Because there is sodium bicarbonate to maintain the amount of carbon dioxide, the extra .4ml must be produced oxygen, and there is only .4ml of oxygen. Then the photosynthetic rate is the result that .8ml/h (calculated by the amount of oxygen released) can be converted into other units. In addition, vaseline is coated on the leaves to seal the pores, so the leaves can't absorb carbon dioxide, which naturally affects the dark reaction.