Early human selection of crops' wild progenitors explains the acquisitive physiology of modern cultivars.

Crops have resource-acquisitive leaf traits, which are usually attributed to the process of domestication. However, early choices of wild plants amenable for domestication may also have played a key role in the evolution of crops' physiological traits. Here we compiled data on 1,034 annual herbs to place the ecophysiological traits of 69 crops' wild progenitors in the context of global botanical variation, and we conducted a common-garden experiment to measure the effects of domestication on crop ecophysiology. Our study found that crops' wild progenitors already had high leaf nitrogen, photosynthesis, conductance and transpiration and soft leaves. After domestication, ecophysiological traits varied little and in idiosyncratic ways. Crops did not surpass the trait boundaries of wild species. Overall, the resource-acquisitive strategy of crops is largely due to the inheritance from their wild progenitors rather than to further breeding improvements. Our study concurs with recent literature highlighting constraints of crop breeding for faster ecophysiological traits.

Leaf structure and water relations of an allotetraploid Mediterranean fern and its diploid parents.

Allopolyploidy is a common speciation mechanism in plants; however, its physiological and ecological consequences in niche partitioning have been scarcely studied. In this sense, leaf traits are good proxies to study the adaptive capacity of allopolyploids and diploid parents to their respective environmental conditions. In the present work, leaf water relations (assessed through pressure-volume curves) and structural and anatomical traits of the allotetraploid fern Oeosporangium tinaei and its diploid parents, Oeosporangium hispanicum and Oeosporangium pteridioides, were studied under controlled conditions in response to a water stress (WS) cycle. O. hispanicum showed the lowest osmotic potential at turgor loss point (πtlp ) and leaf capacitance, together with higher leaf mass per area (LMA), leaf thickness (LT), leaf density (LD), and leaf dry matter content (LDMC), whereas O. pteridioides presented the opposite set of traits (high πtlp and capacitance, and low LMA, LT, LD, and LDMC). O. tinaei showed an intermediate position for most of the studied traits. The responsiveness (osmotic and elastic adjustments) to WS was low, although most of the traits explained the segregation of the three species across a range of drought tolerance according to the rank: O. hispanicum > O. tinaei > O. pteridioides. These trait differences may underlie the niche segregation among coexisting populations of the three species in the Mediterranean basin.

Ecophysiological Differentiation among Two Resurrection Ferns and Their Allopolyploid Derivative.

Theoretically, the coexistence of diploids and related polyploids is constrained by reproductive and competitive mechanisms. Although niche differentiation can explain the commonly observed co-occurrence of cytotypes, the underlying ecophysiological differentiation among cytotypes has hardly been studied. We compared the leaf functional traits of the allotetraploid resurrection fern Oeosporangium tinaei (HHPP) and its diploid parents, O. hispanicum (HH) and O. pteridioides (PP), coexisting in the same location. Our experimental results showed that all three species can recover physiological status after severe leaf dehydration, which confirms their 'resurrection' ability. However, compared with PP, HH had much higher investment per unit area of light-capturing surface, lower carbon assimilation rate per unit mass for the same midday water potential, higher non-enzymatic antioxidant capacity, higher carbon content, and lower contents of nitrogen, phosphorus, and other macronutrients. These traits allow HH to live in microhabitats with less availability of water and nutrients (rock crevices) and to have a greater capacity for resurrection. The higher assimilation capacity and lower antioxidant capacity of PP explain its more humid and nutrient-rich microhabitats (shallow soils). HHPP traits were mostly intermediate between those of HH and PP, and they allow the allotetraploid to occupy the free niche space left by the diploids.

Leaf wettability, anatomy and ultra-structure of Nothofagus antarctica and N. betuloides grown under a CO2 enriched atmosphere.

Increasing CO2 air concentration may affect wettability, anatomy and ultra-structure of leaves of Patagonian forest species, evergreen and deciduous plants potentially responding differently to such CO2 increases. In this study, we analysed the wettability, anatomy and ultra-structure of leaves of Nothofagus antarctica (deciduous) and N. betuloides (evergreen) grown under high CO2 concentrations. Leaf wettability was affected by increasing CO2, in different directions depending on species and leaf side. In both species, soluble cuticular lipid concentrations per unit leaf area raised with higher CO2 levels. Stomatal parameters (density, size of guard cells and pores) showed different responses to CO2 increasing depending on the species examined. In both species, leaf tissues showed a general trend to diminish with higher CO2 concentration. Cuticle thickness was modified with higher CO2 concentration in N. betuloides, but not in N. antarctica leaves. In both species, chloroplasts were often damaged with the increase in CO2 concentration. Our results show that several surface and internal leaf parameters can be modified in association with an increase in atmospheric CO2 concentration which may very among plant species.

