We have examined the effect of gabaculine (3-amino-2,3-dihydrobenzoic acid) on chlorophyll (Chl) binding photosystem (PS) apoproteins in thylakoid ineinbranes relative to the effect on pigment accumulations and grana thylakoid stacking in wheat {Triticum aesíivum L.) seedling leaves. The accumulations of critical PSI, PS2, light- harvesting complex (LHCl and LHC2) apoproteins differed in response to gabaculine treatment. The reaction center proteins. Dl of PS2 and the 68 kDa PSI, were not detected in either primary or secondary leaf chloroplasts of treated etiolated seedlings after greening or secondaiy leaf chloroplasts of treated green seedlings. However, the LHC apoproteins accumulated at reduced levels relative to the Controls and LHCl was more sensitive to treatment than LHC2. Gabaculine decreased plastid pigment levels in primary leaf chloroplasts of treated etiolated seedlings by only 50 % and in secondaiy leaf chloroplasts by more than 85 % of that in the Controls. The accumulations of Chl o, Chl h, and the carotenoids, neoxanthin and í3-carotene, were most sensitive to treatment and were decreased in a similar manner. Granal thylakoid development was more sensitive to treatment than stroma thylakoid development and LHC2 apoprotein accumulations enriched stroma thylakoids in some treatments. The lack of starch and critical reaction center apoproteins in these chloroplasts suggest a loss of photosynthetic competency.