Characterization of acclimation of Hordeum vulgare to high irradiation based on different responses of photosynthetic activity and pigment composition

Light Drives and Temperature Modulates: Variation of Phenolic Compounds Profile in Relation to Photosynthesis in Spring Barley

Accumulation and metabolic profile of phenolic compounds (PheCs; serving as UV-screening pigments and antioxidants) as well as carbon fixation rate (A_n) and plant growth are sensitive to irradiance and temperature. Since these factors are naturally co-acting in the environment, it is worthy to study the combined effects of these environmental factors to assess their possible physiological consequences. We investigated how low and high irradiance in combination with different temperatures modify the metabolic profile of PheCs and expression of genes involved in the antioxidative enzyme and PheCs biosynthesis, in relation to photosynthetic activity and availability of non-structural carbohydrates (NSC) in spring barley seedlings. High irradiance positively affected A_n, NSC, PheCs content, and antioxidant activity (AOX). High temperature led to decreased A_n, NSC, and increased dark respiration, whilst low temperature was accompanied by reduction of UV-A shielding but increase of PheCs content and AOX. Besides that, irradiance and temperature caused changes in the metabolic profile of PheCs, particularly alteration in homoorientin/isovitexin derivatives ratio, possibly related to demands on AOX-based protection. Moreover, we also observed changes in the ratio of sinapoyl-/feruloyl- acylated flavonoids, the function of which is not yet known. The data also strongly suggested that the NSC content may support the PheCs production.

A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance

By means of meta-analyses we determined how 70 traits related to plant anatomy, morphology, chemistry, physiology, growth and reproduction are affected by daily light integral (DLI; mol photons m^-2  d^-1 ). A large database including 500 experiments with 760 plant species enabled us to determine generalized dose-response curves. Many traits increase with DLI in a saturating fashion. Some showed a more than 10-fold increase over the DLI range of 1-50 mol m^-2  d^-1 , such as the number of seeds produced per plant and the actual rate of photosynthesis. Strong decreases with DLI (up to three-fold) were observed for leaf area ratio and leaf payback time. Plasticity differences among species groups were generally small compared with the overall responses to DLI. However, for a number of traits, including photosynthetic capacity and realized growth, we found woody and shade-tolerant species to have lower plasticity. We further conclude that the direction and degree of trait changes adheres with responses to plant density and to vertical light gradients within plant canopies. This synthesis provides a strong quantitative basis for understanding plant acclimation to light, from molecular to whole plant responses, but also identifies the variables that currently form weak spots in our knowledge, such as respiration and reproductive characteristics.

Induction of phenolic compounds by UV and PAR is modulated by leaf ontogeny and barley genotype

We investigated the effect of leaf ontogeny and barley genotype on the accumulation of phenolic compounds (PhCs) induced by ultraviolet (UV) and photosynthetically active radiation (PAR). We hypothesized that different groups of PhCs are induced in leaves differing in ontogeny, and that this has consequences for protective functions and the need for other protection mechanisms. Generally, lower constitutive contents of PhCs (under conditions of UV exclusion and reduced PAR) were found in a UV-sensitive genotype (Barke) compared to a tolerant genotype (Bonus). However, UV and PAR induced accumulation of PhCs exceeded the constitutive amounts several fold. Specifically, lutonarin, 3-feruloylquinic acid, unidentified hydroxycinnamic acid and luteolin derivatives were markedly enhanced by high PAR and UV irradiances. Leaves developed during UV and PAR treatments had higher PhCs contents than mature leaves already fully developed at the onset of the UV and PAR treatment. UV and PAR treatments had, however, a minor effect on saponarin and unidentified apigenin derivatives which occur particularly in mature leaves of the tolerant genotype Bonus. In addition, high UV and PAR intensities increased the total content of xanthophylls (VAZ), while chlorophyll content was reduced, particularly in developing leaves. A redundancy analysis revealed positive associations between most of PhCs and VAZ and a negative association between total chlorophylls and carotenoids. Non-linear relationships between VAZ and lutonarin and other PhCs indicate that VAZ accumulation can compensate for the insufficient efficiency of anti-oxidative protection mediated by PhCs. Accordingly, we conclude that UV and PAR-induced accumulation of PhCs is affected by leaf ontogeny, however, this effect is compound-specific.