Aerial and underground organs display specific metabolic strategies to cope with water stress under rising atmospheric CO2 in Fagus sylvatica L.

Beech is known to be a moderately drought-sensitive tree species, and future increases in atmospheric concentrations of CO2 ([CO2 ]) could influence its ecological interactions, also with changes at the metabolic level. The metabolome of leaves and roots of drought-stressed beech seedlings grown under two different [CO2 ] (400 (aCO2 ) and 800 (eCO2 ) ppm) was analyzed together with gas exchange parameters and water status. Water stress estimated from predawn leaf water potential (Ψpd ) was similar under both [CO2 ], although eCO2 had a positive impact on net photosynthesis and intrinsic water use efficiency. The aerial and underground organs showed different metabolomes. Leaves mainly stored C metabolites, while those of N and P accumulated differentially in roots. Drought triggered the proline and N-rich amino acids biosynthesis in roots through the activation of arginine and proline pathways. Besides the TCA cycle, polyols and soluble sugar biosynthesis were activated in roots, with no clear pattern seen in the leaves, prioritizing the root functioning as metabolites sink. eCO2 slightly altered this metabolic acclimation to drought, reflecting mitigation of its effect. The leaves showed only minor changes, investing C surplus in secondary metabolites and malic acid. The TCA cycle metabolites and osmotically active substances increased in roots, but many other metabolites decreased as if the water stress was dampened. Above- and belowground plant metabolomes were differentially affected by two drivers of climate change, water scarcity and high [CO2 ], showing different chemical responsiveness that could modulate the tree adaptation to future climatic scenarios.

Towards a statistically robust determination of minimum water potential and hydraulic risk in plants.

Minimum water potential (Ψmin ) is a key variable for characterizing dehydration tolerance and hydraulic safety margins (HSMs) in plants. Ψmin is usually estimated as the absolute minimum tissue Ψ experienced by a species, but this is problematic because sample extremes are affected by sample size and the underlying probability distribution. We compare alternative approaches to estimate Ψmin and assess the corresponding uncertainties and biases; propose statistically robust estimation methods based on extreme value theory (EVT); and assess the implications of our results for the characterization of hydraulic risk. Our results show that current estimates of Ψmin and HSMs are biased, as they are strongly affected by sample size. Because sampling effort is generally higher for species living in dry environments, the differences in current Ψmin estimates between these species and those living under milder conditions are partly artefactual. When this bias is corrected using EVT methods, resulting HSMs tend to increase substantially with resistance to embolism across species. Although data availability and representativeness remain the main challenges for proper determination of Ψmin , a closer look at Ψ distributions and the use of statistically robust methods to estimate Ψmin opens new ground for characterizing plant hydraulic risks.

Specific leaf metabolic changes that underlie adjustment of osmotic potential in response to drought by four Quercus species.

Osmotic adjustment is almost ubiquitous as a mechanism of response to drought in many forest species. Recognized as an important mechanism of increasing turgor under water stress, the metabolic basis for osmotic adjustment has been described in only a few species. We set an experiment with four species of the genus Quercus ranked according to drought tolerance and leaf habit from evergreen to broad-leaved deciduous. A cycle of watering deprivation was imposed on seedlings, resulting in well-watered (WW) and water-stressed (WS) treatments, and their water relations were assessed from pressure-volume curves. Leaf predawn water potential (Ψpd) significantly decreased in WS seedlings, which was followed by a drop in leaf osmotic potential at full turgor (Ψπ100). The lowest values of Ψπ100 followed the ranking of decreasing drought tolerance: Quercus ilex L. < Quercus faginea Lam. < Quercus pyrenaica Willd. < Quercus petraea Matt. Liebl. The leaf osmotic potential at the turgor loss point (ΨTLP) followed the same pattern as Ψπ100 across species and treatments. The pool of carbohydrates, some organic acids and cyclitols were the main osmolytes explaining osmotic potential across species, likewise to the osmotic adjustment assessed from the decrease in leaf Ψπ100 between WW and WS seedlings. Amino acids were very responsive to WS, particularly γ-aminobutyric acid in Q. pyrenaica, but made a relatively minor contribution to osmotic potential compared with other groups of compounds. In contrast, the cyclitol proto-quercitol made a prominent contribution to the changes in osmotic potential regardless of watering treatment or species. However, different metabolites, such as quinic acid, played a more important role in osmotic adjustment in Q. ilex, distinguishing it from the other species studied. In conclusion, while osmotic adjustment was present in all four Quercus species, the molecular processes underpinning this response differed according to their phylogenetic history and specific ecology.

