Table of Contents
Photosynthesis does not always run at the same speed. Its rate depends on both external (environmental) and internal (plant-related) factors. In this chapter, we focus on how these factors influence the rate of photosynthesis, assuming that the overall process and its light- and dark-reactions are already known.
We will mostly consider C3 plants (the common case) and then briefly mention special adaptations (C4 and CAM).
1. Light as a Limiting Factor
1.1 Light Intensity (Irradiance)
The light-dependent reactions of photosynthesis directly depend on light. At low light levels, the rate of photosynthesis is limited by the number of photons absorbed.
- Low light:
- Few photons → limited excitation of chlorophyll.
- Light is the limiting factor, so if light intensity increases, the photosynthesis rate increases almost proportionally.
- Intermediate light:
- The rate still rises with increasing light, but the relationship becomes less steep as other factors (e.g., CO₂, temperature, enzyme capacity) begin to limit.
- High light:
- A plateau is reached: the light saturation point.
- Above this point, further increases in light do not increase the rate because something else is limiting (often CO₂ or enzyme capacity in the Calvin cycle).
At very high light intensities, especially under stress (drought, nutrient deficiency), photoinhibition can occur:
- Excess light energy cannot be used or safely dissipated.
- Damage to photosystems (especially PSII) and pigments.
- Decrease in photosynthetic rate.
Plants adapt structurally and physiologically:
- Sun leaves / sun plants:
- Thicker leaves, more palisade cells, higher chlorophyll content per area.
- Higher light saturation point, higher maximum photosynthesis rate.
- Shade leaves / shade plants:
- Thinner leaves, larger surface area per unit mass, more chlorophyll b, often a lower light compensation point (see below).
- Adapted to use low light efficiently but more easily photoinhibited at very high light.
1.2 Light Quality (Wavelength)
Chlorophylls and accessory pigments do not absorb all wavelengths equally.
- Most effective wavelengths:
- Blue light (~430–470 nm) and red light (~640–680 nm) are absorbed strongly and drive photosynthesis efficiently.
- Less effective wavelengths:
- Green light (~500–550 nm) is mostly reflected or transmitted, which is why leaves appear green.
- Far-red and UV:
- Far-red (>700 nm) has lower energy per photon and is less effective for driving the usual photosynthetic reactions.
- UV can damage DNA, proteins, and pigments; plants produce protective compounds (e.g., flavonoids) but UV is not a major driver of photosynthesis.
The quality of light reaching leaves can change:
- In forests, the understory receives filtered light enriched in green and far-red wavelengths.
- Water columns selectively absorb certain wavelengths, so aquatic plants and algae often have different pigment complements.
1.3 Light Compensation Point
The light compensation point is the light intensity at which the rate of photosynthesis (CO₂ fixation and O₂ release) is exactly balanced by the rate of respiration (CO₂ release and O₂ consumption):
- Net gas exchange = 0.
- Below this point: the plant consumes more O₂ (in respiration) than it produces and releases more CO₂ than it fixes → net loss of stored energy.
- Above this point: net photosynthesis is positive.
Shade-adapted plants generally have a lower light compensation point than sun-adapted plants, allowing them to survive in darker environments.
2. Carbon Dioxide Concentration
CO₂ is the immediate carbon source for the Calvin cycle. The enzyme RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate, but it is also capable of binding O₂ (photorespiration).
2.1 CO₂ as a Substrate
If light and temperature are favorable:
- At very low CO₂:
- The rate of CO₂ fixation is low; CO₂ is the limiting factor.
- As CO₂ concentration increases:
- The rate of photosynthesis rises, often roughly linearly at first.
- At moderate to high CO₂:
- A plateau is reached where CO₂ is no longer limiting; further increases have little effect if light or temperature are limiting.
This is why increasing CO₂ concentration in greenhouses (e.g., to 600–1000 ppm) can noticeably increase yield under sufficient light and nutrients.
2.2 Interaction With Oxygen and Photorespiration (C3 vs. C4 and CAM)
In C3 plants, RuBisCO catalyzes two competing reactions:
- Carboxylation: CO₂ fixation → beneficial.
- Oxygenation: O₂ fixation → leads to photorespiration, consuming energy and releasing some of the already fixed CO₂.
Conditions that increase photorespiration and thus reduce net photosynthetic efficiency:
- Low internal CO₂ concentration.
- High O₂ concentration.
- High temperature (which changes the CO₂:O₂ solubility ratio and RuBisCO’s selectivity).
