O. V. Dymova

Pigment Apparatus in Ajuga reptans Plants as Affected by Adaptation to Light Growth Conditions

Mechanisms of adaptation of the photosynthetic apparatus at the level of pigment complex in a shade-tolerant bugle plant (Ajuga reptans L.) grown at full solar irradiation in an open plot were studied. In “sun” plants, the content of photosynthetic pigments decreased markedly as compared to “shade” plants grown under a forest canopy at 5–10% of the full solar irradiation. In leaves of sun plants, the portion of β-carotene and lutein in the carotenoid spectrum was higher than in shade plant leaves, antheraxanthin and zeaxanthin were present, and de-epoxidation of violaxanthin was by an order of magnitude higher in sun plant leaves reaching 40%. The data obtained indicate the role of the violaxanthin cycle in the protection of photosynthetic apparatus in a shade-tolerant plant against destruction under excessive irradiation.

Functional plasticity of photosynthetic apparatus and its resistance to photoinhibition in Plantago media

Morphological and functional characteristics of Plantago media L. leaves were compared for plants growing at different light regimes on limestone outcrops in Southern Timan (62°45′N, 55°49′E). The plants grown in open areas under exposure to full sunlight had small leaves with low pigment content and high specific leaf weight; these leaves exhibited high photosynthetic capacity and elevated water use efficiency at high irradiance. The maximum photochemical activity of photosystem II (Fv/Fm) in leaves of sun plants remained at the level of about 0.8 throughout the day. The photosynthetic apparatus of sun plants was resistant to excess photosynthetically active radiation, mostly due to non-photochemical quenching of chlorophyll fluorescence (qN). This quenching was promoted by elevated deepoxiation of violaxanthin cycle pigments. Accumulation of zeaxanthin, a photoprotective pigment in sun plant leaves was observed already in the morning hours. The plant leaves grown in the shade of dense herbage were significantly larger than the sun leaves, with pigment content 1.5–2.0 times greater than in sun leaves; these leaves had low qN values and did not need extensive deepoxidation of violaxanthin cycle pigments. The data reveal the morphophysiological plasticity of plantain plants in relation to lighting regime. Environmental conditions can facilitate the formation of the ecotype with photosynthetic apparatus resistant to photoinhibition. Owing to this adjustment, hoary plantain plants are capable of surviving in ecotopes with high insolation.

Violaxanthin cycle pigment de-epoxidation and thermal dissipation of light energy in three boreal species of evergreen conifer plants

We studied carotenoid composition and chlorophyll fluorescence in two-year-old needles from Siberian spruce (Picea obovata (L.) Karst.), Siberian fir (Abies sibirica L.), and common juniper (Juniperus communis L.). The highest values of maximum PSII photochemical activity (Fv/Fm) equaling 0.82–0.85 were observed in July–September. The decrease in Fv/Fm in December–March was more pronounced in juniper (down to 0.15) than in spruce and fir (0.45–0.50). In May, we observed a nearly complete recovery in maximum PSII photochemical activity in fir and spruce (0.72–0.77), while in juniper, the Fv/Fm value was notably lower (0.65–0.67). The amount of thermal dissipation of energy absorbed by PSII LHC did not exceed 30% in summer and equaled 60–90% in winter and early spring. The carotenoid pool consisted mainly of xanthophylls, among which lutein (70%), neoxanthin (7–10%), and a violaxanthin cycle (VXC) component — violaxanthin (3–15%) were constantly present. The accumulation of two other VXC pigments—zeaxanthin and antheraxanthin, was noted in December–March. In July, these xanthophylls were not identified. We discovered a direct connection between VXC pigment de-epoxidation level and light energy thermal dissipation in boreal conifer leaves. Such association reflects the non-species-specific character of the mechanism for quenching zeaxanthin-dependent nonphotochemical chlorophyll fluorescence in PSII LHC in winter and spring.

The Effect of Light Regime on Source–Sink Relations in the Shade-Enduring Ajuga reptans Plant

A quantitative approach to the evaluation of source–sink relations in Ajuga reptans plants grown under the forest canopy (shade plants) and on an open plot (sun plants) was worked out in terms of growth characteristics, CO2 exchange, and carbon balance. Shade plants developed leaves with the relative and specific areas twice exceeding those of sun plants. Sun plants assimilated more carbon, using a significant part of it for the development of numerous runners. During a day, shade and sun plants produced 0.03 and 0.67 g of substrate, respectively. At the same time, forest (shade) plants spent 48% of assimilates for the respiration, in comparison with plants from the open plot that spent almost 70% of assimilates for respiration as they were greater in size. It was concluded, that light controls source–sink relations, which is a way of realization of the life strategy and a coordination mechanism of functional integrity of the plant organism. Light not only controls photosynthesis (source activity) but morphophysiological characteristics of plants with their hierarchical structure of sinks too.

