The low-light reduction in the quantum yield of photosynthesis: Potential errors and biases when calculating the maximum quantum yield
Photosynthesis-irradiance (P-E) curves are widely used to describe photosynthetic efficiency and potential. Contemporary models assume maximal photosynthetic quantum yield (φ) at low irradiances. But P-E observations made with both oxygen evolution and carbon uptake techniques show that this is not always the case. Using new and published data in conjunction with modeling exercises, we demonstrate that regardless of the mechanism there can be reductions in φ at low irradiances that are not readily observable using conventional P-E analyses. We also show that analytical errors, such as inaccurate estimation of dark oxygen consumption or carbon uptake, can markedly affect the structure of φ-E curves with negligible effect on P-E curve structure. Whether from respiration 'corrections' or other mechanisms, these deviations in φ at low light levels from the maximum quantum yield of photosynthesis (φmax) can lead to significant errors (> 50%) in the estimation of the linear portion of the P-E curve and ultimately φmax. Non-linear models of P-E, such as the rectangular hyperbola, quadratic, exponential and hyperbolic tangent that are commonly used to estimate the initial slope (α) of the P-E curve assume that φ is maximal at low light levels and therefore can err in the estimation of φmax when φ is reduced at low light levels. Using a diverse data set of 622 P-E curves with a total of 7623 points, we show that although model skills are high (r2 = 0.96 ± 0.05, 0.97 ± 0.04, 0.97 ± 0.04 and 0.97 ± 0.04, respectively), a large fraction of the model-predicted φmax differ by greater than 10% from true φmax values (91%, 50%, 82% and 46%, respectively). Data from these observations and modeling exercises lead us to suggest that φmax be determined by directly estimating the true maximum of a φ-E curve rather than using the more conventional methodology employing the initial slope of the P-E curve.
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