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Plant and Cell Physiology, 1991, Vol. 32, No. 3 419-426
© 1991

Article

Irradiance Effects on Thylakoid Membranes of the Red Alga Porphyridium cruentum. An Immunocytochemical Study

Francis X. Cunningham, Jr.1, László Mustárdy2 and Elisabeth Gantt1,3

1Department of Botany, University of Maryland College Park, MD 20742, U.S.A.
2Institute of Plant Physiology, Biological Research Center, Hungarian Academy of Sciences P.O. Box 521, Szeged, Hungary H-6701

3 To whom correspondence should be addressed

Immunogold labelling on ultrathin sections of the red alga Porphyridium cruentum (ATCC 50161) was used to assess changes in the density and distribution of polypeptide components of photosystem I, photosystem II, phycobilisomes, and ATP synthase within the thylakoid membrane as a function of growth irradiance. In cells grown under a low, limiting quantum flux (6 microeinsteins per square meter per second of continuous white light) thylakoid membrane density and total thylakoid area per cell are 2 1/2 times greater than in cells grown under a high, saturating quantum flux (280 microeinsteins per square meter per second). Immunogold labelling data indicate that concentrations of photosystem I, photosystem II and phycobilisomes in thylakoids of low light-grown cells are only slightly greater than in cells grown under high light. In contrast, the concentration of ATP synthase within the thylakoid membrane is nearly ten times greater in high light-grown cells. Photosystem I polypeptides were detected in those portions of the thylakoid membrane which traverse the pyrenoid, but photosystem II and phycobilisomes appeared to be absent from these membranes. Ribulose-l,5-bisphosphate carboxylase was restricted primarily to the pyrenoid, and its concentration in the stroma or pyrenoid was little affected by the photon flux density. Quantitative estimates of photosystems I and II, phycobilisomes, and ATP synthase by spectroscopy or by immunoelectrophoresis are in accord with the immunogold results and lend support to the use of immunogold labelling for quantifying changes in relative amounts of membrane proteins.

(Received October 29, 1990; Accepted February 4, 1991)
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