Adaptive changes in pigment complex of Pinus sylvestris needles upon cold acclimation

We studied seasonal changes in the content and ratio between photosynthetic pigments in one-yearold needles of Scotch pine (Pinus sylvestris L.) growing in Central Yakutia. Maximum accumulation of chlorophylls in developed young needles occurred in July when light and temperature conditions were favorable. In this period, the needles were notable for a relatively high level of β-carotene and neoxanthin and a reduced content of lutein and the pigments of violaxanthin cycle (VXC). In the course of autumn hardening, the content of chlorophylls decreased two times. Total content of carotenoids remained the same, but pigment composition considerably changed when plants progressed from a vegetating to frost-resistant state. We revealed time and temperature ranges of variation for individual carotenoids. In the beginning of hardening at reduced and low abovezero temperatures, the content of β-carotene in the needles decreased, the pigment-protein complexes (PPC) became enriched in lutein, the pigment pool of VXC gradually increased, and the content of neoxanthin transiently rose. When average daily air temperature further decreased to near- zero values, the content of zeaxanthin sharply rose. In winter, high levels of lutein and zeaxanthin were maintained. Main changes in pigment complex of the needles of P. sylvestris were completed before the coming of steady below-zero temperatures. The obtained data suggested that, upon seasonal decrease in temperature in early stages of hardening, a decrease in the level of chlorophyll promotes a reduction in the quantity of absorbed radiant energy. Apparently, this is accompanied by activation of the role of lutein and neoxanthin that perform specific photoprotective functions in antenna PPC associated with a gradual decrease in plants’ ability to quench singlet energy of excited chlorophyll. Accumulation of zeaxanthin as a result of inhibition of back reaction of epoxidation at near-zero temperatures creates necessary prerequisites for turning on the mechanisms of steady dissipation of absorbed light energy, which do not depend on transmembrane proton gradient of thylakoids. At the same time, zeaxanthin can perform antioxidant functions both in PPC and in the lipid phase of thylakoid membranes. The obtained data point to an adaptive nature of the observed reactions and a specific role of individual pigments in structural and functional reorganization of photosynthetic machinery in the course of development of frost-resistance in the needles.

Photoprotective Mechanisms in Photosystem II of Ephedra monosperma during Development of Frost Tolerance

Dissipation of light energy absorbed by photosystem II (PSII) in assimilating shoots of an evergreen shrub Ephedra monosperma was investigated during its transition from the vegetative to frost-tolerant state under natural conditions of Central Yakutia. The dynamics of modulated chlorophyll fluorescence and carotenoid content was analyzed during seasonal decrease in ambient temperature. The seasonal cooling was accompanied by a stepwise decrease in photochemical activity of PSII (Fv/Fm = (Fm − F0)/Fm). The decrease in Fv/Fm occurred from the beginning of September to the end of October, when the temperature was lowered from 10 to −8°C. During winter period the residual activity of PSII was retained at about 30% of the summer values. The seasonal decrease in temperature was accompanied by a significant stimulation of pH-independent dissipative processes in reaction centers and antenna of PSII. The increase in energy losses was paralleled by a proportional increase in zeaxanthin content on the background of decreasing content of violaxanthin and β-carotene as possible zeaxanthin precursors. At the same time, inhibition of light-induced non-photochemical quenching in the PSII antenna was observed. The results suggest that principal photoprotective mechanisms during seasonal lowering of temperature are: (1) inactivation of PSII and dissipation of excitation energy in PSII reaction centers and (2) zeaxanthin-mediated energy dissipation in the antenna complexes. The first mechanism seems to prevail at early stages of seasonal cooling, whereas both mechanisms are recruited from the onset of sustained freezing temperatures.

