Agronomy and Horticulture Department



Herbicides That Act through Photosynthesis

Date of this Version


Document Type



Plant and Soil Sciences eLibrary (PASSeL) Lesson


Copyright © 2003 John Markwell and Deana Namuth. Used by permission.

Peer reviewed web lesson JNRLSE approved 2003

This project was supported in part by the National Research Initiative Competitive Grants CAP project 2011-68002-30029 from the USDA National Institute of Food and Agriculture, administered by the University of California-Davis and by the National Science Foundation (NSF), Division of Undergraduate Education, National SMETE Digital Library Program, Award #0938034, administered by the University of Nebraska. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the USDA or NSF.

Development of this lesson was supported in part by Cooperative State Research, Education, & Extension Service, U.S. Dept of Agriculture under Agreement Number 98-EATP-1-0403 administered by Cornell University and the American Distance Education Consortium (ADEC). Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture. This manuscript has been assigned Journal Series NO. 03-11, College of Agricultural Sciences and natural Resources, University of Nebraska.



This lesson will examine herbicides that adversely affect light-related processes, thereby causing damage to plants. There are four basic mechanisms that will be studied in this class of agents: herbicides that inhibit or block synthesis of Protoporphyrin IX; herbicides that inhibit synthesis of carotenoids; herbicides that block Photosystem II electron transfer; and herbicides that divert electrons from Photosystem I. All share the ability to cause cellular damage in the presence of light.


At the completion of this lesson, students will be able to differentiate between herbicides with modes of action based on:

  • Inhibition of carotenoid biosynthesis
  • Inhibition of protoporphyrinogen channeling
  • Inhibition of Photosystem II electron transfer
  • Uncoupling of Photosystem I electron transfer