Light, Temperature and Redox Control of the Development of the Photosynthetic Apparatus

The absorption of light through the primary photochemical reactions of photosystem II (PSII) and PSI occurs at least 15 orders of magnitude faster than temperature-dependent redox reactions of intersystem electron transport and subsequent enzyme-catalyzed carbon assimilation. It is imperative that photosynthetic organisms maintain a balance between energy supplied through photochemistry and energy consumed through electron transport and metabolism [1].According to Durnford and Falkowski [2],a balance between energy absorbed and energy utilized is attained when $$\sigma PSII \cdot I = n \cdot {{\tau }^{{ - 1}}} $$ (1) Where σPSII is the functional absorption cross section of PSII, I is the incident radiation, n is the number of electron sinks and τ-1 is the turnover rate of these sinks. Thus, an energy imbalance will occur whenever $$\sigma PSII\cdot I > n\cdot {{\tau }^{{ - 1}}} $$ (2) This will be manifested when organisms are exposed to excessive irradiance (I) at normal temperatures or, alternatively upon exposure to low temperature and normal irradiance since τ-1 is temperature dependent. An energy imbalance will be reflected in a change in the redox state of PSII which reflects an increase in PSII excitation pressure [1]. This may be estimated by Chl a fluorescence as 1-qp, where qp is the photochemical quenching parameter [3].