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Plant water content is a simple and promising parameter for monitoring drought‐driven plant mortality risk. However, critical water content thresholds leading to cell damage and plant failure are still unknown. Moreover, it is unclear whether whole‐plant or a specific organ water content is the most reliable indicator of mortality risk. We assessed differences in dehydration thresholds in leaf, stem and root samples, hampering the organ‐specific rehydration capacity and increasing the mortality risk. We also tested eventual differences between a fast experimental dehydration of uprooted plants, compared to long‐term water stress induced by withholding irrigation in potted plants. We investigated three species with different growth forms and leaf habits i.e., Helianthus annuus (herbaceous), Populus nigra (deciduous tree) and Quercus ilex (evergreen tree). Results obtained by the two dehydration treatments largely overlapped, thus validating bench dehydration as a fast but reliable method to assess species‐specific critical water content thresholds. Regardless of the organ considered, a relative water content value of 60% induced significant cell membrane damage and loss of rehydration capacity, thus leading to irreversible plant failure and death. Summary statements: Across three species with different growth forms and leaf habits, a relative water content (RWC) of 60% increased the risk of mortality. The loss of rehydration capacity and cell membrane damage were correlated to RWC, that turned out as a reliable indicator of incipient plant death under drought. [ABSTRACT FROM AUTHOR]

Element content and expression of genes of interest on single cell types, such as stomata, provide valuable insights into their specific physiology, improving our understanding of leaf gas exchange regulation. We investigated how far differences in stomatal conductance (gs) can be ascribed to changes in guard cells functioning in amphistomateous leaves. gs was measured during the day on both leaf sides, on well‐watered and drought‐stressed trees (two Populus euramericana Moench and two Populus nigra L. genotypes). In parallel, guard cells were dissected for element content and gene expressions analyses. Both were strongly arranged according to genotype, and drought had the lowest impact overall. Normalizing the data by genotype highlighted a structure on the basis of leaf sides and time of day both for element content and gene expression. Guard cells magnesium, phosphorus, and chlorine were the most abundant on the abaxial side in the morning, where gs was at the highest. In contrast, genes encoding H+‐ATPase and aquaporins were usually more abundant in the afternoon, whereas genes encoding Ca2+‐vacuolar antiporters, K+ channels, and ABA‐related genes were in general more abundant on the adaxial side. Our work highlights the unique physiology of each leaf side and their analogous rhythmicity through the day. Guard cell analyses revealed a strong genotypic difference of chlorine content. Moreover, H+‐ATPase, ABA‐related genes, K+ channels, and aquaporins expression displayed marked differences between leaf sides and time of day. [ABSTRACT FROM AUTHOR]

Belowground (BG) herbivory can influence aboveground (AG) herbivore performance and food preference via changes in plant chemistry. Most evidence for this phenomenon derives from studies in herbaceous plants but studies in woody plants are scarce. Here we investigated whether and how BG herbivory on black poplar (Populus nigra) trees by Melolontha melolontha larvae influences the feeding preference of Lymantria dispar (gypsy moth) caterpillars. In a food choice assay, caterpillars preferred to feed on leaves from trees that had experienced attack by BG herbivores. Therefore, we investigated the effect of BG herbivory on the phytochemical composition of P. nigra trees alone and in combination with AG feeding by L. dispar caterpillars. BG herbivory did not increase systemic AG tree defences like volatile organic compounds, protease inhibitors and salicinoids. Jasmonates and salicylic acid were also not induced by BG herbivory in leaves but abscisic acid concentrations drastically increased together with proline and few other amino acids. Leaf coating experiments with amino acids suggest that proline might be responsible for the caterpillar feeding preference via presumptive phagostimulatory properties. This study shows that BG herbivory in poplar can modify the feeding preference of AG herbivores via phytochemical changes as a consequence of root‐to‐shoot signaling. In trees, the role of belowground herbivory for aboveground phytochemistry and accompanying consequences for herbivore preference are poorly understood. Our study demonstrates that belowground herbivory in young black poplar (Populus nigra) trees by Melolontha melolontha larvae does not increase systemic aboveground defences but causes symptoms resembling water stress. Generalist gypsy moth (Lymantria dispar) caterpillars prefer feeding on leaves from trees infested belowground possibly due to increased proline concentrations. [ABSTRACT FROM AUTHOR]

