Compensation point light

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... 

The electrical circuit in Figure 8-17 and Equations 8.30 and 8.31c developed from it are able to portray the CO2 compensation point, the light compensation point, as well as the general interrelations of the fluxes of CO2 for photosynthesis, photorespiration, and respiration in the light and in the dark. Our discussion and Figure 8-17 are for C3 plants—to apply an electrical circuit analog to C4 and Crassulacean acid metabolism (CAM)... 

We next consider the light compensation point for CO2 fixation by leaves. As we mentioned in Chapter 8 (Section 8.4D), light compensation generally occurs at a PPF of about 10 pmol m-2 s-1 for a leaf temperature near 20°C... 

A. If the light compensation point for CO2 fixation is at a PPF of 8 pmol m-2 s-1, what are the cumulative leaf areas per unit ground area for light compensation when 2000, 200, 20, and 0 pmol m-2 s-1 are incident on the canopy ... 

D. The maximum upward flux of CO2 will occur for the level at the light compensation point (leaves below this level have a net evolution of C02)- At 200 pmol m-2 s-1 incident on the canopy, light compensation occurs at F= 4.6 (calculated in A). Because the leaf area index is 8.0, Fincreases by 1.0 every 2.0 m (16 m/8.0). Therefore, an F of 4.6 occurs at 9.2 m from the top of the canopy, or 6.8 m above the ground. 

The results of light saturating specific growth rate change and Tamiya plot analysis [6] are shown in Fig. 1. The maximum specific growth rate (/Umax), li t intensity at 1/2/u max and light compensation point were 0.32 h, 112 n E/hiVs and 8.15 /uE/mVs, respectively. 

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 ... 

The light dependence of photochemical recovery was studied at photon flux densities of 6, 36, 120 and 300 pmol m s. No difference in the rate of recovery was observed at these light levels (data not shown). A photon flux densitiy of 6 pmol m s was below the light compensation point of photosynthesis in all measurements. Still that light level was enough to induce a rapid recovery. The recovery of the photochemical efficiency therefore seems to be a light activated rather than a light driven process. 

Light compensation point the light intensity at which the rate of photosynthesis (COj incorporation) and the rate of respiration (CO2 production) are balanced. See C02-compensation point. 

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