Photosynthesis is the biological process that enables plants, algae, and certain bacteria to convert light energy into chemical energy, primarily sugars. This conversion uses carbon dioxide (\(text{CO}_2\)) and water as raw materials, releasing oxygen (\(text{O}_2\)) as a byproduct. The “rate of photosynthesis” quantifies the speed of this process, typically measured by the rate of \(text{CO}_2\) consumed or \(text{O}_2\) produced. This rate is governed by a dynamic interplay of external environmental conditions, each of which can become the limiting factor.
How Light Intensity Controls the Rate
Light powers the light-dependent reactions of photosynthesis. The relationship between light intensity and the photosynthetic rate follows a distinct curve. It begins at the light compensation point, where the plant’s \(text{CO}_2\) uptake exactly balances its \(text{CO}_2\) release from respiration. Below this point, the plant consumes more energy than it produces.
As light intensity increases past the compensation point, the rate of photosynthesis rises proportionally. This increase continues until the plant reaches the light saturation point, where further light increases do not result in a faster rate. At saturation, the photosynthetic machinery operates at maximum capacity, and the rate becomes limited by other factors, such as \(text{CO}_2\) availability or enzyme speed. Different plant species have varying saturation points, with sun-loving plants having a higher threshold than shade-tolerant species.
The quality of light, or its wavelength, also influences the rate because chlorophyll pigments absorb specific wavelengths most efficiently. Chlorophyll absorbs red light (600–700 nm) and blue light (400–500 nm) most effectively for driving the light reactions. Green light is largely reflected, giving leaves their characteristic color, but it can still be used for photosynthesis, particularly in the lower canopy.
The Impact of Carbon Dioxide Concentration
Carbon dioxide is the raw material for the Calvin cycle, where sugars are synthesized. Atmospheric \(text{CO}_2\) concentration is a limiting factor, currently averaging around 420 parts per million (ppm). At very low \(text{CO}_2\) levels, a plant reaches a \(text{CO}_2\) compensation point, where the carbon fixed by photosynthesis equals the carbon lost through respiration and photorespiration.
For \(text{C}_3\) plants, which are the majority of species, the compensation point is typically around 50 ppm. Increasing the \(text{CO}_2\) concentration above this level dramatically increases the photosynthetic rate until light or temperature becomes the constraint. The enzyme RuBisCO, which catalyzes \(text{CO}_2\) fixation, can also bind with oxygen, leading to inefficient photorespiration, which is more pronounced at lower \(text{CO}_2\) concentrations. This explains why \(text{C}_3\) plants respond strongly to \(text{CO}_2\) enrichment.
\(text{C}_4\) species, such as corn and sugarcane, have evolved a mechanism to concentrate \(text{CO}_2\) around the RuBisCO enzyme, making them efficient at utilizing low atmospheric \(text{CO}_2\) levels. \(text{C}_4\) plants use PEP carboxylase to fix \(text{CO}_2\) into a four-carbon compound. This compound is then shuttled to specialized bundle sheath cells where the Calvin cycle occurs, effectively suppressing photorespiration. This adaptation allows \(text{C}_4\) plants to maintain a high photosynthetic rate even in hot, dry environments where \(text{CO}_2\) intake is restricted.
Temperature and Enzyme Activity
The rate of photosynthesis depends on temperature because the process, particularly the Calvin cycle, is controlled by enzymes. As temperature rises, molecular kinetic energy increases, leading to more frequent collisions between enzyme active sites and substrates. This relationship is quantified by the \(text{Q}_{10}\) temperature coefficient, which shows that for every \(10^circ text{C}\) rise within the optimal range, the photosynthetic rate approximately doubles.
This acceleration continues up to an optimum temperature, which varies widely depending on the plant species (e.g., \(text{C}_3\) versus \(text{C}_4\)). Beyond this optimum, the rate decreases sharply because high thermal energy disrupts the structure of photosynthetic enzymes. This process, known as thermal denaturation, alters the enzyme’s active site and reduces its ability to bind substrates.
A major point of thermal instability is the enzyme RuBisCO activase (RCA), which maintains the carbon-fixing enzyme, RuBisCO, in an active state. RCA is significantly more sensitive to heat than RuBisCO itself, with its activity declining sharply above \(44^circ text{C}\). The thermal denaturation of RCA is a primary cause of the sudden drop in photosynthetic efficiency observed under moderate heat stress.
Real-World Measurement and Application
Understanding the influence of light, \(text{CO}_2\), and temperature allows scientists and growers to measure and manipulate the photosynthetic rate for practical applications in controlled-environment agriculture. Laboratory methods involve measuring the net exchange of gases. The rate of \(text{O}_2\) production can be quantified using an oxygen sensor, while \(text{CO}_2\) consumption is measured using an infrared gas analyzer (IRGA). The IRGA accurately monitors the change in \(text{CO}_2\) concentration within a sealed chamber containing the plant.
These measurements are applied directly in commercial settings, such as greenhouses, to optimize plant growth and maximize yield. Growers often employ \(text{CO}_2\) enrichment, raising the concentration from ambient levels (\(sim\)420 ppm) to 800 to 1,000 ppm. This ensures carbon is not the limiting factor when light is abundant.
Supplemental lighting, often using LED fixtures, is used to extend the photoperiod and ensure light intensity is near the plant’s saturation point. Diffuse glass coatings help scatter light deeper into the plant canopy, increasing the leaf area exposed to light. The careful control of temperature and humidity completes this controlled environment, supporting the highest possible rate of sugar production.