and are valuable for cultivating excellent varieties and molecular marker-assisted breeding (Zhang et al. 2020). Leaf-color mutants with yellow leaves, variegated leaves, stripes, and other characteristics have a high ornamental value in landscaping. Among them, a yellow-leaf mutant is a common mutation type. The paulownia (Paulownia fortnnei) mutant with white flowers presented light green leaves at the seedling stage, they turned yellow at the mature stage, and had a certain ornamental value (Ru et al. Highlights• Higher photooxidative damage was reported in tree peony yl1 mutant compared to the wild type. • The function and coordination of two photosystems in yl1 mutant decreased significantly. • The yl1 mutant increased the contents of antioxidants and osmoregulants and heat dissipation capacity in response to photooxidative stress.