Low measuring temperature induced artifactual increase in chlorophyll a fluorescence.

Measurement of in vivo chlorophyll a fluorescence at temperatures lower than 20 degrees C can cause an artifactual, nonphotochemically related overestimation of variable fluorescence leading to the calculation of negative values for the nonphotochemical quenching parameter and an underestimation of the photochemical quenching parameter. This artifact is observed only upon exposure of the leaf sample to actinic light. We suggest that a temperature differential between the fiber-optic probe and the leaf sample results in the deposition of water vapor on the probe that distorts the light path such that an increased modulated fluorescence signal is observed. This artifact is eradicated by ensuring that the end of the fiber-optic probe is kept free of condensation.