Plant transpiration at high elevations: Theory,field measurements,and comparisons with desert plants

Summary The influence of elevational changes on plant transpiration was evaluated using leaf energy balance equations and well-known elevational changes in the physical parameters that influence water vapor diffusion. Simulated transpirational fluxes for large leaves with low and high stomatal resistances to water vapor diffusion were compared to small leaves with identical stomatal resistances at elevations ranging from sea level to 4 km. The specific influence of various air temperature lapse rates was also tested. Validation of the simulated results was accomplished by comparing actual field measurements taken at a low elevation (300 m) desert site with similar measurements for a high elevation (2,560 m) mountain research site. Close agreement was observed between predicted and measured values of transpiration for the environmental and leaf parameters tested.Substantial increases in solar irradiation and the diffusion coefficient for water vapor in air (D _wv) occurred with increasing elevation, while air and leaf temperatures, the water vapor concentration difference between the leaf and air, longwave irradiation, and the thermal conductivity coefficient for heat in air decreased with increasing elevation. These changes resulted in temperatures for sunlit leaves that were further above air temperature at higher elevations, especially for large leaves. For large leaves with low stomatal resistances, transpirational fluxes for low-elevation desert plants were close to those predicted for high-elevation plants even though the sunlit leaf temperatures of these mountain plants were over 10°C cooler. Simulating conditions with a low air temperature lapse rate (0.003° C m^-1 and 0.004° C m^-1) resulted in predicted transpirational fluxes that were greater than those calculated for the desert site. Transpiration for smaller leaves decreased with elevation for all lapse rates tested (0.003° C m^-1 to 0.010° C m^-1). However, transpirational fluxes at higher elevations were considerably greater than expected for all leaves, especially larger leaves, due to the strong influence of increased solar heating and a greater D _wv. These results are discussed in terms of similarities in leaf structure and plant habit observed among low-elevation desert plants and high-elevation alpine and subalpine plants.     

Isolate-specific effects of ultraviolet radiation on photosynthesis,growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium <Emphasis Type="Italic">Microcoleus </Emphasis><Emphasis Type="Italic">chthonoplastes</Emphasis>

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12–15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 μmol photon m⁻² s⁻¹) = P; PAR + UV-A (8 W m⁻²) = PA; PAR + UV-A + UV-B (0.4 W m⁻²) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F _v/F _m) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F _v/F _m during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F _v/F _m sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.     

Xylem water flow in tropical vines as measured by a steady state heating method

Summary A method for determining the mass flow rate of xylem water in thin stems under natural field conditions is presented. Diurnal courses of xylem water flow and stomatal conductance of the vines Entadopsis polystachya, Cyclanthera multifoliolata, and Serjania brachycarpa were examined in a tropical deciduous forest on the west coast of Mexico. E. polystachya (leaf area 23.6 m²) had a maximum water flow rate of 6.50 kg h^-1 or 1.44 kg cm^-2 _{stem basal area} h^-1; daily water use was 2.00 kg m^-2 _{leaf area} day^-1. S. brachycarpa (leaf area 4.5 m²) and C. multifoliolata (leaf area 3.6 m²) had a maximum water flow rate of 0.72 and 0.19 kg h^-1 or 0.63 and 0.92 kg cm^-2 _{stem basal area} h^-1. Daily water use was 1.26 and 0.39 kg m^-2 _{leaf area} day^-1, respectively. The daily courses of xylem water flow were strongly influenced by the orientation of the leaf area to irradiance and its intensity. While leaves of E. polystachya had a constant high stomatal conductance during the day, S. brachycarpa had a maximum stomatal opening in the morning followed by continuous closure during the rest of the day. In contrast to the woody species, the herbaceous C. multifoliolata exhibited a strong midday depression of stomatal conductance and wilting of its leaves. The leaf biomass accounted for 8% (Entadopsis), 16% (Serjania), and 23% (Cyclanthera) of above-ground biomass. The relation of sapwood area to leaf area supplied (Huber value) was 0.19 (Entadopsis), 0.18 (Serjania), and 0.06 (Cyclanthera) cm² m^-2     

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO₂ uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m^-2 day^-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m^-2 day^-1). Maximum net CO₂ uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO₂ diffusion conductances and the mesophyll surface available for CO₂ absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m^-2 day^-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m^-2 day^-1).     

