Sugar accumulation in sorghum (Sorghum bicolor (L.) Moench) stems is a complex trait that is particularly plastic in response to photoperiod. This study investigated sucrose accumulation in a sterile (no grain filling) and fertile nearisogenic line of the photoperiod-sensitive cultivar IS2848 in two greenhouse experiments. Variable phenology was induced by applying a short (12-h PP) and a long (13-h PP) photoperiod. Dynamics of plant growth, phenology, sugar accumulation and related enzyme activities in internodes were investigated. Under 13-h PP, plants flowered 28 days later and attained threefold higher sucrose concentration at anthesis compared with those under 12-h PP. Sucrose accumulation in individual internodes was driven by organ physiological age, not by plant phenology. Competition with grain filling was marginal but greater under 12-h PP (i.e. when sucrose accumulation in internodes occurred after flowering). Enzyme activities showed marked developmental patterns but contributed little to explaining differences between treatments and genotypes. The study demonstrates that sucrose storage physiology in sweet sorghum resembles that of sugarcane (Saccharum spp.) but is more complex due to photoperiod effects on phenology. It confirms the field results on 14 sorghum genotypes contrasting for phenology and photoperiod sensitivity presented in a companion paper. Perspectives for developing sorghum ideotype concepts for food and fuel crops are discussed.