A new minimum fluorescence parameter, as generated using pulse frequency modulation, compared with pulse amplitude modulation: Fα versus Fo

Abstract

The minimum fluorescence parameter (F α), generated using the new pulse frequency modulation (PFM) technology, was compared with the minimum fluorescence parameter (F o), generated by pulse amplitude modulation (PAM), in response to a reversible low-oxygen stress in 'Honeycrisp'™ (HC) apples (Malus domestica) and an irreversible osmotic stress induced by water loss in two grape (Vitis spp.) cultivars ('L'Acadie' (LAc) and 'Thompson Seedless' (TS)). The minimum fluorescence values produced by both fluorometer types in response to a reversible low-oxygen stress in apples were indistinguishable: both F o and F α increased when O2 levels were lowered below the anaerobic compensation point (ACP); when gas levels returned to normoxia both parameters dipped below, then returned to, the original fluorescence baseline. The two parameters also responded similarly to the irreversible osmotic stress in grapes: in both cultivars, F α and F o first decreased before reaching an inflection point at approximately 20% mass loss and then increased towards a second inflection point. However, the two parameters were not analogous under the irreversible osmotic stress; most notably, the relative F α values appeared to be lower than F o during the later stages of dehydration. This was likely due to the influence of the F m parameter and an overestimation of F α when measuring the fluorescence from healthy and responsive chloroplasts as found in grapes experiencing minimal water loss, but not in grapes undergoing moderate to severe dehydration. An examination of the data during a typical PFM scan reveals this fluorometer system may yield new fluorescence information with interesting biological applications. © 2008 The Author(s).