Abstract
Phloem failure has recently been recognized as one of the mechanisms causing tree mortality under drought, though direct evidence is still lacking. We combined 13C pulse-labelling of ...8-year-old beech trees (Fagus sylvatica L.) growing outdoors in a nursery with an anatomical study of the phloem tissue in their stems to examine how drought alters carbon transport and phloem transport capacity. For the six trees under drought, predawn leaf water potential ranged from −0.7 to −2.4 MPa, compared with an average of −0.2 MPa in five control trees with no water stress. We also observed a longer residence time of excess 13C in the foliage and the phloem sap in trees under drought compared with controls. Compared with controls, excess 13C in trunk respiration peaked later in trees under moderate drought conditions and showed no decline even after 4 days under more severe drought conditions. We estimated higher phloem sap viscosity in trees under drought. We also observed much smaller sieve-tube radii in all drought-stressed trees, which led to lower sieve-tube conductivity and lower phloem conductance in the tree stem. We concluded that prolonged drought affected phloem transport capacity through a change in anatomy and that the slowdown of phloem transport under drought likely resulted from a reduced driving force due to lower hydrostatic pressure between the source and sink organs.
13CO2 pulse-labelling experiments were performed in situ on adult beeches (Fagus sylvatica) and pines (Pinus pinaster) at different phenological stages to study seasonal and interspecific short-term ...dynamics and partitioning of recently assimilated carbon (C) in leaves.
Polar fraction (PF, including soluble sugars, amino acids and organic acids) and starch were purified from foliage sampled during a 10-d chase period. C contents, isotopic compositions and 13C dynamics parameters were determined in bulk foliage, PF and starch.
Decrease in 13C amount in bulk foliage followed a two-pool exponential model highlighting 13C partitioning between ‘mobile’ and ‘stable’ pools, the relative proportion of the latter being maximal in beech leaves in May. Early in the growing season, new foliage acted as a strong C sink in both species, but although young leaves and needles were already photosynthesizing, the latter were still supplied with previous-year needle photosynthates 2 months after budburst. Mean 13C residence times (MRT) were minimal in summer, indicating fast photosynthate export to supply perennial organ growth in both species. In late summer, MRT differed between senescing beech leaves and overwintering pine needles.
Seasonal variations of 13C partitioning and dynamics in field-grown tree foliage are closely linked to phenological differences between deciduous and evergreen trees.
Estimating symbiotic di‐nitrogen (N2) fixation is challenging, especially when working with woody N2 fixers in field trials. Fortunately, isotope methods based on 15N natural abundance or on 15N ...artificial enrichment (dilution method) make it possible to estimate the proportion of nitrogen derived from the atmosphere (Ndfa) in N2‐fixing species. These methods have been extensively used in the field for herbaceous species, much less for tree species such as alder and acacia, and rarely for black locust (Robinia pseudoacacia). The objectives of this study were to characterize the fixation potential of black locust in a plantation by using the two 15N isotope methods in the field, and to document values of isotope fractionation occurring during N2 fixation (the B value). B values were estimated both by growing trees on an N‐free medium in controlled conditions (Blab) and by making Ndfa calculated with the natural abundance method converge with Ndfa calculated with the 15N dilution method in the field (Bfield). The two methods gave consistent estimates of the B value. B values ranging between –1.4 and –3.2‰ were found, varying with the age of the plant material. Up to 76% of the N in the black locust trees came from the atmosphere, representing more than 45 kg N ha−1 over five years and confirming that black locust may be well adapted to N‐poor soils.
Key message
Black locust is an interesting option for biomass production at sites prone to water shortage because the species combines water-use efficiency and a biomass yield largely superior to ...that of poplars under the conditions of the study.
Context
Black locust (
Robinia pseudoacacia
L.) is an interesting tree species for woody biomass production. However, its potential for this purpose has been much less studied and characterized than that for species from the Salicaceae family (i.e., poplar and willow).
Aims
The objective of our study was to evaluate the potential of black locust for biomass production as compared to that of poplar.
Methods
We estimated biomass production, growth habit, and efficiency of water use of two provenances of black locust (1) compared to those of poplar (
Populus deltoides
Bartr. ex Marsh. ×
P. nigra
L.), (2) at two planting densities, and (3) 2 and 4 years after planting.
Results
Black locust had a very different growth habit, much higher biomass production, and larger water-use efficiency than poplar. These differences were exacerbated during the driest year of the experiment. However, black locust responded very badly to harvesting.
Conclusion
Black locust was more productive and more efficient in terms of water use than poplar, especially during the driest year of the experiment.
