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Low temperature in winter depresses rates of photosynthesis, which, in evergreen plants, can exacerbate imbalances between light absorption and photochemical light use. Damage that could result from increased excess light absorption is minimized by the conversion of excitation energy to heat in a process known as energy dissipation, which involves the de-epoxidized carotenoids of the xanthophyll cycle. Overwintering evergreens employ sustained forms of energy dissipation observable even after lengthy periods of dark acclimation. Whereas most studies of photoprotective energy dissipation examine one or a small number of species; here, we measured the levels of sustained thermal energy dissipation of seventy conifer taxa growing outdoors under common-garden conditions at the Red Butte Garden in Salt Lake City, Utah, U.S.A. (forty nine taxa were also sampled for needle pigment content). We observed an extremely wide range of wintertime engagement of sustained energy dissipation; the percentage decrease in dark-acclimated photosystem II quantum efficiency from summer to winter ranged from 6 to 95%. Of the many pigment-based parameters measured, the magnitude of the seasonal decrease in quantum efficiency was most closely associated with the seasonal increase in zeaxanthin content expressed on a total chlorophyll basis, which explained only slightly more than one-third of the variation. We did not find evidence for a consistent wintertime decrease in needle chlorophyll content. Thus, the prevailing mechanism for winter decreases in solar-induced fluorescence emitted by evergreen forests may be decreases in fluorescence quantum yield, and wintertime deployment of sustained energy dissipation likely underlies this effect. [ABSTRACT FROM AUTHOR]

As the Arctic warms, tundra vegetation is becoming taller and more structurally complex, as tall deciduous shrubs become increasingly dominant. Emerging studies reveal that shrubs exhibit photosynthetic resource partitioning, akin to forests, that may need accounting for in the 'big leaf' net ecosystem exchange models. We conducted a lab experiment on sun and shade leaves from S. pulchra shrubs to determine the influence of both constitutive (slowly changing bulk carotenoid and chlorophyll pools) and facultative (rapidly changing xanthophyll cycle) pigment pools on a suite of spectral vegetation indices, to devise a rapid means of estimating within canopy resource partitioning. We found that: (1) the PRI of dark-adapted shade leaves (PRI) was double that of sun leaves, and that PRI was sensitive to variation among sun and shade leaves in both xanthophyll cycle pool size (V + A + Z) ( r = 0.59) and Chl a/b ( r = 0.64); (2) A corrected PRI (difference between dark and illuminated leaves, ΔPRI) was more sensitive to variation among sun and shade leaves in changes to the epoxidation state of their xanthophyll cycle pigments (dEPS) ( r = 0.78, RMSE = 0.007) compared to the uncorrected PRI of illuminated leaves (PRI) ( r = 0.34, RMSE = 0.02); and (3) the SR680 index was correlated with each of (V + A + Z), lutein, bulk carotenoids, (V + A + Z)/(Chl a + b), and Chl a/ b ( r range = 0.52-0.69). We suggest that ΔPRI be employed as a proxy for facultative pigment dynamics, and the SR680 for the estimation of constitutive pigment pools. We contribute the first Arctic-specific information on disentangling PRI-pigment relationships, and offer insight into how spectral indices can assess resource partitioning within shrub tundra canopies. [ABSTRACT FROM AUTHOR]

Moss samples from the Fluxnet-Canada western peatland flux station in the Boreal Region of Alberta were measured in the laboratory to obtain the net photosynthesis rate and chlorophyll fluorescence of the moss under controlled environmental conditions, including the regulation of moss water content, simultaneously with measurements of moss spectral reflectance. One objective was to test whether the photochemical reflectance index (PRI) detected changes in moss photosynthetic light-use efficiency that were consistent with short-term (minutes to hours) changes in xanthophyll cycle pigments and associated changes in non-photochemical quenching (NPQ), as recorded by chlorophyll fluorescence. The rate of net photosynthesis was strongly inhibited by water content at values exceeding approximately 9 (fresh weight/dry weight) and declined as the water content fell below values of approximately 8. Chlorophyll fluorescence measurements of maximum photosystem II efficiency generally remained high until the water content was reduced from the maximum of about 20 to values of approximately 10–11, and then declined with further reductions in moss water content. A significant linear decline in NPQ was observed as moss water content was reduced from maximum to low water content values. There was a strong negative correlation between changes in NPQ and PRI. These data suggest that PRI measurements are a good proxy for short-term shifts in photosynthetic activity in Sphagnum moss. A second objective was to test how accurately the water band index (WBI, ratio of reflectance at 900 and 970 nm) recorded changes in moss water content during controlled laboratory studies. Strong linear relationships occurred between changes in moss water content and the WBI, although the slopes of the linear relationships were significantly different among sample replicates. Therefore, WBI appeared to be a useful tool to determine sample-specific water content without destructive measurements. [ABSTRACT FROM AUTHOR]

