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The portable chlorophyll meter (SPAD-502) has been successfully used for a rapid and direct estimation of total chlorophyll content (TCHL) in the leaves of some crops. In this work, SPAD-502 meter readings and TCHL concentration were compared for the leaves of Amaranthus vlitus L., a common weed. SPAD readings were linearly and positively correlated to TCHL concentration in the leaves. A linear correlation was also shown between SPAD-502 readings and some physiological parameters of the leaves, such as photosynthesis, transpiration, and stomatal conductance.