Rising [CO2] effect on leaf drought-induced metabolome in Pinus pinaster Aiton: Ontogenetic- and genotypic-specific response exhibit different metabolic strategies.

Rising atmospheric CO2 concentrations ([CO2]) together with water deficit can influence ecological interactions of trees through an array of chemically driven changes in plant leaves. In four drought stressed Pinus pinaster genotypes, grown under two levels of atmospheric [CO2] (ambient (aCO2) and enriched (eCO2)) the metabolome of adult and juvenile needles was analyzed to know if the metabolic responses to this environmental situation could be genotype-dependent and vary according to the stage of needle ontogeny. Drought had the highest incidence, followed by needle ontogeny, being lower the eCO2 effect. The eCO2 reduced, eliminated or countered the 50 (adult needles) - 44% (juvenile) of the drought-induced changes, suggesting that CO2-enriched plants could perceived less oxidative stress under drought, and proving that together, these two abiotic factors triggered a metabolic response different from that under single factors. Genotype drought tolerance and ontogenetic stage determined the level of metabolite accumulation and the plasticity to eCO2 under drought, which was mainly reflected in antioxidant levels and tree chemical defense. At re-watering, previously water stressed plants showed both, reduced C and N metabolism, and a "drought memory effect", favoring antioxidants and osmolyte storage. This effect showed variations regarding genotype drought-tolerance, needle ontogeny and [CO2], with remarkable contribution of terpenoids. Chemical defense and drought tolerance were somehow linked, increasing chemical defense during recovery in the most drought-sensitive individuals. The better adaptation of trees to drought under eCO2, as well as their ability to recover better from water stress, are essential for the survival of forest trees.

Correlated evolution of morphology, gas exchange, growth rates and hydraulics as a response to precipitation and temperature regimes in oaks (Quercus).

It is hypothesised that tree distributions in Europe are largely limited by their ability to cope with the summer drought imposed by the Mediterranean climate in the southern areas and by their competitive potential in central regions with more mesic conditions. We investigated the extent to which leaf and plant morphology, gas exchange, leaf and stem hydraulics and growth rates have evolved in a coordinated way in oaks (Quercus) as a result of adaptation to contrasting environmental conditions in this region. We implemented an experiment in which seedlings of 12 European/North African oaks were grown under two watering treatments, a well-watered treatment and a drought treatment in which plants were subjected to three cycles of drought. Consistent with our hypothesis, species from drier summers had traits conferring more tolerance to drought such as small sclerophyllous leaves and lower percent loss of hydraulic conductivity. However, these species did not have lower growth rates as expected by a trade-off with drought tolerance. Overall, our results revealed that climate is an important driver of functional strategies in oaks and that traits have evolved along two coordinated functional axes to adapt to different precipitation and temperature regimes.

Leaf ecophysiological and metabolic response in Quercus pyrenaica Willd seedlings to moderate drought under enriched CO2 atmosphere.