Photoacclimation of photosynthesis in the Eustigmatophycean Nannochloropsis gaditana

Nannochloropsis is an eukaryotic alga of the phylum Heterokonta, originating from a secondary endosymbiotic event. In this work, we investigated how the photosynthetic apparatus responds to growth in different light regimes in Nannochloropsis gaditana. We found that intense illumination induces the decrease of both photosystem I and II contents and their respective antenna sizes. Cells grown in high light showed a larger capacity for electron transport, with enhanced cyclic electron transport around photosystem I, contributing to photoprotection from excess illumination. Even when exposed to excess light intensities for several days, N. gaditana cells did not activate constitutive responses such as nonphotochemical quenching and the xanthophyll cycle. These photoprotection mechanisms in N. gaditana thus play a role in acclimation to fast changes in illumination within a time range of minutes, while regulation of the electron flow capacity represents a long-term response to prolonged exposure to excess light.

Stress Memory and the Inevitable Effects of Drought: A Physiological Perspective

Plants grow and develop by adjusting their physiology to changes in their environment. Changes in the abiotic environment occur over years, seasons, and days, but also over minutes and even seconds. In this ever-changing environment, plants may adjust their structure and function rapidly to optimize growth and reproduction. Plant responses to reiterated drought (i.e., repeated cycles of drought) differ from those to single incidences of drought; in fact, in nature, plants are usually exposed to repeated cycles of drought that differ in duration and intensity. Nowadays, there is increased interest in better understanding mechanisms of plant response to reiterated drought due, at least in part, to the discovery of epigenomic changes that trigger drought stress memory in plants. Beyond epigenomic changes, there are, however, other aspects that should be considered in the study of plant responses to reiterated drought: from changes in other "omics" approaches (transcriptomics, proteomics, and metabolomics), to changes in plant structure; all of which may help us to better understand plant stress memory and its underlying mechanisms. Here, we present an example in which reiterated drought affects the pigment composition of leaves in the ornamental plant Silene dioica and discuss the importance of structural changes (in this case in the photosynthetic apparatus) for the plant response to reiterated drought; they represent a stress imprint that can affect plant response to subsequent stress episodes. Emphasis is placed on the importance of considering structural changes, in addition to physiological adjustments at the "omics" level, to understand stress memory in plants better.

Protective effect of UV-A radiation during acclimation of the photosynthetic apparatus to UV-B treatment

We examined the acclimation response of the photosynthetic apparatus of barley (Hordeum vulgare L.) to a combination of UV-A and UV-B radiation (UVAB) and to UV-B radiation alone. Our aim was to evaluate whether UV-A radiation prevents UV-B-induced damage to the photosynthetic apparatus and whether UV-A pre-acclimation is required to mitigate the negative influence of UV-B radiation. Barley plants were grown from seeds under low photosynthetically active radiation (50 μmol m(-2) s(-1)) either in the absence or presence of UV-A radiation (UVA- and UVA+ plants, respectively). After 8 days of development, plants were exposed simultaneously to UV-A and UV-B radiation for the next 6 days. Additionally, UVA- plants were exposed to UV-B radiation alone. The UVA+ plants had a higher CO2 assimilation rate near the light-saturation region (A(N)) and a higher content of both total chlorophylls (Chls) and total carotenoids than the UVA- plants. Chls content, A(N), the potential quantum yield of photosystem II (PSII) photochemistry (F(V)/F(M)), the capacity of light-induced thermal energy dissipation and the efficiency of excitation energy transfer within PSII remained the same or even increased in both UVA+ and UVA- plants after UVAB treatment. On the contrary, exposure of UVA- plants to UV-B radiation itself led to a reduction in all these characteristics. We revealed that the presence of UV-A radiation during UVAB treatment not only mitigated but completely eliminated the negative effect of UV-B radiation on the functioning of the photosynthetic apparatus and that UV-A pre-acclimation was not crucial for development of this UV-A-induced resistance against UV-B irradiation.

Ultraviolet and photosynthetically active radiation can both induce photoprotective capacity allowing barley to overcome high radiation stress

The main objective of this study was to determine the effects of acclimation to ultraviolet (UV) and photosynthetically active radiation (PAR) on photoprotective mechanisms in barley leaves. Barley plants were acclimated for 7 days under three combinations of high or low UV and PAR treatments ([UV-PAR-], [UV-PAR+], [UV+PAR+]). Subsequently, plants were exposed to short-term high radiation stress (HRS; defined by high intensities of PAR - 1000 μmol m(-2) s(-1), UV-A - 10 W m(-2) and UV-B 2 W m(-2) for 4 h), to test their photoprotective capacity. The barley variety sensitive to photooxidative stress (Barke) had low constitutive flavonoid content compared to the resistant variety (Bonus) under low UV and PAR intensities. The accumulation of lutonarin and 3-feruloylquinic acid, but not of saponarin, was greatly enhanced by high PAR and further increased by UV exposure. Acclimation of plants to both high UV and PAR intensities also increased the total pool of xanthophyll-cycle pigments (VAZ). Subsequent exposure to HRS revealed that prior acclimation to UV and PAR was able to ameliorate the negative consequences of HRS on photosynthesis. Both total contents of epidermal flavonols and the total pool of VAZ were closely correlated with small reductions in light-saturated CO2 assimilation rate and maximum quantum yield of photosystem II photochemistry caused by HRS. Based on these results, we conclude that growth under high PAR can substantially increase the photoprotective capacity of barley plants compared with plants grown under low PAR. However, additional UV radiation is necessary to fully induce photoprotective mechanisms in the variety Barke. This study demonstrates that UV-exposure can lead to enhanced photoprotective capacity and can contribute to the induction of tolerance to high radiation stress in barley.

