Contrasting physiological strategies explain heterogeneous responses to severe drought conditions within local populations of a widespread conifer.

Understanding how trees prioritize carbon gain at the cost of drought vulnerability under severe drought conditions is crucial for predicting which genetic groups and individuals will be resilient to future climate conditions. In this study, we investigated variations in growth, tree-ring anatomy as well as carbon and oxygen isotope ratios to assess the sensitivity and the xylem formation process in response to an episode of severe drought in 29 mature white spruce (Picea glauca [Moench] Voss) families grown in a common garden trial. During the drought episode, the majority of families displayed decreased growth and exhibited either sustained or increased intrinsic water-use efficiency (iWUE), which was largely influenced by reduced stomatal conductance as revealed by the dual carbon­oxygen isotope approach. Different water-use strategies were detected within white spruce populations in response to drought conditions. Our results revealed intraspecific variation in the prevailing physiological mechanisms underlying drought response within and among populations of Picea glauca. The presence of different genetic groups reflecting diverse water-use strategies within this largely-distributed conifer is likely to lessen the negative effects of drought and decrease the overall forest ecosystems' sensitivity to it.

Breeding for adaptation to climate change: genomic selection for drought response in a white spruce multi-site polycross test.

With climate change, increasingly intense and frequent drought episodes will be affecting water availability for boreal tree species, prompting tree breeders and forest managers to consider adaptation to drought stress as a priority in their reforestation efforts. We used a 19-year-old polycross progeny test of the model conifer white spruce (Picea glauca) replicated on two sites affected by distinct drought episodes at different ages to estimate the genetic control and the potential for improvement of drought response in addition to conventional cumulative growth and wood quality traits. Drought response components were measured from dendrochronological signatures matching drought episodes in wood ring increment cores. We found that trees with more vigorous growth during their lifespan resisted better during the current year of a drought episode when the drought had more severe effects. Phenotypic data were also analyzed using genomic prediction (GBLUP) relying on the genomic relationship matrix of multi-locus gene SNP marker information, and conventional analysis (ABLUP) based on validated pedigree information. The accuracy of predicted breeding values for drought response components was marginally lower than that for conventional traits and comparable between GBLUP and ABLUP. Genetic correlations were generally low and nonsignificant between drought response components and conventional traits, except for resistance which was positively correlated to tree height. Heritability estimates for the components of drought response were slightly lower than for conventional traits, but similar single-trait genetic gains could be obtained. Multi-trait genomic selection simulations indicated that it was possible to improve simultaneously for all traits on both sites while sacrificing little on gain in tree height. In a context of rapid climate change, our results suggest that with careful phenotypic assessment, drought response may be considered in multi-trait improvement of white spruce, with accelerated screening of large numbers of candidates and selection at young age with genomic selection.

Combining QTL Mapping and Transcriptomics to Decipher the Genetic Architecture of Phenolic Compounds Metabolism in the Conifer White Spruce.

Conifer forests worldwide are becoming increasingly vulnerable to the effects of climate change. Although the production of phenolic compounds (PCs) has been shown to be modulated by biotic and abiotic stresses, the genetic basis underlying the variation in their constitutive production level remains poorly documented in conifers. We used QTL mapping and RNA-Seq to explore the complex polygenic network underlying the constitutive production of PCs in a white spruce (Picea glauca) full-sib family for 2 years. QTL detection was performed for nine PCs and differentially expressed genes (DEGs) were identified between individuals with high and low PC contents for five PCs exhibiting stable QTLs across time. A total of 17 QTLs were detected for eight metabolites, including one major QTL explaining up to 91.3% of the neolignan-2 variance. The RNA-Seq analysis highlighted 50 DEGs associated with phenylpropanoid biosynthesis, several key transcription factors, and a subset of 137 genes showing opposite expression patterns in individuals with high levels of the flavonoids gallocatechin and taxifolin glucoside. A total of 19 DEGs co-localized with QTLs. Our findings represent a significant step toward resolving the genomic architecture of PC production in spruce and facilitate the functional characterization of genes and transcriptional networks responsible for differences in constitutive production of PCs in conifers.

Connecting tree-ring phenotypes, genetic associations and transcriptomics to decipher the genomic architecture of drought adaptation in a widespread conifer.

