High relative humidity mitigates the adverse effects of excessive light by protecting photosynthetic machinery in flowering Chinese cabbage.

Increasing canopy relative air humidity (RH) facilitates the resistance of flowering Chinese cabbage plants against excessive light exposure in a plant factory with artificial lighting (PFAL), thereby completely inhibiting leaf burn occurrence. To clarify this high RH-mediated resistance mechanism, we further analyzed the transcriptomes, gas exchange parameters, and chlorophyll fluorescence of flowering Chinese cabbage plants subjected to two levels of canopy RH (70% and 90%). Transcriptomic data revealed a significant enrichment of photosynthesis antenna proteins pathway, which was notably downregulated in both the cotyledons and the first true leaves of the plants grown at 70% RH from the 10th to the 14th day after sowing. However, the downregulation of the photosynthesis-antenna proteins pathway in the first true leaves was notably attenuated by increasing the RH from 70% to 90%, consequently inhibiting the down-regulated expression of all light-harvesting complex I- and II-subunit-encoding mRNAs. The 70% RH-treated seedlings exhibited serious photoinhibition, as indicated by relatively lower non-photochemical quenching (NPQ), maximal quantum yield of photosystem I and II, and adaptation of the photosynthetic apparatus to high irradiance. The 90% RH treatment greatly lessened the reduction of NPQ and relative electron transport rates, which favored dissipating the excess excited energy, protected the photosynthetic apparatus against photodamage, and ultimately completely inhibited leaf burn occurrence. Overall, our results indicate that high canopy RH could improve the resistance of plants to intense light by maintaining efficient photosynthesis, thereby minimizing reliance on supplemental artificial lighting.