The observed decrease in respiration during rosette leaf maturation of Nicotiana sylvestris wild-type (WT) plants was shown to be because of a decline in the cytochrome oxidase (COX) pathway ...activity, measured by ¹⁸O/¹⁶O oxygen discrimination, while the alternative oxidase pathway (AOX) remained stable. This suggests a higher contribution of the COX pathway to growth respiration than to maintenance respiration. Mitochondrial superoxide dismutase (MnSOD) activity paralleled the decrease in COX activity with leaf age, whereas chloroplastic FeSOD activity increased. Age-dependent respiratory changes were much less apparent in the Cytoplasmic Male Sterile II (CMSII) mitochondrial mutant devoid of respiratory complex I and previously shown to possess increased AOX content and enhanced respiration but lower photosynthesis in mature leaves. Respiration declined less rapidly with leaf age in CMSII than in the WT, and was significantly higher in the mutant when compared with the WT in mature leaves only. In contrast, photosynthesis was lower in the mutant than in the WT at all leaf stages. The higher respiration of mature CMSII leaves was supported exclusively by enhanced COX activity, in association with an increased mitochondrial MnSOD activity. Steady-state levels of AOX1 transcripts increased in maturing WT leaves, and the CMSII mutant had higher amounts of coxI, AOX1 and MnSOD transcripts than the WT. Enhanced activity of the proton-pumping COX route in the mutant can be viewed as a compensation for the lack of the first coupling site of the respiratory chain. However, this is not quite sufficient to ensure normal growth rates in the mutant.
Nicotiana sylvestris leaves challenged by the bacterial elicitor harpin NEa were used as a model system in which to determine the respective roles of light, oxygen, photosynthesis, and respiration in ...the programmed cell death response in plants. The appearance of cell death markers, such as membrane damage, nuclear fragmentation, and induction of the stress-responsive element Tnt1, was observed in all conditions. However, the cell death process was delayed in the dark compared with the light, despite a similar accumulation of superoxide and hydrogen peroxide in the chloroplasts. In contrast, harpin-induced cell death was accelerated under very low oxygen (<0.1% O2) compared with air. Oxygen deprivation impaired accumulation of chloroplastic reactive oxygen species (ROS) and the induction of cytosolic antioxidant genes in both the light and the dark. It also attenuates the collapse of photosynthetic capacity and the respiratory burst driven by mitochondrial alternative oxidase activity observed in air. Since alternative oxidase is known to limit overreduction of the respiratory chain, these results strongly suggest that mitochondrial ROS accumulate in leaves elicited under low oxygen. We conclude that the harpin-induced cell death does not require ROS accumulation in the apoplast or in the chloroplasts but that mitochondrial ROS could be important in the orchestration of the cell suicide program.
Many studies have highlighted the value of mixed plantations and their advantages over monocultures. The success of mixed plantations is usually assessed by measuring the increase in biomass and/or ...plant production compared to the corresponding monocultures. Among the structural determinants of growth, the vertical distribution of branches and crown shape are important to take into account because they directly impact access to light, which conditions tree growth. We evaluated the effect of two types of species mixtures in northeastern France (agroforestry and a forest mixture) on the crown architecture of poplars (
Populus deltoides
×
P. nigra
) and alder
(Alnus glutinosa
) after seven growing seasons and compared the mixtures to their respective monocultures. Four tree architecture variables (height, crown depth, crown projection area, crown volume) were evaluated. Our study shows that the poplars in the agroforestry plot altered their crown morphology through a true mixture effect due to the presence of clover, an N
2
-fixing species, as well as to reduced competition for light due to larger spacing compared to the monoculture. In the forest mixture, despite a stratification of the canopy suggesting an optimized sharing of the aerial niche, thus possibly creating an additive effect, poplar crown morphology was not different compared to the monoculture. Finally, the different types of mixtures did not affect alder crown morphology. From an agronomic perspective, the more important crown development that occurred when the poplar was associated with an N
2
-fixing crop makes this type of mixture a very promising way to increase the contribution of biomass to the renewable energy mix in Europe.
•Intercrops such as alfalfa could significantly improve the growth of poplars.•Competition for light and soil resources seems to be lower for poplars in mixtures.•Poplar growth rates were higher in ...agroforestry than in forest plots for a dry year.•Stem growth of alder was not affected by its association with poplar or graminoids.