As boreal forests face significant threats from climate change, understanding evolutionary trajectories of coniferous species has become fundamental to adapting management and conservation to a drying climate. We examined the genomic architecture underlying adaptive variation related to drought tolerance in 43 populations of a widespread boreal conifer, white spruce (Picea glauca [Moench] Voss), by combining genotype-environment associations, genotype-phenotype associations, and transcriptomics. Adaptive genetic variation was identified by correlating allele frequencies for 6,153 single nucleotide polymorphisms from 2,606 candidate genes with temperature, precipitation and aridity gradients, and testing for significant associations between genotypes and 11 dendrometric and drought-related traits (i.e., anatomical, growth response and climate-sensitivity traits) using a polygenic model. We identified a set of 285 genes significantly associated with a climatic factor or a phenotypic trait, including 110 that were differentially expressed in response to drought under greenhouse-controlled conditions. The interlinked phenotype-genotype-environment network revealed eight high-confidence genes involved in white spruce adaptation to drought, of which four were drought-responsive in the expression analysis. Our findings represent a significant step toward the characterization of the genomic basis of drought tolerance and adaptation to climate in conifers, which is essential to enable the establishment of resilient forests in view of new climate conditions.

Strong overestimation of water-use efficiency responses to rising CO2 in tree-ring studies.

The carbon isotope ratio (δ13 C) in tree rings is commonly used to derive estimates of the assimilation-to-stomatal conductance rate of trees, that is, intrinsic water-use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO2 concentrations (Ca ), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree-ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising Ca . We found that increases in tree size and stand age greatly influenced iWUE. The effect of Ca on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to Ca , mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO2 effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO2 effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.

Adaptive genetic variation to drought in a widely distributed conifer suggests a potential for increasing forest resilience in a drying climate.

Drought intensity and frequency are increasing under global warming, with soil water availability now being a major factor limiting tree growth in circumboreal forests. Still, the adaptive capacity of trees in the face of future climatic regimes remains poorly documented. Using 1481 annually resolved tree-ring series from 29-yr-old trees, we evaluated the drought sensitivity of 43 white spruce (Picea glauca (Moench) Voss) populations established in a common garden experiment. We show that genetic variation among populations in response to drought plays a significant role in growth resilience. Local genetic adaptation allowed populations from drier geographical origins to grow better, as indicated by higher resilience to extreme drought events, compared with populations from more humid geographical origins. The substantial genetic variation found for growth resilience highlights the possibility of selecting for drought resilience in boreal conifers. As a major research outcome, we showed that adaptive genetic variation in response to changing local conditions can shape drought vulnerability at the intraspecific level. Our findings have wide implications for forest ecosystem management and conservation.

Tree rings provide a new class of phenotypes for genetic associations that foster insights into adaptation of conifers to climate change.

Local adaptation in tree species has been documented through a long history of common garden experiments where functional traits (height, bud phenology) are used as proxies for fitness. However, the ability to identify genes or genomic regions related to adaptation to climate requires the evaluation of traits that precisely reflect how and when climate exerts selective constraints. We combine dendroecology with association genetics to establish a link between genotypes, phenotypes and interannual climatic fluctuations. We illustrate this approach by examining individual tree responses embedded in the annual rings of 233 Pinus strobus trees growing in a common garden experiment representing 38 populations from the majority of its range. We found that interannual variability in growth was affected by low temperatures during spring and autumn, and by summer heat and drought. Among-population variation in climatic sensitivity was significantly correlated with the mean annual temperature of the provenance, suggesting local adaptation. Genotype-phenotype associations using these new tree-ring phenotypes validated nine candidate genes identified in a previous genetic-environment association study. Combining dendroecology with association genetics allowed us to assess tree vulnerability to past climate at fine temporal scales and provides avenues for future genomic studies on functional adaptation in forest trees.

Sawdust and Bark-Based Substrates for Soilless Strawberry Production: Irrigation and Electrical Conductivity Management.

The objective of this work was to optimize a soilless growing system for producing bare-root strawberry transplants in three organic substrates. Three trials were conducted in the Quebec City area to determine the productivity potential of a peat-sawdust mixture (PS25) and an aged bark (AB) material compared to conventional coconut fiber (CF) substrate. A first experiment was carried out to define appropriate irrigation set points for each substrate that allowed optimal plant growth and fruit yields. For all substrates, wetter conditions (irrigation started at -1.0 kPa for CF; -1.5 kPa for AB and PS25, relative to -1.5 kPa for CF; -2.5 kPa for AB and PS25) enhanced plant growth and fruit production. The second trial was carried out to test the productivity potential for commercial production of the three substrates using high-tunnels. After the addition of an initial fertilizer application to PS25, we successfully established bare-root plants that gave similar fruit yields than those in CF and AB. The productivity potential of PS25 and AB were further confirmed during a third trial under greenhouse conditions. The critical factor for plant establishment in PS25 was attributed to consistent N, P and S immobilization by microorganisms, as well as the retention of other elements (Mg2+, K+) in the growth media. Taken together, our results showed that PS25 and AB are promising alternative substrates to coconut coir dust for strawberry cultivation. This paper also provides a useful guide for strawberry cultivation in Quebec, and suggests future research that might be conducted to optimize soilless systems for cold-climate strawberry production in Northern America.

