1. Temperature:
When CO2, light and other factors are not limiting, the rate of photosynthesis increases with a rise in temperature, over a range from 6°C to about 37°C. Above this temperature, there is an abrupt fall in the rate and the tissue dies at 43°C. High temperatures cause the inactivation of enzymes and therefore affect the enzymatically controlled ‘dark’ reactions of photosynthesis.
Given other factors are limiting, the rate of photosynthesis follows Vant Hoffs rule between 6°C-30°C to 35°C i.e., it doubles with each increase of 10°C.
The optimum temperature falls between 20-30°C but varies with the plant species
In nature the maximum rate of photosynthesis due to temperature is not realized, because light or CO2 or both are limiting
2. Carbon Dioxide Concentrations:
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.
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
3. Light:
The photosynthetically active region of the spectrum of light is at wavelengths from 400-700 nm. Green light (550 nm) plays an important role in photosynthesis. Light supplies energy for the process.
Light varies in intensity, quality and duration. A brief account on these three aspects is given as follows:
i. light intensity:
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 different plant species. 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
ii. Light Quality:
The action spectrum for photosynthesis in leaves shows two major peaks, one in the red and the other one in the blue
It is of interest to note that plants show high photosynthesis in the blue and red light while red algae do so in green light and brown algae in blue light. The blue-green algae have action spectrum peak in yellow or orange light.
iii. light Duration:
Plant will accomplish more photosynthesis when exposed to long periods of light. It has also been found that uninterrupted and continuous photosynthesis for relatively long periods of time, may be sustained without any visible damage to the plant.
4. Oxygen:
The rate of photosynthesis increases by 30-50% when the concentration of oxygen in air is reduced from 20% to 0.5%
Oxygen is inhibitory to photosynthesis because it would favour a more rapid respiratory rate utilizing common intermediates, thus reducing photosynthesis.
Secondly, oxygen may compete with CO2 and hydrogen becomes reduced in place of CO2.
Thirdly, O2 destroys the excited (triplet) state of chlorophyll and thus inhibits photosynthesis.
It may be stated that direct effect of O2 on photosynthesis remains to be understood.
5. Water:
Water is an essential raw material in carbon assimilation. Less than 1% of the water absorbed by a plant is used in photosynthesis. The decrease in water contents of the soil from field capacity to the permanent wilting point results in the decreased photosynthesis.
The inhibitory effect is attributed to increased dehydration of protoplasm and also stomatal closure. The removal of water from the protoplasm also affects its colloidal state, impairs enzymatic efficiency, inhibits vital processes like respiration, photosynthesis etc. Dehydration may even damage the micro-molecular structure of the chloroplasts. Stomatal closure also reduces CO2 absorption. Water deficiency may cause drying of the cell walls of mesophyll cells, reducing their permeability to CO2. Water deficiency may accumulate sugars and thus increase respiration and decrease photosynthesis.
6. Mineral Elements:
As discussed earlier, several minerals are essential for plant growth. These include Mg, Fe, Cu, CI, Mn, P and are closely associated with reactions of photosynthesis.
7. Air Pollutants:
Gaseous and metallic pollutants decrease photosynthetic activity. These include ozone, SO2, oxidants, hydrogen fluorides, etc.
8. Chemical Compounds:
Compounds like HCN, H2S, etc. when present even in small quantities, depress the rate of photosynthesis by inhibiting enzymes. In addition chloroform, ether etc., also stop photosynthesis and the effect is reversible at low concentrations. However, at high concentrations the cells die.
9. Chlorophyll Contents:
The rate of photosynthesis in two varieties of barley having normal green leaves and yellow leaves was studied. The green-leaved variety contains ten times more chlorophyll than the yellow one. Clearly, the chlorophyll in the green leaves is surplus. Leaves having high chlorophyll content do not photosynthesize
rapidly since they lack the enzymes or co-enzymes to use the products of the light reactions to reduce available CO2.
10. Accumulation of Carbohydrates:
Accumulation of photosynthate in the plant cells, if not translocate, slows down and finally stops the process. The accumulated products increase the rate of respiration. Sugar is also converted into starch and the accumulation of starch in chloroplasts reduces their effective surfaces and the process slows down.
Anganifelix answered the question on May 9, 2018 at 09:44