Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Authors

  • D. J. ALLEN,

    1. Department of Biological and Chemical Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK
    Search for more papers by this author
  • I. F. MCKEE,

    1. Department of Biological and Chemical Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK
    Search for more papers by this author
  • P. K. FARAGE,

    1. Department of Biological and Chemical Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK
    Search for more papers by this author
  • N. R. BAKER

    Corresponding author
    1. Department of Biological and Chemical Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK
      Professor N.R. Baker, Department of Biological and Chemical Sciences, John Tabor Laboratories, University of Essex, Colchester, Essex CO4 3SQ, UK.
    Search for more papers by this author

Professor N.R. Baker, Department of Biological and Chemical Sciences, John Tabor Laboratories, University of Essex, Colchester, Essex CO4 3SQ, UK.

ABSTRACT

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m−2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO2-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO2 assimilation.

Abbreviations
A

net CO2 assimilation rate

Asat

light-saturated net CO2 assimilation rate

ci

intercellular CO2 concentration

F0

Fm, Fv, minimal, maximal and variable fluorescence yields

Fm

Fv′, Fs, maximal, variable and steady-state fluorescence yields in a light-adapted state

Jmax, RuBP

maximum potential rate of electron transport contributing to RuBP regeneration

Jmax, PSH

rate of PSII electron transport at saturating light and CO2

PPFD

photosynthetically active photon

Ancillary