Wood Quality, Carbon and Nitrogen Partitioning, and Gene Expression Profiling in <i>Populus</i> Exposed to Free Air CO<sub>2</sub> Enrichment (FACE) and N-fertilization

Abstract

Atmospheric CO2 concentrations ([CO2]) are continuously rising since the beginning of the industrialization. At the same time, N-deposition is also increasing. The influences of elevated CO2 and fertilization on the physiology and development of forest trees have been intensively studied. However, the effects of these environmental factors on wood quality, carbon and nitrogen allocation to long- and short-term C-N pools in wood of forest trees are not clear yet. To shed light on these questions, Populus × euramericana, P. alba, and P. nigra clones were grown in ambient air (about 370 ppm CO2) and in air with elevated [CO2] (about 550 ppm CO2) using Free-Air CO2 Enrichment (FACE) technology in central Italy. FACE was maintained for five years. After three growing seasons, the plantation was coppiced and one half of each experimental plot was fertilized with nitrogen. In secondary sprouts, this investigation was carried out.To characterise wood quality in response to elevated CO2 and N-fertilization, growth and wood anatomy of the three poplar clones were investigated. In the three poplar genotypes, most of anatomical traits showed no uniform response pattern to elevated CO2 or N-fertilization. In P. × euramericana, N-fertilization resulted in significant reductions in fiber lengths. In all three genotypes, N-fertilization caused significant decreases in cell wall thickness. In P. × euramericana and P. alba, elevated CO2 also caused decreases in wall thickness, but less pronounced than nitrogen. In P. nigra and P. × euramericana, elevated CO2 induced increases in vessel diameters. The combination of elevated CO2 and N-fertilization resulted in overall losses in cell wall area of 5 12% in all three clones suggesting that in future climate scenarios, the negative effects on wood quality may be anticipated.To quantify carbon allocation between short- and long-term pools in wood in response to elevated CO2 and N-fertilization, in P. nigra, carbon concentrations and stocks were quantified. Although elevated CO2, N-fertilization and season had significant tissue-specific effects on carbon partitioning to the fractions of structural carbon, soluble sugars and starch as well as to residual soluble carbon, the overall magnitude of these shifts was small. The major effect of elevated CO2 and N-fertilization was on biomass production, resulting in about 30% increases in above ground stocks of cell wall mass. Relative C-partitioning between mobile and immobile C-pools was not significantly affected by elevated CO2 or N-fertilization. These data demonstrate high metabolic flexibility of P. nigra to maintain C-homeostasis under changing environmental conditions.To characterise secondary metabolites and internal N-pools responding to elevated CO2 and N-fertilization, carbon-based secondary compounds, concentrations of total N and Klason lignin-bound N were measured in P. nigra. Elevated CO2 had no influence on lignin, cell wall-bound phenolics and soluble condensed tannins. Higher N-supply slightly but markedly stimulated formation of carbon-based secondary compounds. Elevated CO2 decreased internal N-pools in wood, but external N-supply increased the internal N-pools. In wood, 17 26% of N was bound to Klason lignin forming a resistant N-fraction. Neither elevated CO2 nor higher N-supply altered N-partitioning between lignin-bound N and other N-containing compounds. Positive correlations existed between the biosynthesis of proteins and secondary compounds in P. nigra. These data imply that the growth and defense of forest trees are well orchestrated.To elucidate the molecular mechanism causing changes in wood properties in response to elevated CO2 and N-fertilization, wood anatomy, Klason lignin, calorific value, Fourier transform infrared (FT-IR) spectra of wood were analysed and gene expression profiling in the differentiating xylem was performed in P. × euramericana. Elevated CO2 significantly stimulated the annual ring width in the second year after coppicing. However, elevated CO2 significantly decreased the cell wall, ray parenchyma and vessel lumen fractions, which was mainly due to a significant increase in the fraction of fiber lumina and decreased thickness of fiber walls. Higher N-supply also significantly decreased the cell wall fraction which was due to a marked decrease in the thickness of fiber walls and increases in fiber lumen fraction and fiber lumen diameter. Elevated CO2 and N-fertilization together stimulated lignin formation. This was also confirmed by mapping of lignin distribution and FT-IR spectra in wood. The calorific value of wood was unaffected by elevated CO2 or N-fertilization, whereas N-fertilization significantly enhanced the energy potential of the plantation by 16 69% due to the stimulation of aboveground biomass.Gene expression profiling revealed that only few transcripts were markedly affected by elevated CO2 and/or N-fertilization. Under most conditions, only one transcript was significantly affected on the array containing 3 444 expressed sequence tags (ESTs). When comparing the effect of N-fertilization on gene expression under elevated CO2, 15 transcripts were significantly up-regulated, including two genes, caffeic acid-3-O-methyltransferase 1 (COMT-1) and ferulate-5-hydroxylase (F5H), which are tightly linked to lignin biosynthesis. This observation corresponds well to the findings in Klason lignin analysis, mapping of lignin distribution and FT-IR spectra. Among the 15 transcripts, the enhanced expression of a tubulin gene associated with the cytoskeleton may be related to the altered anatomical properties (diameters of fiber and vessel lumen) in the developing xylem of P. × euramericana under these conditions.Based on the above results, it is concluded that in future climate scenario, the negative effects on wood quality may be anticipated. However, non-structural carbon compounds can be utilized more rapidly for structural growth under elevated atmospheric CO2 in fertilized agro-forestry systems. The growth and defense of forest trees were homeostatically balanced even if increases in atmospheric [CO2] were accompanied by increased N availability.