Characteristics and Species-Dependent Employment of Flexible Versus Sustained Thermal Dissipation and Photoinhibition

Diversity in Xanthophyll Cycle Pigments Content and Related Nonphotochemical Quenching (NPQ) Among Microalgae: Implications for Growth Strategy and Ecology

Xanthophyll cycle-related nonphotochemical quenching (NPQ), which is present in most photoautotrophs, allows dissipation of excess light energy. Xanthophyll cycle-related NPQ depends principally on xanthophyll cycle pigments composition and their effective involvement in NPQ. Xanthophyll cycle-related NPQ is tightly controlled by environmental conditions in a species-/strain-specific manner. These features are especially relevant in microalgae living in a complex and highly variable environment. The goal of this study was to perform a comparative assessment of NPQ ecophysiologies across microalgal taxa in order to underline the specific involvement of NPQ in growth adaptations and strategies. We used both published results and data acquired in our laboratory to understand the relationships between growth conditions (irradiance, temperature, and nutrient availability), xanthophyll cycle composition, and xanthophyll cycle pigments quenching efficiency in microalgae from various taxa. We found that in diadinoxanthin-containing species, the xanthophyll cycle pigment pool is controlled by energy pressure in all species. At any given energy pressure, however, the diatoxanthin content is higher in diatoms than in other diadinoxanthin-containing species. XC pigments quenching efficiency is species-specific and decreases with acclimation to higher irradiances. We found a clear link between the natural light environment of species/ecotypes and quenching efficiency amplitude. The presence of diatoxanthin or zeaxanthin at steady state in all species examined at moderate and high irradiances suggests that cells maintain a light-harvesting capacity in excess to cope with potential decrease in light intensity.

Depth-specific fluctuations of gene expression and protein abundance modulate the photophysiology in the seagrass Posidonia oceanica

Here we present the results of a multiple organizational level analysis conceived to identify acclimative/adaptive strategies exhibited by the seagrass Posidonia oceanica to the daily fluctuations in the light environment, at contrasting depths. We assessed changes in photophysiological parameters, leaf respiration, pigments, and protein and mRNA expression levels. The results show that the diel oscillations of P. oceanica photophysiological and respiratory responses were related to transcripts and proteins expression of the genes involved in those processes and that there was a response asynchrony between shallow and deep plants probably caused by the strong differences in the light environment. The photochemical pathway of energy use was more effective in shallow plants due to higher light availability, but these plants needed more investment in photoprotection and photorepair, requiring higher translation and protein synthesis than deep plants. The genetic differentiation between deep and shallow stands suggests the existence of locally adapted genotypes to contrasting light environments. The depth-specific diel rhythms of photosynthetic and respiratory processes, from molecular to physiological levels, must be considered in the management and conservation of these key coastal ecosystems.

Frequently asked questions about in vivo chlorophyll fluorescence: practical issues

The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.

Modulating the light environment with the peach 'asymmetric orchard': effects on gas exchange performances, photoprotection, and photoinhibition

The productivity of fruit trees is a linear function of the light intercepted, although the relationship is less tight when greater than 50% of available light is intercepted. This paper investigates the management of light energy in peach using the measurement of whole-tree light interception and gas exchange, along with the absorbed energy partitioning at the leaf level by concurrent measurements of gas exchange and chlorophyll fluorescence. These measurements were performed on trees of a custom-built 'asymmetric' orchard. Whole-tree gas exchange for north-south, vertical canopies (C) was similar to that for canopies intercepting the highest irradiance in the morning hours (W), but trees receiving the highest irradiance in the afternoon (E) had the highest net photosynthesis and transpiration while maintaining a water use efficiency (WUE) comparable to the other treatments. In the W trees, 29% and 8% more photosystems were damaged than in C and E trees, respectively. The quenching partitioning revealed that the non-photochemical quenching (NPQ) played the most important role in excess energy dissipation, but it was not fully active at low irradiance, possibly due to a sub-optimal trans-thylakoid DeltapH. The non-net carboxylative mechanisms (NC) appeared to be the main photoprotective mechanisms at low irradiance levels and, probably, they could facilitate the establishment of a trans-thylakoid DeltapH more appropriate for NPQ. These findings support the conclusion that irradiance impinging on leaves may be excessive and can cause photodamage, whose repair requires energy in the form of carbohydrates that are thereby diverted from tree growth and productivity.

Resonance Raman spectroscopy

Resonance Raman spectroscopy may yield precise information on the conformation of, and on the interactions assumed by, the chromophores involved in the first steps of the photosynthetic process, whether isolated in solvents, embedded in soluble or membrane proteins, or, as shown recently, in vivo. By making use of this technique, it is possible, for instance, to relate the electronic properties of these molecules to their structure and/or the physical properties of their environment, or to determine subtle changes of their conformation associated with regulatory processes. After a short introduction to the physical principles that govern resonance Raman spectroscopy, the information content of resonance Raman spectra of chlorophyll and carotenoid molecules is described in this review, together with the experiments which helped in determining which structural parameter each Raman band is sensitive to. A selection of applications of this technique is then presented, in order to give a fair and precise idea of which type of information can be obtained from its use in the field of photosynthesis.

