UNIT 2.1.3

C3, C4, and CAM Plants

Photosynthetic adaptations to environment

🎯 After this unit, you will be able to:

Compare C3, C4, and CAM photosynthetic pathways
Explain the problem of photorespiration
Describe how C4 and CAM plants concentrate CO₂
Match photosynthetic types to their optimal environments

🌍 Photosynthetic Adaptations to Environment

While all green plants perform photosynthesis, they have evolved different strategies to deal with environmental challenges—especially hot, dry conditions that promote photorespiration. The three main photosynthetic pathways are C3, C4, and CAM .

Key concept: The names come from the first stable carbon compound produced: a 3-carbon molecule (3-PGA) in C3 plants, a 4-carbon molecule (oxaloacetate/malate) in C4 plants, and CAM plants also produce 4-carbon compounds but at night .

🌾

C3 Plants

Rice, wheat, soybean, potato

~85% of plant species
First product: 3-PGA (3C)

🌽

C4 Plants

Maize, sugarcane, sorghum

~3% of species
First product: oxaloacetate (4C)

🌵

CAM Plants

Cacti, succulents, pineapple

~10% of species
Night CO₂ fixation

⚠️ The Problem: Photorespiration

Photorespiration is a wasteful process that occurs when rubisco fixes O₂ instead of CO₂. This happens because rubisco's active site cannot perfectly distinguish between CO₂ and O₂—it's a relic from when Earth's atmosphere had much higher CO₂ levels .

🔄 [Diagram: Photorespiration pathway showing rubisco oxygenase activity — to be inserted]

When rubisco fixes O₂:

It produces phosphoglycolate (a toxic compound) instead of two 3-PGA
The plant must "recycle" phosphoglycolate through a costly pathway
This consumes ATP and releases previously fixed CO₂

🌡️ Did you know? Photorespiration increases dramatically at high temperatures because rubisco's affinity for O₂ increases more than for CO₂. At 35°C, photorespiration can reduce photosynthetic efficiency by 30-50% in C3 plants .

Photorespiration is essentially the opposite of photosynthesis—it wastes energy and carbon. This is why C4 and CAM plants evolved mechanisms to concentrate CO₂ and suppress photorespiration .

🌾 C3 Plants: The Default Pathway

C3 plants use only the Calvin cycle for carbon fixation. The name comes from the first stable product: the 3-carbon compound 3-phosphoglycerate (3-PGA) .

🌾 [Diagram: C3 photosynthesis - all in mesophyll cells — to be inserted]

Characteristics of C3 Plants:

Fix CO₂ directly via rubisco in mesophyll cells
No special anatomy to concentrate CO₂
Optimal temperature: 15-25°C
Strongly affected by photorespiration in hot, dry conditions
Examples: wheat, rice, barley, oats, soybean, peanut, cotton, most trees

🌾 Ethiopian Context: Wheat and Teff

Both wheat and teff are C3 plants. In the Ethiopian highlands, where temperatures are moderate, they grow well. However, as climate change increases temperatures, photorespiration may reduce yields. This is why breeding for heat tolerance and exploring C4 options (like sorghum) in warmer areas is important .

🌽 C4 Plants: The CO₂ Concentrators

C4 plants evolved a two-stage system that concentrates CO₂ in specialized cells, suppressing photorespiration. The name comes from the first product: the 4-carbon oxaloacetate (which is quickly converted to malate or aspartate) .

🌽 [Diagram: C4 photosynthesis - Kranz anatomy with mesophyll and bundle sheath cells — to be inserted]

Kranz Anatomy

C4 plants have distinctive leaf structure called Kranz anatomy (German for "wreath"):

Mesophyll cells: Outer layer where CO₂ is initially fixed into a 4C compound using PEP carboxylase (which has high affinity for CO₂ and no oxygenase activity)
Bundle sheath cells: Inner layer where CO₂ is released and fixed by rubisco in the Calvin cycle

The C4 Pathway in Two Steps:

In mesophyll cells: CO₂ + PEP → oxaloacetate → malate (using PEP carboxylase)
Malate transported to bundle sheath cells → releases CO₂ → enters Calvin cycle

Energy cost: C4 plants use more ATP than C3 plants (30 ATP per glucose vs. 18 ATP) but gain the advantage of suppressed photorespiration in hot conditions .

Characteristics of C4 Plants:

High water use efficiency (can keep stomata partially closed)
Optimal temperature: 30-45°C
Thrive in high light, high temperature environments
Examples: maize, sugarcane, sorghum, millet, many tropical grasses

🌵 CAM Plants: Night Shift Workers

CAM (Crassulacean Acid Metabolism) plants take a different approach: they open stomata at night to fix CO₂ and store it as malic acid, then use it during the day for the Calvin cycle .

🌵 [Diagram: CAM photosynthesis - day/night cycle of stomatal opening and acid accumulation — to be inserted]

The CAM Cycle:

🌙 Night: Stomata open, CO₂ fixed by PEP carboxylase → malic acid stored in vacuoles

☀️ Day: Stomata close, malic acid released → CO₂ enters Calvin cycle

Characteristics of CAM Plants:

Extremely high water use efficiency (lose 50-100 g water per g CO₂ vs. 250-300 for C3)
Stomata open at night to reduce water loss
Can survive in deserts and arid regions
Some plants can switch between C3 and CAM depending on conditions
Examples: cacti, succulents, pineapple, agave, vanilla, some orchids

🍍 Pineapple: An Important CAM Crop

Pineapple is one of the few commercially important CAM crops. Its CAM metabolism allows it to thrive in tropical regions with dry seasons. Pineapple plantations in Ethiopia (e.g., in the lowlands) benefit from this adaptation—the plants can survive periods of drought that would kill C3 crops .