Seasonal differences in the efficiency of open PSII units ( Fv/ Fm), leaf pigment composition and xanthophyll cycle conversion (Z+A)/(V+A+Z), leaf adenylate status, and photosynthetic capacity were investigated in Pinus ponderosa (Ponderosa pine) and Malva neglecta. In P. ponderosa, acclimation to winter involved a lower photosynthetic capacity, higher carotenoid to chlorophyll ratio, persistent reductions in Fv/ Fm corresponding to persistent retention of Z+A, and no change in foliar ATP/ADP ratios. In contrast, M. neglecta characterized in winter exhibited higher rates of photosynthesis than in summer with no change in carotenoid to chlorophyll ratio, while small nocturnally persistent reductions in Fv/ Fm were observed exclusively on colder winter nights when nocturnal retention of Z+A, and high ATP/ADP ratios were also present. Upon removal of winter-stressed leaves or needles from the field to room temperature, a portion of Fv/ Fm relaxed within 15 min of warming and recovery was completed within 5 h in M. neglecta but required 100 h in P. ponderosa. In M. neglecta, the entire recovery of Fv/ Fm correlated with decreases in the foliar ATP/ADP ratio, while in P. ponderosa this ratio remained unchanged. Possible ATP-dependent forms of sustained (Z+A)-dependent energy dissipation are discussed including a nocturnally retained pH gradient on cold winter nights. The slow recovery in pine involved not only retention of Z+A, but apparently also a persistent engagement of Z+A for energy dissipation via an unidentified mechanism. [ABSTRACT FROM AUTHOR]

We investigated differences between summer and winter in photosynthesis, xanthophyll cycle-dependent energy dissipation, and antioxidant systems in populations of Mahonia repens (Lindley) Don growing in the eastern foothills of the Colorado Rocky Mountains in deep shade, full exposure, and under a single-layered canopy of Pinus ponderosa (partially shaded). In summer, increasing growth irradiance (from deep shade to partial shade to full exposure) was associated with increased xanthophyll cycle-dependent energy dissipation in PSII and an increased capacity to detoxify reactive reduced oxygen species, as measured by increases in the activities of ascorbate peroxidase, superoxide scavenging, glutathione reductase, and monodehydroascorbate reductase, as well as increases in leaf ascorbate and glutathione content. Leaves of exposed and partially shaded plants exhibited decreased capacities for photosynthetic O2 evolution in winter compared to summer, while in the deeply shaded plants this parameter did not differ seasonally. Seasonal differences in the levels of antioxidants generally exhibited an inverse response to photosynthesis, being higher in winter compared to summer in the exposed and partially shaded populations, but remaining unchanged in the deeply shaded population. In addition, total pool size and conversion state of the xanthophyll cycle were higher in winter than in summer in all populations. These trends suggest that both xanthophyll cycle-dependent energy dissipation in PSII and the capacity to detoxify reactive reduced oxygen species responded to the level of excess light absorption. [ABSTRACT FROM AUTHOR]

The photochemical reflectance index (PRI), derived from narrow-band reflectance at 531 and 570 nm, was explored as an indicator of photosynthetic radiation use efficiency for 20 species representing three functional types: annual, deciduous perennial, and evergreen perennial. Across species, top-canopy leaves in full sun at midday exhibited a strong correlation between PRI and ΔF/Fm′, a fluorescence-based index of photosystem II (PSII) photochemical efficiency. PRI was also significantly correlated with both net CO2 uptake and radiation use efficiency measured by gas exchange. When species were examined by functional type, evergreens exhibited significantly reduced midday photosynthetic rates relative to annual and deciduous species. This midday reduction was associated with reduced radiation use efficiency, detectable as reduced net CO2 uptake, PRI, and ΔF/Fm′ values, and increased levels of the photoprotective xanthophyll cycle pigment zeaxanthin. For each functional type, nutrient deficiency led to reductions in both PRI and ΔF/Fm′ relative to fertilized controls. Laboratory experiments exposing leaves to diurnal courses of radiation and simulated midday stomatal closure demonstrated that PRI changed rapidly with both irradiance and leaf physiological state. In these studies, PRI was closely correlated with both ΔF/Fm' and radiation use efficiency determined from gas exchange at all but the lowest light levels. Examination of the difference spectra upon exposure to increasing light levels revealed that the 531 nm Δ reflectance signal was composed of two spectral components. At low irradiance, this signal was dominated by a 545-nm component, which was not closely related to radiation use efficiency. At progressively higher light levels above 100 μmol m−2 s−1, the 531-nm signal was increasingly dominated by a 526-nm component, which was correlated with light use efficiency and with the conversion of the xanthophyll pigment violaxanthin to antheraxanthin and zeaxanthin. Further consideration of the two components composing the 531-nm signal could lead to an index of photosynthetic function applicable over a wide range of illumination. The results of this study support the use of PRI as an interspecific index of photosynthetic radiation use efficiency for leaves and canopies in full sun, but not across wide ranges in illumination from deep shade to full sun. The discovery of a consistent relationship between PRI and photosynthetic radiation use efficiency for top-canopy leaves across species, functional types, and nutrient treatments suggests that relative photosynthetic rates could be derived with the “view from above” provided by remote reflectance measurements if issues of canopy and stand structure can be resolved. [ABSTRACT FROM AUTHOR]

