Light photosynthetic compensation point

Figure 8-19. Idealized hyperbolic relationship between the photosynthetic photon flux incident on the upper leaf surface and the net C02 uptake rate for a C3 plant. The intercept on the ordinate (y-axis) indicates the net COz flux by respiration in the dark (-1 pmol m-2 s 1), the intercept on the dashed line indicates the light compensation point (a PPF of 15 pmol m 2s l), the essentially linear initial slope (37co2 ppf) indicates the quantum yield (Eq. 4.16) for photosynthesis [(5 - 0 pmol m 2 s l)/(115 -15 pmol m-2 s l) = 0.05 mol C02/mol PPF], and the maximum Jco2reached asymptotically at high PPF indicates the light-saturated net C02 uptake rate (about 12 (xrnol m-2 s l often designated AmaK or Amax). Here the quantum yield is based on incident photons, but more appropriately it should be based on absorbed photons.

If we reduce the amount of light incident on a leaf from the value for direct sunlight, we eventually reach a PPF for which there is no net CO2 uptake (Fig. 8-19). This PPF for which Jcch is zero is known as the light compensation point for photosynthesis. Because photorespiration depends on photosynthetic products, both photorespiration and gross photosynthesis decrease as the PPF is lowered. Hence, the light compensation point for leaves is approximately the same for C3 and C4 plants—at 20° C and 380 pmol CO2 mol-1 in the turbulent air near a leaf, light... 

What is the light intensity at 1-m depth in a lake, given an intensity of 3000 microeinsteins (/xE)/(m2 sec) just beneath the lake s surface and an extinction coefficient of 0.6/m If an aquatic plant has a light compensation point (the light intensity at which respiration rate equals photosynthetic rate) of 150 fiE/(m2 sec), what is the maximum depth at which the plant may be expected to grow ... 

Respiration is essentially the converse of the process of photosynthesis. If the two processes occur concurrently, then the rate of photosynthesis as actually observed (net photosynthesis) will be less than the total rate (gross photosynthesis) by an amount equal to the rate of consumption of the products of photosynthesis in respiration. At certain low light intensities or carbon dioxide concentrations the two processes balance so that there is no net gas exchange and the photosynthetic organism is then said to be at its compensation point. It has usually been assumed, according to Fogg... 

Table A smnmary of the photosynthetic and respiratory O2 exchange of leaves grown at different canopy positions. Data points were en from a combination of four separate hght curves per canopy level. Light saturation points were estimated visually, and maximum net photosynthesis values are Wed on rates at 1800 /iEi m s PAR. Apparent quantum yields, hght compensation points, and dark respiration rates were determined by a linear equation fitted to hght intensity points ranging from 0 to 102 /iEi m s (r > 0.9 in ah cases). Absorbed photon yields can be estimated by dividing apparent quantum yields by the average leaf absorbancy.

Osmond and Bjorkman (1975) found that light-dependent CO2 fixation in K. daigremontiana was inhibited by O2 in the same manner as C3 plants, whereas dark CO2 fixation was independent of 4%, 26%, or 36% O2. Further, O2 inhibition of photosynthetic CO2 fixation is accompanied by a high CO2 compensation point (Allaway et al., 1974a). Unlike C3 plant photorespiration, however, the O2 inhibition of photosynthetic CO2 fixation is not eliminated by high CO2 tension, and the postillumination CO2 burst is present at low O2 (Osmond and Bjorkman, 1975 see Fig. 5.13). 

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