An experimental facility for free air humidity manipulation (FAHM) can alter water flux through deciduous tree canopy

A facility for free air humidity manipulation (FAHM) was established to investigate the effect of increased air humidity on trees’ performance and their canopy functioning with respect to rising air humidity predicted for Northern Europe. The FAHM system enables air relative humidity (RH) to be increased up to 18 units (%) over the ambient level during mist fumigation, depending on the wind speed inside the experimental stand. Water was dispersed inside 14 × 14 m experimental plots in the form of mist with an average particle size of 50 μm from June to August in 2008, and from May to September in 2009. The average increase in RH was 7 units (%) over the whole period of humidification in 2008 (P < 0.05). The average diurnal stem sap flux density per unit projected leaf area (F) in silver birch (Betula pendula Roth.) trees was 24.8% (P < 0.05) and 27.2% (P < 0.01) higher in control (C) plots compared to humidification (H) plots during misting in 2008 and 2009, respectively. However, the difference between C and H plots was statistically insignificant (P > 0.05) in silver birch on the days without misting. In hybrid aspen (Populus tremula L. × P. tremuloides Michx.) the average difference in F between C and H plots was 61.1% (P < 0.001) during mist fumigation in the summer of 2009. Nevertheless, the difference was considerable (38.8%; P < 0.001) also on the days without misting, reflecting the impact of plant inner factors on F as a result of long-term acclimation to fumigation. The leaves of silver birch in a humidified plot demonstrated up to 2.4 °C lower (P < 0.05) leaf temperature (T_L) compared to the control plot in 2009. The decline in T_L decreased the humidity gradient between leaf and air by about 1/3, whereas 2/3 of the effect was caused directly by changes in air humidity in the leaf boundary layer. Our preliminary data suggest that the FAHM experimental facility enables water fluxes through a deciduous tree canopy to be reduced and this effect is attributable both to the increased air humidity and decreased leaf temperature. Changes in these two basic factors may create considerable differences in the physiology, anatomy and nutrition of a whole tree, also affecting forest functioning in the light of global climate change.     

IMPACT OF ULTRAVIOLET-B RADIATION ON PHOTOSYSTEM II ACTIVITY AND ITS RELATIONSHIP TO THE INHIBITION OF CARBON FIXATION RATES FOR ANTARCTIC ICE ALGAE COMMUNITIES1

One goal of the Icecolors 1993 study was to determine whether or not photosystem II (PSII) was a major target site for photoinhibition by ultraviolet-B radiation (Q_UVB, 280–320 nm) in natural communities. Second, the degree to which Q_UVBinhibition of PSII could account for Q_UVBeffects on whole cell rates of carbon fixation in phytoplankton was assessed. On 1 October, 1993, at Palmer Station (Antarctica), dense samples of a frazil ice algal community were collected and maintained outdoors in the presence or a bsence of Q_UVBand l or ultraviolet-A (Q_UVB, 320–400 nm) radiation. Samples were then collected at intervals over the day to track the time course of UV inhibition of primary production. The ice algae were assessed for changes in pigment composition and rates of carbon fixation. The quantum yield of PSII (Ø_IIc°) was measured by P ulse A mplitude M odulated fluorometry. Over the day, Ø_IIc° declined due to increasing time-integrated dose exposure of Q_UVB. The Q_UVB-driven inhibition of Ø_IIc° increased from 4% in the early morning hours to a maximum of 23% at the end of the day. The Q_UVB photoinhibition of PSII quantum yield did not recover by 6 h after sunset. In contrast, photoinhibition by Q_UVA and photosynthetically available radiation (Q_PAR, 400–700 nm) recovered during the late afternoon. Flourescence-based estimates of carbon fixation rates were linearly correlated (P =0.002, r²=0.45) with measured carbon fixation. Fluorescence overestimated the observed Q_UVB inhibition in measured carbon fixation rates by 8% in the morning hours; however, the discrepancy increased during the afternoon. Therefore, researchers should be cautious in using fluorescence measurements to infer ultraviolet inhibition for rates of carbon fixation until there is a greater understanding of the coupling of carbon metabolism to PSII activity for natural populations. Despite these current limitations, fluorescence-based technologies represent powerful tools for studying the impact of the ozone hole on natural populations on spatial/ temporal scales not possible using conventional productivity techniques.     

Foliar Spraying with Zineb Increases Fruit Productivity and Alleviates Oxidative Stress in Two Tomato Cultivars

The effects of foliar spraying of the dithiocarbamate zineb on two cultivars of tomato grown in the field in a site with high ozone concentrations were studied by means of biomass assessment, antioxidant enzyme assays, lipid peroxidation, and chlorophyll fluorescence measurements. Zineb prevented the peroxidation of membrane lipids and decreased the activity of scavenging enzymes, which suggests that plants sprayed with zineb are subjected to lower oxidative stress than controls. The beneficial effects of zineb protection is the utilization of a larger fraction of absorbed radiant energy in photosynthesis and a larger fruit yield in plants of both cultivars.     

Metabolic-dependent changes in plant cell redox power after ozone exposure

The tropospheric level of the phytotoxic air pollutant ozone has increased considerably during the last century, and is expected to continue to rise. Long-term exposure of higher plants to low ozone concentrations affects biochemical processes prior to any visible symptoms of injury. The current critical level of ozone used to determine the threshold for damaging plants (biomass loss) is still based on the seasonal sum of the external concentration above 40 nl·l⁻¹ (AOT40). Taking into account stomatal conductance and the internal capacity of leaf defences, a more relevant concept should be based upon the 'effective ozone flux', the balance between the stomatal flux and the intensity of cellular detoxification. The large decrease in the Rubisco/PEPc ratio reflects photosynthetic damage from ozone, and a large increase in activity of cytosolic PEPc, which allows increased malate production. Although the direct detoxification of ozone (and ROS produced from its decomposition) is carried out primarily by cell wall ascorbate, the existing level of this antioxidant is not sufficient to indicate the degree of cell sensitivity. In order to regenerate ascorbate, NAD(P)H is needed as the primary supplier of reducing power. It is hypothesised that increased activity of the catabolic pathways and associated shunts (glucose-6-phosphate dehydrogenase, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase, isocitrate dehydrogenase and malic enzyme) can provide sufficient NAD(P)H to maintain intracellular detoxification. Thus, measurement of the level of redox power would contribute to determination of the 'effective ozone dose', serving ultimately to improve the ozone risk index for higher plants.