Diurnal changes in titratable acidity, photosynthesis, energy dissipation activity, and the carotenoid composition of differently oriented cladodes of the cactus Opuntia macrorhiza were characterized during exposure to full sunlight in the field. Four cladode faces were chosen such that each was exposed to maximum photon flux densities (PFD) at different times of the day in addition to receiving different daily integrated PFDs. The sum of all carotenoids per chlorophyll was found to increase with increasing exposure to PFD, with the carotenoids of the xanthophyll cycle present in the most exposed face at more than twice the concentration found in the least exposed face. All faces exhibited large increases in xanthophyll cycle-dependent energy dissipation as the sun rose in the morning, even those receiving only minimal levels of diffuse radiation. The transient high levels of energy dissipation in those faces that did not receive direct sunlight in the morning may have been due to low temperature inhibition of photosynthesis (predawn low of 2°C). For the two faces receiving peak PFDs in the morning hours (north and east faces), the level of energy dissipation activity increased rapidly during exposure to direct sunlight in the early morning, gradually declining in the late morning under warm temperatures, and was negligible during the afternoon low light conditions. Changes in the xanthophyll cycle paralleled the changes in energy dissipation with the majority of the cycle present as violaxanthin (V) prior to sunrise, largely de-epoxidized to zeaxanthin (Z) and antheraxanthin (A) during exposure to direct sunlight, and reconverted to V during the afternoon. For the two faces receiving peak PFDs in the afternoon (south and west faces), energy dissipation activity increased dramatically during the early morning low light period, subsequently decreasing during midday as decarboxylation of malic acid proceeded maximally (providing a high concentration of CO2 for photosynthesis), and then increased to the highest level in the late afternoon as the supply of malic acid was depleted and rates of photosynthetic electron transport declined. The xanthophyll cycle, largely present as Z and A prior to sunrise in the south and west faces, was de-epoxidized to the greatest extent in the late afternoon, followed by epoxidation back to the predawn level by sunset. In all cladode faces high levels of energy dissipation activity were accompanied by decreases in the intrinsic efficiency of photosystem II (PSII), indicative of a regulatory process that diverted the excess energy away from the reaction centers during periods of excess light. Furthermore, the overnight retention of Z and A by the south and west faces was accompanied by a sustained reduction in PSII efficiency (i.e., “photoinhibition”). We suggest that this “photoinhibition” represents the sustained engagement of nocturnally retained Z and A in the photoprotective down-regulation of PSII. [ABSTRACT FROM AUTHOR]

The changes in photosynthetic efficiency and photosynthetic pigments during dehydration of the resurrection plant Selaginella lepidophylla (from the Chiuhahuan desert, S.W. Texas, USA) were examined under different light conditions. Changes in the photosynthetic efficiency were deduced from chlorophyll a fluorescence measurements (F, F, and F) and pigment changes were measured by HPLC analysis. A small decrease in F/F was seen in hydrated stems in high light (650 μmol photons·m·s) but not in low light (50 μmol photons·m·s). However, a pronounced decline in F/F was observed during dehydration in both light treatments, after one to two hours of dehydration. A rise in F was observed only after six to ten hours of dehydration. Concomitant with the decrease in photosynthetic efficiency during dehydration a rise in the xanthophyll zeaxanthin was observed, even in low-light treatments. The increase in zeaxanthin can be related to previously observed photoprotective non-photochemical quenching of fluorescence in dehydrating stems of S. lepidophylla. We hypothesize that under dehydrating conditions even low light levels become excessive and zeaxanthin-related photoprotection is engaged. We speculate that these processes, as well as stem curling and self shading (Eickmeier et al. 1992), serve to minimize photoinhibitory damage to S. lepidophylla during the process of dehydration. [ABSTRACT FROM AUTHOR]

