Abstract
Poplars are fast-growing, high-yielding forest tree species, whose cultivation as second-generation biofuel crops is of increasing interest and can efficiently meet emission reduction goals. Yet, breeding elite poplar trees for drought resistance remains a major challenge. Worldwide breeding programs are largely focused on intra/interspecific hybridization, whereby Populus nigra L. is a fundamental parental pool. While high-throughput genotyping has resulted in unprecedented capabilities to rapidly decode complex genetic architecture of plant stress resistance, linking genomics to phenomics is hindered by technically challenging phenotyping. Relying on unmanned aerial vehicle (UAV)-based remote sensing and imaging techniques, high-throughput field phenotyping (HTFP) aims at enabling highly precise and efficient, non-destructive screening of genotype performance in large populations. To efficiently support forest-tree breeding programs, ground-truthing observations should be complemented with standardized HTFP. In this study, we develop a high-resolution (leaf level) HTFP approach to investigate the response to drought of a full-sib F2 partially inbred population (termed here ‘POP6’), whose F1 was obtained from an intraspecific P. nigra controlled cross between genotypes with highly divergent phenotypes. We assessed the effects of two water treatments (well-watered and moderate drought) on a population of 4603 trees (503 genotypes) hosted in two adjacent experimental plots (1.67 ha) by conducting low-elevation (25 m) flights with an aerial drone and capturing 7836 thermal infrared (TIR) images. TIR images were undistorted, georeferenced, and orthorectified to obtain radiometric mosaics. Canopy temperature (Tc) was extracted using two independent semi-automated segmentation techniques, eCognition- and Matlab-based, to avoid the mixed-pixel problem. Overall, results showed that the UAV platform-based thermal imaging enables to effectively assess genotype variability under drought stress conditions. Tc derived from aerial thermal imagery presented a good correlation with ground-truth stomatal conductance (gs) in both segmentation techniques. Interestingly, the HTFP approach was instrumental to detect drought-tolerant response in 25% of the population. This study shows the potential of UAV-based thermal imaging for field phenomics of poplar and other tree species. This is anticipated to have tremendous implications for accelerating forest tree genetic improvement against abiotic stress.
Highlights
Fast growing Populus clones are among the most common lignocellulosic feedstocks for second-generation bioenergy production in Europe (Amichev et al, 2010; Sannigrahi et al, 2010; Sabatti et al, 2014; Djomo et al, 2015)
Stomatal Conductance Ground-truthing To validate the unmanned aerial vehicle (UAV)-based approach, we studied the relationship between ground-based midday stomatal conductance and Tc collected from the UAV for selected genotypes, see Figure 1C
While ground-truthing required 3 h to obtain gs data on 24 trees, by performing only two low-elevation UAV flights, we were able to capture the response to drought of 6716 trees with a spatial resolution of 6 cm × 6 cm, that is, at POP6 sub-leaf definition
Summary
Fast growing Populus clones are among the most common lignocellulosic feedstocks for second-generation bioenergy production in Europe (Amichev et al, 2010; Sannigrahi et al, 2010; Sabatti et al, 2014; Djomo et al, 2015). A recent report of the International Poplar Commission indicates that the total area of short rotation coppice (SRC) Populus across Europe is about 23,502 ha (FAO, 2016). Compared to alternative bioenergy crops, SRC plantations offer versatile year-round coppice cycles and high yield to input ratio (Sims and Venturi, 2004). Short rotation coppice Populus clones are selected from worldwide breeding programs based on interspecific hybridization, whereby black poplar (Populus nigra L.) is a fundamental parental pool (Stanton et al, 2013); being widely and naturally spread in Europe, typically associated with riparian ecosystems, and characterized by large phenotypic and genetic variability (van der Schoot et al, 2000; Richardson et al, 2014; DeWoody et al, 2015). While first-generation hybridization (F1) is adopted to obtain heterosis for growth rate (Stettler et al, 1988), advanced generation breeding, such as F2 hybridization, among closely related Populus species, has proved to be an efficient strategy toward genetic improvement (Stanton et al, 2010)
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