Characterization of pigment apparatus in winter-green and summer-green leaves of a shade-tolerant plant Ajuga reptans

Comparative study was performed to assess the content and proportions of photosynthetic pigments and the violaxanthin cycle (VXC) activity in winter-green and summer-green leaves of bugleweed (Ajuga reptans L.) plants grown in shaded (photosynthetically active radiation, PAR 150 μmol/(m2 s)) and sunny (PAR 1200 μmol/(m2 s)) habitats in the Botanic Garden of Jagiellonian University (Krakow, Poland). In overwintered and newly formed leaves of shade plants, the content of green and yellow pigments was two times higher than in leaves of sun plants. The shade plants were distinguished by accumulation of β-carotene, while lutein was predominant in leaves of sun plants. Under the action of strong light (2000 μmol/(m2s)), the level of violaxanthin deepoxidation in winter-green leaves of shade and sun plants increased five- to sixfold, whereas it changed insignificantly in summer-green leaves of shade plants. It is concluded that, in a shadetolerant species A. reptans, the photosynthetic apparatus of winter-green leaves in sun and shade plants and of summer-green leaves in sun plants is protected against excess insolation by high activity of VXC. The carotenoids of summer-green leaves in shade plants are supposed to function mainly as light-harvesting pigments.

Photosynthetic activity of vascular bundles in Plantago media leaves.

Photosynthetic processes in the leaf lamina and midribs of Plantago media were investigated using plants grown in high light (HL) or low light (LL) conditions. The fluorescence parameters, which indicate photochemical/photosynthetic activity, were different in HL and LL grown plants, but no major differences between lamina and midribs were found. An OJIP test (chlorophyll a fluorescence transient induction) of LL grown plants, indicative of the chloroplast electron transport chain, also showed both tissues to be similar. In HL plants, a partial blockage of electron flow between QA (the primary plastoquinone electron acceptor of PSII) and QB (the secondary plastoquinone acceptor of PSII) was found, and this was less visible in midribs. The effective dissipation of quantum energy per reaction center (DI0/RC) was similar in both tissues of HL grown plants, while in the midribs of LL leaves, this process seemed to be less effective. Measurements of 13C discrimination showed that the midrib tissues of LL and HL leaves effectively used β-carboxylation products to accumulate their biomass. Thus, the well protected activity of electron transport in midribs with their limited capacity to fix CO2 from the air may indicate the involvement of this tissue in β-carboxylation, transport or signaling. Carbon accumulated in roots showed a lower 13C discrimination value (more negative) than the values observed in lamina. This could indicate that roots are supplied with assimilates mostly during the light phase of the day cycle with intensive C3 photosynthesis.

The role of pigment system of an evergreen dwarf shrub Ephedra monosperma in adaptation to the climate of Central Yakutia

Adaptive reactions of the pigment system in assimilating shoots of evergreen dwarf shrub Ephedra monosperma J.G. Gmel. ex C.A. Mey. were studied under natural conditions of Central Yakutia. Seasonal changes in the content and ratio of green and yellow pigments were revealed; their relation to the stage of plant development and formation of cold tolerance was shown. The decrease in chlorophyll content started in September when the natural photoperiod became shorter and the air temperature lower; the chlorophyll concentration in winter was 30% lower than in summer. The content of β-carotene decreased twofold. The xanthophyll cycle pigments increased in content and deepoxidation level by a factor of 1.7 and 3.6, respectively. In peripheral cells of assimilating parenchyma, accumulation of a secondary carotenoid, rhodoxanthin was noted. In the period of active plant growth (from June to August), rhodoxanthin was absent, while its concentration in shoots in winter was 75 μg/g dry wt. It is concluded that changes in the pigment pool reflect structural and functional reorganization of photosynthetic machinery and are an indispensable part of the intricate process of plant hardening. Activation of energy-dissipating and antioxidant pigment systems, together with accumulation of the light-screening secondary carotenoid rhodoxanthin, promote the retention of photosynthetic apparatus and the survival of Ephedra monosperma plants under extreme conditions of cryolithozone of Yakutia.

Seasonal variations of leaf chlorophyll–protein complexes in the wintergreen herbaceous plant Ajuga reptans L.

The chlorophyll and carotenoid content, and the spectra of low-temperature fluorescence of the leaves, chloroplasts and isolated pigment-protein complexes in the perennial herbaceous wintergreen plant Ajuga reptans L. (bugle) in different seasons of the year were studied. During winter, these plants downregulate photosynthesis and the PSA is reorganised, including the loss of chlorophyll, possible reductions in the number of functional reaction centres of PSII, and changes in aggregation of the thylakoid protein complexes. We also observed a restructuring of the PSI-PSII megacomplex and the PSII-light-harvesting complex II supercomplex in leaves covered by snow. After snowmelt, the monomeric form of the chl a/b pigment-protein complex associated with PSII (LHCII) and the free pigments were also detected. We expect that snow cover provides favourable conditions for keeping photosynthetic machinery ready for photosynthesis in spring just after snowmelt. During winter, the role of the zeaxanthin-dependent protective mechanism, which is responsible for the dissipation of excess absorbed light energy, is likely to increase.