Signatures based on chlorophyll fluorescence and foliar pigment composition can serve as indicators of plant status with respect to growth, productivity, and stress exposure. These signatures are impacted by both environmental and species-specific factors, and consideration of these factors should thus be applied to the interpretation. There is a close relationship between fluorescence and pigment signatures, and both vary over seasons and with species-dependent responses to seasonal change. The magnitude of chlorophyll-fluorescence or pigment signatures provides information on the degree of imbalance between absorbed light and excitation-energy utilization by the plant. Recent progress confirmed the direct involvement of the xanthophyll zeaxanthin in two mechanisms of excess-energy dissipation from chlorophyll and resulting non-photochemical fluorescence quenching. In addition, the kinetics of changes in chlorophyll fluorescence or pigment composition from day-to-day inform about the cause of the utilization limitation. The presentation will address how different types of limitations to plant function can be identified from fluorescence and pigment signatures. Specifically, transient limitations experienced by annual and biennial mesophytes will be contrasted with lasting limitations seen in evergreens upon sudden exposure to very high light levels as well as overwintering evergreens that undergo seasonal growth arrest and photosynthetic downregulation.
Results/Conclusions
Temporary limitations in energy utilization due to, e.g., intermittent low leaf temperature or low stomatal conductance are associated with temporary changes in fluorescence and pigment signatures. Intermittent low leaf temperature leads to maintenance of a low intra-thylakoid pH and retention of zeaxanthin and non-photochemical fluorescence quenching. As soon as leaf temperature rises again, such signatures disappear, as seen in overwintering annual or biennial mesophytes. In contrast, lasting energy-utilization limitations in overwintering evergreens undergoing season-long growth arrest are associated with lasting deviations from standard fluorescence and pigment signatures, as the light-harvesting/photochemical system is converted to a state with continuously high retained zeaxanthin and non-photochemical fluorescence quenching. Even if the leaf temperature of such overwintering evergreens rises temporarily on intermittent warmer days, neither growth arrest nor these signatures will disappear. Monitoring these signatures at several points over a period of days with varying air temperatures allows determination of whether vegetation maintains a metabolically active state with at least moderate photosynthetic capacity and sink demand by the whole plant. Pigment and fluorescence signatures do not allow a determination of the nature of this sink demand, i.e., whether the demand results from vegetative growth, energy-carrier storage, enhanced energy utilization, or investment into reproduction.