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CSEC Biology: Factors Affecting Photosynthesis, the Limiting Factor and Minerals in Plant Nutrition

You may already be familiar with the conditions necessary for photosynthesis:

  1. Carbon dioxide, which must diffuse into the leaf via the stomata

  2. Sunlight, which is absorbed by chlorophyll in the chloroplasts and converted to chemical energy

  3. Water, which is carried up via the xylem from the roots

  4. Chlorophyll, which absorbs light energy

  5. Enzymes present in chloroplasts necessary to execute the light independent stage of photosynthesis

  6. Suitable temperature, between 5°C and 40°C

Plants also require specific mineral ions such as nitrate and magnesium from the soil so that they can manufacture proteins like chlorophyll and enzymes.

Based on this, you can already begin to think about the factors affecting the rate at which photosynthesis occurs.

Four main factors affect the rate of photosynthesis: light, water, carbon dioxide supply and temperature.

If all of these factors are at an ideal level, then photosynthesis occurs at its maximal rate. However, this is rarely the case. It's like an easily distracted person trying to study- there always seems to be something not quite right that stops them from studying at an optimal rate. For photosynthesis, the factor in shortest supply limits the rate at which it occurs. This factor of shortest supply is known as the limiting factor (law of limiting factors). The limiting factor varies based on season and/or time of day:

  1. Light will be the limiting factor between dusk and dawn (nighttime), and also during winter months.

  2. Water will limit the rate of photosynthesis in the dry season/summer months, and also when the ground is frozen in colder climates.

  3. Temperature is the limiting factor during cold winter months.

  4. Carbon dioxide is the limiting factor during the day in most climates, as the concentration of CO2 is very low (approx. 0.04%)

The graphs for the other factors look very similar to this one, except for the graph of rate photosynthesis against temperature:

As you can see, the ideal temperature is right in the center, and as the temperature increases or decreases beyond this, the rate of photosynthesis decreases. This is the sort of 'sweet spot' is the temperature at which enzyme activity is greatest, so another factor must be the limiting factor. (We will discuss enzymes in a future post.)

Minerals in Plant Nutrition

Plants need several inorganic minerals for their healthy function. These minerals are absorbed through the roots of the plant as mineral ions dissolved in water.

  1. Nitrogen is absorbed as nitrate ions. They are used in making proteins for plant growth and chlorophyll. If nitrogen is deficient, the plant will experience poor growth, chlorosis (the yellowing of leaves) and underdeveloped leaves.

  2. Magnesium is absorbed as magnesium ions- it is used in the synthesis of chlorophyll. In the deficiency of magnesium, the plant experiences chlorosis.

  3. Phosphorous, absorbed as phosphate ions, is used in the formation of ATP and some proteins. If phosphorous is deficient, the plant will experience stunted stem and root growth, and dull/purplish leaves with browned edges.

  4. Potassium is absorbed as potassium ions. It is used in maintaining salt balance in cells (i.e. controlling osmotic pressure) and helping in photosynthesis. If potassium is low in supply, the leaves will die prematurely and have yellow-brown margins and brown spots.

  5. Sulfur, obtained as sulphate, is used in protein synthesis. Without it, the plant experiences stunted growth and chlorosis.

  6. Calcium is absorbed as calcium ions. Calcium is used in making cell walls in the tips of growing roots and shoots (meristems). Without it, the plant's growth is stunted, with poor bud development and the death of root and shoot tips.

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