Short-term response to waterlogging in Quercus petraea and Quercus robur: A study of the root hydraulic responses and the transcriptional pattern of aquaporins.

We characterized the short-term response to waterlogging in Quercus petraea (Matt.) Liebl. and Quercus robur L. as the initial response towards their known long-term differences in tolerance to waterlogging. One-month old seedlings were subjected to hypoxic stress and leaf gas exchange, shoot water potential (Ψs) and root hydraulic conductivity (Lpr) were measured. In parallel, the expression of nine aquaporins (AQPs) along the primary root was analysed by quantitative RT-PCR. Results showed a similar reduction in net assimilation (A) and stomatal conductance (gs) for the two species. Notably, the response of Lpr differed temporally between the two species. Q. robur seedlings exhibited a significant early decline of Lpr within the first 5 h that returned to control levels after 48 h, whereas Q. petraea seedlings showed a delayed response with a significant decrease of Lpr exhibited only after 48 h. Transcriptional profiling revealed that three genes (PIP1;3, TIP2;1 and TIP2;2) were differentially regulated under stress conditions in the two oak species. Taken together, these results suggested species-specific responses to short-term waterlogging in terms of root water transport.

Identification and expression of nine oak aquaporin genes in the primary root axis of two oak species, Quercus petraea and Quercus robur.

Aquaporins (AQPs) belong to the Major Intrinsic Protein family that conducts water and other small solutes across biological membranes. This study aimed to identify and characterize AQP genes in the primary root axis of two oak species, Quercus petraea and Quercus robur. Nine putative AQP genes were cloned, and their expression was profiled in different developmental root zones by real-time PCR. A detailed examination of the predicted amino acid sequences and subsequent phylogenetic analysis showed that the isolated AQPs could be divided into two subfamilies, which included six plasma membrane intrinsic proteins (PIPs) and three tonoplast intrinsic proteins (TIPs). We characterized the anatomical features of the roots and defined three developmental root zones: the immature, transition and mature zones. Expression analysis of the AQPs was performed according to these root developmental stages. Our results showed that the expression of PIP2;3 and TIP1 was significantly higher in Quercus petraea compared with Quercus robur in the three root zones. However, PIP2;1 and TIP2;1 were found to be differentially expressed in the mature zone of the two oak species. Of the nine AQP genes identified and analyzed, we highlighted four genes that might facilitate a deeper understanding of how these two closely related tree species adapted to different environments.

Hydraulic efficiency and coordination with xylem resistance to cavitation, leaf function, and growth performance among eight unrelated Populus deltoidesxPopulus nigra hybrids.

Tests were carried out to determine whether variations in the hydraulic architecture of eight Populus deltoides×Populus nigra genotypes could be related to variations in leaf function and growth performance. Measurements were performed in a coppice plantation on 1-year-old shoots under optimal irrigation. Hydraulic architecture was characterized through estimates of hydraulic efficiency (the ratio of conducting sapwood area to leaf area, A(X):A(L); leaf- and xylem-specific hydraulic conductance of defoliated shoots, k(SL) and k(SS), respectively; apparent whole-plant leaf-specific hydraulic conductance, k(plant)) and xylem safety (water potential inducing 50% loss in hydraulic conductance). The eight genotypes spanned a significant range of k(SL) from 2.63 kg s(-1) m(-2) MPa(-1) to 4.18 kg s(-1) m(-2) MPa(-1), variations being mostly driven by k(SS) rather than A(X):A(L). There was a strong trade-off between hydraulic efficiency and xylem safety. Values of k(SL) correlated positively with k(plant), indicating that high-pressure flowmeter (HPFM) measurements of stem hydraulic efficiency accurately reflected whole-plant water transport efficiency of field-grown plants at maximum transpiration rate. No clear relationship could be found between hydraulic efficiency and either net CO(2) assimilation rates, water-use efficiency estimates (intrinsic water-use efficiency and carbon isotope discrimination against (13)C), or stomatal characteristics (stomatal density and stomatal pore area index). Estimates of hydraulic efficiency were negatively associated with relative growth rate. This unusual pattern, combined with the trade-off observed between hydraulic efficiency and xylem safety, provides the rationale for the positive link already reported between relative growth rate and xylem safety among the same eight P. deltoides×P. nigra genotypes.