Chlorophyll a Fluorescence Predicts Total Photosynthetic Electron Flow to CO₂ or NO₃⁻/NO₂⁻ under Transient Conditions

Abstract

A model which predicts total photosynthetic electron flow from a linear regression of the relationship between corrected steadystate quantum yield and nonphotochemical quenching (E Weis, JA Berry [1987] Biochem Biophys Acta 894: 198-208) was formulated for N-limited cells of the green alga Selenastrum minutum. Unlike other models based on net CO2 fixation, our model is based on total photosynthetic electron flow measured as gross 02 evolution. This allowed for the prediction of total photosyn- Chlorophyll a fluorescence provides a sensitive indicator of photochemical processes (12,14). Qualitative relations between fluorescence emission and photochemistry have frequently been demonstrated, but these relations have been obscured by the fact that there are two major mechanisms of fluorescence quenching (10, 13). Differentiation of these mechanisms via the light doubling procedure of Bradbury and Baker (2, 3) and subsequent development of the 'saturation pulse' method (22) and modulation techniques (9,15,20,22) have allowed the formulation of models predicting rates of photosynthetic electron transport based upon quenching analysis of fluorescence emission (22,31,32 (photochemical quenching; Qq). Oxidized QA allows excitation energy to be used for photochemistry, preventing the reemission of light energy as fluorescence (i.e. 'quenching' the fluorescence). Quenching may also result from nonphotochemical processes (nonphotochemical quenching; QNP). Although there are potentially several mechanisms accounting for QNP, it is generally believed that increases in the transthylakoid pH gradient may result in structural changes which are thought to increase thermal dissipation ofabsorbed light energy. While Qq is a measure of the oxidation state of QA, and therefore one indicator of the ability of PSII to utilize excitation energy to perform work, it has been repeatedly shown that QNP quenching is negatively correlated with quantum yield of PSII (10,11,17,18,31,32). This suggests an important role for thylakoid membrane energization in the regulation of PSII activity. Thus, models for the estimation of photosynthetic electron transport from fluorescence must take into account both of these quenching mechanisms.Weis and Berry (31) have shown that quenching analysis can be used to estimate total electron transport rates (J) from fluorescence emission according to the equation:where I is the incident PAR, and m and b are empirically derived constants. Rates of steady-state photosynthetic electron transport calculated using these fluorescence-derived parameters were highly correlated to electron transport rates calculated from net CO2 exchange (23,31,32).In this report we utilize gross photosynthetic 02 evolution, measured via mass spectrometry, to derive the constants m and b, and to confirm fluorescence-based estimates the total photosynthetic electron transport chain activity.

Regulation of photosynthetic light harvesting by nitrogen assimilation in the green alga Selenastrum minutum

“…The increase in F, reflects reduction of Qa, most likely due to a build up of NADPH resulting from the decreased requirement for reductant during NHs+ assimilation. The initial oscillation of (F,)s reflects a transient increase in non-photochemical quenching [12]. Over the next 20 min both F, and (I;v)s declined at a similar rate.…”

“…For NH; this decline in carbon fixation decreases the demands for photo-generated reductant drastically as illustrated by the decline in gross 02 evolution. However, there is no change in gross 02 evolution after addition of NO:, because NOT/NO; replaces CO2 as the terminal electron acceptor in noncyclic electron flow [12].…”

A model of isoprene emission based on energetic requirements for isoprene synthesis and leaf photosynthetic properties for Liquidambar and Quercus

“…Several chloroplastic biochemical pathways are competitive with respect to reducing equivalents, for example, a significant part of photosynthetic electron flow is used for nitrate assimilation, and dependent on measurement conditions, enhanced nitrate reduction may result in decreased carbon assimilation rates (Bloom et al 1989;Holmes, Weger & Turpin 1989).…”

Dark Ammonium Assimilation Reduces the Plastoquinone Pool of Photosystem II in the Green Alga Selenastrum minutum

“…These authors (22) suggested that this alteration aids in balancing the higher ATP/NAD(P)H requirement of NH4' assimilation relative to CO2 fixation through PSI-supported cyclic photophosphorylation. N03-assimilation, on the other hand, requires more NAD(P)H relative to ATP for its assimilation than NH4', and did not cause a suppression in 02 evolution nor introduce any alteration in fluorescence characteristics of PSII or PSI (3,4,8,22,23).…”

“…These authors (22) suggested that this alteration aids in balancing the higher ATP/NAD(P)H requirement of NH4' assimilation relative to CO2 fixation through PSI-supported cyclic photophosphorylation. N03-assimilation, on the other hand, requires more NAD(P)H relative to ATP for its assimilation than NH4', and did not cause a suppression in 02 evolution nor introduce any alteration in fluorescence characteristics of PSII or PSI (3,4,8,22,23).NH4' assimilation in light influences the respiratory and photosynthetic carbon metabolism in an integrated fashion (18,21,24). In dark, N-limited S. minutum cells are capable of assimilating NH4' at a rate equivalent to that in light (21,24).…”