Light acclimation maintains the redox state of the PSII electron acceptor Q(2) within a narrow range over a broad range of light intensities

Chrysanthemum inducum-hybrid `Coral Charm', Hibiscus rosa-sinensis L. `Cairo Red' and Spathiphyllum wallisii Regel `Petit' were grown in natural light in a greenhouse at three levels of irradiance using permanent shade screens. Light acclimation of photosynthesis was characterized using modulated chlorophyll a fluorescence of intact leaves. A close correlation was found between the degree of reduction of the primary electron acceptor Q A of Photosystem II (PS II) approximated as the fluorescence parameter 1-q P, and light acclimation. The action range of 1-q P was 0-0.4 from darkness to full irradiance around noon, within the respective light treatments in the greenhouse, indicating that most PS II reaction centres were kept open. In general, the index for electron transport (ETR) measured by chlorophyll fluorescence was higher for high-light (HL) than intermediate-(IL) and low-light (LL) grown plants. However, HL Chrysanthemum showed 40% higher ETR than HL Hibiscus at light saturation, despite identical redox states of Q A. The light acclimation of the non-radiative dissipation of excess energy in the antenna, NPQ, varied considerably between the species. However, when normalized against q P, a strong negative correlation was found between thermal dissipation and ETR measured by chlorophyll fluorescence. To be able to accommodate a high flux of electrons through PS II, the plants with the highest light-saturated ETR had the lowest NPQ/q P. The possibility of using chlorophyll fluorescence for quantification of the energy balance between energy input and utilization in PS II in intact leaves is discussed.