photosystem (?f??t???s?st?m) n (Botany) botany either of two pigment-containing systems, photosystem I or II, in which the light-dependent chemical reactions of photosynthesis occur in the chloroplasts of plantsCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014