Paul W. Barnes

PHOTOSYNTHESIS DAMAGE AND PROTECTIVE PIGMENTS IN PLANTS FROM A LA1l11UDINAL ARCTIC/ALPINE GRADIENT EXPOSED TO SUPPLEMENTAL UV-B RADIATION IN THE FIELD

Arctic and alpine plants grown from seed collected from different locations along a latitudinal gradient were studied to determine if populations inhabiting different solar UV-B radiation environments would show differential photosynthetic inhibition under supplemental UV-B irradiation in the field. In general, plants collected from equatorial, alpine sites where solar UV-B irradiance is high, showed no UV-B induced damage of either light-saturated or light-limited photosynthesis, as measured by intact-leaf gas exchange. Photosynthetic inhibition was detected in some but not all ecotypes or species collected from higher latitude locations where effective UV-B irradiance is lower. When exposed to supplemental UV-B irradiation under a full solar spectrum in the field, significant accumulation of UV-absorbing leaf pigments occurred only in populations from higher latitudes. Most alpine populations were apparently protected from UV-B damage without additional pigment accumulation, whereas increased pigment levels did not necessarily act to completely protect populations from higher latitudes. Photosynthesis damage in these species was also not related to leaf weight/leaf area ratios. These results provide additional corroborative evidence for the existence of an appreciable latitudinal gradient in UV-B radiation but suggest that the variation in UV-B sensitivity within and between species cannot be attributed solely to differences in the shielding of UV-B radiation by UV-absorbing leaf pigments and/or leaf structure.

Diurnal changes in epidermal UV transmittance of plants in naturally high UV environments.

Studies were conducted on three herbaceous plant species growing in naturally high solar UV environments in the subalpine of Mauna Kea, Hawaii, USA, to determine if diurnal changes in epidermal UV transmittance (T(UV)) occur in these species, and to test whether manipulation of the solar radiation regime could alter these diurnal patterns. Additional field studies were conducted at Logan, Utah, USA, to determine if solar UV was causing diurnal T(UV) changes and to evaluate the relationship between diurnal changes in T(UV) and UV-absorbing pigments. Under clear skies, T(UV), as measured with a UV-A-pulse amplitude modulation fluorometer for leaves of Verbascum thapsus and Oenothera stricta growing in native soils and Vicia faba growing in pots, was highest at predawn and sunset and lowest at midday. These patterns in T(UV) closely tracked diurnal changes in solar radiation and were the result of correlated changes in fluorescence induced by UV-A and blue radiation but not photochemical efficiency (F(v)/F(m)) or initial fluorescence yield (F(o)). The magnitude of the midday reduction in T(UV) was greater for young leaves than for older leaves of Verbascum. Imposition of artificial shade eliminated the diurnal changes in T(UV) in Verbascum, but reduction in solar UV had no effect on diurnal T(UV) changes in Vicia. In Vicia, the diurnal changes in T(UV) occurred without detectable changes in the concentration of whole-leaf UV-absorbing compounds. Results suggest that plants actively control diurnal changes in UV shielding, and these changes occur in response to signals other than solar UV; however, the underlying mechanisms responsible for rapid changes in T(UV) remain unclear.

Adaptation to Water Stress in the Big Bluestem-Sand Bluestem Complex

Reciprocal transplant experiments and comparative water relations studies were con- ducted to determine the mechanisms controlling habitat assortment in the big bluestem (Andropogon gerardii)-sand bluestem (Andropogon hallii) complex along a local dune/meadow-soil moisture gra- dient in the Nebraska Sandhills. For bluestems growing in their natural habitats, midday leaf con- ductances and leaf water potentials during 1982 and 1983 were consistently lower in sand bluestem on dry, upland sand dunes, than in big bluestem in adjacent, naturally subirrigated, wet meadows. Hybrids at dune/meadow transition zones where soil moisture levels were intermediate between dunes and meadows showed intermediate leaf conductances, but midday leaf water potentials were consis- tently lower than other bluestem types in other habitats. In common gardens and under similar drought conditions, sand bluestem transplants maintained significantly higher midday leaf water potentials than adjacent big bluestem and hybrid transplants. A greater drought resistance in sand bluestem appeared to be related to a greater ability to control transpirational losses under drought conditions, such that higher leaf water potentials were maintained for longer periods of time. In contrast, under drought conditions, leaves of big bluestem continued to lose water, even at water potentials of -4 MPa. Data from the field and a laboratory drought experiment indicated that the bluestems did not differ in osmoregulation capabilities or stomatal sensitivity to vapor pressure deficits; there were major differences in leaf rolling characteristics, which together with variation in epicuticular wax accumulation may be important in controlling water loss under drought conditions. These physiological data cor- related well with results from seedling and adult rhizome transplant experiments, which showed rapid and massive mortality of big bluestem and hybrids following brief dry periods on dune sites, and suggest that variation in drought resistance within the bluestem complex could be an important mechanism of segregating bluestem types along local dune-meadow soil moisture gradients.

