The study looked at how doubling the amount of CO2 in the air and adding nitrogen affected the photosynthesis and chlorophyll levels in winter wheat. They grew two types of wheat in special chambers and found that with more CO2, the wheat had lower chlorophyll levels and less efficient photosynthesis. However, when nitrogen was added, the negative effects of the extra CO2 were reduced. This means that increasing CO2 levels could harm the growth of winter wheat, but adding nitrogen could help counteract this effect.