Nitrogen form and silicon nutrition effects on resistance to blast disease of rice

The effect of N form and Si nutrition on rice (Oryza sativa L.) susceptibility to blast disease (caused by Pyricularia oryzae Cav.) was assessed in the greenhouse with nutrient solution culture. The N form supplied to the susceptible cultivar IR50 affected the relative infection efficiency (RIE) of P. oryzae measured as lesions/cm² leaf. Plants given NO₃ ^- were more susceptible than plants receiving NH₄ ⁺-N. This result may partially explain why plants grown in nonflooded soil, where NO₃ ^- is the main source of inorganic N, are more susceptible to blast than plants grown in flooded soils, where NH₄ ⁺ is the main inorganic N source. Nitrate-N and Mn concentration were higher in leaf blades of plants grown with NO₃ ^-. Total-N, Si, and Fe concentration were not affected by N form. The addition of Si significantly increased IR50 resistance to blast. With 2.2 mol m^-3 Si in solution, RIE values were lower by more than 90% than the control with no Si added in solution. The effect of Si accumulation in leaves at various positions was further studied in cultivars having differing levels of resistance (IR50, IR36, and IAC165). Silicon addition significantly reduced RIE in the three cultivars. Silicon concentration in the topmost leaves (the only leaves showing typical blast lesions) was not significantly different among the three cultivars when 2.2 mol m^-3 Si was used. Silicon was an important component in the mechanism of resistance to blast and it was effective regardless of the original level of resistance of the cultivar used. Contribution from the Agronomy Unit, Agronomy-Physiology-Agroecology Division, International Rice Research Institute (IRRI), P.O. Box 933, 1099 Manila, Philippines, and Colegio de Postgraduados, Mexico. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. degree.     

Gas exchange and water relations of two Rocky Mountain shrub species exposed to a climate change manipulation

Gas exchange and water relations responses to warming were compared for two shrub species, Artemisia tridentata spp. vaseyana (Asteraceae), a widely distributed evergreen species of the Great Basin and the western slope of the Rocky Mountains, and Pentaphylloides floribunda (Rosaceae), a deciduous shrub limited in distribution to moist, high-elevation meadows. Plants were exposed to an in situ infrared (IR) climate change manipulation at the Rocky Mountain Biological Laboratory, near Crested Butte, CO. Measurements of gas exchange and water relations were made on the two species in July and August, 1993 from plants growing in situ in infrared-heated and control plots. Carbon dioxide uptake, water loss, leaf temperature, water use efficiency, and water potential were compared to test the hypothesis that leaf and soil responses to IR will cause leaf level changes in photosynthesis. Photosynthetic CO₂ uptake and water use efficiency increased for A. tridentata (2.9 vs. 1.9 mol m^–2 s^–1 and 1.2 vs. 0.7 mmol C/mol H₂O) in the heated plots compared to the controls, while water potential was significantly lower in the heated plots (–1.1 vs. –0.5 MPa). The heating treatment decreased rates of photosynthesis for P. floribunda, but not significantly so. For A. tridentata, the results are consistent with the community-level changes observed with heating. Taken together, the evidence suggests that global warming is likely to result in increasing dominance of A. tridentata in subalpine meadow habitat now dominated by forbs.     

