Abstract
Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis characterized by improved water use efficiency mediated by major nocturnal CO2 fixation. Due to its inherent metabolic plasticity CAM represents a successful physiological strategy for plant adaptation to abiotic stress. The present study reports on the impact of drought stress and different light intensities (PPFD 50 and 200 μmol m–2 s–1) on the photosynthetic performance of the obligate CAM orchid Phalaenopsis “Edessa” by integrating diel gas exchange patterns with assessments of the light reactions by analyzing fast chlorophyll a fluorescence induction. Parameters such as PIabs (performance index), different energy fluxes per active reaction centre (RC) reflecting the electron flow from photosystem II to photosystem I and the energetic communication between PSII complexes defined as connectivity were considered for the first time in a CAM plant. A higher PS II connectivity for plants grown under low light (p ∼ 0.51) compared to plants grown under high light (p ∼ 0.31) brought about similar specific energy fluxes of light absorbance, dissipation and processing through the electron transport chain, irrespective of the light treatment. With a 25% higher maximum quantum yield and comparable biomass formation, low light grown plants indeed proved to process light energy more efficiently compared to high light grown plants. The performance index was identified as a very reliable and sensitive parameter to indicate the onset and progress of drought stress. Under restricted CO2 availability (due to closed stomata) leaves showed higher energy dissipation and partial inactivation of PSII reaction centres to reduce the energy input to the electron transport chain and as such aid in avoiding overexcitation and photodamage. Especially during CAM idling there is a discrepancy between continuous input of light energy but severely reduced availability of both water and CO2, which represents the ultimate electron acceptor. Taken together, our results show a unique flexibility of CAM plants to optimize the light reactions under different environmental conditions in a dual way by either attenuating or increasing energy flux.
Highlights
Plants thrive in a variety of environments, each associated with certain characteristics and limitations
Diel Crassulacean acid metabolism (CAM) has been defined within a four-phase framework to describe the photosynthetic performance (Osmond, 1978): (i) phase I – open stomata in the dark and external CO2 fixation via phosphoenolpyruvate carboxylase (PEPC) into C4 acids; (ii) phase 2 – open stomata at the start of the light period and external CO2 fixation by combination of PEPC and ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO); (iii) phase 3 – closed stomata in the middle of the day whilst malic acid is decarboxylated [catalyzed by NAD(P)malic enzyme (ME) or phosphoenolpyruvate carboxykinase] and refixation of CO2 by RubisCO; and (iv) phase 4 – stomata open toward the end of the day and external CO2 is fixed via RubisCO and/or PEPC
Maximum quantum efficiency of primary photosystem II (PSII) photochemistry; maximum efficiency at which light absorbed by PSII is used for reduction of QA Probability with which a PSII trapped electron is transferred from reduced QA to QB
Summary
Plants thrive in a variety of environments, each associated with certain characteristics and limitations (e.g., rainforest, desert, and arctic conditions). Due to their sessile nature they need to be extremely adaptable to their continuously changing environment. Crassulacean acid metabolism (CAM), which is characterized by an optimized water use efficiency by taking up CO2 predominantly at night is an important physiological strategy for plant adaptation (Cushman and Borland, 2002; Borland et al, 2011; Ceusters et al, 2017). When experiencing severe drought stress CAM plants enter the stage of CAM-idling, showing no net CO2 uptake and recycling respiratory CO2 behind closed stomates during the complete diel cycle to minimize any further water loss (Ting, 1985; Ceusters et al, 2009a,b)
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