Flower architecture and sex determination: how does Atriplex halimus play with floral morphogenesis and sex genes?

•  Atriplex halimus , a monoecious Chenopodiaceae , produces flowers displaying two basic architectures. •  The first architectural pattern is made of staminate pentamerous flowers with an external whorl of yellowish tepals and an internal whorl of stamens. The second architectural pattern consists of female flowers with a single carpel enclosed within two opposite bracts. In both architectures, bisexual flowers and flowers of the un-expected sex were detected leading to the occurrence of up to six floral phenotypes on the same individual. •  Daylength and light intensity affected sex ratio and flower distribution between both architectural patterns. Short days and low light irradiance promoted femaleness and bracteate floral architecture. Flower position on a reproductive axis and geographical origin of the plant (genotype) also affected sex and architecture ratios. •  Thus, all the genetic information required for the production of both floral architecture and sexual organ types is present in each A. halimus plant but endogenous and environmental cues determine the fate of the floral meristems. These results are discussed in relation to classical models of genetic control of floral morphogenesis.