Tissue structure and respiration of stems of Betula pendula under contrasting ozone exposure and nutrition

Tissue structure and respiration (Rs) of stems were analyzed in Betula pendula grown throughout the growing season in either filtered air (control) or 90/40 nl O-3 l(-1) (day/night) Both regimes were split into high and low nutrient supply High nutrition increased tissue and cell sizes within the stem xylem phloem and periderm whereas ozone (O-3) tended to reduce tissue widths inhibiting in particular the cambial activity of xylem growth in low fertilized O-3 exposed plants (O-3/LF) Callose deposition was enhanced in the phloem sieve plates and tannins tended to condense in vacuoles of parenchyma cells under O-3 stress Decline occurred close to lenticels related to O-3 impact during shoot differentiation and was probably exacerbated by the limited assimi late translocation Radial stem growth ceased 4 weeks earlier than in control plants however the area based Rs was enhanced during intense growth in high fertilized O-3 exposed plants Photosynthetic CO2 refixation of stems reached about 50% of their dark respiration rate and the relative growth rate (RGR) did not differ between treatments At high nutrition RGR enhanced Rs to levels twice as high as the maintenance level Unit construction costs appeared to be similar in each treatment although CO2 release on a volume increment basis was lowered by 45% in O-3/LF plants This latter effect is ascribed to lowered maintenance demands of a xylem remaining reduced in width by 50% The high respiratory costs in the carbon balance of O-3/LF plants result from an enhanced leaf rather than stem respiration given the high demand for stress compensation in the foliage.