Elevational and age gradients in hawaiian montane rainforest: foliar and soil nutrients

Summary Soils and plants were sampled along an elevational gradient from 265–1675 m on a 133-and a 3100-year-old lava flow on Mauna Loa, Hawai'i. Soil organic matter and nutrients accumulated more rapidly at low elevation on the young flow, but reached higher levels at higher elevation on the old flow. Foliar nitrogen and phosphorus concentrations were less and specific leaf weight greater for Metrosideros polymorpha leaves collected at high versus low elevations and on the young versus the old flow. Foliar ¹³C was strongly correlated with specific leaf weight across the range of sites sampled.Published as a contribution to the Tropical Mountain Ecosystem Program of the IUBS Decade of the Tropics     

Circadian rhythms in Kalanchoë: effects of irradiance and temperature on gas exchange and carbon metabolism

Gas exchange in K. blossfeldiana shows a circadian rhythm in net CO₂ uptake and transpiration when measured under low and medium irradiances. The period length varies between 21.4 h at 60 W m^-2 and 24.0 h at 10 W m^-2. In bright light (80 W m^-2) or darkness there are no rhythms. High leaf temperatures result in a fast dampening of the CO₂-uptake rhythm at moderate irradiances, but low leaf temperatures can not overcome the dampening in bright light. The rhythm in CO₂ uptake is accompanied by a less pronounced and more rapidly damped rhythm in transpiration and by oscillations in malate levels with the amplitude being highly reduced. The oscillations in starch content, usually observed to oscillate inversely to the acidification in light-dark cycles, disappear after the first cycle in continuous light. The balance between starch and malate levels depends in continuous light on the irradiance applied. Leaves show high malate and low starch content at low irradiance and high starch and low malate in bright light. During the first 12 h in continuous light replacing the usual dark period, malate synthesis decreases with the increasing irradiance. Up to 50 W m^-2 starch content decreases; at higher irradiances it increases above the values usually measured at the end of the light period of the 12:12 h light-dark cycle.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - PEP phosphoenolpyruvate     

Leaf movement in Calathea lutea (Marantaceae)

Summary Calathea lutea is a broad-leaved, secondary successional plant which shows complex leaf movements involving both elevation and folding of the leaf surface about the pulvinus. In the plants studied, mean leaf elevation increased from approximately 34 degrees in the early morning to 70 degrees at noon while the angle of leaf folding increased from 13 degrees to 50 degrees over the same time period. During the period from early morning to noon, these movements resulted in a significant decrease in the cosine of the angle of incidence, a measure of the direct solar radiation intercepted. The observed changes in elevational angle significantly reduce the cosine of angle of incidence while folding does not significantly reduce the fraction of direct solar radiation intercepted during the period of direct exposure of the leaf surface to the solar beam. Since elevational changes seem to account for the reduction in exposure to direct solar radiation, the role of folding remains unclear.     

Leaf thickness and UV-B absorbing pigments of plants in relation to an elevational gradient along the Blue Mountains,Jamaica

Terrestrial plant species vary widely in their adaptation to (increasing) solar UV-B radiation. Among the various responses of higher plants to enhanced UV-B are increasing leaf thickness and increasing concentrations of UV-B absorbing compounds. In some (UV-B resistant) plant species increased leaf thickness and UV-B absorbance may form part of mechanisms protecting plants from UV-B damage. However, in UV-B sensitive plant species leaf thickness and UV-B absorbance may increase as well with enhanced UV-B radiation. In the latter case however, this response cannot prevent plant damage and disturbance. In the present field study the relationship between these plant parameters and a natural elevational UV-B gradient on the tropical island of Jamaica was described. Four plant species of the Blue Mountain Tropical Montane Forest, occurring on open forest sites along the roadside and paths were studied along an elevational gradient. Plant species studied are Redbush (Polygonum chinense), Wild ginger (Hedychium gardneranum), John Crow Bush (Bocconia frutescens) and White clover (Trifolium repens). The elevational sites were at 800, 1000, 1200, 1400 and 1600 m above sea level. Leaf thickness was measured of leaves of intact plants around midday in the field. Leaf disks (5 mm) were sampled and extracted with a methanol/HCl mixture. UV-B absorption of these leaf extracts was measured spectrophotometrically. For all species leaves from higher elevations were thicker than those from lower elevations. In addition, the absorption of UV-B of leaf extracts increased with increasing elevations. It is assumed that the calculated gradient of the UV-B_BE from 800 m above sea level: 9.45 kJ m^-2 day^-1 to 9.75 kJ m^-2 day^-1 at 1600 m is related to the measured increase of leaf thickness and UV-B absorbing compounds. The responsiveness of these plant parameters to the elevational gradient does not necessarily imply that the plant species are UV-B resistant. One possibility is that the species studied, which are growing on open, disturbed sites on the forest floor and along mountain-roads, are relatively sensitive to UV-B. In addition to clear sky conditions, mist and clouds occur frequently in this tropical mountane forest at Jamaica. Also, the low nutrient status of the soil (low pH, nutrient deficiency) and the high content of polyphenols in leaves of many plant species of the tropical montane rain forest may relate to the marked response of the species studied with increasing elevation. Abbreviations: asl – above sealevel, UV-B – ultraviolet-B radiation (280–320 nm), TMCF – Tropical Montane Cloud Forest.     

