Photosynthetic Response to Irradiance in Valeriana jatamansi Jones,a Threatened Understorey Medicinal Herb of Western Himalaya

Net photosynthetic rate (P _N) of Valeriana jatamansi plants, grown under nylon net shade or under different tree canopies, was saturated with photons at 1 000 mol m^–2 s^–1 photosynthetic photon-flux-density (PPFD), whereas open-grown plants were able to photosynthesise even at higher PPFD, e.g. of 2 000 mol m^–2 s^–1. Plants grown under net shade had higher total chlorophyll (Chl) content per unit area of leaf surface. However, Chl a/b ratio was maximal in open-grown plants, but remained unchanged in plants grown in nylon net shade and under different tree canopies. Sun-grown plants had thicker leaves (higher leaf mass per leaf area unit), higher wax content, and higher P _N than shade grown plants. Thus V. jatamansi is able to acclimate to high PPFD and therefore this Himalayan species may be cultivated in open habitat to meet the ever-increasing industrial demand.     

Arabidopsis Thylakoid Formation 1 Is a Critical Regulator for Dynamics of PSII-LHCII Complexes in Leaf Senescence and Excess Light

In higher plants, photosystem II （PSII） is a large pigment-protein supramolecular complex composed of the PSII core complex and the plant-specific peripheral light-harvesting complexes （LHCil）. PSli-LHCII complexes are highly dynamic in their quantity and macro-organization to various environmental conditions. In this study, we reported a critical factor, the Arabidopsis Thylakoid Formation 1 （THF1） protein, which controls PSII-LHCII dynamics during dark- induced senescence and light acclimation. Loss-of-function mutations in THF1 lead to a stay-green phenotype in path- ogen-infected and senescent leaves. Both LHCII and PSll core subunits are retained in dark-induced senescent leaves of thfl, indicative of the presence of PSII-LHCII complexes. Blue native （BN）-polyacrylamide gel electrophoresis （PAGE） and immunoblot analysis showed that, in dark- and high-light-treated thfl leaves, a type of PSII-LHCII megacomplex is selec- tively retained while the stability of PSII-LHCII supercomplexes significantly decreased, suggesting a dual role of THF1 in dynamics of PSII-LHCII complexes. We showed further that THF1 interacts with Lhcb proteins in a pH-dependent manner and that the stay-green phenotype of thfl relies on the presence of LHCII complexes. Taken together, the data suggest that THF1 is required for dynamics of PSII-LHCII supramolecular organization in higher plants.     

Tradeoffs Between Irradiance Capture and Avoidance in Semi-arid Environments Assessed with a Crown Architecture Model

Plants in arid environments cope with stress from excessiveirradiance by physiological photoprotection of the photosyntheticapparatus and by structurally reducing the leaf area exposedto the sun (structural photoprotection). We assessed the ecologicalrelevance of structural photoprotection in two plants of contrastingarchitecture co-occurring in a semi-arid environment, usingthe three-dimensional canopy model YPLANT. We compared the roleof crown geometry in avoiding excessive radiation, analysedthe costs of structural photoprotection in terms of reductionof potential carbon gain, and compared these costs with thosedue to seasonal constraints of photosynthesis and tissue ageing.The results of the model simulations indicated that canopy architectureofStipa tenacissima(a tussock grass) andRetama sphaerocarpa(aleafless leguminous shrub) minimized the risk of overheatingand photo-oxidative destruction of the photosynthetic apparatuswith steeply oriented foliage and moderate self-shading. Butthis structural photoprotection imposed an increased cost interms of potential carbon gain. Diurnal and seasonal patternsof light interception by the crown of these plants translatedinto a simulated potential carbon gain only half that of anequivalent, horizontal photosynthetic surface. This reductionin potential carbon gain, due to irradiance avoidance, was similarto that imposed by water shortage.S. tenacissima,which ceasesphotosynthetic activity during periods of drought, exhibitedmore structural avoidance of irradiance thanR. sphaerocarpa,whichremains active throughout the year. This illustrates the influenceof the capacity of plants to utilize light for carbon fixationon the trade-offs between irradiance capture and avoidance.Structural avoidance of excessive radiation efficiently preventsthe risk of damage by intense irradiance, has no special maintenancecosts, and is biomechanically cheaper than enhanced light harvestingby a horizontal canopy, which points to structural photoprotectionas a very effective strategy to cope with high irradiance stressin poor and adverse habitats.Copyright 1999 Annals of BotanyCompany Canopy model, functional architecture, irradiance avoidance, light harvesting, photoinhibition, photoprotection, photosynthesis model, plant architecture,Retama sphaerocarpa,semi-arid environment,Stipa tenacissima,structural photoprotection.     

