Two millimeter apical segments of maize (cv. LG 11) primary roots were analysed in relation to the effects of light and gravity on adenine nucleotide content. Adenosine triphosphate (ATP) content is very sensitive to these stimuli. ATP levels are lower in roots exposed to light than in those kept in the dark. The energy charge (E.C.) decreases markedly after exposure to light and gravity. For the vertical roots E.C. is stable. Present data confirm the fact that light and gravity may act on cell metabolism, modifying the energy requirements. This will be discussed in relation to some hormone action
