Analysis of changes in minimal and maximal fluorescence yields with irradiance and o(2) level in tobacco leaf tissue

The responses of minimal and maximal fluorescence yields of chlorophyll a to irradiance of actinic white light were determined by pulse modulated fluorimetry in leaf discs from tobacco, Nicotiana tabacum, at 1.6, 20.5, and 42.0% (v/v) O₂. Steady-state maximal fluorescence yield (F_m′, measured during a saturating light pulse) declined with increasing irradiance at all O₂ levels. In contrast, the steady-state minimal fluorescence yield (F_o′, measured during a brief dark interval) increased with irradiance relative to that recorded for the fully dark-adapted leaf (F_o) or that observed after 5 minutes of darkness (F_o^*). The relative magnitude of this increase was somewhat greater and extended to higher irradiances at the elevated O₂ levels compared with 1.6% O₂. Suppression of F_o′ was only observed consistently at saturating irradiance. The results are interpreted in terms of the occurrence of photosystem II units possessing exceedingly slow turnover times (i.e. “inactive” units). Inactive units play an important role, along with thermal deactivation of excited chlorophyll, in determining the response of in vivo fluorescence yield to changes in irradiance. Also, a significant interactive effect of O₂ concentration and the presence or absence of far red light on oxidation of photosystem II acceptors in the dark was noted.