Compensation point

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image icon Light and CO2 curves for notoginseng plants at different nitrogen levels. The compensation points is where the photosynthetic rate becomes zero.

The light compensation point (Ic) is the light intensity on the

O2
by respiration is equal to the photosynthetic release of oxygen. The concept of compensation points in general may be applied to other photosynthetic variables, the most important being that of CO2 concentration – CO2 compensation point (Γ).Interval of time in day time when light intensity is low due to which net gaseous exchange is zero is called as compensation point.

In assimilation terms, at the compensation point, the net carbon dioxide assimilation is zero. Leaves release CO2 by photorespiration and cellular respiration, but CO2 is also converted into carbohydrate by photosynthesis. Assimilation is therefore the difference in the rate of these processes. At a given partial pressure of CO2 (0.343 hPa in 1980 atmosphere[1]), there is an irradiation at which the net assimilation of CO2 is zero. For instance, in the early morning and late evenings, the light compensation point Ic may be reached as photosynthetic activity decreases and respiration increases. The concentration of CO2 also affects the rates of photosynthesis and photorespiration. Higher CO2 concentrations favour photosynthesis whereas low CO2 concentrations favor photorespiration, producing a CO2 compensation point Γ for a given irradiation.[2]

Light compensation point

As defined above, the light compensation point Ic is when no net carbon assimilation occurs. At this point, the organism is neither consuming nor building biomass. The net gaseous exchange is also zero at this point.

Ic is a practical value that can be reached during early mornings and early evenings. Respiration is relatively constant with regard to light, whereas photosynthesis depends on the intensity of sunlight.

Depth

For aquatic plants where the level of light at any given depth is roughly constant for most of the day, the compensation point is the depth at which light penetrating the water creates the same balanced effect.

CO2 compensation point

The CO2 compensation point (Γ) is the CO2 concentration at which the rate of photosynthesis exactly matches the rate of respiration. There is a significant difference in Γ between C3 plants and C4 plants: on land, the typical value for Γ in a C3 plant ranges from 40–100 μmol/mol, while in C4 plants the values are lower at 3–10 μmol/mol. Plants with a weaker

C2 photosynthesis, may display an intermediate value at 25 μmol/mol.[3]
: 463 

The μmol/mol unit may alternatively be expressed as the

Rubisco. It may be measured by whole-leaf isotopic gas exchange, or be estimated in the Laisk method using an intermediate "apparent" value of C* with correction.[4] C* approximates Γ* in the absence of carbon refixation, i.e. carbon fixation from photorespiration products. In C4 plants, both values are lower than their C3 counterparts. In C2 plants that operate by refixation, only C* is significantly lower.[5]

As it is not yet common to routinely change the CO2 concentration of air, the concentration points are largely theoretical derived from modeling and extrapolation, though they do hold up well in these applications. Both Γ and Γ* are linearly related to the partial pressure of oxygen (p(O2)) due to the side reaction of Rubisco. Γ is also related to temperature due to the temperature-dependence of respiration rates. It is also related to irradiation, as light is required to produce

RuBP (ribulose-1,5-bisphosphate), the electron acceptor for Rubisco. At normal irradiation, there would almost always be enough RuBP; but at low irradiation, lack of RuBP decreases the photosynthetic activity and therefore affects Γ.[2]

The marine environment

Main article: Critical depth

Respiration occurs by both plants and animals throughout the water column, resulting in the destruction, or usage, of organic matter, but photosynthesis can only take place via photosynthetic algae in the presence of light, nutrients and CO2.[6] In well-mixed water columns plankton are evenly distributed, but a net production only occurs above the compensation depth. Below the compensation depth there is a net loss of organic matter. The total population of photosynthetic organisms cannot increase if the loss exceeds the net production.[6][7]

The compensation depth between photosynthesis and respiration of phytoplankton in the ocean must be dependent on some factors: the illumination at the surface, the transparency of the water, the biological character of the plankton present, and the temperature.[7] The compensation point was found nearer to the surface as you move closer to the coast.[7] It is also lower in the winter seasons in the Baltic Sea according to a study that examined the compensation point of multiple photosynthetic species.[8] The blue portion of the visible spectrum, between 455 and 495 nanometers, dominates light at the compensation depth.

A concern regarding the concept of the compensation point is it assumes that phytoplankton remain at a fixed depth throughout a 24-hour period (time frame in which compensation depth is measured), but phytoplankton experience displacement due to isopycnals moving them tens of meters.[9]

See also

References

  1. ^ ESRL / Mauna Loa CO2 annual mean data, [1], [2]
  2. ^ a b Farquhar, G. D.; et al. (1982). "Modelling of Photosynthetic Response to Environmental Conditions". In Lange, O.L.; et al. (eds.). Physiological Plant Ecology II. Water Relations and Carbon Assimilation. New York: Springer-Verlag. pp. 556–558.
  3. S2CID 243028576
    .
  4. PMID 23630324
    .
  5. doi:10.1111/gcbb.12819
    .
  6. ^
    doi:10.1093/icesjms/18.3.287
    .
  7. ^
    doi:10.1139/f35-012
    .
  8. S2CID 85197994
    .
  9. doi:10.1016/j.dsr.2014.07.011
    .
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