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Herbicides That Act Through Photosynthesis: Introduction for Herbicides that Act Through Photosynthesis John Markwell Department of Agronomy and Horticulture at University of Nebraska-Lincoln, USA Deana Namuth Department of Agronomy and Horticulture at University of Nebraska-Lincoln, USA Lesson Navigation Tips: - Click on 'Animations' button found to the left in order to view the animation which supplements this lesson. You can also click on the animation icon within the text. - Click once on figures to see enlarged versions. - Click once on words in color to bring up their definitions. Overview This lesson will examine herbicides that affect light-related cellular processes, thereby causing damage to plants. There are four basic mechanisms that will be studied: 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. Objectives 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 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.