No abstract

No abstract

Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) is widely distributed in the western USA. We report the lack of stomatal closure at night in early summer for ponderosa pine at two of three sites investigated. Trees at a third site with lower nitrogen dioxide and nitric acid exposure, but greater drought stress, had slightly open stomata at night in early summer but closed stomata at night for the rest of the summer. The three sites had similar background ozone exposure during the summer of measurement (2001). Nighttime stomatal conductance (gs) ranged from one tenth to one fifth that of maximum daytime values. In general, pole-sized trees (< 40 years old) had greater nighttime gs than mature trees (> 250 years old). In late summer, nighttime gs was low (< 3.0 mmol H2O m(-2) s(-1)) for both tree size classes at all sites. Measurable nighttime gs has also been reported in other conifers, but the values we observed were higher. In June, nighttime ozone (O3) uptake accounted for 9, 5 and 3% of the total daily O3 uptake of pole-sized trees from west to east across the San Bernardino Mountains. In late summer, O3 uptake at night was < 2% of diel uptake at all sites. Nocturnal O3 uptake may contribute to greater oxidant injury development, especially in pole-sized trees in early summer.

An experiment in open-top chambers was carried out in summer 2008 at Curno (Northern Italy) in order to study the effects of ozone and mild water stress on poplar cuttings (Oxford clone). In this experiment direct fluorescence parameters (JIP-test) were measured in leaves from different sections of the crown (L: lower; M: medium; U: upper parts of the crown). The parameters considered were calculated at the different steps of the fluorescence transient, and include maximum quantum yield efficiency in the dark-adapted state (F(v)/F(M)); the L-band, at 100 ∝ s, that expresses the stability of the tripartite system reaction centre-harvesting light complex-core antenna; the K-band, at 300 ∝ s, that expresses the efficiency of the oxygen-evolving complex; the J-phase, at 2 ms, that expresses the efficiency with which a trapped exciton can move an electron into the electron transport chain from Q(A)(-) to the intersystem electron acceptors; the IP-phase, which expresses the efficiency of electron transport around the photosystem 1 (PSI) to reduce the final acceptors of the electron transport chain, i.e., ferredoxin and NADP; and finally the performance index total (PItot) for energy conservation from photons absorbed by PSII to the reduction flux of PSI end acceptors. The main results are: (i) different dynamics were observed between leaves in the lower section, whose PItot decreased over time, and those in the upper sections in which it increased, with a dynamic connected to the leaf age; (ii) ozone depressed all the considered fluorescence parameters in basal leaves of well-watered plants, while it had little or no damaging effect on medium-level or upper-section leaves; (iii) PItot and IP-phase increased in upper leaves of plants subjected to ozone stress, as well as the net photosynthesis; (iv) water stress increased PItot of leaves in all levels of the crown. The results suggest that ozone-damaged poplar plants compensate, at least partially, for the loss of photosynthesis with higher photosynthetic rates in young leaves (in the upper section of the crown), more efficient to fix carbon.

This paper reports the findings of an open‐top chamber experiment carried out in northern Italy (Forest nursery at Curno), during the 2004 and 2005 growth seasons, on Fagus sylvatica and Quercus robur seedlings and on Populus nigra cuttings, in order to test their photosynthesis response to ambient ozone. The experimental protocols were non‐filtered air (NF), charcoal‐filtered air (CF) and open air (OA). Tests performed included morphological features of leaves; development of foliar symptoms; chlorophyll content, determined by non‐destructive means; chlorophyll fluorescence (direct fluorescence and JIP test) and gas exchanges and net photosynthesis (PN). Main findings were as follows: (1) symptoms occurred early and were extensive in P. nigra, and they occurred later in F. sylvatica, whereas early degeneration of chlorophyll occurred in late summer in Q. robur; (2) in conditions of ozone exposure, the three species all presented a decline in photosynthesis efficiency and a decrease in PN, regardless of the symptomatology they displayed; (3) leaf traits are predictors of species‐specific sensitivity to ozone—the high density of Q. robur foliar tissues prevents this species from developing visible symptoms and reduces the extent of physiological responses and (4) physiological responses varied from year to year in the same species—responses were lower in the second year of the experiment, when plants had become better acclimatized to plot conditions.

No abstract

Fluorescence transient (FT) analysis (fast kinetics) was carried out on sun and shade leaves of beech seedlings in an experimental field at Curno (North Italy), during the 2008 growing season (June-August). The aim of the research was to determine: (1) morphology in sun leaves as long term acclimation to high light; (2) short term responses to high light, i.e. during the course of the day; (3) evolution of responses during the growing season and the ageing process. Sun leaves differ from shade leaves by their lower trapping capacity (expressed by the ratio F V /F M ) and higher capacity to reduce end acceptors beyond PSI (expressed by phase I-P of the FT). These features were assessed at pre-dawn, when the entire plant is fully dark-adapted. Short term responses (differences between midday and pre-dawn assessments) occur especially in sun leaves, indicating that the photosynthetic machinery reacts to the high excitation pressure by increasing photochemical and non-photochemical de-excitation processes. Other responses concern the inactivation of the oxygen evolving system and a stability loss of the tripartite system reaction centre (RC) -harvesting light complex -core antenna, expressed by the onset of K and L-bands at midday. Sun leaves are well acclimated to high light from a structural and functional point of view, however, the irradiance at midday provokes conditions of instability in the photosynthetic machinery, possibly through reactive oxygen species (ROS) production at both PSII and PSI sides. The temporal patterns across the growing season indicate the progression of senescence processes, that are more pronounced in sun leaves which present a permanent (chronic) condition of photoinhibition. Differences between sun and shade leaves decrease over time.

An open-top chamber experiment was carried out in Curno (Northern Italy) in 2004 and 2005 on seedlings of Fagus sylvatica (FS), Quercus robur (QR), and an ozone-sensitive Populus (POP) clone, to investigate the role of two stress factors: tropospheric ozone and water shortage. Treatments were filtered air to achieve a 50% reduction in the environmental ozone concentrations (charcoal filtered, CF); and non-filtered air, with a 5% reduction in the environmental ozone concentrations (non-filtered, NF). Overall ozone exposure (AOT40) in open air (April-September) was 26,995 ppb h in 2004 and 25,166 ppb h in 2005. The plants were either watered (W)or not watered (dry, D). We investigated the above-ground biomass, tree-ring growth, stable carbon isotopes ratio, i.e. d 13 C of tree rings, and the photosynthetic parameter Driving forces (DF ABS ), derived from chlorophyll a fluorescence analysis. Ozone-induced growth reduction (in terms of biomass) in POP, and that reduction was more pronounced in D plots. A synergistic effect of ozone and drought stress was evidenced by DF ABS in POP and QR, but not in FS. The water availability was revealed as the main factor influencing the isotopic ratio d 13 C. In droughtstressed seedlings, the increase in d 13 C value was accompanied by the reduction in stomatal conductance and increased DF ABS . Fast-growing plant species with high water requirements are more susceptible to ozone and drought stress.