Changes in photosystem II (PSII) thermotolerance during drought and recovery were studied under controlled conditions in three Mediterranean cedar species (Cedrus brevifolia Henry, C. libani Loudon and C. atlantica Manetti). The temperature at which the quantum yield of PSII photochemistry was reduced by 15% of its value at 25 degrees C was 3 to 4 degrees C higher in drought-treated plants than in well-watered plants. The drought-induced increase in PSII thermotolerance was already evident 8 days after water had been withheld from the seedlings, when net CO(2) assimilation was still at 80% of its initial value, and was visible for up to 12 days after re-watering. When seedlings of the three species were exposed to temperatures above 45 degrees C for 5 h, both maximal quantum yield of PSII photochemistry and net CO(2) assimilation rate were significantly reduced in unconditioned seedlings, whereas drought-preconditioned seedlings were almost unaffected by the heat treatment. Drought-preconditioned seedlings still exhibited a higher tolerance to heat stress than unconditioned seedlings 60 days after re-watering, although the transient, drought-induced osmotic adjustment had fully disappeared. Among species, C. atlantica was the most heat sensitive, whereas the heat treatment had no significant effect on the parameters measured in C. brevifolia.
We studied hydraulic traits of young plants of the Mediterranean cedar species Cedrus atlantica (Endl.) G. Manetti ex Carrière (Luberon, France), C. brevifolia (Hook. f.) Henry (Cyprus), C. libani A. Rich (Hadeth El Jebbe, Lebanon) and C. libani (Armut Alani, Turkey). With an optimum water supply, no major differences were observed among species or provenances in either stem hydraulic conductivity (Ks) or leaf specific conductivity (Kl) measured on the main shoot. A moderate soil drought applied for 10 weeks induced marked acclimation through a reduction in Ks, particularly in the Lebanese provenance of C. libani, and a decrease in tracheid lumen size in all species. Cedrus atlantica, which had the smallest tracheids, was the species most vulnerable to embolism: a 50% loss in hydraulic conductivity (PsiPLC50) occurred at a water potential of -4.4 MPa in the well-watered treatment, and at -6.0 MPa in the moderate drought treatment. In the other species, PsiPLC50 was unaffected by moderate soil drought, and only declined sharply at water potentials between -6.4 and -7.5 MPa in both irrigation treatments. During severe drought, Ks of twigs and stomatal conductance (g(s)) were measured simultaneously as leaf water potential declined. For all species, lower vulnerability to embolism based on loss of Ks was recorded on current-year twigs. The threshold for stomatal closure (10% of maximum g(s)) was reached at a predawn water potential (Psi(pd)) of -2.5 MPa in C. atlantica (Luberon) and at -3.1 MPa in C. libani (Lebanon), whereas the other provenance and species had intermediate Psi(pd) values. Cedrus brevifolia, with a Psi(pd) (-3.0 MPa) close to that of C. libani (Lebanon), had the highest stomatal conductance of the study species. The importance of a margin of safety between water potential causing stomatal closure and that causing xylem embolism induction is discussed.
Controlled, localized heating was applied to the trunks of young Aleppo pines (Pinushalepensis Mill.) during the winter. Three levels of burning were applied to the circumference of the trunks (zero, partial, or total), and the effects on tree physiology and growth were monitored during the 7 months following heating. Effects of the heat treatment on tree physiology were assessed by measuring predawn water potential, sap flux density, microvariations in trunk diameter, electrical resistance of the cambium, and gas exchanges from the needles (photosynthesis and stomatal conductance). Morphological response measurements included height and radial growth of the trees and morphological characteristics of the needles. The amount of chlorotic foliage was also measured. Aleppo pine was highly resistant to trunk injury, surviving when 80% of the trunk circumference was destroyed by fire. The behaviour of partially burnt pines was not significantly different from that of controls, for all variables studied. Trees whose trunks were totally burnt died from 1 to 5 months after treatment. Sap flux density of totally burnt trees was already significantly lower than that of other trees in the first week following the treatment. A decrease in net photosynthesis and stomatal conductance in the still surviving totally burnt trees was also recorded during the first measurements made in July. Treatment differences in predawn water potential, microvariations in trunk diameter, and electrical resistance of the cambium only appeared a few days before death of the totally burnt trees.