Poplar genotypes differ in transpiration efficiency ( TE) at leaf and whole-plant level under similar conditions. We tested whether atmospheric vapour pressure deficit ( VPD) affected TE to the same extent across genotypes. Six P opulus nigra genotypes were grown under two VPD. We recorded (1) 13 C content in soluble sugars; (2) 18 O enrichment in leaf water; (3) leaf-level gas exchange; and (4) whole-plant biomass accumulation and water use. Whole-plant and intrinsic leaf TE and 13 C content in soluble sugars differed significantly among genotypes. Stomatal conductance contributed more to these differences than net CO2 assimilation rate. VPD increased water use and reduced whole-plant TE. It increased intrinsic leaf-level TE due to a decline in stomatal conductance. It also promoted higher 18 O enrichment in leaf water. VPD had no genotype-specific effect. We detected a deviation in the relationship between 13 C in leaf sugars and 13 C predicted from gas exchange and the standard discrimination model. This may be partly due to genotypic differences in mesophyll conductance, and to its lack of sensitivity to VPD. Leaf-level 13 C discrimination was a powerful predictor of the genetic variability of whole-plant TE irrespective of VPD during growth. [ABSTRACT FROM AUTHOR]

After herbivory, plants release volatile organic compounds from damaged foliage as well as from nearby undamaged leaves that attract herbivore enemies. Little is known about what controls the volatile emission differences between damaged and undamaged tissues and how these affect the orientation of herbivore enemies. We investigated volatile emission from damaged and adjacent undamaged foliage of black poplar ( P opulus nigra) after herbivory by gypsy moth ( L ymantria dispar) caterpillars and determined the compounds mediating the attraction of the gypsy moth parasitoid G lyptapanteles liparidis ( Braconidae). Female parasitoids were more attracted to gypsy moth-damaged leaves than to adjacent non-damaged leaves. The most characteristic volatiles of damaged versus neighbouring undamaged leaves included terpenes, green leaf volatiles and nitrogen-containing compounds, such as aldoximes and nitriles. Electrophysiological recordings and olfactometer bioassays demonstrated the importance of nitrogenous volatiles. Under field conditions, parasitic Hymenoptera were more attracted to traps baited with these substances than most other compounds. The differences in volatile emission profiles between damaged and undamaged foliage appear to be regulated by jasmonate signalling and the local activation of volatile biosynthesis. We conclude that characteristic volatiles from damaged black poplar foliage are essential cues enabling parasitoids to find their hosts. [ABSTRACT FROM AUTHOR]

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides× Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance ( Ψ50) varied from −1.60 to −2.40 MPa. Drought-acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Ψ50 being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Ψ50 was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water-use efficiency. At the whole-plant level, increased safety was associated with higher shoot growth potential under well-irrigated regime only. We conclude that common trade-offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales. [ABSTRACT FROM AUTHOR]

Drought is expected to become an increasingly important factor limiting tree growth caused by climate change. Two divergent clones of Populus nigra (58-861 and Poli) originating from contrasting environments were subjected to water limitation (WL) to elucidate whether they differ in tolerance to drought, which mechanisms to avoid stress they exhibit and whether drought has an impact on the interactions between roots and shoots. Limiting water availability caused photosynthetic rate and total non-structural carbohydrate (TNC) levels to decrease in 58-861. However, starch-degrading enzyme activity and gene expression were induced in roots, and soluble sugar levels were higher than in well-watered (WW) plants. These data suggest that assimilation and partitioning of carbon to the roots are decreased, resulting in mobilization of stored starch. In contrast, the photosynthetic rate of Poli was reduced only late in the treatment, and carbohydrate levels in WL plants were higher than in WW plants. Superoxide dismutase (SOD) activity and gene expression were higher in Poli than in 58-861, even in WW plants, leading to a higher capacity to defend against oxidative stress. [ABSTRACT FROM AUTHOR]

