A farmer decided not to use a concentration of carbon dioxide higher than 0.08% to grow tomato plants.

Suggest two reasons for the farmer's decision. Use information from Table 2 and your own knowledge.

1. **Evidence from the table:** The table shows that at 0.08% CO2, the rate of photosynthesis is 20 arbitrary units. At 0.10% and 0.12%, the rate remains at 20. This means that increasing the CO2 beyond 0.08% has no further effect on the rate of photosynthesis. The rate has plateaued, likely because another factor (such as light intensity or temperature) has become the limiting factor. 2. **Economic reason:** To increase the CO2 concentration in a commercial greenhouse, a farmer would need to buy and burn fuel (like paraffin heaters) or use cylinders of CO2 gas, both of which cost money. Since there is no increase in the rate of photosynthesis (and therefore no faster growth or bigger yield of tomatoes) above 0.08%, spending money on extra CO2 would be uneconomical and would reduce the farmer's profit.