Cellulose and lignin biosynthesis is altered by ozone in wood of hybrid poplar (Populus tremula × alba)

Abstract

Wood formation in trees is a dynamic process that is strongly affected by environmental factors. However, the impact of ozone on wood is poorly documented. The objective of this study was to assess the effects of ozone on wood formation by focusing on the two major wood components, cellulose and lignin, and analysing any anatomical modifications. Young hybrid poplars (Populus tremula × alba) were cultivated under different ozone concentrations (50, 100, 200, and 300 l l(-1)). As upright poplars usually develop tension wood in a non-set pattern, the trees were bent in order to induce tension wood formation on the upper side of the stem and normal or opposite wood on the lower side. Biosynthesis of cellulose and lignin (enzymes and RNA levels), together with cambial growth, decreased in response to ozone exposure. The cellulose to lignin ratio was reduced, suggesting that cellulose biosynthesis was more affected than that of lignin. Tension wood was generally more altered than opposite wood, especially at the anatomical level. Tension wood may be more susceptible to reduced carbon allocation to the stems under ozone exposure. These results suggested a coordinated regulation of cellulose and lignin deposition to sustain mechanical strength under ozone. The modifications of the cellulose to lignin ratio and wood anatomy could allow the tree to maintain radial growth while minimizing carbon cost.

Fig. 1.

Height (A) and radial (B) growth of hybrid poplars since the beginning of the treatment, i.e. height and diameter differences compared with the height and diameter at time zero. Trees were cultivated under control conditions or different ozone concentrations. Data represent means ±SD (n=6).

Fig. 2.

SuSy (A, B) and UGPase (C, D) activity in hybrid poplar wood after 46 d of culture under control conditions or different ozone concentrations. Analyses were performed in tension wood (TW) and opposite wood (OW) at different stem levels. Data represent means ±SD (n=3). *Significant differences (P <0.05) between control and ozone treatments.

Fig. 3.

SHDH (A, B), PAL (C, D), and CAD (E, F) activity in hybrid poplar wood after 46 d of culture under control conditions or different ozone concentrations. Analyses were performed in tension wood (TW) and opposite wood (OW) at different stem levels. Data represent means ±SD (n=3). *Significant differences (P <0.05) between control and ozone treatments.

Fig. 4.

Cellulose content (A, B), lignin content (C, D), and cellulose to lignin ratio (E, F) in hybrid poplar wood after 46 d of culture under control conditions or different ozone concentrations. Analyses were performed in tension wood (TW) and opposite wood (OW) at different stem levels. Cellulose and lignin content are expressed as a percentage of cell wall residue (CWR). Data represent means ±SD (n=3). *Significant differences (P <0.05) between control and ozone treatments.

Fig. 5.

Environmental scanning electron microphotographs of the transverse surface of tension wood (A, C) and opposite wood (B, D) in stems (LS) of hybrid poplars cultivated for 46 d under control conditions (A, B) or 200 nl l⁻¹ ozone (C, D). V, vessel; F, fibre; GF, G-layer fibre. Scale bars=200 μm.