Many external and internal factors affect the rate of photosynthesis. The external or environmental factors at:A light intensity, carbon dioxide concentration and temperature. The internal factor influencing the photosynthesis is chlorophyll content of the leaves and protoplasmic factors.
Light is essential for photosynthesis. Photosynthesis does not take place in dark. The sun is the main source of light energy. Both quality and
intensity of light are important for photosynthesis. •
(a) Light Quality: The light consists of rays of different wavelengths.
Only red and blue light are effective for photosynthesis. Green light is reflected or transmitted. Therefore, it does not play role in photosynthesis. Light of wavelength longer than 700 run is not effective for photosynthesis for green plants. Experiments of Engelmann proved that maximum photosynthesis occurs in the red and blue part of the spectrum.
(b) Light Intensity: Photosynthesis begins at very low intensity. It becomes maximum at bright daylight. But it decreases in strong light. Different plants require different intensity of light. Most of light reaching green leaves is reflected or transmitted. Thus only a small part of light is absorbed. Thus only about 0.5 to 1.5% of light energy is in photosynthesis. Thus light is not a limiting factor at high intensity.
Light is a limiting factor at low intensity. Thus the rate of photosynthesis increases with an increase in light intensity. High light intensities affect the rate of photosynthesis. It increases the temperature of the leaves. Therefore, rate of transpiration increases. The stomata are closed. It stops the . entry of CO2. Thus photosynthesis is stopped. Light also cause photorespiration. Photorespiration reduces the yield.
- 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, CO, remain a limiting factor. The increase in the amount of carbon dioxide increases the photosynthesis. This increase is more rapid up to I % of carbon dioxide 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
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)Failure of carbon dioxide to diffuse rapidly.
(iv) Increased consumption of the photosynthate in photorespiration
(v) Destructive effect of high temperatures on chlorophyll.
Water is one of the raw materials of photosynthesis. The amount of water actually used in photosynthesis is very small. Less than 1 percent water is absorbed by the plant. Therefor, it cannot be a limiting factor directly. But the water content of the leaf often acts as a limiting factor indirectly. The limiting effect of water is indirect. It maintains the turgor of the assimilatory cells. The rate of photosynthesis decreases in the cells which have lost their turgor, The loss of turgor of guard cells closed the stomata. It reduces the rate of photosynthesis.
Photosynthesis does not take plate 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
Many…factors of leaf anatomy affect the rate of photosynthesis. These factors influences the diffusion of carbon dioxide thorough the stomata. They also effect on the amount of light reaching the chlorenchyma. These factors are:
(i) Different leaves have different thickness of the cuticle and epidermis.
(ii) They develop palisade. They have different sizes and distributions of the intercellular spaces.
(iii)They have difference sizes, positions and distributions of the stomata.
(iv) They develop different types of chlorenchyma and the vascular tissues.
7. CHLOROPHYLL CONTENT
Chlorophyll is essential for photosynthesis. Etiolated plants and non-green tissues do not show photosynthesis. The green cells produce starch in variegated leaves. There are two views about the affect of chlorophyll on photosynthesis:
(i) Willstatter and Stoll: They believe that the rate of photosynthesis is not proportional to the amount of chlorophyll content. The rate of photosynthesis depends on the concentration of enzymes and chlorophyll.
(ii) Emerson (1927): He found a direct relationship between the amount of food formed and the chlorophyll content.
8. PROTOPLASMIC FACTOR
Photosynthesis does not start immediately after the appearance of chlorophyll in very young leaves. It starts after some time. Same thing happens when plants are transferred from dark to light. Thus some internal factor is present in the protoplasm of the cells. This is called “the unknown factor” or the “protoplasmic factor” It is enzymatic in nature. Arnon (1954) has demonstrated that cell free chloroplasts are capable of carrying out photosynthesis. This indicates that protoplasm is not necessary for photosynthesis. Thus the chloroplasts are the complete photosynthetic units. They contain all the necessary enzymes.