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IS Ih

Light intensity (i)

Figure 2. Light-response curve of photosynthesis (known as P-I curve). The intercept on the vertical axis is the measure of O2 uptake due to dark respiration. Ic, light compensation point; Is, light saturation intensity; Ih, light intensity value at which photoinhibition occurs. For details refer to the text.

IS Ih

Light intensity (i)

Figure 2. Light-response curve of photosynthesis (known as P-I curve). The intercept on the vertical axis is the measure of O2 uptake due to dark respiration. Ic, light compensation point; Is, light saturation intensity; Ih, light intensity value at which photoinhibition occurs. For details refer to the text.

with which the organism uses low light intensities. The relative quantum yield is expressed as moles of O2 evolved per mole of photons of incident PAR. Plotting the absorbed light (incident light minus reflected and transmitted light) instead of incident light will give the absolute quantum yield. The reciprocal of the quantum yield (i.e., the number of photons required to evolve one molecule of oxygen) is called the quantum requirement of photosynthesis. In the range of intensities where P does not vary with I, photosynthesis is said to be light saturated and the light intensity is said to be saturating (IJ\ P reaches its maximum value (Pmax) in this range. If light intensity further increases, P may decrease again. This phenomenon involves damage to some components of the photosynthetic apparatus and is referred to as photoinhibition. At low light intensities, P is limited by the light reactions of photosynthesis (thermodynamic constraint). At saturating light intensities, P (Pmax) is limited by the so-called dark reactions, in which the products of light reactions are used to fix CO2 (metabolic constraint). P-I curves are species specific and must be determined under conditions such that all the cells receive the same photon flux and are not mutually shaded.

Hill and Bendall's widely accepted Z-scheme of oxygenic photosynthesis incorporates two photosystems (PSII and PSI), and establishes that it is necessary to move four electrons through each photosystem to evolve one molecule of O2 and fix one molecule of CO2. Since the transfer of each electron requires two photons (one in PSII and one in PSI), it follows that the theoretical minimal quantum requirement cannot be lower than 8 (i.e., not less than eight photons are needed to move four electrons from water to NADP through the two photosystems) and the maximal quantum efficiency cannot be higher than 0.125 (1/8).

The curve relating growth to light intensity has a pattern similar to the P-I curve (with growth rate increasing proportionally to light intensity when intensity is limiting and being independent of intensity when saturating values are reached), since phototrophic growth is a direct consequence of the photosynthetic rate.

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