(5) An introduction to what factors affect the rate of photosynthesis?

The rate of photosynthesis is usually limited by three main environmental conditions - factors:

• (i) Shortage of light (usually lack of sunlight) slows photosynthesis - since the greater the light intensity, the greater the rate of photosynthesis in the green chloroplast in plant cells.

• (ii) Low temperature, slows down the rate of photosynthesis - a general rule for all chemical reactions

  • A combination of both (i) and (ii) will cause very different rates between photosynthesis in winter (less sunlight time, less intense light, slower) compared to summer (more sunlight time, more intense light, faster).

  • At night, light is the limiting factor, in winter its usually the temperature in daylight.

  • If the temperature gets too high photosynthesis will slow down due to enzyme damage.

• (iii) A shortage of carbon dioxide will also slow down the rate of photosynthesis but you can artificially increase it by pumping CO₂ into a greenhouse structure.

  • If there is sufficient light and the temperature not too low, the ambient carbon dioxide concentration becomes the limiting factor.

• So, three factors affecting the rate of photosynthesis that can be investigated in the laboratory - see Parts (9) to (11).

• (iv) However, under some circumstances the essential green pigment chlorophyll might be the limiting factor too.

  • Lack of chlorophyll/chloroplasts in the plant cells reduce the plant's capacity to photosynthesise.

    • Stressed or damaged plants may turn pale yellow or develop spots from a fungus, bacteria or virus.

    • Plants maybe affected by disease eg halo blight, tobacco mosaic virus, poor nutrition - lack of vital minerals.

    • Also (v) lack of water denatures cells and plants droop, reducing photosynthesis, and eventually die.

    • Any of these factors can cause damage to chloroplasts or the cell cannot make enough chlorophyll.

    • Therefore the plant cell capacity to absorb sunlight is reduced weakening the plants growth and development.

    • See notes on Plant diseases and defences against pathogens and pests

• (v) Strictly speaking, lack of water is another factor, but that does affect the whole plant.

• Light intensity, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits the speed (rate) photosynthesis.

  • You can relate the principle of limiting factors to the economics of enhancing the following conditions in greenhouses.

  • You can carry out laboratory experiments to measure the rate of photosynthesis under various conditions i.e. changing any of the three factors and keeping the other two factors constant.

  • These experiments and graphical data analysis are discussed in detail in other Parts of this section.

  • For more on this read the section How to successfully operate a commercial  greenhouse!

For graph data and a more detailed discussions see parts

(6) Factors controlling the rate of photosynthesis - detailed discussion of typical data graphs

(8) More complex graphs involving more than one limiting factor controlling photosynthesis

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Keywords, phrases and learning objectives for this part on photosynthesis

Be able to describe and explain factors affecting the rate of photosynthesis including light intensity, carbon dioxide concentration, ambient temperature, formation of chlorophyll in chloroplasts and water limiting environmental conditions.

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