Abstract
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.
Highlights
Sixty years ago Todd [1] described effectively the effects of tropospheric ozone (O3 ) on plants in the Los Angeles region: “Damage to plants from pollutants such as those present in polluted LosAngeles air may be manifested in at least two primary ways: first, as visible oxidant damage to the leaves of susceptible plants, where it is apparent that leaf cells have been killed; and second, as a Forests 2019, 10, 1144; doi:10.3390/f10121144 www.mdpi.com/journal/forestsForests 2019, 10, 1144 decrease in plant growth not accompanied by visible injury”
Various working methods have been adopted to determine the critical levels of O3 for plant protection [2], intended as the maximum acceptable threshold for protecting vegetation against significant adverse effects
It is generally accepted that the responses of plants to O3 do not depend on the ambient concentration of this pollutant, but most likely on the dose absorbed by leaves through stomata, calculated as cumulated O3 stomatal flux or phytotoxic ozone dose (POD)
Summary
Forests 2019, 10, 1144 decrease in plant growth not accompanied by visible injury”. Various working methods have been adopted to determine the critical levels of O3 for plant protection [2], intended as the maximum acceptable threshold for protecting vegetation against significant adverse effects. It is generally accepted that the responses of plants to O3 do not depend on the ambient concentration of this pollutant (i.e., the level of exposure), but most likely on the dose absorbed by leaves through stomata, calculated as cumulated O3 stomatal flux or phytotoxic ozone dose (POD). Growth reduction was chosen as the response indicator of the O3 impact on trees and crops to set O3 critical levels [7] and should be considered as a damage response variable that integrates the various physiological processes responsible for reducing the photosynthesis and biomass production rate of plants. Foliar visible symptoms should be considered as “injuries”, as they consist of single or groups of dead cells in a specific plant organ (the leaf)
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