L'ozone et les conifères du Sud-Est de la France

Dans le Sud-Est de la France des défoliations importantes chez plusieurs espèces de conifères s'observent parallèlement à de fortes concentrations d'ozone. Des recherches récentes permettent de discriminer les symptômes spécifiques d'ozone dans des feuilles ou des aiguilles par rapport aux altérations causées par d'autres facteurs de stress. Cette étude présente les niveaux d'ozone mesurés en 2000 sur le littoral et dans l'arrière-pays niçois et les compare aux taux de décoloration et de défoliation observés sur cinq espèces de conifères. Chez le pin cembro, des aiguilles ont été prélevées pour détecter et vérifier la présence de symptômes spécifiques d'ozone. Les niveaux d'ozone mesurés étaient suffisants pour causer des symptômes visibles et, probablement, pour affecter la croissance des pins étudiés. Des symptômes macroscopiques et microscopiques typiques de l'ozone, observables dans l'arrière pays, indiquent un état de stress et une accélération de la sénescence des aiguilles symptomatiques chez les pins cembro, contribuant probablement à leur mauvais état sanitaire dans les Alpes-Maritimes

Drought and air warming affects abundance and exoenzyme profiles of <em>Cenococcum geophilum</em> associated with <em>Quercus robur</em>, <em>Q.&nbsp;petraea </em>and <em>Q.&nbsp;pubescens</em>.

The present study aimed to elucidate the influence of drought and elevated temperature on relative abundance and functioning of the ectomycorrhizal fungus Cenococcum geophilum on three oak species differing in adaptation to a warm and dry climate. The experiment QUERCO comprised three Quercus species (Q.&nbsp;robur, Q.&nbsp;petraea, Q.&nbsp;pubescens) grown for 3&emsp;years under four treatments: elevated air temperature, drought, a combination of the two, and control. Fine root samples were analysed for relative abundance and potential extracellular enzyme activities of ectomycorrhizae of C.&nbsp;geophilum, a fungal species known to be drought resistant. The relative abundance of C.&nbsp;geophilum on the roots of the oak species was significantly increased by temperature, decreased by drought, but unchanged in the combined treatment compared to the control. Although the extent of treatment effects differed among oak species, no significant influence of tree species on relative abundance of C.&nbsp;geophilum was detected. Exoenzyme activities of C.&nbsp;geophilum on Q.&nbsp;robur and Q.&nbsp;petraea (but not Q.&nbsp;pubescens) significantly increased in the combined treatment, but for all oak species were reduced under drought and air warming alone compared to the control. There was a significant negative correlation between abundance of C.&nbsp;geophilum and its leucine aminopeptidase activity. As this enzyme is not frequent among ectomycorrhizal fungi, this emphasises the functional importance of C.&nbsp;geophilum in the ectomycorrhizal community. Our results indicate that increased temperature and drought will influence the relative abundance and enzyme activity of C.&nbsp;geophilum. However, both the Quercus species and C.&nbsp;geophilum tolerated warming and strong drought

Physiological and biochemical responses of <i>Quercus pubescens</i> to air warming and drought on acidic and calcareous soils

The drought- and thermo-tolerant Quercus pubescens, a tree species growing on both acidic and calcareous soils in the sub-Mediterranean region, was exposed to soil drought (-60% to -80% soil water content) and air warming (+1.2 C daytime temperature), singly and in combination. The experiment was conducted on two natural forest soils with similar texture but different pH (acidic and calcareous soils). The physiological (photosynthesis) and biochemical (antioxidant system) responses of Q. pubescens were investigated. On acidic soil, Q. pubescens had a higher reactive oxygen species (ROS) content than on calcareous soil, confirming that this species is better adapted to the latter soil type. A down-regulation of ascorbate-glutathione cycle enzymes suggests that ROS were used as signalling molecules. Air warming stimulated stomatal opening, while soil drought induced stomatal closure in the late afternoon and reduced Rubisco carboxylation efficiency. Photosynthetic performance in the combined treatment was higher than under single drought stress and similar to control and air warming. Q. pubescens biochemical responses depended on soil pH. On acidic soil, Q. pubescens trees exposed to air warming used ROS as signalling molecules. On calcareous soil, these trees were able to balance both soil drought and air warming stress, avoiding ROS toxic effects by increasing antioxidant enzyme activitiy and maintaining a high enzymatic antioxidant defence. When combined, drought and air warming induced either more severe (higher oxidative pressure and impairment of the light-harvesting complex) or different responses (decline of the thermal energy dissipation capacity) relative to the single stressors. Overall, however, Q. pubescens preserved the functionality of the photosynthetic apparatus and controlled the antioxidant system response, thus confirming its drought and thermo-tolerance and therefore its potential to adapt to the ongoing climate change

