Ultraviolet-B Radiation (UV-B) Relieves Chilling-Light-Induced PSI Photoinhibition And Accelerates The Recovery Of CO2 Assimilation In Cucumber (Cucumis sativus L.) Leaves

Abstract

Ultraviolet-B radiation (UV-B) is generally considered to negatively impact the photosynthetic apparatus and plant growth. UV-B damages PSII but does not directly influence PSI. However, PSI and PSII successively drive photosynthetic electron transfer, therefore, the interaction between these systems is unavoidable. So we speculated that UV-B could indirectly affect PSI under chilling-light conditions. To test this hypothesis, the cucumber leaves were illuminated by UV-B prior or during the chilling-light treatment, and the leaves were then transferred to 25 °C and low-light conditions for recovery. The results showed that UV-B decreased the electron transfer to PSI by inactivating the oxygen-evolving complex (OEC), thereby protecting PSI from chilling-light-induced photoinhibition. This effect advantages the recoveries of PSI and CO₂ assimilation after chilling-light stress, therefore should minimize the yield loss caused by chilling-light stress. Because sunlight consists of both UV-B and visible light, we suggest that UV-B-induced OEC inactivation is critical for chilling-light-induced PSI photoinhibition in field. Moreover, additional UV-B irradiation is an effective strategy to relieve PSI photoinhibition and yield loss in protected cultivation during winter. This study also demonstrates that minimizing the photoinhibition of PSI rather than that of PSII is essential for the chilling-light tolerance of the plant photosynthetic apparatus.

Figure 1. The influence of UV-B to PSI and PSII photoinhibition under chilling-light condition.

The activity of the PSI complex and the maximum quantum yield of PSII (Fv/Fm) in leaves exposed to chilling-light and UV-B conditions for the indicated times. In plots (a,c), the leaves were exposed to approximately 6 μmol m⁻² s⁻¹ UV-B (+UV) or darkness (−UV) at 25 °C for 2 h (shaded part), and the leaves were then transferred to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) for 6 h. In plots (b,d), the leaves were exposed to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) in the presence (+UV) and absence (−UV) of approximately 2 μmol m⁻² s⁻¹ UV-B for 6 h. The means ± SE, n = 12. Different letters indicate significant differences between +UV and −UV treated leaves at P < 0.05.

Figure 2. Immunoblot analysis of thylakoid membranes proteins extracted from leaves.

Line 1, the leaves were exposed to darkness at 25 °C for 2 h; line 2, the leaves were exposed to approximately 6 μmol m⁻² s⁻¹ UV-B at 25 °C for 2 h; line 3, the leaves were exposed to darkness at 25 °C for 2 h and the leaves were then transferred to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) for 6 h; line 4, the leaves were exposed to approximately 6 μmol m⁻² s⁻¹ UV-B at 25 °C for 2 h and the leaves were then transferred to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) for 6 h; line 5, before treatment; line 6, the leaves were exposed to chilling-light conditions for 6 h; line 7, the leaves were exposed to chilling-light conditions in the presence of approximately 2 μmol m⁻² s⁻¹ UV-B for 6 h; Thylakoid membranes (10 μg of chlorophyll) were separated by SDS-PAGE, electroblotted, and probed using specific antibodies against PsbO, PsbA, and PsaA.

Figure 3. The PSI (DCPIP-MV) and PSII (H₂O-BQ) electron transport activities of isolated thylakoid membranes in leaves exposed to chilling-light treatment and UV-B.

In plots (a,c), the leaves were exposed to approximately 6 μmol m⁻² s⁻¹ UV-B (+UV) or darkness (−UV) at 25 °C for 2 h, and the leaves were then transferred to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) for 6 h. In plots (b,d), the leaves were exposed to chilling-light (6 °C, 200 μmol m⁻² s⁻¹) in the presence (+UV) or absence (−UV) of approximately 2 μmol m⁻² s⁻¹ UV-B for 6 h. The means ± SE, n = 12. Different letters indicate significant differences between +UV and −UV treated leaves at P < 0.05.

Figure 4

The maximum fluorescence (Fm or Fm_DCMU; (a,b)) and maximum quantum yield of PSII (Fv/Fm or Fv/Fm_DCMU; (c,d)) in leaves exposed to chilling-light and UV-B conditions for the indicated times. In plots (a,c), the leaves were exposed to approximately 6 μmol m⁻² s⁻¹ UV-B (+UV) or darkness (−UV) at 25 °C for 2 h (shaded part), and the leaves were then transferred to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) for 6 h. In plots (b,d), the leaves were exposed to chilling-light conditions (6 °C, 200 μmol m⁻² s⁻¹) in the presence (+UV) or absence (−UV) of approximately 2 μmol m⁻² s⁻¹ UV-B for 6 h. After chilling-light and UV-B treatment, the leaves were infiltrated with DCMU or water in the dark for 1 h. The original Fm or the Fm measured in the presence of DCMU (Fm_DCMU) was used to calculate Fv/Fm (or Fv/Fm_DCMU). The means ± SE, n = 12. Different letters indicate significant differences between +UV and −UV treated leaves at P < 0.05.

Figure 5. UV-B during chilling-light treatment accelerated the following recovery of the activity of the PSI complex and photosynthetic rate.

The activity of the PSI complex (a) and photosynthetic rate (Pn; (b)) in leaves exposed to chilling-light conditions in the presence (+UV) or absence (−UV) of approximately 2 μmol m⁻² s⁻¹ UV-B for 6 h (shaded part) and subsequent repair process at 25 °C under low-light conditions (15 μmol m⁻² s⁻¹) for 72 h. The means ± SE, n = 12 (a) or 6 (b). Different letters indicate significant differences between +UV and −UV treated leaves at P < 0.05.

Figure 6. Scheme of UV-B protects PSI from photoinhibition.

OEC, oxygen-evolving complex; Fd, ferredoxins.

Figure 7. Spectral transmissions of the two spectral filters used in this study.

Mylar film excluding most UV-B or Aclar film characterized by high UV-B transmittance. The variation between multiple samples of identical films was negligible, and the SEs are too small to present.