📊 Comparison: C3 vs. C4 vs. CAM

Feature	C3	C4	CAM
First stable product	3-PGA (3C)	Oxaloacetate (4C)	Malic acid (4C, at night)
CO₂ fixation enzyme	Rubisco only	PEP carboxylase (mesophyll) Rubisco (bundle sheath)	PEP carboxylase (night) Rubisco (day)
Anatomy	No special arrangement	Kranz anatomy (two cell types)	Succulent tissue, large vacuoles
Stomata open	Day	Day	Night
Water use efficiency	Low	High	Very high
Optimal temperature	15-25°C	30-45°C	Wide range (adapted)
ATP per glucose	18	30	Similar to C4? Actually ~30
Photorespiration	High	Low (suppressed)	Low (suppressed)
Examples	Rice, wheat, soybean	Maize, sugarcane, sorghum	Cactus, pineapple, agave

🌍 Where Do We Find Each Type?

C3 plants dominate in cool, moist environments with moderate light. They're the majority of plant species because they evolved first and are perfectly adequate in non-stressful conditions .

C4 plants thrive in hot, sunny environments with seasonal rainfall. They're common in tropical grasslands, savannas, and agricultural systems in warm climates. Only about 3% of plant species are C4, but they account for ~25% of global primary productivity because they're so efficient in their environments .

CAM plants dominate in arid deserts and epiphytic niches (like tree branches in rainforests). Their ability to survive extreme drought makes them essential in dryland ecosystems .

🌾 Did you know? Some of the world's most important food crops are C4: maize, sugarcane, sorghum, and millet. These crops are particularly important in tropical regions where C3 crops like wheat would suffer from photorespiration .

🧑‍🌾 Horticultural Implications

Choosing the Right Crop for Your Environment

Understanding photosynthetic pathways helps farmers and horticulturists select appropriate crops for their climate:

In cool highlands (like Ethiopian highlands above 2000m), C3 crops like wheat, barley, and potato are suitable
In warm lowlands (like Ethiopian Rift Valley), C4 crops like maize, sorghum, and millet are better adapted
In arid regions with long dry seasons, CAM crops like cactus pear (opuntia) or drought-tolerant varieties may be appropriate

Management Differences

Practice	C3	C4	CAM
Irrigation need	Higher (more water loss)	Lower (better water efficiency)	Very low (succulent)
CO₂ enrichment	Highly beneficial	Less responsive (already concentrated)	Variable
Heat stress risk	High (photorespiration)	Low (adapted)	Low (adapted)
Shade tolerance	Better (lower light saturation point)	Poor (need high light)	Variable

🇪🇹 Ethiopian Application: Sorghum vs. Wheat

In Ethiopia's lowlands (e.g., around 1000m elevation), sorghum (C4) is a traditional staple because it reliably produces grain even in hot, dry conditions. Wheat (C3) grown in these areas suffers from photorespiration and produces low yields. As temperatures rise with climate change, C4 crops may become increasingly important even in areas where C3 crops are currently grown .

🔬 Recent Research: Engineering C4 Rice

One of the most ambitious projects in plant science is the C4 Rice Project, an international effort to introduce C4 photosynthesis into rice (a C3 crop). If successful, this could:

Increase rice yields by 30-50%
Improve water and nitrogen use efficiency
Help rice cope with higher temperatures from climate change

🌾→🌽 [Diagram: C4 rice concept - introducing Kranz anatomy to rice leaves — to be inserted]

The project involves introducing genes for Kranz anatomy and the C4 biochemical pathway into rice. While challenging, progress has been made, and some prototype lines exist. This could revolutionize rice production in tropical regions .

🌾 Did you know? If C4 rice is successful, it could help feed billions of people in Asia and Africa where rice is a staple, while reducing agriculture's water footprint .

📌 Unit Summary

C3 plants: Use Calvin cycle only, first product 3-PGA, suffer from photorespiration in heat/drought
C4 plants: Concentrate CO₂ using Kranz anatomy, first product oxaloacetate, high water use efficiency
CAM plants: Fix CO₂ at night, store as malic acid, use during day, extremely water efficient
Photorespiration: Wasteful process when rubisco fixes O₂ instead of CO₂; increases at high temperatures
Adaptation: Each pathway suited to different environments—cool (C3), hot (C4), arid (CAM)

Reflection question: Consider your region in Ethiopia (or another area you know well). What photosynthetic types dominate among local crops and wild plants? How does the climate explain this pattern? If temperatures rise by 2-3°C due to climate change, which crops might become more difficult to grow, and which might become more viable?

📌 Key terms introduced

C3 plant C4 plant CAM plant Photorespiration Kranz anatomy PEP carboxylase Bundle sheath cells Mesophyll cells Malic acid Water use efficiency

✅ Check your understanding

What is photorespiration, and why is it a problem for C3 plants in hot conditions?
Explain how Kranz anatomy helps C4 plants suppress photorespiration.
Why do CAM plants open their stomata at night rather than during the day?
A farmer in a hot, dry region wants to grow a grain crop. Would you recommend wheat (C3) or sorghum (C4)? Why?
Pineapple is a CAM plant. What advantage does this give it during a drought?

Discuss your answers in the course forum.

Plant Biochemistry for Horticulture · HORT 202 · Dilla University · Last updated March 2026