Impact of drought under enriched CO2 atmosphere on ecophysiological and leaf metabolic response of the sub-mediterranean Q. pyrenaica oak was studied. Seedlings growing in climate chamber were submitted to moderate drought (WS) and well-watered (WW) under ambient ([CO2]amb =400 ppm) or CO2 enriched atmosphere ([CO2]enr =800 ppm). The moderate drought endured by seedlings brought about a decrease in leaf gas exchange. However, net photosynthesis (Anet) was highly stimulated for plants at [CO2]enr. There was a decrease of the stomatal conductance to water vapour (gwv) in response to drought, and a subtle trend to be lower under [CO2]enr. The consequence of these changes was an important increase in the intrinsic leaf water use efficiency (WUEi). The electron transport rate (ETR) was almost a 20 percent higher in plants at [CO2]enr regardless drought endured by seedlings. The ETR/Anet was lower under [CO2]enr, pointing to a high capacity to maintain sinks for the uptake of extra carbon in the atmosphere. Impact of drought on the leaf metabolome, as a whole, was more evident than that from [CO2] enrichment of the atmosphere. Changes in pool of non-structural carbohydrates were observed mainly as a consequence of water deficit including increases of fructose, glucose, and proto-quercitol. Most of the metabolites affected by drought back up to levels of non-stressed seedlings after rewetting (recovery phase). It can be concluded that carbon uptake was stimulated by [CO2]enr, even under the stomatal closure that accompanied moderate drought. In the last, there was a positive effect in intrinsic water use efficiency (WUEi), which was much more improved under [CO2]enr. Leaf metabolome was little responsible and some few metabolites changed mainly in response to drought, with little differences between [CO2] growth conditions.

Increased root investment can explain the higher survival of seedlings of 'mesic' Quercus suber than 'xeric' Quercus ilex in sandy soils during a summer drought.

In Mediterranean-type ecosystems, drought is considered the main ecological filter for seedling establishment. The evergreen oaks Quercus ilex L. and Quercus suber L. are two of the most abundant tree species in the Mediterranean Basin. Despite their shared evergreen leaf habit and ability to resist low soil water potentials, traditionally it has been suggested that Q. ilex is better suited to resist dry conditions than Q. suber. In this study, we examined how seedlings of Q. ilex and Q. suber grown in sandy soils responded to different levels of water availability using natural dry conditions and supplemental watering. Specifically, we estimated survival and water status of seedlings and explored the role of acorn mass and belowground biomass in seedling performance. To our surprise, Q. suber was better able to survive the summer drought in our experiment than Q. ilex. Nearly 55% of the Q. suber seedlings remained alive after a 2-month period without rain or supplemental water, which represents almost 20% higher survival than Q. ilex over the same period. At the end of the dry period, the surviving seedlings of Q. suber had strikingly higher water potential, potential maximum quantum yield of photosystem II (Fv/Fm) and stomatal conductance (gs) than those of Q. ilex. Acorn mass was associated with the probability of survival under dry conditions; however, it did not explain the differences in survival or water status between the species. In contrast, Q. suber had a higher root ratio and root:shoot ratio than Q. ilex and these traits were positively associated with predawn leaf water potential, Fv/Fm, gs and survival. Taken together, our results suggest that the higher relative investment in roots by Q. suber when growing in a sandy acidic substrate allowed this species to maintain better physiological status and overall condition than Q. ilex, increasing its probability of survival in dry conditions.

Metabolic response to elevated CO2 levels in Pinus pinaster Aiton needles in an ontogenetic and genotypic-dependent way.

Global climate changes involve elevated atmospheric [CO2], fostering the carbon allocation to tree sink tissues, partitioning it into metabolic pathways. We use metabolomics analysis in adult and juvenile needles of four Pinus pinaster genotypes exposed to two levels of growth [CO2]: ambient (400 µmol mol-1) and enriched (800 µmol mol-1), to know if the metabolic responses are genotype-dependent and vary according to the stage of needle ontogeny. The eCO2-induced changes in the needle metabolomes are more significant in secondary metabolism pathways and especially meaningful in juvenile needles. The heteroblasty has important consequences in the expression of the metabolome, and on the plasticity to CO2, determining the level of specific metabolite accumulation, showing an interdependence between adult and juvenile needles. The P. pinaster needle metabolomes also show clear quantitative differences linked to genotype, as well as regarding the metabolic response to eCO2, showing both, common and genotype-specific biochemical responses. Thus, the changes in flavonol levels are mainly genotype-independent, while those in terpenoid and free fatty acids are mainly genotype-dependent, ratifying the importance of genotype to determine the metabolic response to eCO2. To understand the adaptation mechanisms that tree species can develop to cope with eCO2 it is necessary to know the genetically distinct responses within a species to recognize the CO2-induced changes from the divergent approaches, what can facilitate knowing also the possible interrelation of the physiological and metabolic responses. That could explain the controversial effects of eCO2 on the carbon-based metabolite in conifers, at the inter- and intra-specific level.

