Environmental stress reduces yield and quality in crop plants. Understanding these stresses is an essential enabler for mitigating breeding strategies and becomes more important as the frequency of extreme weather events increases due to climate change. This study analyses the response of barley (Hordeum vulgare L.) to a heat wave during grain filling in three distinct stages: the heat wave itself, the return to a normal temperature regime, and the process of maturation and desiccation. The properties and structure of the starch produced was followed throughout maturational stages. Further, key enzymes involved in the carbohydrate supply of grains were monitored. Clear differences in starch structure were found, with well separated effects due to the heat wave itself and the senescence process. Heat stress produced marked effects on sucrolytic enzymes in source and sink tissues. Early cessation of plant development as an indirect consequence of the heat wave was identified as the major contributor to final yield loss from the stress, highlighting the importance for functional stay-green traits for the development of heat resistant cereals.