Abstract: With 2-year pot-grown grafted Citrus cv. Olinda as test material, this paper studied the effects of Apatococcus lobatus parasitization on its leaf photosynthesis characteristics. No significant differences were observed between slightly affected and control leaves in their total chlorophyll (Chl a+b) and carotenoid (Car) contents, net photosynthetic rate (P_n), intercellular CO₂ concentration (C_i), primary maximum photochemical efficiency of PS Ⅱ (F_v/F_m), quantum efficiency of noncyclic electron transport of PS Ⅱ (ΦPS Ⅱ), and photochemical quenching (qP), but for medium and severely affected leaves, their Chla+b, Car, P_n,F_v/F_m, ΦPS Ⅱ and qP decreased by 23.85%, 26.49%, 43.3%, 4.5%, 35.1% and 22.5%, and 37.61%, 44.04%, 64.5%, 8.6%, 63.6% and 40.1%, respectively, while C_i increased, with significant differences to the control. It could be concluded that the massive parasitization of A. lobatus caused a stress to orange plant, and non-stomatal limitation was the dominating factor of P_n reduction.

Key words: Stomatal infiltration, K⁺ efflux, ABA disposal, Protection effect