Structural changes in Psidium guajava 'Paluma' leaves exposed to tropospheric ozone Alterações anatômicas em folhas de Psidium guajava 'Paluma' expostas ao ozônio troposférico

Psidium guajava 'Paluma' has being tested as an ozone (O3) bioindicator and responds with pigmentation between the veins on the adaxial surface, due to the accumulation of phenolic compounds. These compounds act as non-enzymatic antioxidants that neutralize reactive oxygen species (ROS), formed from O3. This study aimed to evaluate the leaf structure of plants with and without visible symptoms and to establish these symptoms at the cellular level. Beside this we also aimed to detect structural changes that can minimize the effects of the O3 on the plant. The accumulation of phenolic substances, stomatal density and structural changes in P. guajava 'Paluma' leaf tissues exposed during the four seasons of the year were evaluated. The study was conducted at the Parque Estadual das Fontes do Ipiranga ( PEFI), which is a park in the city of São Paulo that has high levels of O3. Leaves with symptoms showed, on the adaxial surface, anthocyanin accumulation in the vacuoles of epidermal cells and hypodermis. When the symptoms were more intense this accumulation was observed even in the first three layers of palisade parenchyma. Comparing symptomatic and asymptomatic leaves, there was higher accumulation of phenolic compounds in the symptomatic leaves. Some parenchyma cells adjacent to substomatal chambers showed intrusive growth towards the stomatal pore, promoting its occlusion, which could reduce the entry of O3 in the leaf. The accumulation of anthocyanins and other phenolic compounds, in addition to the occlusion of the chamber, protect the plant against O3 effects. These features and the compact arrangement of the mesophyll contribute to why Psidium guajava 'Paluma' does not present cell death, a symptom usually observed in species sensitive to O3.<br>Psidium guajava 'Paluma' vem sendo testada como bioindicadora de ozônio (O3) e responde com pigmentações, entre as nervuras, na superfície adaxial, decorrente do acúmulo de compostos fenólicos. Tais compostos atuam como antioxidantes não enzimáticos que neutralizam espécies reativas de oxigênio (ROS), formadas a partir do O3. O estudo objetivou comparar a estrutura foliar de plantas com e sem sintomas visíveis para estabelecer como esses sintomas se apresentam em nível celular. Para tanto, avaliaram-se acúmulo de substâncias fenólicas, densidade estomática e alterações estruturais nos tecidos foliares de P. guajava 'Paluma', exposta durante as quatro estações do ano, no Parque Estadual das Fontes do Ipiranga (PEFI), área verde da cidade de São Paulo, com altos índices de O3. Folhas com sintomas apresentaram, na superfície adaxial, antocianina nos vacúolos das células da epiderme, da hipoderme e, quando o sintoma foi mais intenso, em até três primeiras camadas do parênquima paliçádico, além de maior acúmulo de outros compostos fenólicos, em comparação com folhas assintomáticas. Algumas células parenquimáticas contíguas à câmara subestomática mostraram crescimento intrusivo em sua direção, promovendo a oclusão da câmara, o que pode contribuir para reduzir a entrada de O3 na folha. O acúmulo de antocianina e outros compostos fenólicos, além da oclusão da câmara subestomática são características que podem conferir proteção à planta contra efeitos do O3. Tais características, somadas à compactação do mesofilo, contribuem para que Psidium guajava 'Paluma' não apresente morte celular, sintoma frequente em espécies sensíveis ao O3