Final answer:
The net carbon uptake per unit time by a plant is directly related to the difference between photosynthetic rate and respiration rate.
Step-by-step explanation:
The net carbon uptake per unit time by a plant is directly related to the difference between photosynthetic rate and respiration rate. This is because photosynthetic rate represents the rate at which plants fix carbon dioxide into organic compounds, while respiration rate represents the rate at which plants release carbon dioxide back into the atmosphere. The net carbon uptake is calculated by subtracting the respiration rate from the photosynthetic rate.