Response of Tradescantia albiflora to growth irradiance: Change versus changeability

Most chloroplasts undergo changes in composition, function and structure in response to growth irradiance. However, Tradescantia albiflora, a facultat ive shade plant, is unable to modulate its light-harvesting components and has the same Chi a/Chl b ratios and number of functional PS II and PS I rea ction centres on a Chi basis at all growth irradiances. With increasing gro wth irradiance, Tradescantia leaves have the same relative amount of chloro phyll-proteins of PS II and PS I, but increased xanthophyll cycle component s and more zeaxanthin formation under high light. Despite high-light leaves having enhanced xanthophyll cycle content, all Tradescantia leaves acclima ted to varying growth irradiances have similar non-photochemical quenching. These data strongly suggest that not all of the zeaxanthin formed under hi gh light is necessarily non-covalently bound to major and minor light-harve sting proteins of both photosystems, but free zeaxanthin may be associated with LHC II and LHC I or located in the lipid bilayer. Under the unusual ci rcumstances in light-acclimated Tradescantia where the numbers of functiona l PS II and PS I reaction centres and their antenna size are unaltered duri ng growth under different irradiances, the extents of PS II photoinactivati on by high irradiances are comparable. This is due to the extent of PS II p hotoinactivation being a light dosage effect that depends on the input (pho ton exposure, antenna size) and output (photosynthetic capacity, non-radiat ive dissipation) parameters, which in Tradescantia are not greatly varied b y changes in growth irradiance.