Light- and cold-stress effects on the greening process in epicotyls and young stems of red oak (Quercus rubra) seedlings.

Protochlorophyllide (Pchlide) and protochlorophyll (Pchl) were found in epicotyls of 14-day-old dark-germinated seedlings and in 100-day-old dark-grown stems of red oak (Quercus rubra L.). Fluorescence spectroscopy measurements of epicotyls at 77 K showed that the majority of Pchlide and Pchl is present as a shorter wavelength-emitting monomer with a fluorescence emission maximum at 629-631 nm. A small amount of a monomeric form emitting at 635-636 nm was also present. Minor amounts of Pchlide were aggregated into larger complexes with fluorescence emission maxima at 640, 644-646 and 652-654 nm, as seen in etiolated leaves. Flash illumination transformed the 652-654-nm-emitting form to chlorophyllide, but not those forms with emission maxima at 629-631, 635-636 and 644-646 nm. These shorter wavelength-emitting forms were transformed to chlorophyllide by continuous illumination, but the process took several hours. Epicotyls and young stems were light sensitive, with exposure to full daylight causing strong pigment bleaching and tissue destruction. Complete greening took place only at low irradiances. Light sensitivity was greater at 4 degrees C than at room temperature. We conclude that the monomeric arrangement of the pigments accounted for the light and temperature sensitivity of the greening process in epicotyls and stems.