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Abstract Background In the violaxanthin (V) cycle, V is de-epoxidized to zeaxanthin (Z) when strong light or light combined with other stressors lead to an overexcitation of photosystems. However, plants can also suffer stress in darkness and recent reports have shown that dehydration triggers V-de-epoxidation in the absence of light. In this study, we used the highly stress-tolerant brown alga Pelvetia canaliculata as a model organism, due to its lack of lutein and its non-photochemical quenching independent of the transthylakoidal-ΔpH, to study the triggering of the V-cycle in darkness induced by abiotic stressors. Results We have shown that besides desiccation, other factors such as immersion, anoxia and high temperature also induced V-de-epoxidation in darkness. This process was reversible once the treatments had ceased (with the exception of heat, which caused lethal damage). Irrespective of the stressor applied, the resulting de-epoxidised xanthophylls correlated with a decrease in Fv/Fm, suggesting a common function in the down-regulation of photosynthetical efficiency. The implication of the redox-state of the plastoquinone-pool and of the differential activity of V-cycle enzymes on V-de-epoxidation in darkness was also examined. Current results suggest that both violaxanthin de-epoxidase (VDE) and zeaxanthin-epoxidase (ZE) have a basal constitutive activity even in darkness, being ZE inhibited under stress. This inhibition leads to Z accumulation. Conclusion This study demonstrates that V-cycle activity is triggered by several abiotic stressors even when they occur in an absolute absence of light, leading to a decrease in Fv/Fm. This finding provides new insights into an understanding of the regulation mechanism of the V-cycle and of its ecophysiological roles.

Members of the cosmopolitan streptophycean genus Klebsormidium live in various habitats, including sand dunes and polar/alpine environments. To survive in these harsh conditions they must possess an array of adaptive physiological and structural mechanisms, for example, to deal with chilling and photochilling stresses. Since these mechanisms have not been studied in detail, the objectives of this study were (i) to determine the physiological and biochemical responses of Klebsormidium cf. flaccidum (K. cf. flaccidum) to chilling (low temperature [LT]) and photochilling (LT in combination with high light [HL]) stresses; and (ii) to understand the cross‐link between biochemical parameters and cellular ultrastructural changes. The results indicated that 5°C is a temperature threshold (i.e., at 5°C) but not at higher temperatures, physiological changes were observed (Fv/Fm and ETR decreased and energy‐partitioning distribution changed, with an increase in Y[NPQ] under LT and an increase in Y[NO] under HL‐LT). Also, pigment contents changed significantly, with increased concentrations of photoprotective pigments such as antheraxanthin, zeaxanthin, and total carotenes. All of these responses occurred under LT and, to a greater extent, under LT‐HL, indicating that the two stresses (temperature and light) are additive. The cold treatment applied here induced the formation of spores under both LL and HL. The degree of photoinhibition was higher in spores than in vegetative cells, indicating that spores are less susceptible to photodamage. This study demonstrated a broad acclimation potential in different developmental stages of K. cf. flaccidum, which helps to explain the ecological success of this genus. [ABSTRACT FROM AUTHOR]

Members of the cosmopolitan streptophycean genus Klebsormidium live in various habitats, including sand dunes and polar/alpine environments. To survive in these harsh conditions they must possess an array of adaptive physiological and structural mechanisms, for example, to deal with chilling and photochilling stresses. Since these mechanisms have not been studied in detail, the objectives of this study were (i) to determine the physiological and biochemical responses of Klebsormidium cf. flaccidum (K. cf. flaccidum) to chilling (low temperature [LT]) and photochilling (LT in combination with high light [HL]) stresses; and (ii) to understand the cross‐link between biochemical parameters and cellular ultrastructural changes. The results indicated that 5°C is a temperature threshold (i.e., at 5°C) but not at higher temperatures, physiological changes were observed (Fv/Fm and ETR decreased and energy‐partitioning distribution changed, with an increase in Y[NPQ] under LT and an increase in Y[NO] under HL‐LT). Also, pigment contents changed significantly, with increased concentrations of photoprotective pigments such as antheraxanthin, zeaxanthin, and total carotenes. All of these responses occurred under LT and, to a greater extent, under LT‐HL, indicating that the two stresses (temperature and light) are additive. The cold treatment applied here induced the formation of spores under both LL and HL. The degree of photoinhibition was higher in spores than in vegetative cells, indicating that spores are less susceptible to photodamage. This study demonstrated a broad acclimation potential in different developmental stages of K. cf. flaccidum, which helps to explain the ecological success of this genus. [ABSTRACT FROM AUTHOR]