AbstractBrassinosteroids (BRs) are essential regulators of growth and development. BR‐deficient mutants such as cpd/cbb3 and dwf4 display extreme dwarfism due to a failure in cell elongation. To avoid the severe pleiotropic effects caused by the extreme growth defect, transgenic Arabidopsis lines carrying a construct for antisense inhibition of CPD gene expression were established and subjected to physiological analysis. The CPD‐antisense (α‐CPD) lines display characteristic phenotypic alterations of BR‐deficient plants such as reduced stem and petiole growth, smaller leaves, and a slightly delayed development. The observed changes are intermediate between the corresponding loss‐of‐function mutant (cbb3) and wild‐type plants. In the present study, the primary carbon metabolism of the transgenic lines as well as the BR‐deficient cbb1 (dwf1‐6/dim) mutant was analysed. Gas exchange measurements indicated a reduced assimilatory capacity of the α‐CPD plants. Soil‐grown α‐CPD as well as cbb1 (dwf1‐6) mutant plants show a clear reduction in starch content. The metabolic alterations are accompanied by altered enzyme activities such as reduced invertase and cytosolic β‐amylase activity, and altered expression patterns of genes such as Atbfruct1, Asus1, and ct‐Bmy (encoding a cell wall invertase, sucrose synthase, and plastidic β‐amylase, respectively). The impaired carbon assimilation, as well as the altered enzyme activities and gene expression patterns in the α‐CPD and cbb1 (dwf1‐6) plants, demonstrate the necessity of normal CPD and DIM expression for proper carbon uptake and metabolism and may point to an essential function of BRs. The impaired growth of BR‐deficient plants may be (at least in part) due to reduced photosynthesis.