Comparative Photobiology of Growth Responses to Two UV-B Wavebands and UV-C in Dim-red-light- and White-light-grown Cucumber (Cucumis sativus) Seedlings: Physiological Evidence for Photoreactivation†

Abstract We examined the influence of short-term exposures of different UV wavebands on the elongation and phototropic curvature of hypocotyls of cucumbers (Cucumis sativus L.) grown in white light (WL) and dim red light (DRL). We evaluated (1) whether different wavebands within the ultraviolet B (UV-B) region elicit different responses; (2) the hypocotyl elongation response elicited by ultraviolet C (UV-C); (3) whether irradiation with blue light–enriched white light (B/WL) given simultaneous with UV-B treatments reversed the effect of UV in a manner indicative of photoreactivation; and (4) whether responses in WL-grown plants were similar to those grown in DRL. Responses to brief (1–100 min) irradiations with three different UV wavebands all induced inhibition of elongation measured after 24 h. When WL-grown seedlings were irradiated with light containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm), inhibition of hypocotyl elongation was induced at a threshold of 0.5 kJ m−2, whereas exposure to UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation at a threshold of 1.6 kJ m−2. The UV-C treatment induced reduction in elongation at a threshold of <0.01 kJ m−2 for DRL-grown plants and <0.03 kJ m−2 for WL-grown plants. B/WL caused 50% reversal of the short-wavelength UV-B–induced inhibition of elongation in DRL-grown seedlings but did not reverse the effect of long-wavelength UV-B. B/WL caused 30% reversal of the UV-C–induced inhibition of elongation in WL-grown seedlings but did not affect the response to short-wavelength UV-B. Short-wavelength UV-B also induced positive phototropic curvature in both types of seedlings, and this was reversed 60% or completely in DRL-grown and WL-grown seedlings, respectively. The similarity of responses between the etiolated (DRL-grown) and de-etiolated (WL-grown) seedlings indicates that the short-wavelength specific response may be relevant to natural light environments, and the apparent photoreactivation implicates DNA damage as the sensory mechanism for the response.

Sunlight and Soil–Litter Mixing: Drivers of Litter Decomposition in Drylands

Decomposition of leaf litter is a key component of biogeochemical cycles but the mechanisms driving it in arid and semiarid ecosystems (drylands) remain unresolved. Here, we review recent findings that demonstrate dual roles of solar radiation (ultraviolet and photosynthetically active radiation) and soil–litter mixing as drivers of decomposition in drylands. We focus on the known and potential mechanisms by which these factors influence leaf litter decomposition, explore how the importance of these two drivers may shift over time, and propose possible avenues by which these factors may interact. Special attention is given to UV in sunlight, as this radiation is known to have multiple roles in influencing decomposition and has received considerable recent research attention. We also identify important uncertainties and challenges and offer a generalized conceptual model to guide future research aimed at enhancing our mechanistic understanding and quantitative modeling of the processes by which soil deposition and solar radiation together influence leaf litter decomposition rates in globally extensive dryland ecosystems.

Rapid modulation of ultraviolet shielding in plants is influenced by solar ultraviolet radiation and linked to alterations in flavonoids

The accumulation of ultraviolet (UV)-absorbing compounds (flavonoids and related phenylpropanoids) and the resultant decrease in epidermal UV transmittance (TUV ) are primary protective mechanisms employed by plants against potentially damaging solar UV radiation and are critical components of the overall acclimation response of plants to changing solar UV environments. Whether plants can adjust this UV sunscreen protection in response to rapid changes in UV, as occurs on a diurnal basis, is largely unexplored. Here, we use a combination of approaches to demonstrate that plants can modulate their UV-screening properties within minutes to hours, and these changes are driven, in part, by UV radiation. For the cultivated species Abelmoschus esculentus, large (30-50%) and reversible changes in TUV occurred on a diurnal basis, and these adjustments were associated with changes in the concentrations of whole-leaf UV-absorbing compounds and several quercetin glycosides. Similar results were found for two other species (Vicia faba and Solanum lycopersicum), but no such changes were detected in Zea mays. These findings reveal a much more dynamic UV-protection mechanism than previously recognized, raise important questions concerning the costs and benefits of UV-protection strategies in plants and have practical implications for employing UV to enhance crop vigor and quality in controlled environments.

Adjustments in epidermal UV-transmittance of leaves in sun-shade transitions.

Epidermal UV transmittance (TUV ) and UV-absorbing compounds were measured in sun and shade leaves of Populus tremuloides and Vicia faba exposed to contrasting light environments under field conditions to evaluate UV acclimation potentials and regulatory roles of photosynthetically active radiation (PAR) and UV in UV-shielding. Within a natural canopy of P. tremuloides, TUV ranged from 4 to 98% and showed a strong nonlinear relationship with mid-day horizontal fluxes of PAR [photon flux density (PFD) = 6-1830 µmol m⁻²  s⁻¹]; similar patterns were found for V. faba leaves that developed under a comparable PFD range. A series of field transfer experiments using neutral-density shade cloth and UV blocking/transmitting films indicated that PAR influenced TUV during leaf development to a greater degree than UV, and shade leaves of both species increased their UV-shielding when exposed to full sun; however, this required the presence of UV, with both UV-A and UV-B required for full acclimation. TUV of sun leaves of both species was largely unresponsive to shade either with or without UV. In most, but not all cases, changes in TUV were associated with alterations in the concentration of whole-leaf UV-absorbing compounds. These results suggest that, (1) moderate-to-high levels of PAR alone during leaf development can induce substantial UV-protection in field-grown plants, (2) mature shade leaves have the potential to adjust their UV-shielding which may reduce the detrimental effects of UV that could occur following sudden exposures to high light and (3) under field conditions, PAR and UV play different roles in regulating UV-shielding during and after leaf development.

