Light Reactions vs. Dark Reactions in Photosynthesis: …
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.