The lowland rainforests in Xishuangbanna of Yunnan province account for the largest proportion of tropical rainforests in China and, at 21–22°N, mark the northern limit of tropical rainforests in Asia. Dense radiation fog is frequent in the cool dry season, persisting from night until midday when the air temperature warms up. Fog has been hypothesized to play a key role in buffering chilling and drought effects, thereby allowing tropical rainforests and tropical crop production to exist in this region, but the physiological mechanisms for this hypothesis have not been well studied.

Dr. ZHANG Yongjiang, who graduated from Xishuangbananna Tropical Botanical Garden (XTBG) and is now a postdoc in Harvard University, conducted a study at XTBG(21°41′N, 101°25′E, elevation 570 m) in Mengla County, Yunnan Province, SW China. Eleven woody plant species (including three native tree species, and eight tree and shrub species introduced from lower latitudes) were selected for this study. The research focused on understanding the effects of fog on protecting leaves from chilling-induced photodamage by studying well-watered plants.

The researchers measured light-saturated net photosynthetic rate per leaf area (A_a), stomatal conductance (g_s) and intercellular CO₂ concentration (C_i) in situ using a portable photosynthetic system. They used a Dual PAM-100 to determine seasonal changes in leaf chlorophyll fluorescence and P700 redox state. The maximum quantum yield of PSII (F_v/F_m) and maximum P700 changes during quantitative transformation from the fully reduced to the fully oxidized state (P_m; indicating the amount of active PSI complex) were determined at predawn of the next morning in the laboratory. The electrolyte leakage method was used to assess the chilling sensitivities of the 11 species.

The study found that most of the 11 studied tropical plants significantly decreased their light-saturated net photosynthetic rate (A_a) in the cool dry season (November to early March), which was associated with both lower stomatal conductance and photoinhibition. However, they maintained considerably high net CO₂ assimilations. Shaded leaves had much higher quantum yield of Photosystem II than the sun-exposed leaves in the cool season, comparable to the values in the warm season.  A combination treatment of low temperature and photoinhibitory light caused much faster decline in maximum P700 changes and quantum yield of Photosystem II compared with low temperature treatment alone.

 Because fog persistence in the morning prevents leaves from experiencing high light when the temperatures are still low or at least shortened the time period with combined high light and low temperatures, fog could avoid or reduce chilling-induced photodamage to leaves. The research results provided support for the hypothesis that fog mitigated chilling-induced photodamage of plants and played a role in maintaining both tropical rainforests and tropical crop production in SW China. The researchers thus suggested that the potential impact of forest and land use management on fog persistence should be taken into account in the future.