Fluorescence and Photochemical Properties of Plants with Defective Photosystem II

Photosystem II, Low Temperature Fluorescence, Tobacco, Neottia The mykotrophic orchid Neottia nidus-avis does not evolve oxygen in the light but is able to perform photophosphorylation. The low temperature fluorescence emission spectrum lacks the 680 and 690 nm bands. Hence, the spectroscopic chlorophyll a forms which are attributed to photosystem II do not occur in plastids of this orchid. The low temperature excitation spectrum of photosystem I fluorescence exhibits a maximum at 666 nm. The position of this maximum ap­ pears not to be influenced by energy transfer and corresponds to the absorption maximum of the chlorophyll form which emits the photosystem I fluorescence. Energy migration, however, occurs from carotenoids whose absorption spectrum is shifted to longer wavelengths and which cause the yellow-brown color of the Neottia plastids. Room temperature fluorescence emission shows after the onset of light no variable part. Despite the fact that plastids of the tobacco mutant NC 95 at most evolve only traces of oxygen the low temperature emission spectrum shows the three bands which are usually observed with fully functioning chloroplasts. However, the two bands at 680 and 690 nm are distinctly lower than with the wild type. The variable portion of room temperature fluorescence is barely detectable. In line with the very low capacity for oxygen evolution, rates of electron transport partial reactions in the region of photosystem II are extremely low. In agreement with this observation no 690 nm absorption change signal is detected. However, a normal P7+00 signal is seen. In the presence of electron donors like reduced phenazine methosulfate the decay time of the P7̂ 0 signal is faster than with the wild type. The yellow tobacco mutant Su/su var. aurea which exhibits at high light intensities higher rates of photosynthesis than the wild type shows at low temperature an emission spectrum with stronger photosystem II bands than the wild type.