From Forest to Farmland: Species Richness Patterns of Trees and Understorey Plants along a Gradient of Forest Conversion in Southwestern Cameroon

Vegetation surveys were carried out at 24 sampling stations distributed over four land use types, namely near-primary forest, secondary forest, agroforestry systems and annual crop lands in the northeastern part of the Korup region, Cameroon, to assess the impact of forest conversion on trees and understorey plants. Tree species richness decreased significantly with increasing level of habitat modification, being highest and almost equal in secondary and near-primary forests. Understorey plant species richness was significantly higher in annual crop lands than in other land use types. The four land use types differed in tree and understorey plant species composition, the difference being smaller among natural forests. Tree and understorey plant density differed significantly between habitat types. Density was strongly correlated with species richness, both for trees and understorey plants. Five tree and 15 understorey plant species showed significant responses to habitat. A 90% average drop in tree basal area from forest to farmland was registered. Our findings support the view that agroforestry systems with natural shade trees can serve to protect many forest species, but that especially annual crop lands could be redesigned to improve biodiversity conservation in agricultural landscapes of tropical rainforest regions.     

Dorsiventrality in Photosynthetic Light Response Curves of a Leaf

Terashima, I. 1986. Dorsiventrality in photosynthetic lightresponse curves of a leaf.—J. cxp. Bot. 37 399–405 The photosynthetic light response curve of a leaf of Glycinemax (L.) Merrill obtained by illuminating the adaxial side layabove that obtained by illuminating the abaxial side. However,after inverting the leaf for 11 d, the curve obtained by illuminatingthe abax.ial side came to lie slightly above that obtained byilluminating the adaxial side. The difference in the shape oflight response curves is satisfactorily explained only whenthe intra-leaf heterogeneities in light absorption and in photosyntheticactivity are taken into account. Key words: Photosynthetic rate, direction of illumination     

Evidence for Light Wavelength-Specific Photoelectrophysiological Signaling and Memory of Excess Light Episodes in Arabidopsis

Although light is essential for photosynthesis, excess light can damage the photosynthetic apparatus and deregulate other cellular processes. Thus, protective integrated regulatory responses that can dissipate excess of absorbed light energy and simultaneously optimize photosynthesis and other cellular processes under variable light conditions can prove highly adaptive. Here, we show that the local and systemic responses to an excess light episode are associated with photoelectrophysiological signaling (PEPS) as well as with changes in nonphotochemical quenching and reactive oxygen species levels. During an excess light incident, PEPS is induced by quantum redox changes in photosystem II and in its proximity and/or by changes in glutathione metabolism in chloroplasts. PEPS is transduced, at least in part, by bundle sheath cells and is light wavelength specific. PEPS systemic propagation speed and action potential are dependent on ASCORBATE PEROXIDASE2 function. Excess light episodes are physiologically memorized in leaves, and the cellular light memory effect is specific for an excess of blue (450 nm) and red (650 nm) light of similar energy. It is concluded that plants possess a complex and dynamic light training and memory system that involves quantum redox, reactive oxygen species, hormonal, and PEPS signaling and is used to optimize light acclimation and immune defenses.     

