Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2012
Publication Date: 4/15/2012
Citation: Harrison Jr, H.F., Wechter, W.P., Kousik, C.S. 2012. Inhibition of bacterial, fungal and plant growth by testa extracts of Citrullus genotypes. HortScience. 47:448-451.
Interpretive Summary: Germinating watermelon seeds release substances that inhibit the growth of weeds, bacteria and fungi. The objective of this study was to determine if extractable substances present in watermelon seed coats are responsible for the inhibition. Fatty substances extracted from seed coats with dichloromethane inhibited the growth of a pathogenic bacterium but did not inhibit Proso millet seedling root growth. Water soluble components extracted with 70% methanol were inhibitory to both species. In each experiment, there were large differences between watermelon varieties and wild watermelon relatives in the strength of inhibition. The results of this study demonstrates that watermelon seed coats contain inhibitory compounds that may function to protect the embryo prior to seed germination and during the early stages of growth. The differences between varieties observed in all experiments suggest that inhibitory compounds may contribute to pest resistance of some of the wild watermelon varieties. If the content of the substances causing the inhibition and resistance to pests are linked, then they could be used to identify tolerant lines which will enhance efforts to breed watermelons for pest resistance.
Technical Abstract: Watermelon (Citrullus lanatus var. lanatus (Thunb.) Matsum & Nakai) seed exudates inhibit germination and seedling growth of several plant species and growth of pathogenic fungi and bacteria. This study was conducted to determine if extractable components in testae contribute to the inhibition. The nonpolar testa components of eight genetically diverse Citrullus genotypes extracted with dichloromethane were not inhibitory in a Proso millet radicle growth bioassay; however, they were highly inhibitory to the fruit blotch pathogen Acidovorax avenae subsp. citrulli (Aac). The more polar components extracted in 70 % methanol inhibited proso millet radicle and Aac growth. The greatest inhibition of radicle growth was found with 70% methanol extracts from two wild watermelon relatives, C. lanatus v. citroides (PI 532738) and C. colocynthis (PI 432337). Extracts from testa of this genotype were also highly inhibitory to Acc. Extracts from testa of other accessions from C. colocynthis and C. lanatus v. citroides and from watermelon genotypes were less inhibitory. All dichloromethane extracts were highly inhibitory to Aac except those from a watermelon breeding line, 406-1-x7 and a C. lanatus var. citroides accession, PI 500354. The 70% methanol extracts of several genotypes partially inhibited Acc, but the citroides accession, PI 532738 was the only genotype with 70 % methanol extracts that completely inhibited the bacterium. These results indicate that the protective and autotoxic properties of germinating watermelon seed exudates is at least partially due to extractable components in seed testae.