Artificial photosynthesis is highly desired to enhance natural photosynthesis, but it still faces enormous challenges stemming from the sluggish water oxidation process. Herein, biocompatible g-C₃N₄ nanosheets decorated by ultra-small Ru-Oxo clusters (Ru-COS-CN) were synthesized using tailored chitosan oligomers, and were applied to foliar spray for promoting photosynthesis of tobacco. The tobacco growth exhibited a positive correlation with the water oxidation performance of the photocatalysts, leading to the growth promotion of 36.1%, 36.8%, 27.4%, and 15.9% per 10 days for average fresh weight, dry weight, plant length, and leaf area with Ru-COS-CN treatment, respectively. The promotion is attributed to the produced long-lifetime photoelectrons from the utilized light range extension and the accelerated photocatalytic water oxidation by means of in situ µs-transient absorption spectra, and to the effective electron transfer into the chloroplasts with an enhanced electron transfer rate in the electron transport chain based on the results of Hill reaction.