From Tutor vista .com
Photosynthesis is influenced by two categories of factors – external or environmental and internal or plant factors. According to the law of limiting factors, put forward by F.F.Blackman in 1905, photosynthesis is limited by the most limiting factor. This means that, at any given time, only one factors is the most limiting factor among all and this factor determines the rate of photosynthesis.
1) Carbon dioxide
In C3 plants, the rate of photosynthesis increases with an increase. In CO2 concentration upto 500 ml.l-1.
In C4 plants, photosynthetic rate increases with an increase in CO2 concentration. However these plants attain saturation at around 360 ml.l-1 (as compared to C3 plants which is around 500 ml.l-1)
The current level of CO2 in the atmosphere is about 360-400 ml.l-1.
Response of C3 and C4 plants to CO2 concentration measured as rate of CO2 uptake
Photosynthesis could be enhanced by increasing the concentration of CO2.
Compensation Point or Threshold Value
It represents that CO2 concentration at which CO2 fixed during photosynthesis is equal to the volume of CO2 evolved during respiration.
2) Irradiance or Light Intensity
Both quality and intensity of light influence the rate of photosynthesis.
As the intensity of light increases, the rate of photosynthesis also increases. At a very high light intensity the rate of photosynthesis decreases as chlorophyll itself is destroyed due to high intensity. This phenomenon is called as photooxidation as it occurs in the presence of O2.
In moderate light intensities the rate of photosynthesis is directly proportional to irradiance.
High light intensity
Beyond saturation point, light intensity reduces photosynthesis. This effect is called solarization. It can be caused by photo-inhibition and photo-oxidation. (For grass mis management of 1000w HPS lamps can cause this).
Photo-inhibition
Highlight intensity increases the rate of transpiration and reduction in hydration of green cells. Stomata may close. Photo inhibition is not influenced by O2 concentration and temperature.
Photo-oxidation
Due to O2 there occurs destruction or oxidation of photosynthetic pigments and intermediate.
Wavelength of light
Wavelength of light between 400 nm and 700 nm is most effective for photosynthesis. This light is called photosynthetically active radiation (PAR).
Comparatively more photosynthesis occurs is red and blue regions though others have significant net photosynthesis. Light has maximum efficiency in red and minimum in blue region. In both these regions light is absorbed by chlorophylls. Red light favours more carbohydrate accumulation while blue light favours more protein synthesis.
Duration of light
If light intensities are not otherwise harmful, continuous photosynthesis can be sustained throughout 24 hours.
3) Temperature
Overall, the process of photosynthesis is sensitive to higher temperatures. This is because the enzymes involved in photosynthesis becomes inactive at higher temperatures. Low temperatures also inactivate the enzymes. Thus an optimum temperature is preferred.
The optimum temperature lies between 15o – 35 deg C.
For SeeGrow we have found the optimum temperature to be a leaf temperature of 21.5 Deg C
Oxygen
Small quantity of oxygen is required for optimum photosynthetic electron transport.
Aeration is essential
Soil water
If water availability in the soil decreases, plants undergo water stress. Under the condition of water stress, the rate of photosynthesis declines, as istomata fail to open and leaf water potential decreases.
It is observed that the rate of photosynthesis is reduced with the decrease hydration of the leaves. Care must be taken when using high intensity grow lamps for increased periods of time.
5) Minerals
Nutrients
Among nutrients nitrogen influences the rate of photosynthesis. Reduction in nitrogen supply adversely affects photosynthesis, as nitrogen forms the basic constituent of chlorophyll. In general, all essential elements also affect the rate of photosynthesis.
6) Leaf Factors
Leaf age, leaf angle and leaf orientation also effect the rate of photosynthesis.
Mg, Fe, Cu, Mn, Cl, S and K take part in photosynthesis.
Air Pollutants
They decrease the rate of photosynthesis.
Examples: Ozone, hydrogen fluoride, nitrogen oxide, sulphur dioxide, dust particles, smoke reduce light intensity and photosynthesis.
Notes:
- The compensation point is the point at which the rate of uptake of CO2 by a plant is equal to the rate of output of CO2. It indicates the point at which the rate of photosynthesis and rate of respiration are exactly equal.
- A limiting factor is the factor, in a chemical reaction involving several factors, which prevents the reaction rate from increasing because it is present at its minimum level.
- Temperature: At low temperature the activity of RuBisCo will limit the uptake of CO2 by RUBP. Thus the rate of formation of PGA and PGAL will be reduced.
- Light Intensity: Inadequate light intensity, no electrons will flow along the electron carrier chain and no ATP synthesis, no O2, no NADPH will occur.
- CO2: CO2 is a reactant required in the initial chemical reactions of the light independent Calvin cycle.
Steve Noel SeeGrow Developments Ltd
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