The twospotted spider mite Tetranychus urticae Koch is a common pest on soybean plants. To clarify plantarthropod interaction on mite-soybean system, leaf fluorescence, photosynthetic responses to variable carbon dioxide levels, and chlorophyll content were evaluated. Significant photosynthetic rate reduction was observed due to stomatal limitation. Stomatal closure was the major plant physiological response. As a consequence, there was reduction in photosynthetic rates. Surprisingly, plants did not show chlorophyll content reduction associated with photosynthetic impairment. No differences in fluorescence data indicate that T. urticae injury did not impair the function of light harvesting and photoelectron transport. These results showed that T. urticae could be a serious pest of soybean even on lower infestation, at least when photosynthesis was determinant to yield