Tree Leaf Form in Brunei: A Heath Forest and a Mixed Dipterocarp Forest Compared1

Canopy‐top leaves of the dominant tree species from two 0.96‐ha plots in Brunei, northern Borneo, were sampled for structural and chemical analysis. Thirteen species from the mixed dipterocarp forest at Andulau and 14 from the lowland heath forest at Badas were studied. The heath‐forest species had significantly thicker leaves and were lower in nitrogen and ash concentration than those from the mixed dipterocarp forest. There were no significant differences between the two species groups in leaf mass per unit area (LMA), leaf fracture toughness, carbon concentration, 8¹³C, neutral detergent fiber concentration, sclerophylly index, and stomatal density. A significant negative correlation between %C and 8¹³C was found for the species from the mixed dipterocarp forest, but not those from the heath forest. The degree of sclerophylly measured in physical terms overlapped between the two sites to a considerable degree; however, all six species tested that were present in both plots had higher leaf fracture toughness in the heath forest. The possible reasons for the marked sclerophylly in the mixed dipterocarp forest are discussed.     

Myxobacterial Biodiversity in an Established Oak-Hickory Forest and a Savanna Restoration Site

The purpose of this study was to establish the myxobacterial biodiversity of an established oak-hickory forest and a savanna restoration site that has been cut and subsequently burned on four occasions between 1993 and 1998 in an attempt to restore the land to the native savanna ecosystem. Soil and bark samples were processed through standard methods specifically for myxobacteria and numbers and types of species were recorded for both locations. Species were identified through morphology of fruiting bodies, SDS-PAGE of whole cell protein profiles, and 16S rRNA gene sequences. Statistical analyses were employed and suggested that significantly greater numbers and types of myxobacteria are present on the bark of the trees in the established oak-hickory forest than the bark of trees in the savanna restoration site, while little difference in numbers and types of species were observed between the soil samples of the two locations.     

Spectral and Structural Measures of Northwest Forest Vegetation at Leaf to Landscape Scales

We report a multiscale study in the Wind River Valley in southwestern Washington, where we quantified leaf to stand scale variation in spectral reflectance for dominant species. Four remotely sensed structural measures, the normalized difference vegetation index (NDVI), cover fractions from spectral mixture analysis (SMA), equivalent water thickness (EWT), and albedo were investigated using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. Discrimination of plant species varied with wavelength and scale, with deciduous species showing greater separability than conifers. Contrary to expectations, plant species were most distinct at the branch scale and least distinct at the stand scale. At the stand scale, broadleaf and conifer species were spectrally distinct, as were most conifer age classes. Intermediate separability occurred at the leaf scale. Reflectance decreased from leaf to stand scales except in the broadleaf species, which peaked in near-infrared reflectance at the branch scale. Important biochemical signatures became more pronounced spectrally progressing from leaf to stand scales. Recent regenerated clear-cuts (less than 10 years old) had the highest albedo and nonphotosynthetic vegetation (NPV). After 50 years, the stands showed significant decreases in albedo, NPV, and EWT and increases in shade. Albedo was lowest in old-growth forests. Peak EWT, a proxy measure for leaf area index (LAI), was observed in 11- to 30-year-old stands. When compared to LAI, EWT and NDVI showed exponentially decreasing, but distinctly different, relationships with increasing LAI. This difference is biologically important: at 95% of the maximum predicted NDVI and EWT, LAI was 5.17 and 9.08, respectively. Although these results confirm the stand structural variation expected with forest succession, remote-sensing images also provide a spatial context and establish a basis to evaluate variance within and between age classes. Landscape heterogeneity can thus be characterized over large areas—a critical and important step in scaling fluxes from stand-based towers to larger scales.     

