This study is aimed at screening new auxin- and cytokinin-like substances among chemical low-molecular-weight nitrogen-containing heterocyclic compounds, derivatives of thienopyrimidine, to improve growth and enhance photosynthesis of an important agricultural crop - grain sorghum (Sorghum bicolor L.) variety Odeske 202 during the vegetation period. A comparative analysis of the morphometric parameters of sorghum plants, as well as the content of photosynthetic pigments in sorghum plants treated with auxin IAA (1H-indol-3-yl)acetic acid or chemical low-molecular-weight nitrogen-containing heterocyclic compounds, derivatives of sodium salt and potassium salt of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur and Kamethur) or new compounds, thienopyrimidine derivatives at a concentration of 10-7M was carried out. As a result of the screening, the most physiologically active compounds among thienopyrimidine derivatives were identified, which promote growth and intensification of photosynthesis in sorghum plants, and the relationship between their chemical structure and regulatory effect was analyzed. Based on the obtained results, the practical use of selected compounds, derivatives of thienopyrimidine, as new effective plant growth regulators for improving growth and increasing photosynthesis of sorghum plants during the vegetation period is proposed.