2.7 Measuring Photosynthesis and Limiting Factors

Photosynthetic rate can be estimated from oxygen production, carbon dioxide uptake, pH change in aquatic systems, or biomass increase. Each method measures a proxy with limitations. Counting bubbles ignores changing bubble size; collecting gas volume is better but includes gases already dissolved. Carbon dioxide removal raises aquatic pH because less carbonic acid forms, but buffering and other metabolic processes affect the relationship. Biomass integrates growth over time but subtracts respiratory and other losses.

A sound investigation controls plant species and size, temperature, carbon dioxide availability, wavelength, light duration and distance from the lamp while changing one independent variable. Light intensity follows an inverse-square relationship only for an approximately point-like source without major reflections. A heat shield or water bath helps separate the effect of light from heating. Replicates reveal biological variation and allow uncertainty to be shown.

A limiting factor is the variable in shortest effective supply relative to the process's demand. At low light intensity, more photons increase excitation and rate. Once light-dependent capacity no longer limits, the curve plateaus because carbon dioxide, temperature or enzyme capacity has become limiting. Raising the new limiting factor can raise the plateau; increasing a non-limiting factor has little effect.

Carbon dioxide curves also rise and plateau. Temperature curves differ: rate rises as enzyme-controlled reactions accelerate, reaches an optimum, then declines as enzymes lose activity, membranes are affected and stomatal closure may restrict carbon dioxide entry. The whole-plant curve combines biochemistry with water relations and gas exchange, so it need not be a perfect isolated-enzyme curve.

Plant cells respire continuously, including in the light. Gross photosynthesis is total photosynthetic production; net photosynthesis subtracts respiration. At the light compensation point, photosynthetic oxygen production equals respiratory oxygen consumption, so net gas exchange is zero. Below it, the plant releases net carbon dioxide; above it, the plant takes up net carbon dioxide. Zero net exchange does not mean both pathways have stopped.

Over a 24-hour period, successful growth requires cumulative carbon gain in light to exceed respiratory carbon loss across both light and dark periods. A brief measurement at midday cannot establish daily productivity. Shade-adapted plants often have a lower compensation point, while sun-adapted plants may reach a higher maximum rate under strong light.

Growers can enrich carbon dioxide, provide artificial light and heat a greenhouse. The biologically maximum rate is not necessarily the economically optimal rate: energy and carbon dioxide costs rise, while another factor eventually limits growth. Ventilation, water supply, mineral nutrition and crop developmental stage also matter. Good control uses sensors and changes conditions only when the expected value of added growth exceeds the input cost and environmental impact.