Tansley Review No. 36 Excited leaves

Abstract

SUMMARY Photosynthesis is largely to do with energy transduction; the conversion of light energy into electrical energy into chemical energy. Precisely how much light energy is needed to bring about the reduction of one molecule of carbon dioxide and the release of one molecule of oxygen (the quantum requirement) is a matter of fundamental importance and one which has attracted much past controversy. This article concludes that a minimum quantum requirement of eight, as demanded by the Z‐scheme, is obviously consistent with much contemporary work which puts the measured value for C3 leaves close to nine. Moreover, while values of less than eight (obtained in some circumstances with micro‐organisms), are a reminder that nothing is beyond challenge they are not, in the absence of confirmation and extension, sufficiently compelling to demand rejection of either the Z‐scheme or current measuring procedures. This article also shows why, even if the underlying minimum requirement was now accepted beyond all reasonable doubt, there would still be very good reasons for continuing, indefinitely, to measure actual photosynthetic efficiency in the natural environment. It discusses some of the implications of the fact that all plants, if not stressed, appear to photosynthesize at the same rate in low light. It explains the role of fluorescence in its relation to quantum yield, the possibility that the rate of photosynthesis might be determined from fluorescence measurements alone, and that a combination of fluorescence and gas exchange measurements could provide new information about the manner in which ‘dark respiration’ is affected by light. It indicates how contemporary interest in all of these matters has focused attention on the necessity for safe dissipation of excitation energy by leaves and on the manner by which this might be achieved.

Can we measure terrestrial photosynthesis from space directly, using spectral reflectance and fluorescence?

“…4). Chlorophyll fluorescence, combined with NPQ is an expression of the balance between light harvesting (absorption), and light utilization in the photosynthetic process (Krause & Weis, 1991;Walker, 1992;Maxwell & Johnson, 2000). The 690 nm fluorescence signal from leaves and crops is therefore widely used by physiologists and agronomists as a field-based or laboratory-based diagnostic tool for detection of stress.…”

Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation

“…In the absence of competition from photochemistry for available energy in the pigment bed, re-emission of energy as fluorescence is maximized (Krause and Weis 1991); i.e., F M 0 is reached. When F P is low due either to high actinic Q or to feedback downregulation of the light reactions [e.g., when the photosynthetic rate is reduced because of low leaf internal (CO 2 ) or inadequate induction of Calvin cycle enzymes], non-photochemical quenching processes may also compete strongly for excitation energy, and dissipate this energy as heat (Walker 1992;Demmig-Adams and Adams 1996;Laisk et al 1997). Thus, an extremely high pulse intensity (Q 0 ) may be required to achieve complete closure of all PS II reaction centers.…”

Temperature‐induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae

“…It is thought that metabolic disturbances that cause decreases in the rate of electron flow through the transport chain to the dark‐reactions of photosynthesis (Calvin cycle) carry the risk of strongly reducing the acceptor side. The net result of this is that the potential for photodamage at the PSII reaction centre increases (Demmig‐Adams 1990; Walker 1992). For example, a decrease in the quantum yield associated with heat stress in higher plants has been shown to occur after initial damage to electron flow beyond photosystem I (PSI; Schreiber & Bilger 1987).…”