PHOTOINHIBITION IN RED ALGAL SPECIES WITH DIFFERENT CAROTENOID PROFILES(1)

Members of the Rhodophyta present different carotenoid profiles. In a majority of the species, lutein constitutes >50% of the total carotenoid content, while in other species, it is replaced by zeaxanthin or antheraxanthin. Given that carotenoids have specific roles in photoprotection, different carotenoid profiles of red algae species could be related to their capacity to cope with photoinhibitory stress. Therefore, in the present work, the sensitivity to light stress of red algal species with different carotenoid profiles was investigated. Photoinhibition of photosynthesis induced by high-light stress and the subsequent recovery in dim-light conditions was measured using maximal PSII quantum efficiency (Fv /Fm ). The degree of decrease and recovery of Fv /Fm and their respective kinetics were related to the carotenoid profile of the species. Although no relationship between sensitivity to high-light stress and the carotenoid profile was observed, there were clear carotenoid profile-related differences in the decrease and recovery kinetics. In species with zeaxanthin or antheraxanthin as the major carotenoid, Fv /Fm reduction and recovery was principally associated with slowly activated and relaxed processes. In contrast, in species with lutein as the major carotenoid, rapidly activated processes appear to play a major role in the down-regulation of photosynthesis during light-stress conditions. In these species, the repair of D1 is also important during light-stress conditions. This finding could imply differential expression of mechanisms involved in photoprotection in red algae that seems to be related to the carotenoid profile of the species.

Short- and long-term modulation of the lutein epoxide and violaxanthin cycles in two species of the Lauraceae: sweet bay laurel (Laurus nobilis L.) and avocado (Persea americana Mill.)

Short- and long-term responses of the violaxanthin (V) and lutein epoxide (Lx) cycles were studied in two species of Lauraceae: sweet bay laurel (Laurus nobilis L.) and avocado (Persea americana L.). The Lx content exceeded the V content in shade leaves of both species. Both Lx and V were de-epoxidised on illumination, but only V was fully restored by epoxidation in low light. Violaxanthin was preferentially de-epoxidised in low light in L. nobilis. This suggests that Lx accumulates with leaf ageing, partly because its conversion to lutein is limited in shade. After exposure to strong light, shade leaves of avocado readjusted the total pools of alpha- and beta-xanthophyll cycles by de novo synthesis of antheraxanthin, zeaxanthin and lutein. This occurred in parallel with a sustained depression of F(v)/F(m). In Persea indica, a closely related but low Lx species, F(v)/F(m) recovered faster after a similar light treatment, suggesting the involvement of the Lx cycle in sustained energy dissipation. Furthermore, the seasonal correlation between non-reversible Lx and V photoconversions and pre-dawn F(v)/F(m) in sun leaves of sweet bay supported the conclusion that the Lx cycle is involved in a slowly reversible downregulation of photosynthesis analogous to the V cycle.

Photosynthetic acclimation in the context of structural constraints to carbon export from leaves

The potential role of foliar carbon export features in the acclimation of photosynthetic capacity to differences and changes in light environment was evaluated. These features included apoplastic vs. symplastic phloem loading, density of loading veins, plasmodesmatal frequency in intermediary cells, and the ratio of loading cells to sieve elements. In initial studies, three apoplastic loaders (spinach, pea, Arabidopsis thaliana) exhibited a completely flexible photosynthetic response to changing light conditions, while two symplastic loaders (pumpkin, Verbascum phoeniceum), although able to adjust to different long-term growth conditions, were more limited in their response when transferred from low (LL) to high (HL) light. This suggested that constraints imposed by the completely physical pathway of sugar export might act as a bottleneck in the export of carbon from LL-acclimated leaves of symplastic loaders. While both symplastic loaders exhibited variable loading vein densities (low in LL and high in HL), none of the three apoplastic loaders initially characterized exhibited such differences. However, an additional apoplastic species (tomato) exhibited similar differences in vein density during continuous growth in different light environments. Furthermore, in contrast to the other apoplastic loaders, photosynthetic acclimation in tomato was not complete following a transfer from LL to HL. This suggests that loading vein density and loading cells per sieve element, and thus apparent loading surface capacity, play a major role in the potential for photosynthetic acclimation to changes in light environment. Photosynthetic acclimation and vein density acclimation were also characterized in the slow-growing, sclerophytic evergreen Monstera deliciosa. This evergreen possessed a lower vein density during growth in LL compared to HL and exhibited a more severely limited potential for photosynthetic acclimation to increases in light environment than the rapidly-growing, mesophytic annuals.

Photoprotection in an ecological context: the remarkable complexity of thermal energy dissipation

This review places photoprotection into the context of ecology and species diversity. The focus is on photoprotection via the safe removal - as thermal energy - of excess solar energy absorbed by the light collecting system, which counteracts the formation of reactive oxygen species. An update on the surprisingly complex, multiple variations of thermal energy dissipation is presented, placing these different forms into ecological and genetic contexts. Zeaxanthin-facilitated, flexible thermal dissipation associated with the PsbS protein and controlled by the trans-thylakoid pH gradient apparently occurs ubiquitously in plants, and can become sustained (and thus less flexible) at low temperatures. Long-lived, slow-growing plants with low intrinsic capacities for photosynthesis have greater capacities for this flexible dissipation than short-lived, fast-growing species. Furthermore, potent, but inflexible (zeaxanthin-facilitated) thermal dissipation, prominent in evergreen species under prolonged environmental stress, is characterized with respect to the involvement of photosystem II core rearrangement and/or degradation as well as the absence of control by trans-thylakoid pH and, possibly, PsbS. A role of PsbS-related proteins in photoprotection is discussed.