Title: The importance of limiting factors – Higher - Photosynthesis - AQA - GCSE Combined Science Revision - AQA Trilogy - BBC Bitesize
Headings: The importance of limiting factors – Higher
Photosynthesis - AQA
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The importance of limiting factors – Higher
Light
Carbon dioxide
Water
Temperature

Content: Plant growers can: use greenhouses constructed from glass or alternative materials that enable maximum transmission of light
position plants for maximum light absorption
Artificial lighting systems can be used: with increased light intensities
that provide optimum wavelengths of light for photosynthesis
that lengthen a plant's growing season so that crops can be grown in more months of the year
Carbon dioxide
Many plants have the capacity to use additional carbon dioxide. The table shows an example of data collected by Robb and Wittwer (1964) on plants grown in normal, and carbon dioxide-enriched greenhouses. Normal yield
Enriched yield
Lettuce
0.9 kg
1.1 kg
Tomatoes
4.4 kg
6.4 kg
To increase the carbon dioxide concentration in a growing area, plant growers: use paraffin heaters on a small scale
on a larger scale, use carbon dioxide wastes from industrial processes that has been cleaned up before use
can use liquid carbon dioxide, applied through pipes, though this can be expensive
sometimes use fungi, grown as a mycelium in bags around the greenhouse, that add carbon dioxide through their respiration
The concentration of carbon dioxide in greenhouses is often kept at around 0.1 per cent. Above one per cent, carbon dioxide changes cell pH – it makes it more acidic – and it becomes toxic. In greenhouses at higher temperatures, photosynthetic production declines, and carbon dioxide supplements have no effect. Water
Irrigation systems in greenhouses help to increase yields. Plants in greenhouses are often sprayed with water to keep the humidity high.