Stomatal density, anatomy and nutrient concentrations of Scots pine needles are affected by elevated CO2 and temperature

Abstract

Stomatal density, anatomy and nutrient concentrations of Scots pine (Pinus sylvestris L.) needles were studied during 3 years of growth at elevated CO 2 (693 ± 30 µ mol mol−1), at elevated temperature (ambient +2·8–6·2 °C depending on the time of the year) and in a combination of elevated CO 2 and temperature in closed‐top chambers. The treatments were started in August 1996. At elevated temperature, the needles that were grown in the first year (i.e. the 1997 cohort) were thinner, had thinner mesophyll in the abaxial side, thinner vascular cylinder and lower stomatal density than those grown at ambient temperature. The proportion of mesophyll area occupied by vascular cylinder or intercellular spaces were not changed. Lower stomatal density apparently did not lead to decreased use of water, as these needles had higher concentrations of less mobile nutrients (Ca, Mg, B, Zn and Mn), which could indicate increased total transpiration. In the 1997 and 1998 cohorts, elevation of temperature decreased concentrations of N, P, K, S and Cu. In the 1999 cohort, contradictory, higher concentrations of N and S at elevated temperature may be related to increased nutrient mineralization in the soil. Elevation of CO 2 did not affect stomatal density, needle thickness, thickness of epidermis or hypodermis, vascular cylinder or intercellular spaces. Concentrations of N, P, S and Cu decreased at elevated CO 2. Reductions were transient and most distinct in the 1997 cohort. The effects of CO 2 and temperature were in some cases interactive, which meant that in the combined treatment stomatal density decreased less than at elevated temperature, and concentrations of nutrients decreased less than expected on the basis of separate treatments, whereas the thickness of the epidermis and hypodermis decreased more than in the separate treatments. In conclusion, alterations in the anatomy and stomatal density of Scots pine needles were more distinct at elevated temperature than at elevated CO 2. Both elevated CO 2 and temperature‐induced changes in nutrient concentrations that partly corresponded to the biochemical and photosynthetic alterations in the same cohorts (Luomala et al. Plant, Cell and Environment26, 645–660, 2003) Reductions in nutrient concentrations and alterations in the anatomy were transient and more evident in the needle cohort that was grown in the first treatment year.