Water Use, Efficiency, and Stomatal Sensitivity in Eastern Cottonwood and Hybrid Poplar Varietals on Contrasting Sites in the Southeastern United States

The southeastern United States has wide-scale potential to achieve high productivity from elite eastern cottonwood and hybrid poplar varietals to produce renewable bioenergy and bioproducts. In order to determine how environmental drivers impact water use and growth so that individuals can maintain growth during drought periods, varietals that use water efficiently, and/or tolerate water stress conditions, are needed to make planting recommendations across a variety of sites. Additionally, inoculation with nitrogen-fixing endophytic bacteria may improve water stress tolerance. The goals of this research were (1) to determine water use strategies using measurements of diurnal sapflow and differences in leaf retention for three eastern cottonwood (Populus deltoides, ST66, S7C8, and 110412) and three hybrid poplar (two P. deltoides × Populus maximowiczii, 6329 and 8019, and one Populus trichocarpa × P. deltoides, 5077) varietals on contrasting field sites, (2) determine the physiological impact of endophyte inoculation, and (3) determine which physiological parameters were most highly correlated with aboveground biomass. We found that whole-tree water use efficiency (WUE) was similar across varietals at 5.2 g biomass per kg water used and that water use scaled with tree size. We found that water use strategies in terms of scaled stomatal sensitivity to vapor pressure deficit converged across varietals under stressful soil water conditions at both sites, but that varietals 8019 and 110412 tended to exhibit the highest plasticity in stomatal sensitivity exhibiting the largest range in scaled stomatal sensitivity under different soil moisture conditions. Endophyte inoculation increased growth and stomatal sensitivity at the nitrogen-limited site. Leaf area, whole-tree WUE, and plasticity in stomatal sensitivity were correlated with aboveground biomass production across sites and varietals. Overall, these data can be used to model hydrologic impacts of large-scale Populus biofuel production as well as recommend varietals with efficient water use and stomatal sensitivity under a range of soil and atmospheric moisture stress factors.

Genome Assembly of Salicaceae Populus deltoides (Eastern Cottonwood) I-69 Based on Nanopore Sequencing and Hi-C Technologies

Populus deltoides has important ecological and economic values, widely used in poplar breeding programs due to its superior characteristics such as rapid growth and resistance to disease. Although the genome sequence of P. deltoides WV94 is available, the assembly is fragmented. Here, we reported an improved chromosome-level assembly of the P. deltoides cultivar I-69 by combining Nanopore sequencing and chromosome conformation capture (Hi-C) technologies. The assembly was 429.3 Mb in size and contained 657 contigs with a contig N50 length of 2.62 Mb. Hi-C scaffolding of the contigs generated 19 chromosome-level sequences, which covered 97.4% (418 Mb) of the total assembly size. Moreover, repetitive sequences annotation showed that 39.28% of the P. deltoides genome was composed of interspersed elements, including retroelements (23.66%), DNA transposons (6.83%), and unclassified elements (8.79%). We also identified a total of 44 362 protein-coding genes in the current P. deltoides assembly. Compared with the previous genome assembly of P. deltoides WV94, the current assembly had some significantly improved qualities: the contig N50 increased 3.5-fold and the proportion of gaps decreased from 3.2% to 0.08%. This high-quality, well-annotated genome assembly provides a reliable genomic resource for identifying genome variants among individuals, mining candidate genes that control growth and wood quality traits, and facilitating further application of genomics-assisted breeding in populations related to P. deltoides.

From Propagation to Field: Influence of Tray Design on Tree Seedling Quality and Performance1

This study evaluated the effects of low, moderate and high substrate exposure air-pruning propagation trays on eastern cottonwood (Populus deltoides W. Bartram ex Marshall ssp. deltoides) and black cherry (Prunus serotina Ehrh.) seedling root system quality and overall performance. Root system quality was characterized primarily by proportion of coarse root defects within the container imprint. Seedlings were evaluated after a nearly four-month commercial greenhouse production phase and one year after transplanting into a nursery field. Above and below-ground growth were measured at both time points. Proportions of coarse root defects, indicating degree of root deflection in container production, were persistent between greenhouse and field production phases. The Open (high substrate exposure) tray produced seedlings with roughly three times less deflected coarse root weight compared to the Closed Wall (low substrate exposure) tray for both species in both production phases. At neither production phase were there significant differences in above-ground growth among trays. This corroborates findings from other research studies that have found that variable root system quality does not always result in above-ground growth differences; and that when it does, differences in growth may take several years to manifest. Index words:, tree seedling quality, root defects, transplant performance, above-ground growth. Species used in this study: eastern cottonwood (Populus deltoides W. Bartram ex Marshall ssp. deltoides), black cherry (Prunus serotina Ehrh.).

