Abstract:

Aims Imbalance between light energy absorption and utilization causes winter photoinhibition in overwintering evergreens. Many studies have investigated the overwintering photoprotective strategies of temperate and boreal evergreens under freezing temperatures. However, little is known about the photoprotective mechanisms of evergreen broadleaf plants in response to low temperatures dominated by above 0 °C in the subtropical winter. This research aims to explore the photoprotective strategies of overwintering evergreen broadleaf species with different cold tolerances in the subtropical region.

Methods This study was carried out on the campus of Jinggangshan University from October 2022 to March 2023. Three evergreen broadleaf species, Photinia × fraseri (high cold resistance), Magnolia grandiflora(moderate cold resistance), and Ficus concinna (cold sensitive), which were planted more than 10 years ago, were selected. The chlorophyll fluorescence parameters of shade leaves and sun leaves were measured from the Detached leaves using Dual-PAM-100/F.

Important findings The winter photoinhibition (WPI) of photosystem II (PSII) and photosystem I (PSI), as well as the photoprotective mechanisms in shade- and sun-leaves of the three evergreen broadleaf species, displayed distinct temperature response characteristics in relation to low temperatures in winter and warming in early spring. During winter, low temperatures combined with strong light only induced slight reversible photoinhibition of PSII in sun leaves of P. × fraseri, but led to severe photoinhibition of both PSII and PSI in sun leaves of M. grandiflora and F. concinna. Furthermore, the reductions in the maximum photochemical efficiency of PSII (F_v/F_m) and the maximum fluorescence signal of P700 reaction center (P_m) in F. concinna were significantly greater than those in M. grandiflora. Low temperatures combined with strong light triggered increases in heat dissipation (NPQ) and cyclic electron flow around PSI (CEF-PSI) to safeguard PSII and PSI of the sun leaves on P. × fraseri during winter. Moreover, low temperature combined with strong light stimulated the compensatory recovery of PSII and PSI functions in sun leaves of P. × fraseri, as manifested by the photochemical quenching (q_P), and the effective quantum yields of PSII and PSI (Y(II) and Y(I)) recovering to a higher level in the warming condition of early spring (March 2023) compared to those in Autumn (October 2022). The functions of PSII and PSI in sun leaves of M. grandiflora continuously declined during winter, but it adopted a photoprotective mechanism of enhancing CEF-PSI and maintaining a relatively strong heat dissipation capacity to maintain the coordination of PSII and PSI functions. Although CEF-PSI was enhanced, the heat dissipation capacity decreased significantly in sun leaves of F. concinna during winter, and low temperatures combined with strong light caused severe damage to PSII and PSI. Low temperatures did not cause obvious photoinhibition damage to shade leaves of the three evergreen species in the subtropical region during winter. Slight reversible photoinhibition of PSI was observed in shade leaves of P. × fraseriand M. grandiflora during winter. Photinia × fraseri possessed the photoprotective mechanism of maintaining a relatively high photochemical capacity and partially closing the PSII reaction center to reduce light absorption, while M. grandiflora mainly adopted the photoprotective mechanism of enhancing CEF-PSI and heat dissipation capacity. Although low temperatures reduced the heat dissipation capacity of shade leaves of F. concinna, the functions of PSII and PSI were not significantly affected, and slight reversible photoinhibition of PSII and PSI was caused in shade leaves of F. concinna during winter. The results indicated that the degree of WPI was negatively correlated with the cold tolerance of the three evergreen broadleaf species, and it was mainly determined by the tolerance of sun leaves to low temperature with strong light in winter in subtropical region. Evergreen broadleaf plants with high cold tolerance possessed relatively strong capacities of heat dissipation and CEF-PSI, and were able to maintain the coordination of PSII and PSI functions during winter.

Key words: evergreen broadleaf plant, winter photoinhibition, photoprotective mechanism, low temperature combined with strong light, subtropical region