Control of Leaf and Stem Growth in Light-grown Pea Seedlings by Two High Irradiance Responses

The control exerted by light on leaf and stem growth in light-grown Alaska pea seedlings was studied during the main photoperiod. Two high irradiance responses were observed. The action spectrum for one had a single sharp peak at 600 nanometers. The action spectrum for the other showed a broad peak between 440 and 470 nanometers. These two light responses must be activated simultaneously for any inhibition of stem growth or promotion of leaf growth. Both action spectra may be explained in terms of the high irradiance response of phytochrome.     

The Senescence Process in Oat Leaves and its Regulation by Oxygen Concentration and Light Irradiance

The regulation of senescence by oxygen-concentration, lightirradiance and H₂O₂ has been studied in leaf segments of Avenasativa L. cv. Suregrain. The development of the components of the senescence process,for example chlorophyll breakdown, proteolysis (as soluble aminoacids), hydroperoxides (as malondi-aldehyde) and permeability(as conductivity) is accelerated in light as the O₂-tensionincreases. In darkness, 0.3% O₂ accelerates increases in hydroperoxides,permeability and proteolysis and delays the chlorophyll break-down,but 0.0005% O₂ delays all the components studied. In every casethe hydroperoxide content, permeability and proteolysis areclosely related. Any treatment inducing an increase in membranepermeability causes chlorophyll bleaching (photo-oxidation)if leaf segments are then treated with light in an atmospherecontaining oxygen. Light has a modulating effect on the senescenceprocess. An irradiance lower or higher than 40 W m^–2 hasan accelerating effect on the senescence process. (Received September 7, 1985; Accepted July 30, 1985)     

The Polarization of Light in a Tropical Rain Forest1

The light environment within forests presents complex patterns of brightness and spectral distribution of light. The polarized light field is no less complex. Using an imaging polarized light analyzer, we examined the natural fields of linearly polarized light in the tropical rain forest of Guatopo National Park, Venezuela. We found that the celestial polarization pattern remains visible underneath the forest canopy, although cloud and fog coverage may diffuse the light and reduce the polarization signal. We characterized several distinct light environments, each having a characteristic polarized light field. Furthermore, objects throughout the forest reflect light that is polarized in a predictable fashion depending upon the material, structure, and orientation of the reflecting surface. As a consequence of these patterns in the distribution of polarized light, some functions of polarization vision, such as navigation, must be limited to the spaces exposed to several extended portions of the sky, while others, such as remote sensing of surface orientation, object detection, and breaking of camouflage would be useful throughout the forest. The polarization of light adds another dimension to the complexity of the rain forest photic environment.     

Leaf Gas Exchange in Canopy Species of a Venezuelan Cloud Forest

Tropical cloud forests are considered humid ecosystems with frequent cloud cover down to the ground surface. However, seasonal variation in precipitation may induce short-term water stress. For canopy leaves, this water stress may also be a consequence of large atmospheric vapor pressure deficits. The objective of this work was to study five canopy cloud forest species to determine if there are restrictions to leaf gas exchange as a consequence of seasonality in precipitation and to daily water deficit due to air evaporative demand mainly during maximum incoming radiation hours. Seasonal daily courses of microclimatic variables (air temperature, relative humidity, photosynthetic photon flux density) and plant responses (leaf water potential, stomatal conductance, CO₂ assimilation rates, leaf nitrogen concentration) were measured at 2400 m asl in Monterrey, an intermontane valley of the Venezuelan Andes. A gradient in terms of responses to water stress conditions was observed between the species, with Clusia multiflora (a 46% reduction in stomatal conductance between seasons) as the most affected and Miconia resimoides (increased stomatal conductance) responding more favorably to slight water stress conditions. If we consider the limitations of water stress and/or light conditions on CO₂ assimilation we may arrange the species into those in which water stress conditions have a greater impact on leaf carbon gain, those where light conditions are determinant and one in which both water stress and light conditions may affect leaf carbon assimilation.     

