ADAPTATION TO SHADE OF THE LIGHT-HARVESTING APPARATUS IN SILENE-DIOICA

The physiological characteristics and photosystem composition of the photosynthetic apparatus of Silene dioica, a woodland plant, grown in sun and natural shade are examined. As expected, shade leaves exhibited lower chlorophyll a/b ratios, light saturated rates of CO2 assimilation (A(sat)), dark respiration (R(d)) and light compensation points (GAMMA), with both sun and shade leaves having similar absorptances and quantum yields of CO2 assimilation (phi). Shade leaves were able to utilize far-red light for electron transport and carbon assimilation and reach the compensation point. Sun leaves in far-red light had a rate of carbon assimilation equivalent to their dark respiration rate. Chlorophyll fluorescence kinetics from leaves at 77K together with analyses of thylakoid contents of photosystems (PS) I and II and the light-harvesting cholorphyll a/b protein complex associated with PSII (LHCII) demonstrated that the antenna size of PSII was similar in thylakoids of sun and shade leaves, but shade leaves contained ca. 20% more PSII and ca. 12% less PSI complexes. The increased PSII/PSI ratio in shade leaves accounted for their ability to achieve the compensation point in far-red light. An important feature of photosynethic shade adaptation in S. dioica is an increase in the PSII/PSI ratio and not an increase in the antenna size of PSII. The adaptive response of sun leaves when placed in a shade environment was rapid and had a half-time of ca. 18h.