Light and dark reactions in photosynthesis - …

Photosynthetic efficiency in low light confers a selective advantage on shade-adapted plants, but also renders them especially vulnerable to full sun. Accordingly, such species have evolved remarkable features for photoprotection. Their acclimation to sun and shade, together with properties of sun-loving plants, thus reveal an extraordinary plasticity in the photosynthetic apparatus of vascular plants (Section 12.1). Even increased UV-B radiation, commonly associated with global change, and the so called ‘ozone hole’ over Antarctica, elicits responses that offer photoprotection to plants (Section 12.6).

Light Reactions vs. Dark Reactions in Photosynthesis: …

There are two pathways associated with photosynthesis: light and dark reactions.

Dark Reactions in Photosynthesis

Since shade plants have a lower Pmax than sun plants, they experience more excess light at a given photon irradiance above saturation. Additional stresses such as drought, nutrient limitation or temperature extremes can also lead to a reduction in Pmax and thus increase the probability of plants being exposed to excess light. However, even the most hardy sun plant will reach Pmax at less than full sunlight (incident beam normal to leaf surface). At that level (say, 1000 µmol quanta m–2 s–1), approximately 25% of absorbed energy is used to drive photosynthesis, but at full sunlight (c. 2000 µmol quanta m–2 s–1) as little as 10% is used (Long et al. 1994). Individual leaves on plants growing in full sun commonly experience such excess light intensities. This is potentially damaging, and plants adapted to full sunlight have evolved a number of mechanisms for either avoiding excess light or for dissipating the excess absorbed energy.

Difference between Light and Dark Reactions in Photosynthesis ..

In the light-response curves for photosynthesis (Figure 12.7 above), photosynthesis is regarded as light limited in the initial linear region of the curve. However, at higher photon irradiances once the light-saturation point (that is, the maximum rate of photosynthesis) has been reached, further increases in light will exceed the energy-utilising capacity of that photosynthesising leaf. Refer to the dashed lines in Figure 12.7 which represent a continuation of the initial rate of photosynthesis (quantum yield of photosynthesis) and demonstrates the actual light absorption. The extent to which this absorbed light is not ‘gainfully employed’ for photosynthesis is set by Pmax (the light-saturated rate of photosynthesis in normal air). At low light (–2 s–1), both sun and shade leaves use more than 80% of the absorbed light for photosynthesis. However, once Pmax has been reached, all additional absorbed light is in excess of that which can be used in photosynthesis.

Photosynthesis consists of light-dependent and light-independent reactions.
The lightness represents peace and the darkness represents disastrous events or death.

Light and Dark Effecting Photosynthesis and Cellular Res.

which depends on the maximal Rubisco activity and provided the quantitative framework for comparing rates of CO2 assimilations with the amount of Rubisco present in leaves (von Caemmerer and Farquhar 1981). Difference in CO2 assimilation rates observed under different growth conditions could then be explained according to variations in the amount of Rubisco present in leaves. In Figure 2 the dotted line shows a CO2 response curve modelled by Equation 7. Chloroplast CO2 partial pressure was then assumed to be similar to that in the intercellular airspaces. Using on-line discrimination between 13CO2 and 12CO2, and deriving an estimate of CO2 partial pressure at fixation sites within chloroplasts, we subsequently learned that a further draw down can occur, but the general applicability of Equation 7 was not compromised. As an aside, these equations became basic to most photosynthetic models long before the order of the reaction mechanism of Rubisco had been unequivocally established. Had CO2 and O2 bound to Rubisco before RuBP, or the reaction not been ordered, our equations would have been much more complex with both m(CO2) and m(O2) dependent upon RuBP concentration.

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What is the difference betweem the light- and the dark ..

C4 photosynthesis also offers a potential for growth rates almost twice those seen in C3 plants, but this potential will only be seen at higher temperatures and higher light and this will not be evident in all C4 species. With this kind of growth potential, it is not surprising that C4 species also number among the world’s worst weeds!

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Leaf-level photosynthesis responds to changes in light and ..

Patches of sunlight move across the forest floor on bright days, and will illuminate any leaves or parts of leaves which lie in their path (Figure 12.17). Daily total photon irradiance on the occasion of the measurement shown in Figure 12.17 was 6.0 mol quanta m-2 d-1). The abrupt increase from a background level of around 50 µmol quanta m-2 s-1 to 1,750 µmol quanta m-2 s-1 energises photosynthesis, but is counteracted by increased leaf temperature, especially during prolonged exposures. Transpiration cooling is a significant component of heat budgets for such large leaves, so that adequate soil moisture is a prerequisite for continuing leaf gas exchange during a sunfleck.