Chlorophyll is not the primary photoreceptor for the stimulation of P-type H+ pump and growth in variegated leaves of Coleus x hybridus.

There has been persisting controversy over the role of photosynthesis in the stimulation of the plasma membrane H+-ATPase and growth of dicotyledonous leaves by light. To investigate this, we compared the effects of light on growth, H+ net efflux and membrane potential (Vm) of strips which contained either only chlorophyll-free (white) mesophyll cells or chlorophyll-containing (green) cells cut from variegated Coleus leaves. White mesophyll cells responded to white, blue and red light with a hyperpolarization of Vm, an acidification of the apoplast and a promotion of growth, all of which began after a lag of 2-7 min. In contrast, green mesophyll cells showed a biphasic light response in which the hyperpolarization and the acidification were preceded by a rapid depolarization of Vm and an alkalinization of the apoplast. Nevertheless, green and white tissues showed comparable growth promotions in response to light. The light response of the leaf mesophyll is a composite of two separate photosystems. The initial depolarization and alkalinization are mediated by photosynthesis and blocked by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The slower hyperpolarization, acidification and growth response, on the other hand, are clearly in response to light absorption by pigments other than chlorophyll.