Chlorophyll organization in dark-grown and light-grown pine (Pinus brutia) and barley (Hordeum vulgare)

A study was conducted comparing the organization of chlorophyll during development of the photosynthetic apparatus in dark-grown and light-grown pine and barley. The rationale was that gymnosperms, but not angiosperms, have a capacity to synthesize chlorophyll in darkness. Seedlings of Pinus brutia were germinated and grown in darkness or under photoperiodic (day/night) conditions. The low-temperature (77 K) fluorescence spectra of newly-emerging dark-grown seedlings exhibited a single fluorescence band peaking at 678–679 nm, which decayed primarily with a ～5.5 ns lifetime. Over the first few days of growth, the emission shifted to longer wavelengths and a subnanosecond lifetime component became prevalent. After several days of dark growth the emission spectrum and lifetime profile of the low temperature fluorescence came to resemble those of light-grown pine and barley. At room temperature, dark-grown pine showed little variable fluorescence, though addition of DCMU caused a substantial fluorescence rise. Illumination with moderate light for a few hours was sufficient to 'photoinduce' the appearance of normal variable fluorescence. At 77 K, DCMU-treated samples clearly showed a very long-lived (～40 ns) fluorescence lifetime component in light-grown pine and barley. This component was undetectable in dark-grown pine. If, however, dark-grown samples were illuminated either before or after DCMU addition and then frozen to 77 K, the ～40 ns lifetime component appeared at a fluorescence intensity similar to that in light-grown samples. These results are explained primarily in terms of photoactivation of the photosystem II (PSII) donor side. The temporary maintenance of PSII in an inactive, highly-quenched state is suggested to provide an available, yet protected precursor for active PSII.