-Thermostability of photosynthesis was studied in four Mediterranean conifer species growing in southern France, namely Cedrus atlantica and Pinus nigra growing usually on mid elevation areas, and Cupressus sempervirens and Pinus halepensis from coastal areas. Chlorophyll a fluorescence was used and lead to two indices assessing the degree of thermostability of the photosynthetic apparatus: the critical temperature at ground fluorescence breakpoint (T c ) and the temperature threshold inducing a 15% decrease in photochemical efficiency (T 15 ). The two indices were correlated and yielded similar rankings among species, although mean values of T 15 were 6.5°C lower than that of T c . Values of T c were in the range 44 to 52 °C and clear interspecific differences were detected. C. atlantica consistently displayed higher T c than the other species (1-1.5°C difference during a seasonal time course). Among the three other species (C. sempervirens, P. nigra and P. halepensis), the differences were smaller and not always significant. T c also displayed a large intraspecific plasticity, with: (i) a seasonal time-course showing significant increases during summer and lower values during Spring and Autumn; and (ii) large responses to ambient temperatures, with 5-6°C increases in response to a gradual rise of temperature from 10 to 35 °C. The amplitude of the rise was of the same magnitude in all species. Therefore records of thermostability of photosynthesis, whatever the parameter used (T c or T 15 ) need to take into account the large plasticity in this parameter when comparing species or genotypes. The degree of plasticity in response to given changes in micro-environment could be an important functional trait for the tolerance to environmental stresses. high temperature / photosynthesis / Cedrus atlantica / Cupressus sempervirens / Pinus nigra / Pinus halepensis Résumé -Variations saisonnières et potentiel d'acclimatation de la thermostabilité de la photochimie de quatre conifères méditerra-néens. Nous avons analysé la thermostabilité de la photosynthèse de quatre conifères méditerranéens de la forêt française, le cèdre de l'Atlas (Cedrus atlantica ) et le pin noir d'Autriche (Pinus nigra) qui occupent habituellement des zones de montagne, et le cyprès (Cupressus sempervirens) et le Pin d'Alep (Pinus halepensis) qui sont plus spécifiques des zones côtières. La fluorescence de la chlorophylle a a permis d'estimer deux indices de thermostabilité de l'appareil photosynthétique : la température critique à la quelle la fluorescence de base augmente brutalement (T c ), et le seuil de température induisant une baisse de 15 % du rendement quantique de la photochimie (T 15 ). Ces deux indices étaient fortement corrélés et ont conduit au même classement des espèces, bien que les valeurs de T 15 étaient en moyenne plus faibles de 6,5°C que celles de T c . Les valeurs de T c couvraient la gamme de 44 à 52 °C et des différences interspécifiques significatives ont été détectées. C atlantica présentait des valeurs de T c s...
-We studied the xylem hydraulic traits and anatomy of four diverse Mediterranean conifers to determine how these species protect themselves against catastrophic xylem failure. Cedrus atlantica, Cupressus sempervirens, Pinus nigra and P. halepensis seedlings were grown for two years in pots in a greenhouse under well-watered conditions. Measurements were conducted in April and September. The vulnerability to cavitation was lower in April in the two pines and cedar whereas the conductivity was lower in the two pines and cypress. There were also large species differences in vulnerability to cavitation in September: loss of 50% conductivity occurred at -2.8 MPa in P. nigra, -3.8 MPa in C. atlantica, -4.8 MPa in C. sempervirens and -4.9 MPa in P. halepensis. Leaf specific hydraulic conductivity was much higher in Cupressus sempervirens and P. nigra than in Cedrus atlantica and P. halepensis. No trade-off between xylem safety (low vulnerability) and efficiency (high hydraulic conductivity) was found among the four species. Specific conductivity was directly related to "hydraulic mean" tracheid lumen diameter, while xylem vulnerability appeared to be independent of tracheid size. xylem embolism / hydraulic conductivity /xylem anatomy / Mediterranean conifers Résumé -Efficience hydraulique et vulnérabilité de plantules de quatre espèces de conifères méditerranéens (Cedrus atlantica, Cupressus sempervirens, Pinus halepensis et Pinus nigra). Nous avons étudié les caractéristiques hydrauliques et l'anatomie du xylème de quatre espèces de conifères méditerranéens afin de déterminer comment ces espèces se protègent contre un dysfonctionnement catastrophique du xylème. Des plants de Cedrus atlantica, Cupressus sempervirens, Pinus halepensis et P. nigra ont été cultivés en serre pendant deux ans en conditions d'alimentation en eau non limitante. Les mesures ont été effectuées en avril et septembre. La vulnérabili-té à la cavitation a été plus faible en avril chez les deux pins et le cèdre tandis que la conductivité hydraulique a été plus faible chez les deux pins et le cyprès. D'importantes différences de vulnérabilité à la cavitation ont également été observées entre les espèces en septembre : la perte de 50 % de conductivité est intervenue à -2,8 MPa chez P. nigra, -3,8 MPa chez C. atlantica, -4,8 MPa chez C. sempervirens et -4,9 MPa chez P. halepensis. La conductivité spécifique foliaire a été plus élevée chez C. sempervirens et P. nigra que chez C. atlantica et P. halepensis. Aucun compromis n'a été mis en évidence entre la protection du système conducteur (faible vulnérabilité à la cavitation) et l'efficacité de la circulation de la sève (forte conductivité hydraulique) entre les 4 espèces. La conductivité hydraulique spécifique est positivement corrélée avec le diamètre « hydraulique moyen » des lumières des trachéides alors que la vulné-rabilité du xylème semble être indépendante de la taille des trachéides. embolie / conductivité hydraulique / anatomie du xylème / conifères méditerranéens Ann. For. Sci. 59 (20...
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