Zeaxanthin and the Heat Dissipation of Excess Light Energy in Nerium oleander Exposed to a Combination of High Light and Water Stress

Upon termination of watering of plants of Nerium oleander exposed to high light, photochemical efficiency became reduced as leaf water content decreased. Evidence is presented that this type of photoinhibition reflects to a substantial degree radiationless dissipation of excitation energy, probably mediated by the carotenoid zeaxanthin. During the imposition of water stress, the zeaxanthin content of leaves increased at the expense of violaxanthin and β-carotene as a water deficit developed over a period of several days. The increase in zeaxanthin content was linearly related to an increase in the rate of radiationless energy dissipation in the antenna chlorophyll as calculated from the characteristics of chlorophyll a fluorescence measured with a pulse amplitude modulated fluorometer at room temperature. The increase in the rate of radiationless dissipation was also linearly related to a decrease in PSII photochemical efficiency as indicated by the ratio of variable to maximum fluorescence. Leaves of well-watered shade plants of N. oleander exposed to strong light showed a similar increase in zeaxanthin content as sun leaves of the same species subjected to drought in strong light. Shade leaves possessed the same capacity as sun leaves to form zeaxanthin at the expense of both violaxanthin and β-carotene. The resistance of this species to the destructive effects of excess light appears to be related to interconversions between β-carotene and the three carotenoids of the xanthophyll cycle.     

Capture of linear fragments at a double-strand break in yeast

Double-strand breaks (DSBs) are dangerous chromosomal lesions that must be efficiently repaired in order to avoid loss of genetic information or cell death. In all organisms studied to date, two different mechanisms are used to repair DSBs: homologous recombination (HR) and non-homologous end joining (NHEJ). Previous studies have shown that during DSB repair, non-homologous exogenous DNA (also termed ‘filler DNA’) can be incorporated at the site of a DSB. We have created a genetic system in the yeast Saccharomyces cerevisiae to study the mechanism of fragment capture. Our yeast strains carry recognition sites for the HO endonuclease at a unique chromosomal site, and plasmids in which a LEU2 gene is flanked by HO cut sites. Upon induction of the HO endonuclease, a linear extrachromosomal fragment is generated in each cell and its incorporation at the chromosomal DSB site can be genetically monitored. Our results show that linear fragments are captured at the repaired DSB site at frequencies of 10⁻⁶ to 10⁻⁴ per plated cell depending on strain background and specific end sequences. The mechanism of fragment capture depends on the NHEJ machinery, but only partially on the homologous recombination proteins. More than one fragment can be used during repair, by a mechanism that relies on the annealing of small complementary sequences. We present a model to explain the basis for fragment capture.     

Leaf Carbon Metabolism and Metabolite Levels during a Period of Sinusoidal Light

Photosynthesis rate, internal CO₂ concentration, starch, sucrose, and metabolite levels were measured in leaves of sugar beet (Beta vulgaris L.) during a 14-h period of sinusoidal light, which simulated a natural light period. Photosynthesis rate closely followed increasing and decreasing light level. Chloroplast metabolite levels changed in a manner indicating differential activation of enzymes at different light levels. Starch levels declined during the first and last 2 hours of the photoperiod, but increased when photosynthesis rate was greater than 50% of maximal. Sucrose and sucrose phosphate synthase levels were constant during the photoperiod, which is consistent with a relatively steady rate of sucrose synthesis during the day as observed previously (BR Fondy et al. [1989] Plant Physiol 89: 396-402). When starch was being degraded, glucose 1-phosphate level was high and there was a large amount of glucose 6-phosphate above that in equilibrium with fructose 6-phosphate, while fructose 6-phosphate and triose-phosphate levels were very low. Likewise, the regulatory metabolite, fructose, 2,6-bisphosphate was high, indicating that little carbon could move to sucrose from starch by the triose-phosphate pathway. These data cast doubt upon the feasibility of significant carbon flow through the triose-phosphate pathway during starch degradation and support the need for an additional pathway for mobilizing starch carbon to sucrose.     

