Effects of Late Spring Coldness and Recovery Light Intensity on Photosystem II and Photosystem I Function in Mosla chinensis Maxim. cv. Jiangxiangru Seedlings

Late spring coldness (LSC) threatens the spring cultivation of the geo-authentic (Daodi) medicinal herb Mosla chinensis Maxim. cv. Jiangxiangru (McJXR). This study examined the effects of LSC and subsequent recovery light intensity on photosystem II (PSII) and photosystem I (PSI) function in McJXR seedlings. The experiment was conducted during a natural LSC event in the spring of 2025. After LSC, seedlings were subjected to two recovery light regimes: the shading (RLL, daily maximum light intensity of approximately 500 µmol·m−2·s−1) and the full-sunlight (RHL, daily maximum light intensity of approximately 1100 µmol·m−2·s−1) treatments. Chlorophyll fluorescence parameters of PSII and PSI were measured synchronously at various stages using a Dual-PAM-100/F. The results showed that during the LSC period, the maximum photochemical efficiency of PSII (Fv/Fm) and the maximum P700 signal (Pm) showed no significant changes relative to the pre-stress levels (LSC-0d). By contrast, the photochemical quantum yields of PSII (Y(II)) and PSI (Y(I)) declined continuously, decreasing by 61.0% and 48.6%, respectively, by the fourth day of LSC (LSC-4d) compared with the LSC-0d levels. On the first day of LSC (LSC-1d), the photochemical quenching coefficient (qP), the maximum electron transport rate through PSII (Jmax-ETR(II)), and that through PSI (Jmax-ETR(I)) dropped by 51.9%, 70.1%, and 61.1%, respectively, and subsequently remained stable, non-photochemical quenching (NPQ) and the quantum yield of regulated energy dissipation in PSII (Y(NPQ)) increased sharply to 4.4-fold and 4.9-fold of the LSC-0d levels and remained elevated thereafter. Cyclic electron flow (CEF) showed no significant change during the LSC period. During recovery, Fv/Fm decreased under both shading and full sunlight, with a greater decline under full sunlight. However, the recovery of Y(II), qP, Jmax-ETR(II), Y(I), and Jmax-ETR(I) was greater under full sunlight than under shading, and CEF increased markedly under both treatments. By the third day of recovery, Y(I) had fully recovered to the LSC-0d level under both treatments, whereas Y(II) had not. In conclusion, the natural LSC event did not induce photoinhibition of either photosystem, but only a reversible downregulation of their photochemical activity, which was more pronounced for PSII than for PSI. The seedlings relied on photoprotective pathways, including downregulation of PSII activity and enhanced thermal dissipation, to protect the photosystems from irreversible photodamage under LSC. During recovery, the full-sunlight treatment accelerated the recovery of the photochemical activity of both photosystems relative to the shading treatment, although it also increased the degree of PSII photoinhibition; the recovery of PSI was superior to that of PSII, and the rapid decline in thermal dissipation together with the marked increase in cyclic electron flow during recovery may explain this difference. These findings contribute to understanding the responses of the two photosystems of McJXR seedlings to LSC stress.