In recent years, plantations of fast-growing tree species have emerged as a possible way to meet the increasing demand for biomass for renewable energy in Europe. Agroforestry plantations including fast-growing tree species could be an attractive option because they reduce land competition for biomass and food production while providing forest benefits. Today, the species interactions that determine whether a given mixture will be more productive than the corresponding monocultures are still poorly understood. Our objective was to assess the performance of fast-growing trees, i.e. poplar (Populus nigra × P. deltoides) and alder (Alnus glutinosa), in association with herbaceous species either N2-fixing species (succession alfalfa (Medicago sativa) and clover (Trifolium pratense)) or graminoids (succession of wheat (Triticum aestivum) – triticale (Triticosecale) – temporary grassland consisting of mixtures of perennial ryegrass (Lolium perenne) and fescue (Festuca ovina)), respectively, and in association together. We compared tree growth in mixed stands to growth in the respective tree monocultures. An experimental plantation composed of three blocks including the different treatments was set up in 2014 in northeastern France. In the forest mixture, poplar and alder were planted in alternating rows, and in the agroforestry plots, every second line of trees was replaced by the crops. No fertilizers (of any kind) were used during the experiments. During six growing seasons, tree growth (height and diameter at breast height) was monitored monthly in the agroforestry stands, the forest mixture stand and the two tree monocultures. By the end of 2019, poplar stem height in association with N2-fixing crops (alfalfa, then clover) was higher than in the forest mixture and the monoculture. During the warmest growing season with a marked dry period in June-July (2018), poplar height growth rate in the agroforestry treatment was between three and five times higher than for the poplars in the monoculture and the forest mixture. A facilitation process in the agroforestry plantations could explain this result due to a significant enrichment of the soil in nitrogen by the N2-fixing crops. This result only appeared several years after tree planting, and management actions were necessary in order to control the strong competition between crops and trees during the early stages. In the forest mixture, poplars associated with alders were smaller than in their monoculture, in spite of a stratification of the canopies of the two species. For alders, tree growth did not appear to be affected by either an association with graminoids or with poplar.
Sphagnum
mosses are considered peatland engineers because of their ability to create conditions inducing carbon accumulation. Here, we report on a review of the effects of four environmental ...variables (elevated temperature, N and CO
2
and reduced moisture) on the capitulum biomass, length increment, respiration, photosynthetic capability, N and P exchange and content of the 3 most studied
Sphagnum
subgenera (
Acutifolia
,
Cuspidata
,
Sphagnum
). Overall, we observe that, when compared to in situ experiments, laboratory experiments tend to exacerbate length increments and underestimate maximum photosynthesis in most of the studies inventoried. This review underscores some differences among results that can be associated with the used of different protocols (e.g. exposure time, instrumental analysis). Studies that investigated the impact of elevated temperature (2–5 °C) on
Sphagnum
reveal an increase in length, respiration and photosynthesis regardless of the experimental conditions and subgenus. Elevated N (3–23 g Nm
−2
y
−1
) on the other hand appears to reduce the length increment but had contrasting effects on photosynthesis. Some divergent responses are found with
Cuspidata
species because of their tolerance to high doses of N. Low moisture reduces the length increment and photosynthesis of species of the
Cuspidata
and
Sphagnum
subgenera but has different effects on species of the
Acutifolia
subgenus, which are relatively tolerant to water fluctuations. Responses to elevated CO
2
have no clear trends reported. Allelochemical interactions between
Sphagnum
, their microbiome or surrounding mosses or other plants were found to be determinant to
Sphagnum
responses under those variables and reinforce the interest of such investigations.
Abstract
In a global context where water will become a scarce resource under temperate latitudes, managing tree plantations with species associations, i.e., forest mixture or agroforestry, could play ...a major role in optimizing the sustainable use of this resource. Conceptual frameworks in community ecology suggest that, in mixed plantations, environmental resources such as water may be more efficiently used for carbon acquisition and tree growth thanks to niche complementarity among species. To test the hypotheses behind these conceptual frameworks, we estimated water-use efficiency (WUE) for poplar trees grown in a monoculture, in association with alder trees (forest mixture) and in association with clover leys (agroforestry) in an experimental plantation located in northeastern France. Water-use efficiency was estimated (i) at leaf level through gas exchange measurements and analysis of carbon isotope composition, (ii) at wood level through carbon isotope composition and (iii) at tree level with sap flow sensors and growth increment data. We hypothesized that species interactions would increase WUE of poplars in mixtures due to a reduction in competition and/or facilitation effects due to the presence of the N2-fixing species in mixtures. Poplar trees in both mixture types showed higher WUE than those in the monoculture. The differences we found in WUE between the monoculture and the agroforestry treatment were associated to differences in stomatal conductance and light-saturated net CO2 assimilation rate (at the leaf level) and transpiration (at the tree level), while the differences between the monoculture and the forest mixture were more likely due to differences in stomatal conductance at the leaf level and both transpiration and biomass accumulation at the tree level. Moreover, the more WUE was integrated in time (instantaneous gas exchanges < leaf life span < seasonal wood core < whole tree), the more the differences among treatments were marked.
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