Multiple Herbicide Resistance in Wild Oat and Impacts on Physiology, Germinability, and Seed Production

Published in Agron. J. 105:854–862 (2013) doi:10.2134/agronj2012.0428 Copyright © 2013 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. In a review of fitness costs associated with herbicide resistance, Vila-Aiub et al. (2009) described fitness costs as arising when (i) novel, resistance-conferring mutations in herbicide target enzymes interfere with normal plant function or metabolism, (ii) altered ecological interactions occur such as herbicide-resistant plants becoming less attractive to pollinators or more susceptible to disease or herbivores, and (iii) the physiological mechanism(s) of herbicide resistance divert resources away from growth and reproduction. The last scenario, known as the resource-based allocation theory (Coley et al., 1985), results from individuals maximizing their ecological success under different environmental selective pressures by differentially allocating resources among growth, defense, and reproduction processes (Bazzaz et al., 1987; Herms and Mattson, 1992). The resource-based allocation theory provides a framework within which to study the evolutionary dynamics and impacts of herbicide resistance as an evolved defense mechanism that can alter the ecological fitness of resistant biotypes. The expression and magnitude of these fitness tradeoffs are mediated by the abiotic, biotic, and genetic environment where selection occurs (Vila-Aiub et al., 2009). Many field and greenhouse studies support the phenomenon of fitness tradeoffs associated with herbicide resistance. In a greenhouse evaluation of the ecological cost of herbicide resistance due to enhanced metabolism, Vila-Aiub et al. (2005) AbStrAct The evolution of weed biotypes resistant to multiple herbicide modes of action, here termed multiple herbicide resistance, is a growing problem around the world. We investigated two multiple herbicide resistant (MHR) wild oat (Avena fatua L.) populations from Montana and hypothesized that they would exhibit fitness costs compared with two herbicide-susceptible (HS) populations. Dose-response tests showed that the MHR populations were resistant to difenzoquat (a membrane disruptor), imazamethabenz (an acetolactate synthase [ALS] inhibitor), flucarbazone (an ALS inhibitor), and tralkoxydim (an acetyl-CoA carboxylase inhibitor). In greenhouse studies, we assessed differences between MHR and HS populations in seed germination, photosynthetic parameters, plant growth, and reproduction. Seeds of one HS population germinated more at cold temperature (4.9°C) and less at high temperature (29.6°C) compared with the other populations. Plants of this HS population also had lower stomatal conductance (23%), intercellular CO2 concentration (7.5%), and transpiration (15.3%) than the other populations, but there were no differences in photosynthetic rates between any populations. Also, there were no differences in relative growth rate among all HS and MHR populations. The MHR populations initiated seed production several days sooner than the HS populations; however, HS populations produced 67% more tillers, and one HS population ultimately produced 43% more seeds than the MHR populations, indicating a potential fitness cost of resistance. With the exception of seed production differences, our results do not indicate a consistent fitness cost. More research is needed in field settings and with resource competition to further evaluate fitness costs in MHR populations.