Long-term high temperature stress decreases the photosynthetic capacity and induces irreversible damage in chrysanthemum seedlings

To study the effects of long-term and short-term high temperature stress and recovery on the physiological functions and appearance quality of chrysanthemums, a controlled experiment with chrysanthemums was conducted. The treatments were 25 °C for 3 days (T1D3), 25 °C for 9 days (T1D9), 41 °C for 3 days (T2D3) and 41°C for 9 days (T2D9). The results indicated that there is no significant difference between the T1D3 and T1D9 groups. Conversely, the total chlorophyll content (Chl), net photosynthetic rate (PN), and maximum quantum yield of Photosystem II (PSII) (FV/FM) under T2D3 and T2D9 decreased by 27.07%, 43.30%, 5.62%, and 44.85%, 68.22%, 8.29%, respectively. The JIP-test results showed that the T2D9-stressed plants had a lower efficiency and functional antenna size, and a higher activity of the reaction centre than T2D3. The contents of malondialdehyde, soluble protein and proline increased by 3.67 nmol/g FM, 298.75 μg/g, and 192.99 μg/g, and the antioxidant enzymes activities were inhibited significantly under T2D9. After the stress was relieved, Chl, PN, and FV/FM under T2D3 recovered to the same level as T1D3, while T2D9 did not. Furthermore, the diameter of the flowers in T2D3 showed no significant difference with the chrysanthemums under T1D3. However, the plants in T2D9 recovered poorly. Both the diameter of the flowers and the anthocyanin under T2D9 reduced significantly comparing with T1D9, indicating that the damage in the chrysanthemum seedlings caused by long-term high temperature was irreversible.