Variability in susceptibility to simulated sunlight of conidia among isolates of entomopathogenic Hyphomycetes

The influence of simulated sunlight on survival of conidia of 4 species of entomopathogenic Hyphomycetes was investigated. Conidia from 65 isolates ofBeauveria bassiana, 23 ofMetarhizium anisopliae, 14 ofMetarhizium flavoviride and 33 isolates ofPaecilomyces fumosoroseus were irradiated by artificial sunlight (295 to 1,100 nm at an ultraviolet-B irradiance of 0.3 W m^–2) for 0, 1, 2, 4 and 8 h. Survival was estimated by comparing the number of colony forming units (CFU) produced by conidia exposed to irradiation to the number of CFUs produced by an unexposed control. Survival decreased with increased exposure to simulated sunlight; exposure for 2 h or more was detrimental to all isolates tested. Overall, isolates ofM. flavoviride were the most resistant to irradiation followed byB. bassiana andM. anisopliae. Conidia ofP. fumosoroseus were most susceptible. In addition to the large interspecies differences in susceptibility to irradiation, there was also an intraspecies variation indicating that strain selection to irradiation tolerance may be important in the development of microbial control agents where increased persistence in an insolated environment is desirable.Abbreviations CFU Colony forming units - UV-B ultraviolet radiation-B     

Carry-over effects of ozone and water stress on leaf phenological characteristics and bud frost hardiness of <Emphasis Type="Italic">Fagus crenata</Emphasis> seedlings

We examined the carry-over effects of ozone (O₃) and/or water stress on leaf phenological characteristics and bud frost hardiness of Fagus crenata seedlings. Three-year-old seedlings were exposed to charcoal-filtered air or 60 nl l^–1 O₃, 7 h a day, from May to October 1999 in naturally-lit growth chambers. Half of the seedlings in each gas treatment received 250 ml of water at 3-day intervals (well-watered treatment), while the rest received 175 ml of water at the same intervals (water-stressed treatment). All the seedlings were moved from the growth chambers to an experimental field on October 1999, and grown until April 2000 under field conditions. The exposure to O₃ during the growing season induced early leaf fall and reduction in leaf non-structural carbohydrates concentrations in the early autumn, as well as resulting in late bud break and reduction in the number of leaves per bud in the following spring. However, O₃ did not affect bud frost hardiness in the following winter. On the contrary, water stress did not affect leaf phenological characteristics, leaf and bud non-structural carbohydrates concentrations and bud frost hardiness. There were no significant synergistic or antagonistic effects of O₃ and water stress on leaf phenological characteristics, concentrations of leaf and bud non-structural carbohydrates and bud frost hardiness of the seedlings. These results show that the carry-over effects of O₃ can be found on the phenological characteristics and leaf non-structural carbohydrates concentrations, although there are almost no carry-over effects of water stress on phenological characteristics and winter hardiness of the seedlings.     

