Abstract: Effects of extracts, varying in concentration (4, 8, 12, 16, 20mg•L^-1) from Oryza sativa and Thalia dealbata straws inhibiting growth of Spriogyra sp. were studied. Results show that the extracts in all the treatments displayed inhibitory effects on Spirogyra sp.after 96h of exposure, with content of chlorophyll in Spriogyra sp. cells decreasing with the increasing of extract concentration, demonstrating a significant dose-effect relationship. The 96h,EC₅₀ of T. dealbata extract was 6.85mg•L^-1 and of O. sativa extract, 11.27mg•L^-1, obviously showing that the former is higher than the latter in toxicity to Spriogyra sp.. Extremely significant positive relationships were observed between of MDA content in Spriogyra sp. cells and concentration of O. sativa extract (P＜0.01,r=0.977), and between MDA content and concentration of T. dealbata extract (P＜0.01,r=0.964), too. MDA contents were higher in T. dealbata treatments than in O. sativa treatments, regardless of extract concentration. Among all the treatments, the one of T. dealbata extract, 20 mg•L^-1 in concentration, had the highest MDA content, reaching  1.884μmol•g^-1 or being 105.20% higher than that in the treatment of O. sativa extract the same in concentration. A similar trend was also observed with electrolyte exosmosis (R_EL), that is to say, R_EL is higher in all the T. dealbata treatments than in their corresponding O. sativa treatments, except for the treatments 0 or 4 mg•L^-1 in extract concentration. The PSⅡ maximal photochemical efficiency (F_v/F_m), the PSⅡactual photochemical efficiency (Φ_PSⅡ) and the maximal relative electron transport rate (η_e,t,max) of Spriogyra sp. were found to be negatively related to concentration of the extracts. In the treatments of O. sativa extract, F_v/F_m of Spriogyra sp. was 96.58%, Φ_PSⅡ,96.74% lower and η_e,t,max, 84.56% lower in the treatment, 20 mg•L^-1 in concentration than in that 0 mg•L^-1 in concentration, while in the treatments of T. dealbata extract F_v/F_m was 82.24% lower, Φ_PSⅡ,96.17% lower and η_e,t,max, 43.40% lower in the former than in the latter. Alll the above findings suggest that the extracts of the straws of the two plants have some potential value in controlling growth of Spriogyra sp. in water environment.

Key words: plant straw extract, Spirogyra sp., inhibitory effects, chlorophyll fluorescence parameter