FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS :

1. FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS:

2. 1)LIGHT • As light intensity increases, the rate of photosynthesis initially increases, and thereafter, levels off to a plateau.The photosynthetically active region of the spectrum of light is at wavelengths from 400-700 nm. When CO2 and temperature are not limiting and light intensities are low, the rate of photosynthesis increases with an increase in its intensity. At a point saturation may be reached, when further increase in light intensity fails to induce increase in photosynthesis. • Optimum or saturation intensities may vary with diff plant species e.g., C4 and C3. C3 plants become saturated at levels considerably lower than full sunlight but C4 plants are usually not saturated at full sunlight.

3. When the intensity of light falling on a photosynthesizing organ is increased beyond a certain point, the cells of that organ become vulnerable to chlorophyll catalyzed photo-oxidations. Consequently, these organs begin to consume O2 instead of CO2 and CO2 is released. Photo-oxidation is maximal when O2 is present or carotenoids are absent or CO2 concentration is low.

4. This plateau represents the maximum rate of photosynthesis ---as seen in the diagram. • Higher light intensity initially causes more electrons in the chlorophyll molecules to become excited (gain energy). Modern Biology (Holt)

5. As more and more electrons are excited, the light reactions occur more rapidly. • At a certain light intensity, however, all the available electrons are excited and a further increase in light intensity will not increase the rate of photosynthesis. Modern Biology (Holt)

6. What is a limiting factor? What is the ideal combination of factors for the maximum rate of photosynthesis? enough light enough carbon dioxide ideal temperature (not too hot or cold) How is the rate affected if one of these factors is restricted? If one of the factors is restricted, the rate of photosynthesis will be below the maximum possible rate. The restricted factor controls how quickly photosynthesis occurs and so limits the rate.Itiscalledthelimiting factor. Blackman’s Principle of Limiting Factors: “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor.”

7. How does the amount of light affect the rate of photosynthesis on this labelled graph? Photosynthesis and light – limiting factors As the amount of light increases, so does the rate. The limiting factor is light. 1 2 Here,increasingtheamount of light has no affect on the rate. The limiting factor is now carbon dioxide or temperature. What must be increased for the rate to increase? 2 rate of photosynthesis 1 light intensity

8. How does the amount of carbon dioxide affect the rate of photosynthesis on this labelled graph? Photosynthesis and carbon dioxide – limiting factors As the amount of carbon dioxide goes up, so does the rate. The limiting factor is carbon dioxide. 1 2 • Here, increasing the amount of carbon dioxide has no affect on the rate. Light or warmth is now the limiting factor. • What must be increased for the rate to increase? 2 rate of photosynthesis 1 concentration of carbon dioxide

9. Carbon dioxide The atmosphere is the chief source of carbon dioxide. It contains only 0.03 % of the gas by volume. It is very small amount. Therefore, CO2, remains a limiting factor. The increase in the amount of carbon dioxide increases the photosynthesis. concentration. But it slows down beyond this point. Higher concentrations have an inhibitory effect on photosynthesis. It is clear that increase in concentration of CO, increases the yield of plant

10. Nearly 0.032% by volume of carbon dioxide is present in the atmosphere and at this low level it acts as a limiting factor. Under laboratory conditions when light and temperature are not limiting factors, increase in CO2 concentration in the atmosphere from 0.03% to 0.3-1% raises rate of photosynthesis. With the further increase in the concentration of CO2 progressively the rate of carbon assimilation increases slightly and then it becomes independent of CO2 concentration. Thereafter, it remains constant over a wide range of CO2concentrations. Plants vary in their ability to utilize high concentrations of CO2. In tomatoes, high concentration of CO2, above the physiological range, exerts harmful influence causing leaf senescence. During the early period of the earth, the concentration of CO2 in the atmosphere was as high as 20%.

11. 3) Temperature a) Raising the temperature accelerates various chemical reactions of photosynthesis. As a result, the rate of photosynthesis increases, over a certain range.

12. b) The rate of photosynthesis generally peaks at a certain temperature, as seen in the graph. • c)Above this temperature, the rate decreases.

13. Temperature A suitable temperature is necessary for photosynthesis. There are three cardinals of temperature for photosynthesis. (a)   Minimum: It is minimum temperature at which the photosynthesis starts. The plants of cold and temperate regions have lower values of these cardinals. But tropical plants have higher sable of these cardinals. Minimum temperature for many lichens is – 20°C. It is – 350 C for some conifers. Photosynthesis hardly starts at about 5°C in tropical plants. Desert plants like cactus can carry on photosynthesis even at 55°C (b) Optimum: Maximum photosynthesis occurs at that point The optimum temperature also varies greatly. Photosynthesis increases with rise in temperature up to 25°C. This increase follows Vant Hoffs law. According to this law the rate of chemical reaction doubles for every rise of 10°C. This is true only if light or carbon dioxide is not the limiting factors. (c)   Maximum: It is the highest temperature at which photosynthesis can take place. There is an initial increase in the rate of photosynthesis at this temperature. But this is soon followed by a decline. Higher the temperature the more rapid is the decline. The decline may be due to one or more of the following causes: (i)    Accumulation of the end products of photosynthesis. (ii)   Inhibitory effect of high temperature on the activity of enzymes. (iii) Failureof carbon dioxide to diffuse rapidly.

14. In nature the maximum rate of photosynthesis due to temperature is not realized, because light or CO2 or both are limiting. The response curve of net photosynthesis to temperature is different from those for light and CO2. It shows minimum, optimum and maximum temperatures. Between the C3 and C4 plants, while the former species have optimal rates from 20-26°C, the latter species may show optimal rates from 35-40°C. Similarly, temperature also influences the light (optimum, 30-35°C) and dark respiration (optimum 40-45°C).

15. d) As the temperature increases, the stomates begin to close, to limit water loss. This will have the effect of stopping the carbon dioxide from enteringthe leaf. This will also decrease the rate of photosynthesis. (Also: Enzymes do not function well at too high a temperature.)

16. 4) Water • A lack of water will also slow the rate of photosynthesis. Stomata can close from water loss. • Plants such as the cactus have adaptations to prevent water loss in dry, desert climates.

17. 5. OXYGEN Photosynthesis does not take place in cells which lack oxygen. There are two reasons of this. First, the energy produced in oxygen respiration is necessary for photosynthesis. Second. oxygen is required for the production and maintenance of some substance. This substance is essential for photosynthesis. High concentrations of oxygen inhibit the rate of photosynthesis. It promotes photorespiration. 6. LEAF ANATOMY