Reflectance continuum removal spectral index tracking the xanthophyll cycle photoprotective reactions in Norway spruce needles

This laboratory experiment tested the ability of the spectral index called 'area under curve normalised to maximal band depth' (ANMB) to track dynamic changes in the xanthophyll cycle of Norway spruce (Picea abies (L.) Karsten) needles. Four-year-old spruce seedlings were gradually acclimated to different photosynthetic photon flux densities (PPFDs) and air temperature regimes. The measurements were conducted at the end of each acclimation period lasting for 11 days. A significant decline in the chlorophylls to carotenoids ratio and the increase of the amount of xanthophyll cycle pigments indicated a higher need for carotenoid-mediated photoprotection in spruce leaves acclimated to high PPFD conditions. Similarly, the photochemical reflectance index (PRI) changed from positive to negative values after changing light conditions from low to high intensity as a consequence of the increase in carotenoid content. Systematic responses of PRI to the de-epoxidation state of xanthophyll cycle pigments (DEPS) were, however, observed only during high temperature treatments and after the exposition of needles to high irradiance. The ANMB index computed from needle reflectance between 507 and 556nm was able to track dynamic changes in DEPS without any influence induced by changing the content of leaf photosynthetic pigments (chlorophylls, carotenoids).

Molecular dynamics of the diatom thylakoid membrane under different light conditions

During the last years significant progress was achieved in unraveling molecular characteristics of the thylakoid membrane of different diatoms. With the present review it is intended to summarize the current knowledge about the structural and functional changes within the thylakoid membrane of diatoms acclimated to different light conditions. This aspect is addressed on the level of the organization and regulation of light-harvesting proteins, the dissipation of excessively absorbed light energy by the process of non-photochemical quenching, and the lipid composition of diatom thylakoid membranes. Finally, a working hypothesis of the domain formation of the diatom thylakoid membrane is presented to highlight the most prominent differences of heterokontic thylakoids in comparison to vascular plants and green algae during the acclimation to low and high light conditions.

Acclimation of Norway spruce photosynthetic apparatus to the combined effect of high irradiance and temperature

Diurnal courses of photosynthetic gas exchange parameters, chlorophyll a fluorescence characteristics and the de-epoxidation state of the xanthophyll cycle pigments (DEPS) were measured during the gradual acclimation of 4-year-old Norway spruce seedlings to different photosynthetic photon flux density (PPFD) and air temperature (T(air)) regimes, simulating cloudy days with moderate T(air) (LI, maximum PPFD 300 micromol m(-2)s(-1), T(air) range 15-25 degrees C), sunny days with moderate T(air) (HI, maximum PPFD 1000 micromol m(-2)s(-1), T(air) range 15-25 degrees C) and hot sunny days (HI-HT, maximum PPFD 1000 micromol m(-2)s(-1), T(air) range 20-35 degrees C). The plants were acclimated inside a growth chamber and each acclimation regime lasted for 13d. Acclimation to HI conditions led to a strong depression of the net CO(2) assimilation rates (A(N)), particularly during noon and afternoon periods. Exposure to the HI-HT regime led to a further decrease of A(N) even during the morning period. Insufficient stomatal conductance was found to be the main reason for depressed A(N) under HI and HI-HT conditions. Only slight changes of the maximum photosystem II (PSII) photochemical efficiency (F(V)/F(M)), in the range of 0.78-0.82, supported the resistance of the Norway spruce photosynthetic apparatus against PSII photoinhibition during acclimation to both HI and HI-HT conditions. The HI plants showed increased content of xanthophyll cycle pigments (VAZ) and enhanced efficiency of thermal energy dissipation within PSII (D) that closely correlated with the increased DEPS. In contrast, acclimation to the HI-HT regime resulted in a slight reduction of VAZ content and significantly diminished D and DEPS values during the entire day in comparison with HI plants. These results indicate a minor role of the xanthophyll cycle-mediated thermal dissipation in PSII photoprotection under elevated temperatures. The different contributions of the thermal dissipation and non-assimilatory electron transport pathways in PSII photoprotection during acclimation of the Norway spruce photosynthetic apparatus to excess irradiance and heat stresses are discussed.