Rediscovering leaf optical properties: New insights into plant acclimation to solar UV radiation

The accumulation of UV-absorbing compounds (flavonoids and other phenylpropanoid derivatives) and resultant decrease in the UV transmittance of the epidermis in leaves (TUV), is a primary protective mechanism against the potentially deleterious effects of UV radiation and is a critical component of the overall acclimation response of plants to changing UV environments. Traditional measurements of TUV were laborious, time-consuming and destructive or invasive, thus limiting their ability to efficiently make multiple measurements of the optical properties of plants in the field. The development of rapid, nondestructive optical methods of determining TUV has permitted the examination of UV optical properties of leaves with increased replication, on a finer time scale, and enabled repeated sampling of the same leaf over time. This technology has therefore allowed for studies examining acclimation responses to UV in plants in ways not previously possible. Here we provide a brief review of these earlier studies examining leaf UV optical properties and some of their important contributions, describe the principles by which the newer non-invasive measurements of epidermal UV transmittance are made, and highlight several case studies that reveal how this technique is providing new insights into this UV acclimation response in plants, which is far more plastic and dynamic than previously thought.

Spatial Asymmetry in Tree-Shrub Clusters in a Subtropical Savanna

Abstract Plateau live oak (Quercus virginiana var. fusiformis) is thought to act as a nurse plant to other woody species in the upland savannas of the Edwards Plateau of central Texas; however, little is known of the nature, extent and duration of this facilitation. We tested the hypothesis that spatial asymmetry exists in the composition of the understory woody plant community associated with live oak trees that would indicate facilitative effects of the oak canopy on understory microclimate. The central live oaks in 20 discrete tree-shrub clusters sampled at a location in the eastern Edwards Plateau were relatively large and possessed a dense understory of 5 to 11 species of shrubs and small trees. Both adult and juvenile woody plants were nonuniformly distributed around the central live oak with significantly more individuals and species found in the northern than southern halves of clusters. No statistical differences were found between northern (NE vs. NW) or southern (SE vs. SW) quadrants or between eastern and western sides. Similar patterns were found for total cover (canopy and basal diameter) of adults and density and cover of Juniperus ashei, the dominant understory species. Neither the degree of this asymmetry or extent of understory development (i.e., total shrub density or species richness) was related to the size of the central live oak. Results are consistent with the hypothesis that microclimate modification by the live oak overstory is an important and perhaps persistent mechanism of facilitation in these woody clusters. These findings also suggest that processes structuring these live oak clusters differ from those of other tree-shrub clusters in subtropical savannas in this general region.

Photomorphogenic regulation of increases in UV-absorbing pigments in cucumber (Cucumis sativus) and Arabidopsis thaliana seedlings induced by different UV-B and UV-C wavebands.

Brief (1-100 min) irradiations with three different ultraviolet-B (UV-B) and ultraviolet-C (UV-C) wave bands induced increases the UV-absorbing pigments extracted from cucumber (Cucumis sativus L.) and Arabidopsis. Spectra of methanol/1% HCl extracts from cucumber hypocotyl segments spanning 250-400 nm showed a single defined peak at 317 nm. When seedlings were irradiated with 5 kJ m(-2) UV-B radiation containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm; full-spectrum UV-B, FS-UVB), tissue extracts taken 24 h after irradiation showed an overall increase in absorption (91% increase at 317 nm) with a second defined peak at 263 nm. Irradiation with 1.1 kJ m(-2) UV-C (254 nm) caused similar changes. In contrast, seedlings irradiated with 5 kJ m(-2) UV-B including only wavelengths longer than 290 nm (8% of UV-B between 290 and 300 nm; long-wavelength UV-B, LW-UVB) resulted only in a general increase in absorption (80% at 317 nm). The increases in absorption were detectable as early as 3 h after irradiation with FS-UVB and UV-C, while the response to LW-UVB was first detectable at 6 h after irradiation. In extracts from whole Arabidopsis seedlings, 5 kJ m(-2) LW-UVB caused only a 20% increase in total absorption. Irradiation with 5 kJ m(-2) FS-UVB caused the appearance of a new peak at 270 nm and a concomitant increase in absorption of 72%. The induction of this new peak was observed in seedlings carrying the fah1 mutation which disrupts the pathway for sinapate synthesis. The results are in agreement with previously published data on stem elongation indicating the existence of two response pathways within the UV-B, one operating at longer wavelengths (>300 nm) and another specifically activated by short wavelength UV-B (<300 nm and also by UV-C).