Does supplemental UV-B radiation affect gas exchange and RuBisCO activity of Betula pendula Roth. seedlings grown in forest soil under greenhouse conditions?

Abstract

Background: High nutrient supply has been frequently used in ultraviolet-B (UV-B) research, although it is unnatural for many species.

Aims: To study the effect of increased irradiance of UV-B on the photosynthetic capability of silver birch (Betula pendula Roth.) seedlings.

Methods: The seedlings were grown in a greenhouse in a substrate composed of birch forest top soil and sand, without additional supply of mineral nutrients. The increased daily integrated irradiance of UV-B (GPAS) was equivalent to 25% stratospheric ozone depletion under clear sky, and the treatment was started at seedling emergence. Leaf gas exchange was measured under saturating light conditions, and CO₂-assimilation rate, stomatal conductance and intercellular CO₂ concentration were computed. Initial and total activity of RuBisCO and the concentrations of RuBisCO, chlorophylls, and soluble protein were measured in the same leaf. Total nitrogen concentration was also measured, as well as carbon isotope discrimination.

Results: None of the variables measured were affected by UV-B at either of the two sampling dates (58 d and 76 d after the beginning of the treatment) except for stomatal conductance (interaction with time only).

Conclusions: Consequently, it is unlikely that significant damage to photosynthesis of silver birch seedlings will occur in a natural setting under current and projected changes in solar UV-B.

Keywords:

• Betula pendula seedlings
• birch
• Δ¹³C
• carbon isotope discrimination
• forest soil
• mineral nutrition
• photosynthesis
• RuBisCO
• ultraviolet-B
• UV-B

Acknowledgements

We thank Professor Lars Olof Björn for lending us a deuterium standard lamp and for helping with the testing of the Macam spectroradiometer, Ms Marja-Leena Jalkanen and Mr Pekka Voipio for technical assistance during biochemical assays and gas-exchange measurements, and Mr Pekka Piiroinen for his help with the greenhouse. We thank Dr Riitta Tegelberg for thoughtful comments on the manuscript. This study was supported by the Maj and Tor Nessling foundation (Grant to PJA) and the Faculty of Forest Sciences and the Graduate School in Forest Sciences at the University of Joensuu.