Effects of High Temperature Stress on the Physiological and Biochemical Characteristics of Paeonia ostii.

In order to explore the effects of high temperature stress on the physiological characteristics of Paeonia ostii, the Paeonia ostii were subjected to 25 °C, 35 °C, 38 °C, and 40 °C for 7 days. Meanwhile, the physiological indicators of oxidative stress (hydrogen peroxide, H2O2; malondialdehyde, MDA; relative electrical conductivity, REC), antioxidant enzyme activity (superoxide dismutase, SOD; ascorbate peroxidase, APX; catalase, CAT; peroxidase, POD), photosynthetic pigment content (chlorophyll a, Chla; chlorophyll b, Chlb), photosynthetic characteristics (net photosynthetic rate, Pn; intercellular CO2 concentration, Ci; stomatal conductance, Gs; transpiration rate, Tr), and osmoregulatory substances content (soluble protein, SP; soluble sugar, SS) were determined. The results showed that, with the increase in temperature and stress time, the H2O2 content, MDA content, REC value, CAT activity, and APX activity increased, while Chla content, Chlb content, SS content, and SP content decreased. With the extension of stress time, the SOD activity, POD activity, and Tr value of each high temperature stress group first increased and then decreased; Ci first decreased, then increased, and then decreased; meanwhile, Pn and Gs showed an overall downward trend. PLS-DA (partial least squares discriminant analysis) was used to analyze the changes in physiological and biochemical indexes of peony leaves under 40 °C stress for different days. SOD was found to be the biggest factor affecting the changes in physiological and biochemical indexes of peony leaves treated with different days of stress.

Photosynthetic and physiological responses of different peony cultivars to high temperature.

In order to investigate the causes of the differences in heat tolerance ('Lu He Hong' and 'Zhi Hong'), we studied the physiological changes, photosynthetic properties and regulatory mechanism of the two peony cultivars at high temperature. The results showed that the physiological changed of different peony cultivars varied significantly under high temperature stress. With the extension of high temperature stress time, MDA content of 'Lu He Hong' increased,while 'Zhi Hong' rised first and then decreased, SOD activity of 'Lu He Hong' rised first and then decreased, that of 'Zhi Hong' kept rising, POD activity of 'Lu He Hong' kept decreasing, while 'Zhi Hong' rised. The photosynthetic instrument records the change of peony photosynthesis parameters at high temperature; the chlorophyll A (Chla) fluorescence transient is recorded using the plant efficiency analyzer (PEA), analyzed according to the JIP test (O-J-I-P fluorescence transient analysis), and several parameters were derived to explain the photosynthetic efficiency difference between different peony cultivars. The tested cultivars responded differently to the survey conditions, and the PCA analysis showed that the 'Zhi Hong' was more well tolerated and showed better thermal stability of the PSII. The reduced efficiency of the 'Lu He Hong' PSII antenna leads to higher heat dissipation values to increase the light energy absorbed by unit reaction center (ABS/RC), the energy captured by unit reaction center (TR0/RC), and the energy dissipated by unit reaction center (DI0/RC), which significantly leads to its lower total photosynthetic performance (PItotal). The light capture complex of the variety 'Zhi Hong' has high connectivity with its reaction center, less damage to OEC activity, and better stability of the PSII system. The results show that 'Zhi Hong' improves heat resistance by stabilizing the cell membrane, a strong antioxidant system, as well as a more stable photosynthetic system. The results of this study provide a theoretical basis for the screening of heat-resistant peonies suitable for cultivation in Jiangnan area and for the selection and breeding of heat-resistant cultivars.