A study of factors which regulate the membrane appression of lettuce thylakoids in relation to irradiance

Factors that may influence the extent of thylakoid membrane appression have been examined using lettuce (Lactuca sativa cv. Celtuce) grown under different irradiances. Electron microscopy and salt-induced chlorophyll fluorescence suggest that the percentage of membrane appression is increased in plants grown in low light (20 Wm^–2) compared with those grown in high light (150 Wm^–2). In high light plants surface charge, as measured by 9-aminoacridine, was found to be twice that measured in low light plants. There was a similar difference in ATPase activity of CF₁ and in light saturated photophosphorylation. The chlorophyll content of LHC-2 as a proportion of the total chlorophyll was greatest in thylakoids of low light plants. Measurement of non-cyclic photophosphorylation rates suggested that membrane appression has a stimulatory role in the photophosphorylation process. The importance of these inter-related factors for the mechanism of thylakoid appression is discussed.Abbreviations PS photosystem - chl chlorophyll - LHC-2 light harvesting chlorophyll-protein complex serving PS 2 - CF₁ coupling factor 1 - NADP nicotinamide-adenine dinucleotide phosphate