Screening of Some Cotton Varieties for Allelopathic Potential on Clover Broomrape Germination

Published in Agron. J. 104:569–574 (2012) Posted online 16 Feb 2012 doi:10.2134/agronj2011.0372 Copyright © 2012 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. T root parasitic plant broomrape seriously hampers agricultural production in the Mediterranean, Europe, Central America, and some parts of Asia. Broomrapes are holoparasites and completely dependent on their host plants for reduced C, nutrients, and water. In Dingbian County, Shaanxi Province, China, sunflower (Helianthus annuus L.) plantations cover 40,000 hm2 or 25% of the total crop production area. Broomrape invasion resulted in crop losses of up to 48% in 2002 and a 3000 hm2 reduction in the area of sunflower production the following year. Sunflower plantations presently cover around 10,000 hm2 in Dingbian County, but 64% of the fields are parasitized by broomrape. In heavily infested fields, yield losses range between 40 and 50% (Chen, 2010). Broomrape seeds have special germination requirements. The seeds should be kept in a warm and moist environment for several days and then exposed to germination stimulants, which are produced by and released from the roots of host plants (Joel et al., 1995). In the absence of germination stimulants, the seeds will revert to secondary dormancy (Parker and Riches, 1993a, 1993b). Strigolactones are a class of chemicals known to stimulate the germination of broomrape and another root parasitic species, witchweed (Striga)(Sato et al., 2005). Efficient and economical broomrape control is extremely difficult because infestation occurs primarily underground. Broomrape can be controlled using mechanical means, manual removal, or pesticide application. However, these options require significant manpower and material resources. There is also concern about crop phytotoxicity and environmental pollution caused by chemical pesticides. The control of parasitic weeds with trap crops is by far the most economical and practical method for small-scale commercial farming (Aalders and Pieters, 1987). The trap crops consist of nonhost plant species whose roots exude the chemical stimulant required for germination but do not allow the attachment and development of the parasitic weed. This phenomenon results in the suicidal germination of the parasitic weed seeds. Cotton is a nonhost plant that can stimulate clover broomrape germination (Carson, 1988). The production of strigol, later called strigolactone, by cotton was observed by Cook et al. (1966, 1972). The production of two strigolactones (i.e., strigol and strigyl acetate) by cotton was recently confirmed and quantified (Sato et al., 2005). Based on their concentrations in root exudates, the authors concluded that strigol was the main stimulant for broomrape germination. Botanga et al. (2003) showed that the ability to stimulate witchweed germination by cotton is a qualitatively inherited trait. Furthermore, the gene controlling this trait is monogenic and simply inherited. Therefore, the selection and breeding of cotton genotypes that produce large amounts of highly active germination stimulants, while maintaining or improving other agronomic attributes, should be possible. The term allelopathy refers to both detrimental and beneficial reciprocal biochemical interactions among all types of plants and microorganisms (Molisch, 1937) and it was redefined as any direct or indirect harmful or beneficial effect of one plant (including microorganisms) on another through production of chemical compounds that escape into the environment by Rice (1984). Research about the chemical basis of allelopathy has ABSTRACT