Effects of temperature and solar radiation interactions on the survival of quiescent conidia of the entomopathogenic hyphomycetePaecilomyces fumosoroseus (Wize) Brown and Smith

The detrimental effect of solar radiation on the survival of conidia of the entomopathogenic fungusPaecilomyces fumoroseus was studied by monitoring germinability and ability to form colonies (CFU) of conidia irradiated at two temperatures, 25 and 35 °C, harmless to shaded conidia. There was no apparent effect when spores were exposed to a high level of artificial radiation (0.66 W m^–2 UVB). However, at a lower level of irradiance (0.33 W m^–2), effects of radiation occurred more quickly at 35 °C than at 25 °C. Under natural solar radiation, the rate of decrease in germinability or viability was doubled at 35 °C as compared to 25 °C, indicating an interaction between temperature and radiation effects under natural conditions. This interaction was not detected in indoor experiments, indicating that the spectral distribution of UV radiation has to be taken in account as well as its irradiance when studying its effects.Abbreviations CFU Colony Forming Units - UTC Universal Time Coordinates - UVB Ultra Violet B radiation (280–320 nm)     

西藏东达山3种嵩草属植物气孔特征沿海拔的变化

嵩草属(Kobresia)植物是藏东南高山草甸的优势种和建群种,对该区畜牧业发展和维持生态系统平衡起着重要作用。选择西藏左贡县东达山为研究地点,从林线开始,海拔每升高约100m设置1个样带直至高山草甸分布边缘,共8个样带,调查各样带中物种的组成及盖度,并依据相对盖度和相对频度计算3种嵩草植物矮生嵩草(K.humilis)、线叶嵩草(K.capillifolia)和大花嵩草(K.macrantha)在群落中的重要值,同时取样观察它们叶片远、近轴面表皮细胞形态,测量气孔长度及保卫细胞宽度,计算气孔密度,探讨嵩草属植物对海拔梯度的适应性。结果表明:(1)3种嵩草属植物叶表皮细胞均呈波浪状,气孔器仅分布于远轴面,近轴面无气孔器分布。(2)3种嵩草属植物气孔密度沿海拔梯度的变化均呈单峰曲线分布格局,且在海拔4 537m样带处达到最大值,并表现为矮生嵩草(777.6个/mm2)线叶嵩草(476.4个/mm2)大花嵩草(414.3个/mm2)。(3)随海拔的增加,矮生嵩草和线叶嵩草气孔长度显著增大(P0.05),而保卫细胞宽度显著减小;但大花嵩草气孔长度随海拔的升高而显著减小,保卫细胞宽度基本保持不变。(4)矮生嵩草和线叶嵩草气孔密度、长度和保卫细胞宽度与海拔梯度均显著相关,气孔特征对海拔梯度变化的敏感程度高,与其在群落中重要值高的分布特征一致;而大花嵩草仅气孔密度和长度与海拔梯度显著相关,气孔特征对海拔梯度变化的敏感性低,与其在群落中重要值低的分布特征一致;嵩草属植物气孔密度、长度和保护细胞宽度与海拔梯度之间的相关性,反映出它们在海拔梯度上对生境的适应程度。可见,3种嵩草属植物气孔特征对海拔梯度上生境变化的适应性不同,从而影响它们在群落中的分布范围和物种优势度,其中矮生嵩草和线叶嵩草对环境变化敏感,而大花嵩草对环境变化相对不敏感;保卫细胞宽度与气孔长度同样对植物适应环境变化起重要作用。     

Diurnal and spatial patterns of Phytoseiidae in the citrus canopy

Phytoseiid mites were sampled in a grapefruit orchard at various times of the day to study their diurnal and seasonal distributions in the tree canopy. Samples were collected on 14, 20 and 28 October 1999 at 2 h intervals from 0600 to 2200 h. Similar samples were collected in a grapefruit orchard at 3 h intervals from 0600 to 2100 h on 9 and 16 March and on 17 and 24 August 2000 for phytoseiid mites. No differences in numbers of phytoseiid motiles were observed among the hours sampled in any of the three months. However, significant differences were observed in the number of phytoseiids per leaf based on location within the tree (eastern, western sides of the canopy or interior). Interior leaves collected in March and August 2000 had higher numbers of phytoseiids than exterior leaves taken from either the eastern or western sides of the tree canopy. Phytoseiids were more abundant in the March 2000 samples (mean = 1.10 phytoseiids/leaf) than in either October 1999 or August 2000 samples (mean = 0.16 and 0.19 phytoseiids/leaf, respectively). Prevalent phytoseiid species were Typhlodromalus peregrinus (Muma) (42.1%) and Iphiseiodes quadripilis (Banks) (50.4%) in October 1999, Typhlodromalus peregrinus (Muma) (76.2%) in March 2000, and Euseius mesembrinus (Dean) (54%) in August 2000.     