C4 plants (e.g., maize, sugarcane):
- Have a CO₂-concentrating mechanism; they effectively increase CO₂ around RuBisCO.
- Show higher efficiency at high light, high temperature, and low atmospheric CO₂.
- Are less limited by photorespiration under such conditions.
CAM plants (e.g., many succulents):
- Take up CO₂ mostly at night (when stomata are open) and store it as organic acids.
- Release CO₂ internally during the day for the Calvin cycle, with stomata mostly closed → reduced water loss.
- Particularly advantageous in arid environments.
The detailed mechanisms of C4 and CAM are treated elsewhere; for this chapter, note that CO₂ availability at the site of RuBisCO is a critical factor and that different plant types have evolved strategies to optimize this.
3. Temperature
Photosynthesis includes both light-dependent physical processes and enzyme-catalyzed biochemical reactions (e.g., in the Calvin cycle). Enzymes are strongly temperature-dependent.
3.1 Temperature Dependence of Enzymes
- At low temperatures:
- Enzyme activity is low; reaction rates are slow.
- Photosynthetic rate increases with temperature because enzymes work faster.
- At an optimum temperature:
- Photosynthetic rate reaches a maximum.
- The optimum differs among species (e.g., cool-season vs. warm-season plants).
- At high temperatures:
- Enzymes start to denature.
- Membranes become too fluid, and the structure of the photosystems can be destabilized.
- Photosynthetic rate declines and can eventually collapse.
The rate of many biological reactions roughly follows a Q₁₀ rule (within a limited range):
$$
Q_{10} = \frac{\text{reaction rate at } (T + 10^\circ\text{C})}{\text{reaction rate at } T}
$$
For many processes, $Q_{10}$ is around 2, meaning the rate approximately doubles for a 10°C increase, up to the optimum.
3.2 Interaction With Other Factors
Temperature modifies how strongly light and CO₂ can drive photosynthesis:
- At low temperature, even with plenty of light and CO₂, the Calvin cycle is slow, so light is not fully used.
- At high temperature, photorespiration increases in C3 plants, reducing net CO₂ fixation even if light is abundant.
Different plant types:
- Cold-adapted species (e.g., boreal plants, alpine species):
Optimal photosynthesis at relatively low temperatures. - Heat-adapted species (many C4 plants, desert plants):
High temperature optima, better protection of photosynthetic apparatus from heat damage.
4. Water Availability
Water is essential for photosynthesis in several ways:
- It is the electron donor in the light reactions (photolysis of water).
- It is crucial for turgor pressure, which maintains leaf shape and stomatal function.
- It is the main component of the internal medium where biochemical reactions occur.
4.1 Water Stress and Stomatal Closure
Under adequate water supply:
- Stomata open to allow CO₂ diffusion into the leaf.
- Transpiration occurs, helping to cool the leaf and drive water movement from roots.
Under water deficit (drought):
- Loss of turgor and chemical signals (e.g., ABA) cause stomata to close or narrow.
- This reduces water loss but also limits CO₂ uptake, which decreases the rate of the Calvin cycle and net photosynthesis.
- High light intensity during drought can cause strong photoinhibition because absorbed light energy cannot be used effectively when CO₂ is lacking.
Long-term or severe drought can lead to:
- Structural damage to chloroplasts.
- Degradation of pigments.
- Permanent reduction in photosynthetic capacity.
4.2 Too Much Water
Excess water (waterlogging) can also limit photosynthesis:
- Roots may become oxygen-deprived.
- Root respiration and nutrient uptake are impaired.
- Reduced water and mineral transport to leaves indirectly lowers photosynthetic performance.
5. Mineral Nutrients
Photosynthesis depends on several essential elements. Deficiencies alter chloroplast structure, pigment composition, and enzyme function.
5.1 Key Elements for Photosynthesis
- Nitrogen (N):
- Component of chlorophyll, amino acids, and many enzymes (including those in the Calvin cycle).
- Deficiency → chlorosis (yellowing), especially in older leaves; reduced chlorophyll and reduced maximum photosynthesis rate.
- Magnesium (Mg):
- Central atom in the chlorophyll molecule.
- Deficiency → interveinal chlorosis; lowered chlorophyll content, reduced light absorption.
- Iron (Fe):
- Involved in electron transport (cytochromes, ferredoxin).
- Deficiency → strong chlorosis in young leaves; impaired electron transport.