云南松林与常绿阔叶林中枯落叶分解研究

采用分解袋的方法,研究了云南松、滇青冈和元江栲枯叶在针、阔叶林两种生境下的分解及养分动态变化规律。结果表明,滇青冈和元栲枯叶分解速率高于云南松针叶。在阔叶林下这些枯叶的分解系数（０．５５－０．６１ａ＾－１）要比在云南松林下的（０．５０－０．５３ａ＾－１）高。在分解过程中３种枯叶的Ｎ、Ａｌ、Ｆｅ、Ｚｎ元素含量表现出分解前期富积,后期释放的特征,Ｐ、Ｃａ元素在阔叶林地的分解中也有富积过程。而Ｍｇ、Ｋ元     

Analysis of nifH Gene Pool Complexity in Soil and Litter at a Douglas Fir Forest Site in the Oregon Cascade Mountain Range

Nitrogen-fixing microbial populations in a Douglas fir forest on the western slope of the Oregon Cascade Mountain Range were analyzed. The complexity of the nifH gene pool (nifH is the marker gene which encodes nitrogenase reductase) was assessed by performing nested PCR with bulk DNA extracted from plant litter and soil. The restriction fragment length polymorphisms (RFLPs) of PCR products obtained from litter were reproducibly different than the RFLPs of PCR products obtained from the underlying soil. The characteristic differences were found during the entire sampling period between May and September. RFLP analyses of cloned nifH PCR products also revealed characteristic patterns for each sample type. Among 42 nifH clones obtained from a forest litter library nine different RFLP patterns were found, and among 64 nifH clones obtained from forest soil libraries 13 different patterns were found. Only two of the patterns were found in both the litter and the soil, indicating that there were major differences between the nitrogen-fixing microbial populations. A sequence analysis of clones representing the 20 distinct patterns revealed that 19 of the patterns had a proteobacterial origin. All of the nifH sequences obtained from the Douglas fir forest litter localized in a distinct phylogenetic cluster characterized by the nifH sequences of members of the genera Rhizobium, Sinorhizobium, and Azospirillum. The nifH sequences obtained from soil were found in two additional clusters, one characterized by sequences of members of the genera Bradyrhizobium, Azorhizobium, Herbaspirillum, and Thiobacillus and the other, represented by a single nifH clone, located between the gram-positive bacteria and the cyanobacteria. Our results revealed the distinctness of the nitrogen-fixing microbial populations in litter and soil in a Douglas fir forest; the differences may be related to special requirements for degradation and mineralization processes in the plant litter.     

Explaining Leaf Herbivory Rates on Tree Seedlings in a Malaysian Rain Forest

Seedlings of five species of dipterocarp trees were planted in experimental plots in rain forest gaps in Sabah, Malaysia, and the rates of herbivory on their mature leaves recorded over 6 mo. A novel method was used to estimate the feeding pressure exerted by the local insect herbivore community, derived from the relative abundances of the dominant generalist herbivores and their feeding preferences. Characteristics of the leaves related to their defense and nutritional value were measured—phenolic content, laminar fracture toughness, laminar thickness, and nitrogen content. Three main groups of herbivorous insects were present—coleopteran and lepidopteran herbivores, which were sampled by hand from the seedlings, and orthopteran herbivores, which were sampled by sweep netting. The feeding preferences of the main coleopteran and orthopteran herbivores were determined using laboratory feeding trials. Combining variables in a Principal Components Analysis, a clear separation was found between the five seedling species along the first extracted component. This correlated closely with herbivory rates between species. The first extracted component comprised a negative influence of phenolic content and positive effects of nitrogen content, laminar fracture toughness, abundances of coleopteran and lepidopteran herbivores, and estimated feeding pressure of the coleopteran community. Further studies are required to determine the potential applications of the latter measure of estimated herbivore community impact.