Dynamics of the xanthophyll cycle and non-radiative dissipation of absorbed light energy during exposure of Norway spruce to high irradiance

The response of Norway spruce saplings (Picea abies [L.] Karst.) was monitored continuously during short-term exposure (10 days) to high irradiance (HI; 1000micromol m(-2)s(-1)). Compared with plants acclimated to low irradiance (100micromol m(-2)s(-1)), plants after HI exposure were characterized by a significantly reduced CO(2) assimilation rate throughout the light response curve. Pigment contents varied only slightly during HI exposure, but a rapid and strong response was observed in xanthophyll cycle activity, particularly within the first 3 days of the HI treatment. Both violaxanthin convertibility under HI and the amount of zeaxanthin pool sustained in darkness increased markedly under HI conditions. These changes were accompanied by an enhanced non-radiative dissipation of absorbed light energy (NRD) and the acceleration of induction of both NRD and de-epoxidation of the xanthophyll cycle pigments. We found a strong negative linear correlation between the amount of sustained de-epoxidized xanthophylls and the photosystem II (PSII) photochemical efficiency (F(V)/F(M)), indicating photoprotective down-regulation of the PSII function. Recovery of F(V)/F(M) at the end of the HI treatment revealed that Norway spruce was able to cope with a 10-fold elevated irradiance due particularly to an efficient NRD within the PSII antenna that was associated with enhanced violaxanthin convertibility and a light-induced accumulation of zeaxanthin that persisted in darkness.

Do mature shade leaves of tropical tree seedlings acclimate to high sunlight and UV radiation?

Seedlings of neotropical forest trees grown in low light were exposed to 0.5-9 h d^-1 direct sunlight, for up to 3 months, to test the capability of mature shade leaves to acclimate to full solar visible and UV radiation. Photosynthetic pigments and the antioxidant, ascorbate, were analysed in leaves of two pioneer and two late-succession species. Seedlings of one or two of these species were used to assess further acclimative responses. Sun-exposure for 0.5 or 1 h d^-1 resulted in strongly decreased α-carotene and increased β-carotene and lutein levels. The pool size of xanthophyll-cycle pigments (sum of viola-, anthera- and zeaxanthin) was increased and their turnover was enhanced. These changes were associated with an increase in the capacity of non-photochemical fluorescence quenching and its 'energy-dependent' component, qE, and with reduced susceptibility to photoinhibition of PSII. Prolonged exposure to full direct sunlight (approximately 4 or 9 h d^-1) resulted in a marked decrease of chlorophyll a + b content and increase in chlorophyll a / b ratios and the pool of xanthophyll-cycle pigments (based on chlorophyll), leading to extremely high zeaxanthin levels during high-light periods. Contents of ascorbate and UV-B-absorbing substances were substantially increased. PSI activity exhibited a response to full sunlight that is characteristic of sun leaves. Rates of net photosynthetic CO₂ assimilation under saturating light were increased. The data show that mature shade leaves of seedlings of both early- and late-succession tree species can substantially acclimate to full-sunlight conditions by employing similar physiological mechanisms.

Regulation of the excitation energy utilization in the photosynthetic apparatus of chlorina f2 barley mutant grown under different irradiances

Acclimation of the photosynthetic apparatus of chlorophyll b-less barley mutant chlorina f2 to low light (100 micromolm(-2)s(-1); LL) and extremely high light level (1000 micromolm(-2)s(-1); HL) was examined using techniques of pigment analysis and chlorophyll a fluorescence measurements at room temperature and at 77 K. The absence of chlorophyll b in LL-grown chlorina f2 resulted in the reduction of functional antenna size of both photosystem II (by 67%) and photosystem I (by 21%). Chlorophyll fluorescence characteristics of the LL-grown mutant indicated no impairment of the utilization of absorbed light energy in photosystem II photochemistry. Thermal dissipation of excitation energy estimated as non-photochemical quenching of minimal fluorescence (SV(0)) was significantly higher as compared to the wild-type barley grown under LL. Despite impaired assembly of pigment-protein complexes, chlorina f2 was able to efficiently acclimate to HL. In comparison with chlorina f2 grown under LL, HL-grown chlorina f2 was characterized by unaffected maximal photochemical efficiency of photosystem II (F(V)/F(M), doubled content of both beta-carotene and the xanthophyll cycle pigments and considerably reduced efficiency of excitation energy transfer from carotenoids to chlorophyll a. The enormous xanthophyll cycle pool size was however associated with reduced SV(0) capacity. We suggest that the substantial part of the xanthophyll cycle pigments is not bound to the remaining pigment-protein complexes and acts as filter for excitation energy, thereby contributing to the efficient photoprotection of chlorina f2 grown under HL.