- Phosphorus (P):
- Component of ATP and NADPH-related reactions, and sugar phosphates in the Calvin cycle.
- Deficiency → reduced energy transfer capacity; slower regeneration of RuBP and sugar export.
- Manganese (Mn), Copper (Cu), Zinc (Zn):
- Trace elements serving as cofactors in specific parts of the photosynthetic machinery (e.g., Mn in the water-splitting complex of PSII).
5.2 Effects of Nutrient Limitation
- Lower chlorophyll content → less effective light capture.
- Less efficient electron transport → reduced ATP and NADPH production.
- Slower Calvin cycle → decreased CO₂ fixation, even if light and CO₂ are abundant.
In natural ecosystems, nutrient availability (especially nitrogen and phosphorus) often strongly controls primary productivity (the total amount of biomass produced by photosynthesis).
6. Internal Leaf and Plant Factors
Beyond external conditions, several internal features influence photosynthesis.
6.1 Leaf Structure and Age
- Leaf age:
- Young, fully expanded leaves usually have the highest photosynthetic capacity.
- Very young leaves may still be developing chloroplasts and not yet at full capacity.
- Older leaves often show senescence: chlorophyll degradation, reduced enzyme activity, lower photosynthesis.
- Anatomy:
- Thickness of the palisade and spongy mesophyll layers.
- Density and distribution of chloroplasts.
- Arrangement of veins affecting water and nutrient supply.
6.2 Stomatal Density and Conductance
- Stomatal density (number per unit leaf area) and stomatal conductance (how open they are) affect:
- CO₂ diffusion into the leaf.
- Water loss via transpiration.
Plants can, within limits, adjust stomatal density over developmental time in response to long-term light, CO₂, or water conditions.
6.3 Chloroplast Number and Pigment Composition
- More chloroplasts per cell and more chlorophyll per chloroplast generally increase maximum photosynthetic capacity, up to structural and resource limits.
- Ratio of chlorophyll a to chlorophyll b and the presence of accessory pigments (e.g., carotenoids) adjust light capture to the quality of the local light environment.
7. Environmental Interactions and Limiting Factors
7.1 Law of the Limiting Factor
Often, only one factor at a time is truly limiting the rate of photosynthesis, even though many are important. This is summarized by the law of the limiting factor:
- If a process requires several factors, the rate is limited by that factor which is in shortest supply relative to its required amount.
- Increasing a non-limiting factor has no effect until the limiting factor is improved.
Example:
- At low light: increasing CO₂ or temperature has little effect until light is adequate.
- At high light and high CO₂: increasing temperature from too low to optimal strongly increases the rate.
- Once temperature is optimal: further temperature increases do not help and may harm, while CO₂ or nutrient availability might then be limiting.
7.2 Short-Term vs. Long-Term Responses
- Short-term (seconds to hours):
- Stomatal opening/closing.
- Activation/inactivation of enzymes.
- Non-photochemical quenching mechanisms to safely dissipate excess light.
- Intermediate (days to weeks):
- Adjustments in pigment content.
- Changes in leaf thickness or orientation.
- Induction of stress-protective proteins.
- Long-term (developmental, generational):
- Morphological adaptation (e.g., shade vs. sun leaves).
- Evolutionary changes (e.g., development of C4 or CAM photosynthesis in certain lineages).
8. Measuring and Interpreting Photosynthetic Responses
To study how factors influence photosynthesis, several types of measurements are used:
- Gas exchange:
- Net CO₂ uptake or O₂ release under controlled light, CO₂, and temperature.
- Generates curves such as photosynthesis vs. light intensity or vs. CO₂ concentration.
- Chlorophyll fluorescence:
- Gives information on the efficiency of photosystem II and the extent of photoinhibition.
- Chlorophyll content and pigment analysis:
- Indicates adjustments to light environment and nutrient status.
Typical graphs derived from such measurements:
- Light response curve:
- Shows light compensation point, light saturation point, and maximum photosynthetic rate.
- CO₂ response curve (at constant saturating light):
- Shows CO₂-limitation at low CO₂ and saturation at higher CO₂.
These measurements reveal which factors are limiting in a particular situation and how plants acclimate to their environment.
In summary, the rate of photosynthesis is shaped by a network of interacting physical, chemical, and biological factors. Light, CO₂, temperature, water, and nutrients each can become limiting and can alter how effectively plants convert solar energy into chemical energy and biomass. Understanding these influences is essential for interpreting plant performance in natural ecosystems and optimizing productivity in agriculture and horticulture.