Leaves from two species, Euonymus kiautschovicus and Arctostaphylos uva-ursi, with a variety of different orientations and exposures, were examined in the field with regard to the xanthophyll cycle (the interconversion of three carotenoids in the chloroplast thylakoid membranes). East-, south-, and west-facing leaves of E. kiautschovicus were sampled throughout the day and all exhibited a pronounced and progressive conversion of violaxanthin to zeaxanthin, followed by a reconversion of zeaxanthin to violaxanthin later in the day. Maximal levels of zeaxanthin and minimal levels of violaxanthin were observed at the time when each leaf (orientation) received the maximum incident light, which was in the morning in east-facing, midday in southfacing, and in the afternoon in west-facing leaves. A very slight degree of hysteresis in the removal of zeaxanthin compared to its formation with regard to incident light was observed. Leaves with a broader range of orientations were sampled from A. uva-ursi prior to sunrise and at midday. All of the examined pigments (carotenoids and chlorophylls) increased somewhat per unit leaf area with increasing total daily photon receipt. The sum of the carotenoids involved in the xanthophyll cycle, violaxanthin + antheraxanthin + zeaxanthin, increased more strongly with increasing growth light than any other pigment. In addition, the amounts of zeaxanthin present at midday also increased markedly with increasing total daily photon receipt. The percentage of the xanthophyll cycle that was converted to zeaxanthin (and antheraxanthin) at peak irradiance was very large (approximately 80%) in the leaves of both E. kiautschovicus and A. uva-ursi. The daily changes in the components of the xanthophyll cycle that paralleled the daily changes in incident light in the leaves of E. kiautschovicus, and the increasing levels of the xanthophyll cycle components with total daily photon receipt in the leaves of A. uva-ursi, are both consistent with the involvement of zeaxanthin (i.e. the xanthophyll cycle) in the photoprotection of the photosynthetic apparatus against damage due to excessive light. [ABSTRACT FROM AUTHOR]

Cotton ( Gossypium hirsutum L. var. DP 61) was grown at different temperatures during 12-h light periods, with either 1800-2000 μmol photons m s (high photon flux density, PFD) or 1000-1100 μmol m s (medium PFD) incident on the plants. Night temperature was 25°C in all experiments. Growth was less when leaf temperatures were below 30°C during illumination, the effect being greater in plants grown with high PFD (Winter and Königer 1991). Leaf pigment composition and the photon-use efficiency of photosynthesis were analysed to assess whether plants grown with high PFD and suboptimal temperatures experienced a higher degree of high irradiance stress during development than those grown with medium PFD. The chlorophyll content per unit area was 3-4 times less, and the content of total carotenoids about 2 times less, with the proportion of the three xanthophylls zeaxanthin + antheraxanthin + violaxanthin being greater in leaves grown at 20-21°C than in leaves grown at 33-34°C. In leaves from plants grown at 21°C and 1800-2000 μmol photons m s, zeaxanthin accounted for as much as 34% of total carotenoids in the middle of the photoperiod, the highest level recorded in this study. This finding is consistent with a protective role of zeaxanthin under conditions of excess light. At the lower temperatures, the photochemical efficiency of photosystem II, measured as the ratio of variable to maximum fluorescence yield ( F/ F) after 12-h dark adaptation, was 0.76 in medium PFD plants and 0.75 in high PFD plants compared with 0.83 and 0.79, respectively, at the higher temperatures. The photon-use efficiency of O evolution (ϕ) based on absorbed light between 630 and 700nm, decreased with decrease in temperature from 0.102 to 0.07 under conditions of high PFD, but remained above 0.1 at medium PFD. Owing to compensatory reactions in these long-term growth experiments, sustained differences in F/ F and ϕ were much less pronounced than the differences in chlorophyll content and dry matter, particularly in plants which had developed at high PFD and low temperature. In fact, in these plants, which exhibited pronounced photobleaching, a largely functional photosynthetic apparatus was still maintained in cells adjacent to the lower leaf surfaces. This was indicated by measurements of photon use efficiencies of photosynthetic O evolution with leaves illuminated first at the upper, and then at the lower surface. [ABSTRACT FROM AUTHOR]

Sudden illumination of sunflower ( Helianthus annuus L. cv. CGL 208) leaves and canopies led to excess absorbed PFD and induced apparent reflectance changes in the green, red and near-infrared detectable with a remote spectroradiometer. The green shift, centered near 531 nm, was caused by reflectance changes associated with the de-epoxidation of violaxanthin to zeaxanthin via antheraxanthin and with the chloroplast thylakoid pH gradient. The red (685 nm) and near-infrared (738 nm) signals were due to quenching of chlorophyll fluorescence. Remote sensing of shifts in these spectral regions provides non-destructive information on in situ photosynthetic performance and could lead to improved techniques for remote sensing of canopy photosynthesis. [ABSTRACT FROM AUTHOR]