The article discusses the response of carbon (C) expenditure to the plant's carbon-to-nitrogen (C:N) ratio. The key points are:

1. Calculating the difference between the target C:N ratio (target_CN) and the existing C:N ratio (CN_plant) of the plant:
   - CN_plant = (C_leaf + C_leaf,storage) / (N_leaf + N_leaf,storage)
   - δ_CN = CN_plant - target_CN

2. Adjusting the γ_FUN parameter to account for situations where the plant's C:N ratio has deviated too far from the target:
   - If δ_CN is positive (plant C:N ratio is higher than target), increase γ_FUN by 0.5 * (δ_CN / c_flexcn)
   - If δ_CN is negative (plant C:N ratio is lower than target), reduce C spent on N uptake and retain more C for growth by adjusting γ_FUN

3. Restricting the degree to which C expenditure can be reduced to prevent unrealistically high C:N ratios:
   - γ_FUN = max(min(1.0, γ_FUN), 0.5)

The article presents a mathematical approach to modeling the plant's response in carbon expenditure based on the deviation of its C:N ratio from the target value.