This research aimed to study leaf anatomy in the widely common Brazilian weed species Bidens pilosa, Emilia sonchifolia, Ageratum conyzoides and Sonchus asper, to acquire a better understanding of the barriers each species imposes upon herbicide penetration, and provide information on how to overcome these obstacles. The completely expanded leaves from the third to the fifth nodes were collected from plants spontaneously occurring in the field. From the leaves of each species, three samples with approximately 1 cm² were taken at the central portion and used for structure, and clarification studies and scanning-electron microscope (SEM) observations. All species were found to be amphistomatic. The main leaf barriers against herbicide penetration in B. pilosa were found to be high trichome density and low stomatic density in the adaxial side. High content of epicuticular wax, high cuticle thickness in the adaxial side and low stomatic density on both sides were the herbicide barriers observed in the leaves of E. sonchifolia, while in A. conyzoides the barrier was low stomatic density in the adaxial side. S. asper's main leaf barriers were low stomatic density in the adaxial side and high epiderm thickness in the adaxial side.
cuticle; epicuticular wax; stomata; trichome; vascular bunches