The keystone role of leaf-removing crabs in mangrove forests of North Brazil

Principle factors which influence mangroveleaf litter turnover, in particular therole of leaf-removing crabs, were evaluatedin a riverine mangrove site nearBragança (Pará, North Brazil). Ourspecial interest was focussed on the roleof the leaf-removing crab Ucidescordatus. Leaf litter fluxes between themangrove forest and the adjacent estuarywere investigated by estimating the biomassand fate of leaf litter material and propagules. Vegetation is dominated by Rhizophora mangle, with Avicenniagerminans trees, both up to 25 m high,found intermittently. During 1997, Rhizophora trees produced around 1.40 gDW m^-2 d^-1 of leave fall and0.75 g DW m^-2 d^-1 of propagules.Leaf decomposition rates on the ground wereabout 0.06 g DW m^-2 d^-1,irrespective of species, habitat or siteexposure. This amount accounts for <3%of total leaf fall. Average leaf litterbiomass present on the ground was 0.01 gDW m^-2 d^-1. When the mangroveforest was flooded (on average 10 days permonth) the quantity of leaf litterand propagules washed out with the springtide was 10 and 17 times greater thanduring neap tide. Nevertheless, tidalexport and decomposition together made upless than 39 percent of annual leaf litterfall. The bulk of the remaining amount isapparently removed by Ucides. Eachcrab consumed about 1.30 g DW of leaflitter material and propagules per day.Since the average density of these crabswas 1.38 crabs m^-2, it is proposedthat Ucides is a keystone species inBragantinian mangroves.     

Sensitivity to far-red radiation in stomata of Phaseolus vulgaris L.: Rhythmic effects on conductance and photosynthesis

The influence of far-red (FR; 700–800 nm) radiation on steady-state stomatal conductance and net photosynthesis in P. vulgaris has been studied. Whereas FR radiation alone was relatively ineffective, addition of FR to a background of white light (WL; predominantly 400–700 nm) resulted in increased stomatal conductance. Stomata exhibited a marked diurnal sensitivity to FR. The action maximum for enhancing stomatal conductance was near 714 nm. A combination of FR and infra-red (IR; >800 nm) enhanced net photosynthesis when added to a background of WL. When IR alone was added to WL, there was a net decrease in photosynthesis, indicating that it is the FR waveband which is responsible for the observed photosynthetic effects. Naturally occurring levels of FR radiation (235 mol·m^-2·s^-1) in vegetation-canopy shade enhanced net photosynthetic CO₂ gain by 28% when added to a background of 55 mol·m^-2·s^-1 WL.Abbreviations BL blue - FR far-red - IR infra-red - PAR photosynthetically active radiation - R red - WL white light     

Drought responses of conifers in ecotone forests of northern Arizona: tree ring growth and leaf δ<Superscript>13</Superscript>C

