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Assessing the Impact of Ozone on Forest Trees in An Integrative Perspective: Are Foliar Visible Symptoms Suitable Predictors for Growth Reduction? A Critical Review

1
Catholic University of Brescia, Department of Mathematics and Physics, via Musei 41, I-25121 Brescia, Italy
2
University of Firenze, Department of Agriculture, Food, Environment and Forestry (DAGRI), Piazzale delle Cascine 28, 50144 Firenze, Italy
*
Author to whom correspondence should be addressed.
Forests 2019, 10(12), 1144; https://doi.org/10.3390/f10121144
Received: 15 November 2019 / Revised: 11 December 2019 / Accepted: 12 December 2019 / Published: 14 December 2019
(This article belongs to the Special Issue Impacts of Ozone on Forest Plants and Ecosystems)
Plant growth reduction (GR) is the most widely accepted damage parameter to assess the sensitivity of trees to tropospheric ozone (O3) pollution since it integrates different physiological processes leading to loss of photosynthetic activity and distraction of metabolic resources from growth to defense, repair, and recovery pathways. Because of the intrinsic difficulty to assess the actual O3 risk assessment for forests in field conditions, foliar visible symptoms (FVS) induced by O3 have been proposed as a proxy to estimate possible GR in forest trees. The rationale for this assumption is that the onset of FVS implies a reduction of the photosynthetic capacity of plants. In this review, we show that GR and FVS can be the consequences of independent physiological pathways involving different response mechanisms that can cause both FVS without GR and GR without FVS. The onset of FVS may not lead necessarily to significant GR at plant level for several reasons, including the rise of compensatory photosynthesis, the time lag between growth processes and the accumulation of critical O3 dose, and the negligible effect of a modest amount of injured leaves. Plant GR, on the other hand, may be induced by different physiological mechanisms not necessarily related to FVS, such as stomatal closure (i.e., carbon starvation) to avoid or reduce O3 uptake, and the increase of respiratory processes for the production of metabolic defense compounds. Growth reduction and FVS can be interpreted as different strategies for the acclimation of plants to a stressful environment, and do not mean necessarily damage. Growth reduction (without FVS) seems to prevail in species adapted to limiting environmental conditions, that avoid loss and replacement of injured leaves because of the high metabolic cost of their production; conversely, FVS manifestation (without GR) and the replacement of injured leaves is more common in species adapted to environments with low-stress levels, since they can benefit from a rapid foliar turnover to compensate for the decreased rate of photosynthesis of the whole plant. View Full-Text
Keywords: ozone; forest trees; foliar visible symptoms; growth reduction; field surveys; controlled conditions experiments ozone; forest trees; foliar visible symptoms; growth reduction; field surveys; controlled conditions experiments
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Marzuoli, R.; Gerosa, G.; Bussotti, F.; Pollastrini, M. Assessing the Impact of Ozone on Forest Trees in An Integrative Perspective: Are Foliar Visible Symptoms Suitable Predictors for Growth Reduction? A Critical Review. Forests 2019, 10, 1144.

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