Divergent phenological and leaf gas exchange strategies of two competing tree species drive contrasting responses to drought at their altitudinal boundary.

In Mediterranean mountains, Pinus sylvestris L. is expected to be displaced under a warming climate by more drought-tolerant species such as the sub-Mediterranean Quercus pyrenaica Willd. Understanding how environmental factors drive tree physiology and phenology is, therefore, essential to assess the effect of changing climatic conditions on the performance of these species and, ultimately, their distribution. We compared the cambial and leaf phenology and leaf gas exchange of Q. pyrenaica and P. sylvestris at their altitudinal boundary in Central Spain and assessed the environmental variables involved. Results indicate that P. sylvestris cambial phenology was more sensitive to weather conditions (temperature at the onset and water deficit at the end of the growing season) than Q. pyrenaica. On the other hand, Q. pyrenaica cambial and leaf phenology were synchronized and driven by photoperiod and temperatures. Pinus sylvestris showed lower photosynthetic nitrogen-use efficiency and higher intrinsic water-use efficiency than Q. pyrenaica as a result of a tighter stomatal control in response to summer dry conditions, despite its less negative midday leaf water potentials. These phenological and leaf gas exchange responses evidence a stronger sensitivity to drought of P. sylvestris than that of Q. pyrenaica, which may therefore hold a competitive advantage over P. sylvestris under the predicted increase in recurrence and intensity of drought events. On the other hand, both species could benefit from warmer springs through an advanced phenology, although this effect could be limited in Q. pyrenaica if it maintains a photoperiod control over the onset of xylogenesis.

Extreme droughts affecting Mediterranean tree species' growth and water-use efficiency: the importance of timing.

It has been known for a long time that drought intensity is a critical variable in determining water stress of Mediterranean tree species. However, not as much attention has been paid to other drought characteristics, for example the timing of the dry periods. We investigated the impact of the timing and intensity of extreme droughts on growing season length, growth and water-use efficiency of three tree species, Pinus nigra ssp. Salzmannii J.F. Arnold, Quercus ilex ssp. ballota (Desf.) Samp. and Quercus faginea Lam. coexisting in a continental Mediterranean ecosystem. Over the study period (2009-13), intense droughts were observed at annual and seasonal scales, particularly during 2011 and 2012. In 2012, an atypically dry winter and spring was followed by an intense summer drought. Quercus faginea growth was affected more by drought timing than by drought intensity, probably because of its winter-deciduous leaf habit. Pinus nigra showed a lower decrease in secondary growth than observed in the two Quercus species in extremely dry years. Resilience to extreme droughts was different among species, with Q. faginea showing poorer recovery of growth after very dry years. The highest intra- and inter-annual plasticity in water-use efficiency was observed in P. nigra, which maintained a more water-saving strategy. Our results revealed that the timing of extreme drought events can affect tree function to a larger extent than drought intensity, especially in deciduous species. Legacy effects of drought over months and years significantly strengthened the impact of drought timing and intensity on tree function.

Inter-genotypic differences in drought tolerance of maritime pine are modified by elevated [CO2].

Background and Aims: Despite the importance of growth [CO 2 ] and water availability for tree growth and survival, little information is available on how the interplay of these two factors can shape intraspecific patterns of functional variation in tree species, particularly for conifers. The main objective of the study was to test whether the range of realized drought tolerance within the species can be affected by elevated [CO 2 ]. Methods: Intraspecific variability in leaf gas exchange, growth rate and other leaf functional traits were studied in clones of maritime pine. A factorial experiment including water availability, growth [CO 2 ] and four different genotypes was conducted in growth rooms. A 'water deficit' treatment was imposed by applying a cycle of progressive soil water depletion and recovery at two levels of growth [CO 2 ]: 'ambient [CO 2 ]' (aCO 2 400 µmol mol -1 ) and 'elevated [CO 2 ]' (eCO 2 800 µmol mol -1 ). Key Results: eCO2 had a neutral effect on the impact of drought on growth and leaf gas exchange of the most drought-sensitive genotypes while it aggravated the impact of drought on the most drought-tolerant genotypes at aCO2. Thus, eCO2 attenuated genotypic differences in drought tolerance as compared with those observed at aCO2. Genotypic variation at both levels of growth [CO2] was found in specific leaf area and leaf nitrogen content but not in other physiological leaf traits such as intrinsic water use efficiency and leaf osmotic potential. eCO2 increased Δ 13 C but had no significant effect on δ 18 O. This effect did not interact with the impact of drought, which increased δ 18 O and decreased Δ 13 C. Nevertheless, correlations between Δ 13 C and δ 18 O indicated the non-stomatal component of water use efficiency in this species can be particularly sensitive to drought. Conclusions: Evidence from this study suggests elevated [CO 2 ] can modify current ranges of drought tolerance within tree species.

