High light fluence impairs light‐harvesting Chl a/b complex apoprotein syntheses and/or membrane uptake in the CD3 mutant of wheat

Abstract

The CD3 mutant of wheat is a chlorophyll(Chlo‐deficient mutant the phenotype of which depends upon the accumulation of the light‐harvesting Chl a/b protein complex in leaves in response to the intensity of illumination. In the present studies, the rates of synthesis and/or uptake, and degradation of the light‐harvesting Chl apoprotein in chloroplasts of wild‐type wheat (Triticum aestivum L. selection ND 496) and CD3 wheat leaf segments were examined in response to two different intensities of illumination. We were interested particularly in the 21. 23 kDa proteins of the light‐harvesting Chl a/b complex of photosystem I (LHCI) and the 25. 27. 29 kDa proteins of the light‐harvesting Chl a/b complex of photosystem II (LHCII). The accumulation of [35S]‐Met into the light‐harvesting Chl protein of CD3 wheat chloroplasts was impaired by a high but not by a low light fluence. The levels of radiolabel in the supernatant fractions of leaf tissue homogenates from the wild‐type and CD3 wheats were not significantly different over time, suggesting that the cellular uptake of [35S]‐Met was not limiting in the mutant. The high fluence did not enhance the degradation of light‐harvesting Chl protein from CD3 wheat thylakoids. Our data indicate an impairment in the light‐harvesting Chl protein synthesis/membrane uptake system in CD3 wheat leaves under high fluence. A recovery in levels of the inner LHCPII, but not of LHCPI, was observed in the Chl‐deficient wheat mutant after a prolonged (4 days) exposure to high fluence. Under low fluence, LHCP was added to both photosystem II (PSH) and photosystem I (PSI) but only that added to PSI remained in thylakoids after seedlings were switched to high fluence.