Abstract: We analyzed the growth, leaf chlorophyll content, and photosynthetic parameters of a tomato leaf yellowing mutant (Y55) induced by ethyl methane sulfonate (EMS) from the cultivar “Heinz 1706” (WT). Results showed that the plant height, stem diameter, and fresh mass of Y55 significantly . The contents of chlorophyll a, chlorophyll b, carotenoid, total chlorophyll and the chlorophyll a/b ratio of the mutant were significantly lower than those of WT. The contents of all precursor materials of chlorophyll synthesis, especially porphyrinogen III and those involved in the chlorophyll biosynthesis pathway, were significantly lower in Y55 than those in WT. Moreover, the net photosynthesis (P_n), transpiration rate (T_r), intercellular CO₂ concentration (C_i), and conductance to H₂O (g_s) significantly in Y55. The maximum photosynthetic rate, CO₂ saturation and compensation point, and light saturation and compensation point. The F_v/F_m significantly, whereas the F_o significantly in Y55. The photosynthetic electron production and electron transport rates of PSII and PSI also significantly decreased. The total photosynthetic pigment molecules (N_o) and the minimum average lifetime of photosynthetic pigment molecules in the excited state (τ_min) significantly in Y55. All these results suggest that blocking the synthesis of porphyrinogen III ould decrease the chlorophyll content in the mutant Y55. Furthermore, the reduced amount of leaf pigment could affect photosynthesis in leaves and slow down the growth of mutant plants.