The researchers studied how spraying sugar solutions and high CO2 levels affected the sugar and protein content of Betula platyphylla leaves. They found that spraying sugar solutions increased sugar and protein levels under certain CO2 concentrations. Sucrose had the best effect among the sugars tested. However, spraying sugar solution with high CO2 levels limited sugar and protein accumulation. High CO2 levels generally increased sugar and protein content, except when fructose solution was sprayed. Overall, there was a positive relationship between sugar content in leaves and CO2 concentration when the same sugar solution was sprayed.