negatively), indicating a strong influence of canopy properties and stomatal conductance on above-ground growth at the population level. Specifically, a combination of vegetation indices and canopy temperature accounted for about 60% of population variability in stem volume of adult trees. This is the first study to propose UAV remote sensing as an effective tool for screening genetic variation in morphophysiological traits of adult forest trees.
Progress in high-throughput phenotyping and genomics provides the potential to understand the genetic basis of plant functional differentiation. We developed a semi-automatic methodology based on unmanned aerial vehicle (UAV) imagery for deriving tree-level phenotypes followed by genome-wide association study (GWAS). An RGB-based point cloud was used for tree crown identification in a common garden of Pinus halepensis in Spain. Crowns were combined with multispectral and thermal orthomosaics to retrieve growth traits, vegetation indices and canopy temperature. Thereafter, GWAS was performed to analyse the association between phenotypes and genomic variation at 235 single nucleotide polymorphisms (SNPs). Growth traits were associated with 12 SNPs involved in cellulose and carbohydrate metabolism. Indices related to transpiration and leaf water content were associated with six SNPs involved in stomata dynamics. Indices related to leaf pigments and leaf area were associated with 11 SNPs involved in signalling and peroxisome metabolism. About 16-20% of trait variance was explained by combinations of several SNPs, indicating polygenic control of morpho-physiological traits. Despite a limited availability of markers and individuals, this study is provides a successful proof-of-concept for the combination of high-throughput UAV-based phenotyping with cost-effective genotyping to disentangle the genetic architecture of phenotypic variation in a widespread conifer.
Background and Aims
Understanding inter-population variation in the allocation of resources to specific anatomical compartments and physiological processes is crucial to disentangle adaptive patterns in forest species. This work aims to evaluate phenotypic integration and trade-offs among functional traits as determinants of life history strategies in populations of a circum-Mediterranean pine that dwells in environments where water and other resources are in limited supply.
Methods
Adult individuals of 51 populations of Pinus halepensis grown in a common garden were characterized for 11 phenotypic traits, including direct and indirect measures of water uptake at different depths, leaf area, stomatal conductance, chlorophyll content, non-structural carbohydrates, stem diameter and tree height, age at first reproduction and cone production. The population differentiation in these traits was tested through analysis of variance (ANOVA). The resulting populations’ means were carried forward to a structural equation model evaluating phenotypic integration between six latent variables (summer water uptake depth, summer transpiration, spring photosynthetic capacity, growth, reserve accumulation and reproduction).
Key Results
Water uptake depth and transpiration covaried negatively among populations, as the likely result of a common selective pressure for drought resistance, while spring photosynthetic capacity was lower in populations originating from dry areas. Transpiration positively influenced growth, while growth was negatively related to reproduction and reserves among populations. Water uptake depth negatively influenced reproduction.
Conclusions
The observed patterns indicate a differentiation in life cycle features between fast-growing and slow-growing populations, with the latter investing significantly more in reproduction and reserves. We speculate that such contrasting strategies result from different arrays of life history traits underlying the very different ecological conditions that the Aleppo pine must face across its distribution range. These comprise, principally, drought as the main stressor and fire as the main ecological disturbance of the Mediterranean basin.
Water and carbon fluxes in forests are largely related to leaf gas exchange physiology 27 varying across spatiotemporal scales and modulated by plant responses to environmental cues. 28 We quantified the relevance of genetic and phenotypic variation of intrinsic water-use 29 efficiency (WUEi, ratio of net photosynthesis to stomatal conductance of water) in Pinus 30 sylvestris L. growing in the Iberian Peninsula as inferred from tree-ring carbon isotopes. Inter-31 population genetic variation, evaluated in a provenance trial comprising Spanish and German 32 populations, was low and relevant only at continental scale. In contrast, phenotypic variation, 33 evaluated in natural stands (at spatial level) and by tree-ring chronologies (at temporal inter-34 annual level), was important and ten-and threefold larger than the population genetic 35 variance, respectively. These results points to preponderance of plastic responses dominating 36 variability in WUEi for this species. Spatial phenotypic variation in WUEi correlated 37 negatively with soil depth (r=-0.66; p<0.01), while temporal phenotypic variation was mainly 38 driven by summer precipitation. At the spatial level, WUEi could be scaled-up to ecosystem-39 level WUE derived from remote sensing data by accounting for soil water holding capacity 40 (r= 0.63; p<0.01). This outcome demonstrates a direct influence of the variation of leaf-level 41 WUEi on ecosystem water and carbon balance differentiation. Our findings highlight the 42 contrasting importance of genetic variation (negligible) and plastic responses in WUEi (large, 43 with changes of up to 33% among sites) on determining carbon and water budgets at stand 44 and ecosystem scales in a widespread conifer such as Pinus sylvestris.
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