▶ Ozone stress induces little variations in the
F
V/
F
M parameter, but enhances pathways of controlled processes of energy dissipation. ▶ The I–P phase of the fluorescence transient (fast kinetics) ...reveals a block of electrons beyond the PSI in ozone treated plants. ▶ Foliar symptoms are connected to the overexcitation produced from electron not reaching the end acceptors (ferredoxine, NADP+, RuBP). ▶ Ozone stress can temporarily enhance the electron transport capacity and the overall photosynthesis efficiency. ▶ Species-specific responses are evidenced with the behaviour of
F
0.
This paper proposes an overall reassessment of results from fluorescence transient (FT) and modulated fluorescence (MF) analysis on tree species subjected to treatment with ozone. Findings from literature and open-top chamber experiments were used in this review, in order to identify damage mechanisms and repair/avoidance strategies. Main results are summarised below.
(i)
The most widely used parameter to assess the response to ozone treatment was the quantum yield of primary photochemistry in the dark-adapted state (
φ
Po, or
F
V/
F
M). This parameter proved to be very stable in stress and control conditions. Ozone-induced stress – and the related loss of photosynthetic efficiency and performance – was characterized by the change occurring in all parameters connected with the controlled dissipation: reduction of
F
M,
F
V/
F
M, RC/CS
0 (in the fluorescence transient analysis) and NPQ (in the MF analysis). This can be considered as a down-regulation mechanism aimed at lowering the electron supply as a consequence of a reduced demand from the Calvin cycle.
(ii)
The FT analysis revealed a change in the I–P region shape, indicating that events beyond PSI are affected. These events include a lesser density of PSI itself and the compromised ability of the end acceptors of electrons (ferredoxine, NADP
+) and RuBP to manage effectively the flux of electrons. This behaviour may create an imbalance between electrons sent by the electron transport chain and those reaching the acceptors beyond PSI. Free electrons (those coming from PSI, but that do not reach the end acceptors) can activate the oxygen from fundamental to excited status, with production of ROS (Reactive Oxygen Species), thus inducing photo-oxidation processes of the cellular content.
(iii)
In many cases a temporarily enhanced efficiency of electron trapping and transport (expressed by the parameters
F
V/
F
M, ET/TR, PI
ABS, pQ) in PSII system has been observed. That efficiency has been connected to the triggering of repair processes, but when it is connected to a reduced end acceptor capacity in combination with reduced Calvin cycle energy demand lead to over-excitation of the photosynthetic apparatus and initiates response towards visible foliar injury.
(iv)
The behaviour of
F
0 can help us distinguish between different response strategies. The increase of
F
0 observed in some ozone-treated plant species is considered an expression of irreversible damage in PSII, whereas lowered values of this parameter may indicate the activation of PSII in the cells surrounding the damaged one, as part of a compensative process.
Future directions for the research in this field concern: (i) the possibility to combine fluorescence parameters with carbon assimilation and growth to support the study on critical levels and (ii) the analysis of the events concerning the activity of PSI and the events leading to the fixation of CO
2, by using innovative technologies.
Ozone (O
3) is well known to affect photosynthesis on tree seedlings under experimental conditions. In natural conditions, young trees grow under a competitive environment that may alter light ...availability. Such conditions were simulated in an open-top chamber (OTC) experiment carried out in Southern Switzerland (Lattecaldo). The experimental set-up consisted of
Fagus sylvatica (beech) seedling plots (planted within the same chamber) either in a monospecific population (Mono, leaves grown in full sunlight conditions—FL) or under competition with taller plants of
Viburnum lantana (Mixed, shaded leaves—SH). These cultures were treated with ambient air (NF, not filtered, 92% of ozone in ambient air) and with charcoal filtered air (CF, 50% of ozone in ambient air). Measures of net photosynthesis (
P
N) and chlorophyll a fluorescence (with the analysis of the OKJIP transient) were performed over a growing season, from June to September. Net photosynthesis (
P
N) and the maximum quantum yield of electrons reaching the acceptor side (
φ
Ro
=
RE
0/ABS) were higher in Mono conditions (full light leaves). Vice versa, quantum yield of primary photochemistry (
φ
Po
=
TR
0/ABS
=
F
V/
F
M) and Performance Index on absorption basis (PI
ABS) were higher in Mixed plots (shaded leaves). Changes due to ozone occurred only at the end of the growing season (September). Ozone affected mainly
P
N, that was lower in the NF-OTCs, both Mono (−19%) and Mixed (−28%) plots, compared to the CF ones. Fluorescence parameters were affected much more by light conditions (full light vs. shaded leaves) than ozone exposure, but ozone enhanced the action of high light by lowering the values of TR
0/ABS and PI
ABS. The most specific effect of ozone concerned the decrease in RE
0/ABS, that indicates the inactivation of the end acceptors of electrons, so producing an excess of oxidative pressure. Results are discussed in relation to the impact of ozone on the processes of the regeneration of forests.
