The Measurement of Rate of Photosynthesis as a Function of Photon Flux Density and the Significance and Implication of These Measurements
Photosynthesis is driven by light. Particularly in low or moderate light, its rate is therefore largely determined by the rate at which photons reach the photosynthetic apparatus. When the rate of photosynthesis is plotted against the rate of arrival of photons (photon flux density or PFD) the relationship (Fig. la) is full of information. The intercept on the vertical axis is a measure of dark respiration. The intercept on the horizontal axis is the “light compensation point” (LCP). This, again, is largely determined by the rate of dark respiration because it is the PFD at which respiratory O2 uptake and photosynthetic oxygen evolution come into balance. In the region between the LCP and 100 µmole quanta.m-2.s-1 the relationship is nearly linear and this initial slope gives the maximum efficiency of energy transduction; usually expressed as quantum yield (O2 evolved per photon absorbed) or its reciprocal, quantum requirement (number of photons required to bring about the evolution of one molecule of oxygen). As will be seen, other information may be derived from the overall rate v PFD relationship by appropriate analysis. Although the importance of this relationship has been long recognised it is only recently that its measurement has become a practical possibility as a routine procedure in non-specialist laboratories.
KeywordsQuantum Yield Initial Slope Dark Respiration Shade Leave Ribulose Bisphosphate
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