By the end of this lesson
- Describe how glasshouses and polythene tunnels increase yield.
- Link controlled conditions to limiting factors of photosynthesis.
- Evaluate the costs and benefits of protected cropping.
BIG QUESTION
How can tomatoes be grown in northern Europe in early spring?
An enclosed structure traps heat, allows extra carbon dioxide and lighting, and keeps out some pests and weather.
Trapping heat and extending the season
A glasshouseA greenhouse or polythene tunnel that lets light in and traps heat, so temperature, light, carbon dioxide and water for crops can be controlled. (greenhouse) or glasshouseA greenhouse or polythene tunnel that lets light in and traps heat, so temperature, light, carbon dioxide and water for crops can be controlled. lets light in and traps heat, so the air and soil inside are warmer than outside. Crops can be planted earlier, grown later and grown in cooler climates.
Polythene tunnels are cheaper and portable, so they are often used for field crops such as strawberries; glasshouses are more permanent and allow greater control.
Controlling limiting factors
Growers can add heating, artificial lighting and extra carbon dioxide, often from paraffin heaters that release CO₂ as they burn. Watering and mineral nutrients can be controlled precisely, sometimes using hydroponicsGrowing plants without soil, with their roots in a solution of mineral ions..
Raising whichever factor is limiting increases the rate of photosynthesisThe endothermic process in which green plants use light energy, absorbed by chlorophyll, to make glucose from carbon dioxide and water, releasing oxygen., so more glucose is available for growth and more crop is produced.
Protecting the crop
The enclosure protects crops from wind, heavy rain, frost and some pests and birds. Biological control can be used more effectively in an enclosed space.
However, warm, humid conditions can favour fungal disease and pests can spread quickly once inside, so ventilation and monitoring are needed.
Controlled conditions in protected cropping
Link each controlled condition to its effect on yield.
| Factor controlled | How it is controlled | Effect on yield |
|---|---|---|
| Temperature | Trapped heat; heaters. | Faster enzyme-controlled reactions and growth; longer season. |
| Light | Transparent cover; artificial lighting. | Photosynthesis continues for longer each day. |
| Carbon dioxide | Paraffin heaters or CO₂ supply. | Removes CO₂ as a limiting factor. |
| Water and minerals | Irrigation or hydroponics. | Prevents wilting and mineral deficiency. |
| Pests and weather | Physical barrier. | Less crop damage and loss. |
Language that earns precision
- yield
- The amount of useful crop produced per area.
- polythene tunnel
- A tunnel-shaped plastic structure that encloses crops.
- limiting factor
- The factor in shortest supply that restricts the rate of a process.
- hydroponics
- Growing plants in mineral nutrient solutions without soil.
Common misconceptions
Claim: Glasshouses make plants grow by giving them more oxygen.
Correction: They mainly control temperature, light and carbon dioxide for photosynthesis.
Claim: Enclosed crops cannot get pests.
Correction: Pests can enter and may spread quickly in warm, humid conditions.
Claim: Increasing one factor always increases yield.
Correction: Only increasing the limiting factor increases the rate.
Match the answer to the command word
Name the factor and the method of control, then link it to photosynthesis or protection.
Balance extra yield and season length against costs of building, heating and lighting.
Recall, apply, then justify
Describe two ways that growing crops in a glasshouse can increase yield.
Hint
Choose two different factors.
Suggested marking points
- Higher temperature (trapped heat or heating) increases rate of photosynthesis / growth.
- Extra CO₂, lighting or protection from pests/weather (any second distinct way).
Suggest why a farmer uses polythene tunnels rather than a glasshouse for strawberries.
Hint
Cost and flexibility.
Suggested marking points
- Polythene tunnels are cheaper to build.
- They can be moved to different fields / used over large areas.
Evaluate the use of artificial lighting in a tomato glasshouse in winter.
Planning hint
Limiting factors and economics.
What a strong design includes
- Winter light intensity and day length are low.
- Light is likely to be the limiting factor for photosynthesis.
- Extra lighting can increase photosynthesis and yield.
- Electricity costs money and may release CO₂ if from fossil fuels.
- Yield increase only continues until another factor becomes limiting.
- Lighting is worthwhile only if the value of extra crop exceeds the cost.