Light modulation of the gravitropic set-point angle (GSA)

A study has been made of the means by which light influences the gravitropic set-point angle (GSA) of the nodes of Tradescantia and the hypocotyls of the lazy-2 mutant of tomato. In light-grown Tradescantia there is a light-regulated developmental change in the GSA with the magnitude of this change being dependent on the photon flux density of white light. The photosynthetic inhibitor DCMU abolished the effect of white light. Low fluence rates of red light had no significant effect on the GSA of Tradescantia. It was concluded that there is an interaction between photosynthesis and the GSA in Tradescantia. The light-induced reduction of the GSA of the hypocotyl of lazy-2 tomato has previously been assumed to be solely an action of light acting via phytochrome. However, it can be shown that the GSA of hypocotyls of lazy-2 seedlings grown in white light is sensitive to DCMU and norflurazon treatment, hence the light effects on the GSA of an organ can be mediated via both phytochrome and photosynthesis. The implication of these findings to the study of gravitropism is discussed.