We sought to understand differences in tree response to meteorological drought among species and soil types at two ecotone forests in northern Arizona, the pinyon-juniper woodland/ponderosa pine ecotone, and the higher elevation, wetter, ponderosa pine/mixed conifer ecotone. We used two approaches that provide different information about drought response: the ratio of standardized radial growth in wet years to dry years (W:D) for the period between years 1950 and 2000 as a measure of growth response to drought, and ¹³C in leaves formed in non-drought (2001) and drought (2002) years as a measure of change in water use efficiency (WUE) in response to drought. W:D and leaf ¹³C response to drought for Pinus edulis and P. ponderosa did not differ for trees growing on coarse-texture soils derived from cinders compared with finer textured soils derived from flow basalts or sedimentary rocks. P. ponderosa growing near its low elevation range limit at the pinyon-juniper woodland/ponderosa pine ecotone had a greater growth response to drought (higher W:D) and a larger increase in WUE in response to drought than co-occurring P. edulis growing near its high elevation range limit. P. flexilis and Pseudotsuga menziesii growing near their low elevation range limit at the ponderosa pine/mixed conifer ecotone had a larger growth response to drought than co-occurring P. ponderosa growing near its high elevation range limit. Increases in WUE in response to drought were similar for all species at the ponderosa pine/mixed conifer ecotone. Low elevation populations of P. ponderosa had greater growth response to drought than high-elevation populations, whereas populations had a similar increase in WUE in response to drought. Our findings of different responses to drought among co-occurring tree species and between low- and high-elevation populations are interpreted in the context of drought impacts on montane coniferous forests of the southwestern USA.     

UV-B Radiation Mediated Alterations in the Nitrate Assimilation Pathway of Crop Plants 1. Kinetic Characteristics of Nitrate Reductase

The kinetics and other characteristics of nitrate reductase (NR, EC 1.6.6.1) in cowpea [Vigna unguiculata (L.) Walp.] seedlings irradiated with biologically effective UV-B radiation (280-320 nm, 3.2 W m^-2 s^-1) were recorded. The in vivo and in vitro NR activities were inhibited by 34 and 41 % under UV-B treatment, respectively. Both V_max and K_m for the substrate were enhanced by UV-B radiation. The K_m for nitrate increased from 1.2 to 1.7 mM after the UV-B irradiation. The change in K_m for NADH was from 0.12 to 0.17 mM. The increases in K_m indicate that UV-B radiation seriously changes the topology of NR, particularly with respect to the nitrate and NADH binding sites. The rate of NR turnover indicates the extent of damage inflicted by UV-B radiation on the nitrate metabolism. The half-life (t_1/2) of NR was reduced from 7 to 4 h in the UV-B treated seedlings. UV-B also inhibited the kinetics of nitrate uptake by plants: its K_m increased from 0.08 to 0.12 mM. This revised version was published online in June 2006 with corrections to the Cover Date.     

Leaf dynamics and standing stocks of intertidal Zostera noltii Hornem. and Cymodocea nodosa (Ucria) Ascherson on the Banc d'Arguin (Mauritania)

Leaf dynamics and standing stocks of intertidal seagrasses were studied in the Baie d'Aouatif (Parc National du Banc d'Arguin, Mauritania) in April and September 1988. Standing stocks of Zostera noltii Hornem. suggest a unimodal seasonal curve similar to what is found for populations at higher latitudes. Also, leaf growth rates (0.03 cm² cm^–2 day^–1 on average) were similar to those found at higher latitudes in these months. Variation in leaf loss over tidal depth, time and different locations in the Baie d'Aouatif was larger and more often significant than variation in leaf growth. In general, Z. noltii beds in the Baie d'Aouatif had comparable leaf growth rates and standing stocks. In both months losses were almost always higher than or equal to growth.Variation in leaf loss over time was much higher in the plots that were situated high in the intertidal than in lower plots. This is explained by differences in susceptibility to sloughing, which is presumably higher in periods with low tide around noon for shallow depths.In an experiment using artificial shading nets, in situ leaf growth was affected negatively from 94% shading onwards. This shading was observed to reduce the light intensity reaching the seagrass bed to a level below the reported range of light compensation points for Z. noltii. Cymodocea nodosa (Ucria) Ascherson on average had higher leaf area and relative growth rates than Z. noltii and much lower loss rates, resulting in a positive net increase in September. Standing stocks were also higher than for Z. noltii. A mixed seagrass bed containing the above two species and Halodule wrightii Ascherson had the highest observed total biomass: 335 g m^–2 ash-free dry weight.