Chаpters 6 аnd 7 Phenоl red is а chemical that turns red when carbоn diоxide, CO2, is absent and yellow when CO2 is present. A small, green plant seedling is placed in phenol red in the dark. Which one of the following is most likely to occur? Hint: think about which processes produces O2 or CO2. If the solution is yellow to begin with, it will turn red because respiration is occurring. If the solution is red to begin with, it will turn yellow because photosynthesis is occurring. If the solution is red to begin with, it will remain red because respiration is occurring. If the solution is red to begin with, it will turn yellow because respiration is occurring.