Photosystem II Efficiency and Non-Photochemical Fluorescence Quenching in the Context of Source-Sink Balance

Abstract

Products of photosynthesis (reduced carbon, typically in the form of sugars), synthesized by a plant’s mature source leaves, are exported to the plant’s sinks for growth, storage, and/or maintenance respiration. When the rate of sugar production in source leaves exceeds the rate of export (often as a result of insufficient sink activity), sugars and starch accumulate in source leaves. In a feedback response, levels of messenger RNAs, coding for various photosynthetic proteins, of source leaves are repressed along with the levels of these proteins. Furthermore, the overall capacity for photosynthetic CO2 fixation is also decreased. Consequently, these source leaves with decreased rates of photosynthetic electron transport utilize a lessened fraction of the light they absorb for photochemistry, and should thus increase the level of thermal dissipation of (excess) absorbed light. Thermal dissipation can be assessed as increased non-photochemical quenching (NPQ) of chlorophyll fluorescence, as well as decreased intrinsic photosystem II (PS II) efficiency, the ratio of variable to maximal chlorophyll fluorescence Fv/Fm (in darkness) or F v ′ /F m ′ (in the light) (Fv being Fm – Fo; where v stands for variable, m for maximum, and o for minimum). Greater source activity relative to sink activity within the whole plant, resulting in starch and/or sugar accumulation, indeed leads to increases in thermal dissipation and in the concentration of zeaxanthin, a xanthophyll involved in thermal dissipation. Under severe conditions, high zeaxanthin levels, high thermal energy dissipation, and low PS II efficiency in source leaves become locked-in as part of a state of chronic photoinhibition associated with high foliar levels of sugar and starch. A correlation between photoinhibition and foliar non-structural carbohydrate accumulation is a common occurrence. In this chapter, we discuss various potential underlying causes for this correlation, including the possibility that reduced growth of plants under stress conditions leads to sink limitation, foliar starch and sugar accumulation, and photoinhibition as a manifestation of photosynthetic repression under excess light.