Sub-saturating Multiphase Flash Irradiances to Estimate Maximum Fluorescence Yield

Abstract

Many intricacies of leaf-level photosynthesis can be probed by combining infrared gas analysis with pulse-amplitude-modulation chlorophyll a fluorometry. A key fluorescence yield (Φ_F) parameter required for estimating many of the phenomena associated with the light reactions of photosynthesis is referred to as the maximum Φ_F, which is termed F_m′ when measured on a light-adapted leaf. While ubiquitously used to assess many aspects of photosynthesis, F_m′ is problematic because it is prone to being underestimated. This error can be propagated to parameters and phenomena that are based on estimation of F_m′. Theoretical and experimental observations have shown that Φ_F increases hyperbolically in response to increasing irradiance, asymptotically approaching the maximum Φ_F, or F_m′, at extreme irradiances. Importantly, depending upon the convexity of the hyperbolic response, Φ_F exhibits a linear and inverse relationship with the reciprocal of irradiance, a relationship previously referred to as a reciprocal plot. Given the negative slope of the reciprocal plot, estimates of Φ_F at infinite irradiance can be obtained, even over sub-saturating irradiances, by linear regression and extrapolation of the resultant reciprocal plot to the y-intercept. Here, we show how to obtain data from a dynamic multiphase flash of sub-saturating irradiance, occurring within the time span of ~1 s, to generate a reciprocal plot that subsequently provides an accurate estimate of Φ_F at infinite irradiance, or F_m′.