Introduction

Photosynthesis fuels ecosystems and replenishes the Earth's atmosphere with oxygen. Like all enzyme drive reactions, the rate of photosynthesis can be measured by either the disappearance of substrate or the accumulation of the by-products. The rate of photosynthesis can be determined by the production of oxygen or the consumption of carbon dioxide by the reaction.
Because the spongy mesophyll layer of leaves is normally infused with oxygen and carbon dioxide, leaves normally float in water. If the leaf disk is placed in a solution with an alternate source of carbon dioxide in the form of bicarbonate ions, then photosynthesis can occur in a sunken leaf. As photosynthesis proceeds, oxygen accumulates in the air spaces of the spongy mesophyll. These means that indirectly, the rate of photosynthesis can be measured by the rate of the rise of the leaf.
Aerobic respiration will consume oxygen that has accumulated in spongy mesophyll. The two processes counter each other with respect to the accumulation of oxygen in the air spaces of the spongy mesophyll. The buoyancy of the leaf is an indirect measurement of the net rate of photosynthesis occurring in the leaf tissue.