Inhibitory effects of rice seedling extracts and root exudates of Japanese traditional rice varieties

The extracts of eight Japanese traditional rice seedlings, including varieties of Soujaakamai, Tanegashimaakamai and Tushimaakamai which have been observed in the literature for more than several hundreds years, inhibited root and shoot growth of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.) and barnyardgrass (Echinochloa crus-galli L. Beauv.). Tushimaakamai had the greatest inhibitory activity. All root exudates of rice varieties also inhibited the root and shoot growth of test plant species and Awaakamai recorded the greatest inhibitory activity. Therefore, Japanese traditional varieties may produce allelochemicals and release them into rice rhizosphere as root exudates. However, these rice varieties showed different range of the inhibitory activity by the extracts and exudates, and test plants also show different sensitivity. Significant correlation between the inhibitory activity of rice extracts and rice root exudates were only found on alfalfa shoots and lettuce roots. Thus, except for alfalfa shoots and lettuce roots, the responses of these test plants to rice extracts are probably different from those to rice root exudates regarding the growth inhibition, which indicates that inhibitory substances in rice extracts and/or rice plants may not necessarily release into rice rhizosphere.