Evidence is presented for a very specific, seasonally recurring tri-phase carbon isotope pattern in tree rings of broad-leaf deciduous tree species. It is derived from highly resolved intra-annual measurements of 13C/12C ratios of wood and cellulose from tree rings of Fagus sylvatica, Populus nigra, Quercus petraea and Morus alba. Investigations on δ13C from buds and leaves of Fagus sylvatica revealed a similar tri-phase δ13C pattern. At the very beginning of a growing season, the δ13C trend of tree rings and foliage shows a marked increase of up to 5‰. The maximum δ13C-value of each vegetation period always occurs in young heterotrophic leaves shortly after bud burst and persistently in the early wood of each tree ring, when growth depends on carbon reserves. Thereafter, δ13C profiles represent the autotrophic stage of the leaves, which show different patterns of variation, by and large characterized by a decline. The minimum δ13C-value always shows up in the late wood of each tree ring. At the very end of each tree ring δ13C-values start rising again. This increase in δ13C marks the gradual switch-over to storage-dependent growth and can also be observed in senescent leaves. Seasonal changes of more than 4‰ were measured, whereas contiguous δ13C values rarely differed from each other by more than 0.3‰. This tri-phase pattern cannot be explained by the common model of carbon isotope fractionation during photosynthesis. It appears to be primarily an indication of seasonal changes in down-stream processes of the carbohydrate metabolism. Environmental influences on the carbon isotope fractionation during photosynthesis are presumably of secondary importance and expressed by certain peculiarities showing up during the autotrophic phase, i.e. the mid-section of the seasonal δ13C pattern. [ABSTRACT FROM AUTHOR]

Light-saturated net leaf photosynthesis (Asat) CO2 response curves (A/Ci), current photochemical capacity (Fv/Fm) and pigment contents were measured in leaves of Populus nigra (Clone T107) which had been exposed to ozone stress in open-top chambers for the entire growth period. Surprisingly, not only elevated (ao+, i.e. ambient air + 50 mm3 m-3 ozone) but also ambient (aa) ozone concentrations led to a reduction in Asat in comparison with leaves exposed to air containing almost no ozone (cf-, i.e. charcoal filtered ambient air). The very small change in leaf conductance (gl) indicated that the decrease in Asat was not due to stomatal limitation. This finding was supported by the fact that, a decrease in carboxylation efficiency (CE) correlated with a loss in Asat. In comparison to cf- leaves, aa leaves showed no change in current photochemical capacity (Fv/Fm) throughout the whole experiment. However, a marked decline in Fv/Fm in ao+ leaves was observed at a time when Asat and CE were already decreased by about 45% and 60% respectively. As the chlorophyll b content of leaves is known to correlate with the amount of LHC and PSII centres, it was used to normalize fluorescence parameters in relation to PSII centres present. The normalized values for Fm and F0 increased with the dosage of ozone in ao+ leaves but not in aa leaves, indicating a change of the pigment content of PSII in the former, but not in the latter. These data led to the conclusion that ozone interacts primarily with components of the Calvin cycle, which results in a decrease in Asat with subsequent feedback on the current photochemical capacity of PSII centres. [ABSTRACT FROM AUTHOR]

An equation is derived relating tissue water potential to relative water content. This equation may be used to fit a single curve to a set of date such as the standard pressure-volume measurements made with a pressure chamber. From such a single curve-fitting operation, estimates of the parameters involved may be found, and this allows calculation of such quantities as bulk modulus of elasticity, osmotic potential at full turgor and at the turgor loss point, pressure potential, and the weight of symplastic water. The methods of analysis has several advantages, which are illustrated using pressure chamber data obtained from leaves of Lombardy poplar, Populus nigra L. 'italica'. [ABSTRACT FROM AUTHOR]