Xylem and Leaf Functional Adjustments to Drought in Pinus sylvestris and Quercus pyrenaica at Their Elevational Boundary.

Climatic scenarios for the Mediterranean region forecast increasing frequency and intensity of drought events. Consequently, a reduction in Pinus sylvestris L. distribution range is projected within the region, with this species being outcompeted at lower elevations by more drought-tolerant taxa such as Quercus pyrenaica Willd. The functional response of these species to the projected shifts in water availability will partially determine their performance and, thus, their competitive success under these changing climatic conditions. We studied how the cambial and leaf phenology and xylem anatomy of these two species responded to a 3-year rainfall exclusion experiment set at their elevational boundary in Central Spain. Additionally, P. sylvestris leaf gas exchange, water potential and carbon isotope content response to the treatment were measured. Likewise, we assessed inter-annual variability in the studied functional traits under control and rainfall exclusion conditions. Prolonged exposure to drier conditions did not affect the onset of xylogenesis in either of the studied species, whereas xylem formation ceased 1-3 weeks earlier in P. sylvestris. The rainfall exclusion had, however, no effect on leaf phenology on either species, which suggests that cambial phenology is more sensitive to drought than leaf phenology. P. sylvestris formed fewer, but larger tracheids under dry conditions and reduced the proportion of latewood in the tree ring. On the other hand, Q. pyrenaica did not suffer earlywood hydraulic diameter changes under rainfall exclusion, but experienced a cumulative reduction in latewood width, which could ultimately challenge its hydraulic performance. The phenological and anatomical response of the studied species to drought is consistent with a shift in resource allocation under drought stress from xylem to other sinks. Additionally, the tighter stomatal control and higher intrinsic water use efficiency observed in drought-stressed P. sylvestris may eventually limit carbon uptake in this species. Our results suggest that both species are potentially vulnerable to the forecasted increase in drought stress, although P. sylvestris might experience a higher risk of drought-induced decline at its low elevational limit.

Intra-population variability in the drought response of a beech (Fagus sylvatica L.) population in the southwest of Europe.

Phenotypic variability within forest species populations is considered of special relevance for local adaptation under new environments, albeit it has been analyzed to a lesser extent than inter-population phenotypic variability. A common garden study was carried out to assess phenotypic variability in response to water stress in half-sibling families from a marginal population of Fagus sylvatica L. at its south-western range edge distribution in Europe. Two irrigation regimes were applied, well-watered (WW) seedlings and those submitted to weekly cycles of drying-rewatering of growth media. Seedling growth and their leaf functional traits were recorded during the last cycle of water stress. Most of the phenotypic changes were explained by phenotypic plasticity in response to water stress, but there was also a significant effect of family in the expression of some of the studied traits. The relationship of carbon isotope fractioning with gas exchange traits across families under WW conditions did not follow the same pattern as the phenotypic trends. The leaf net photosynthesis across families was modified by the nitrogen content on a leaf mass basis that was in turn correlated positively with leaf nitrogen isotope fractionation. The results point to an important role of leaf nitrogen in determining the intrinsic water-use efficiency (WUE) across families. Variation in WUE was ruled mainly by control of stomatal conductance to water vapor under water stress, but by leaf net photosynthesis under wet conditions. Relatively high inter-family phenotypic variability in growth and functional traits were observed. Within-population phenotypic variability, and the plasticity of some of the studied traits, is of fundamental importance to cope with the harsher environments beech will have to endure in the future at different points in its distribution range.

Drought-induced shoot dieback starts with massive root xylem embolism and variable depletion of nonstructural carbohydrates in seedlings of two tree species.