Adaptation and acclimation are strategies that forests enact to cope with climate change. They consist of genotypic and phenotypic adjustments that allow plants to grow and reproduce successfully in ...a stressful environment. Both these aspects can be actively promoted by reforestation programmes. The key phases are: (1) selection of the most suitable provenances and genotypes; (2) adoption of adequate cultural techniques in nursery; and (3) monitoring of the plantations. Physiological techniques may be useful to assist all the phases of this process. Among these, chlorophyll fluorescence-based techniques, such as the JIP test, are relevant to monitor the stress conditions and the effectiveness of the cultural practices. JIP test is a non-destructive, non-invasive, informative, very fast and inexpensive technique, and can be used to support the cultural decision.
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. δ¹³C of tree rings, and the photosynthetic parameter Driving forces (DFABS), 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 DFABS in POP and QR, but not in FS. The water availability was revealed as the main factor influencing the isotopic ratio δ¹³C. In drought-stressed seedlings, the increase in δ¹³C value was accompanied by the reduction in stomatal conductance and increased DFABS. Fast-growing plant species with high water requirements are more susceptible to ozone and drought stress.
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.
▶ Different surveyors using different instruments for chlorophyll fluorescence analysis may have results not comparable. ▶ The parameters FV/FM is the more consistent across different instruments and ...operators. ▶ The intercalibration of the instruments and the adoption of proper field protocols are prerequisite indispensable before a large survey.
Two field exercises were carried out to compare chlorophyll
a fluorescence measurements taken in the field by field teams working on the same project. In the first exercise (2007, Passo Pura, Ampezzo, Udine, Northern Italy) the operators took measurements on the same leaf areas (maintaining fixed leaf clips); in the second (2009, Monterotondo Marittimo, Grosseto, Central Italy) the teams worked independently, but addressing a common research question. The results of the first exercise showed that: (a)
F
V/
F
M was stable and had little variation among teams and instruments; (b) the results from the different teams correlated well; (c) the most suitable parameters of fast kinetics analysis are those measured on the normalized fluorescence transients. In the second exercise, when the teams worked independently, the results were much more variable and the correlations between measurements of different operators were weak. These results suggest that field chlorophyll
a fluorescence measurements taken by different teams/operators can be comparable only if particular care is taken to the internal variability of the samples and a standardized sampling strategy is applied. A statistically sound representation of a population can be then reached.
Adaptation and acclimation are strategies that forests enact to cope with climate change. They consist of genotypic and phenotypic adjustments that allow plants to grow and reproduce successfully in ...a stressful environment. Both these aspects can be actively promoted by reforestation programmes. The key phases are: (1) selection of the most suitable provenances and genotypes; (2) adoption of adequate cultural techniques in nursery; and (3) monitoring of the plantations. Physiological techniques may be useful to assist all the phases of this process. Among these, chlorophyll fluorescence-based techniques, such as the JIP test, are relevant to monitor the stress conditions and the effectiveness of the cultural practices. JIP test is a non-destructive, non-invasive, informative, very fast and inexpensive technique, and can be used to support the cultural decision. PUBLICATION ABSTRACT
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.
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