The Theme-Rheme system has been employed in synchronic discourse studies to analyze the profile of the research article (RA). However, while diachronic analyses can frame our understanding of discourse, such studies have not received due attention in research on thematicity. This study, therefore, examined thematic choices in the RAs of mechanical engineering and horticulture diachronically. The corpus consisted of 120 RAs from 1965-1975 and 120 RAs from 2005-2015, drawn from eight peer-reviewed journals. From a SFL perspective, the RAs were examined through Halliday’s (1994) model alongside McCabe’s (1999) analytical framework. Results revealed that the two time periods are associated with significant changes in thematic choices. Yet, the patterns of thematic progression (TP) have not altered in either discipline. The findings suggest implications for discourse analysis as well as richer academic writing instruction. We recommend further diachronic discourse studies which can contribute to the research base concerning academic disciplines, science, and genres.