Combining hydraulic- and carbon-related measurements helps to understand drought-induced plant mortality. Here, we investigated the role that plant respiration (R) plays in determining carbon budgets under drought. We measured the hydraulic conductivity of stems and roots, and gas exchange and nonstructural carbohydrate (NSC) concentrations of leaves, stems and roots of seedlings of two resprouting species exposed to drought or well-watered conditions: Ulmus minor (riparian tree) and Quercus ilex (dryland tree). With increasing water stress (occurring more rapidly in larger U. minor), declines in leaf, stem and root R were less pronounced than that in leaf net photosynthetic CO2 uptake (Pn ). Daytime whole-plant carbon gain was negative below -4 and -6 MPa midday xylem water potential in U. minor and Q. ilex, respectively. Relative to controls, seedlings exhibiting shoot dieback suffered c. 80% loss of hydraulic conductivity in both species, and reductions in NSC concentrations in U. minor. Higher drought-induced depletion of NSC reserves in U. minor was related to higher plant R, faster stomatal closure, and premature leaf-shedding. Differences in drought resistance relied on the ability to maintain hydraulic conductivity during drought, rather than tolerating conductivity loss. Root hydraulic failure elicited shoot dieback and precluded resprouting without root NSC reserves being apparently limiting for R.

Organ-specific metabolic responses to drought in Pinus pinaster Ait.

Drought is an important driver of plant survival, growth, and distribution. Water deficit affects different pathways of metabolism, depending on plant organ. While previous studies have mainly focused on the metabolic drought response of a single organ, analysis of metabolic differences between organs is essential to achieve an integrated understanding of the whole plant response. In this work, untargeted metabolic profiling was used to examine the response of roots, stems, adult and juvenile needles from Pinus pinaster Ait. full-sib individuals, subjected to a moderate and long lasting drought period. Cyclitols content showed a significant alteration, in response to drought in all organs examined, but other metabolites increased or decreased differentially depending on the analyzed organ. While a high number of flavonoids were only detected in aerial organs, an induction of the glutathione pathway was mainly detected in roots. This result may reflect different antioxidant mechanisms activated in aerial organs and roots. Metabolic changes were more remarkable in roots than in the other organs, highlighting its prominent role in the response to water stress. Significant changes in flavonoids and ascorbate metabolism were also observed between adult and juvenile needles, consistent with previously proven differential functional responses between the two developmental stages. Genetic polymorphisms in candidate genes coding for a Myb1 transcription factor and a malate dehydrogenase (EC 1.1.1.37) were associated with different concentration of phenylalanine, phenylpropanoids and malate, respectively. The results obtained will support further research on metabolites and genes potentially involved in functional mechanisms related to drought tolerance in trees.

Nucleotide polymorphisms in a pine ortholog of the Arabidopsis degrading enzyme cellulase KORRIGAN are associated with early growth performance in Pinus pinaster.

We have carried out a candidate-gene-based association genetic study in Pinus pinaster Aiton and evaluated the predictive performance for genetic merit gain of the most significantly associated genes and single nucleotide polymorphisms (SNPs). We used a second generation 384-SNP array enriched with candidate genes for growth and wood properties to genotype mother trees collected in 20 natural populations covering most of the European distribution of the species. Phenotypic data for total height, polycyclism, root-collar diameter and biomass were obtained from a replicated provenance-progeny trial located in two sites with contrasting environments (Atlantic vs Mediterranean climate). General linear models identified strong associations between growth traits (total height and polycyclism) and four SNPs from the korrigan candidate gene, after multiple testing corrections using false discovery rate. The combined genomic breeding value predictions assessed for the four associated korrigan SNPs by ridge regression-best linear unbiased prediction (RR-BLUP) and cross-validation accounted for up to 8 and 15% of the phenotypic variance for height and polycyclic growth, respectively, and did not improve adding SNPs from other growth-related candidate genes. For root-collar diameter and total biomass, they accounted for 1.6 and 1.1% of the phenotypic variance, respectively, but increased to 15 and 4.1% when other SNPs from lp3.1, lp3.3 and cad were included in RR-BLUP models. These results point towards a desirable integration of candidate-gene studies as a means to pre-select relevant markers, and aid genomic selection in maritime pine breeding programs.