Response of allelochemicals under pseudo-microgravity in sunflower plant (Helianthus annuus L. cv. Taiyo)

Interactions between organisms and allelochemicals in space have been not well investigated. We employed a 3D-clinorotation device to generate pseudo-microgravity in order to answer this question. If any allelopathy associated biosynthesis, emission, transport and sensing mechanism is found to be gravity dependent, many organisms and ecological systems might exhibit a different behavior under the microgravity experienced in space. The sunflower (Helianthus annuus L.) is known to show strong allelopathic properties against other plant species. The growth of lettuce seedlings, as an index of the allelopathic action, was compared between the 3D-clinostated group and a control group cultivated in normal gravity. The degree of suppression of the lettuce growth by the sunflower plant was found to be less in the experiment group. Sundiversifolide,4,15-dinor-3-hydroxy-1(5)-xanthene-12,8-olide, has been isolated and identified from the exudate of seeds (achenes) of the Taiyo sunflower as a new plant species-specific allelopathic substance (Ohno et al., 2001). The occurrence of this substance was remarkably different under normal gravity vs. the pseudo-microgravity. It was found that sundiversifolide was not a stable compound and it became a non-active substance, which did not have any inhibitory activity on the shoot lengths of lettuce seedlings after adding the extraction from the achenes. These results indicate that the synthesis and release of the allelochemicals are reduced under pseudo-microgravity.