Light regulation of photosynthetic membrane structure, organization, and function

The light environment during plant growth determines the structural and functional properties of higher plant chloroplasts, thus revealing a dynamically regulated developmental system. Pisum sativum plants growing under intermittent illumination showed chloroplasts with fully functional photosystem (PS) II and PSI reaction centers that lacked the peripheral chlorophyll (Ch1) a/b and Ch1 a light-harvesting complexes (LHC), respectively. The results suggest a light flux differential threshold regulation in the biosynthesis of the photosystem core and peripheral antenna complexes. Sun-adapted species and plants growing under far-red-depleted illumination showed grana stacks composed of few (3-5) thylakoids connected with long intergrana (stroma) thylakoids. They had a PSII /PSI reaction center ratio in the range 1.3-1.9. Shade-adapted species and plants growing under far-red-enriched illumination showed large grana stacks composed of several thylakoids, often extending across the entire chloroplast body, and short intergrana stroma thylakoids. They had a higher PSII /PSI reaction center ratio, in the range of 2.2-4.0. Thus, the relative extent of grana and stroma thylakoid formation corresponds with the relative amounts of PSII and PSI in the chloroplast, respectively. The structural and functional adaptation of the photosynthetic membrane system in response to the quality of illumination involves mainly a control on the rate of PSII and PSI complex biosynthesis.