HIGH LIGHT FLUENCE IMPAIRS LIGHT-HARVESTING CHL A B COMPLEX APOPROTEIN SYNTHESES AND/OR MEMBRANE UPTAKE IN THE CD3 MUTANT OF WHEAT/

The CD3 mutant of wheat is a chlorophyll(Chl)-deficient mutant the phe notype of which depends upon the accumulation of the light-harvesting Chl a/b protein complex in leaves in response to the intensity of illu mination. In the present studies, the rates of synthesis and/or uptake , and degradation of the light-harvesting Chl apoprotein in chloroplas ts of wild-type wheat (Triticum aestivum L. selection ND 496) and CD3 wheat leaf segments were examined in response to two different intensi ties of illumination. We were interested particularly in the 21, 23 kD a proteins of the light-harvesting Chl a/b complex of photosystem I (L HCI) and the 25, 27, 29 kDa proteins of the light-harvesting Chl a/b c omplex of photosystem II (LHCII). The accumulation of [S-35]-Met into the light-harvesting Chl protein of CD3 wheat chloroplasts was impaire d by a high but not by a low light fluence. The levels of radiolabel i n the supernatant fractions of leaf tissue homogenates from the wild-t ype and CD3 wheats were not significantly different over time, suggest ing that the cellular uptake of [S-35]-Met was not limiting in the mut ant. The high fluence did not enhance the degradation of light-harvest ing Chl protein from CD3 wheat thylakoids. Our data indicate an impair ment in the light-harvesting Chl protein synthesis/membrane uptake sys tem in CD3 wheat leaves under high fluence. A recovery in levels of th e inner LHCPII, but not of LHCPI, was observed in the Chl-deficient wh eat mutant after a prolonged (4 days) exposure to high fluence. Under low fluence, LHCP was added to both photosystem II (PSII) and photosys tem I (PSI) but only that added to PSI remained in thylakoids after se edlings were switched to high fluence.