A Model to Assess Eastern Cottonwood Water Flow Using Adjusted Vapor Pressure Deficit Associated with a Climate Change Impact Application

Short-rotation woody crops have maintained global prominence as biomass feedstocks for bioenergy, in part due to their fast growth and coppicing ability. However, the water usage efficiency of some woody biomass crops suggests potential adverse hydrological impacts. Monitoring tree water use in large-scale plantations would be very time-consuming and cost-prohibitive because it would typically require the installation and maintenance of sap flux sensors and dataloggers or other instruments. We developed a model to estimate the sap flux of eastern cottonwood (Populus deltoides. Bartr. ex Marsh.)) grown in bioenergy plantations. This model is based on adjusted vapor pressure deficit (VPD) using Structural Thinking and Experiential Learning Laboratory with Animation (STELLA) software (Architect Version 1.8.2), and is validated using the sap flux data collected from a 4-year-old eastern cottonwood biomass production plantation. With R2 values greater than 0.79 and Nash Sutcliffe coefficients greater than 0.69 and p values < 0.001, a strong agreement was obtained between measured and predicted diurnal sap flux patterns and annual sap flux cycles. We further validated the model using eastern cottonwood sap flux data from Aiken, South Carolina, USA with a good agreement between method predictions and field measurements. The model was able to predict a typical diurnal pattern, with sap flux density increasing during the day and decreasing at night for a 5-year-old cottonwood plantation. We found that a 10% increase in VPD due to climate change increased the sap flux of eastern cottonwood by about 5%. Our model also forecasted annual sap flux characteristics of measured cycles that increased in the spring, reached a maximum in the summer, and decreased in the fall. The model developed here can be adapted to estimate sap flux of other trees species in a time- and cost-effective manner.

Bacterial Canker Disease on Populus × euramericana Caused by Lonsdalea populi in Serbia

Populus × euramericana (Dode) Guinier clone (cl.) “I-214” is a fast-growing interspecific hybrid between Eastern cottonwood (P. deltoides Bartr. ex Marsh) and European black poplar (Populus nigra L.). Populus × euramericana was introduced into Serbia in the 1950s and has become one of the most widely grown poplar species. In September 2019, cankers were observed on stems and branches of P. × euramericana cl. “I-214” trees in a two-year-old poplar plantation in the province of Vojvodina, Serbia. The canker tissue was soft and watery, and a colorless fluid that smelled rotten flowed from the cracks in the bark, suggesting possible bacterial disease. After two weeks, diseased trees experienced crown die-back and oozing of foamy, odorous exudates and this study aimed to identify the causal agent of the disease. Canker margins and exudates were collected from 20 symptomatic trees. The associated bacterium was isolated and identified using biochemical characteristics, phylogenetic analyses based on 16S rRNA gene sequences, and multilocus sequence analyses (MLSA) based on partial sequencing of three housekeeping genes (gyrB, infB, and atpD). The pathogen was identified as Lonsdalea populi. Pathogenicity tests were conducted on rooted cuttings of P. × euramericana cl. “I-214” in an environmental test chamber and demonstrated that the isolated bacterial strain was able to reproduce symptoms of softened, water-soaked cankers and exudation. To the best of our knowledge, this is the first report of L. populi causing bacterial canker disease on P. × euramericana cl. “I-214” in Serbia and in southeastern Europe (SEE). It is also the first report of a bacterial disease on hybrid poplars, including P. × euramericana in this country and in SEE. If the disease spreads into new areas, selection for L. populi resistance may need to be integrated into future poplar breeding programs.

Complete Genome Sequence of Starkeya sp. Strain ORNL1, a Soil Alphaproteobacterium Isolated from the Rhizosphere of Populus deltoides

ABSTRACT Starkeya sp. strain ORNL1 is an alphaproteobacterium isolated from the rhizosphere of an Eastern cottonwood tree. Starkeya spp. are physiologically versatile, using a wide range of nutritional and energetic resources and serving important ecological roles in carbon and sulfur cycling. The 6.3-Mb chromosome of Starkeya sp. strain ORNL1 was completely sequenced and will help in understanding nutrient cycles.

Complete Genome Sequence of the Novel Roseimicrobium sp. Strain ORNL1, a Verrucomicrobium Isolated from the Populus deltoides Rhizosphere

ABSTRACT Roseimicrobium sp. strain ORNL1 is a soil bacterium that belongs to the phylum Verrucomicrobia and was isolated from the rhizosphere of a forest Eastern cottonwood tree, Populus deltoides, in Tennessee. Its 7.9-Mb chromosome was completely sequenced using PacBio long reads and is predicted to encode 6,288 proteins and 76 RNAs.

Draft Genome Sequence of Tumebacillus sp. Strain BK434, Isolated from the Roots of Eastern Cottonwood

ABSTRACT A Gram-positive bacterium was isolated from the root of an eastern cottonwood tree (Populus deltoides) in Georgia and identified as a Tumebacillus species with 99% 16S rRNA nucleotide identity to Tumebacillus avium. The genome is 4.6 Mbp and encodes 4,072 proteins and 251 RNAs.

Draft Genome Sequence of Larkinella sp. Strain BK230, Isolated from Populus deltoides Roots

Larkinella sp. strain BK230, a heterotrophic bacterium of the phylum Bacteroidetes, was isolated from the roots of a field-grown eastern cottonwood tree (Populus deltoides) located in Georgia. The draft 7.27-Mb genome has a G+C content of 53.4% and contains 6,026 coding sequences, including 41 tRNA genes.