Analysis of limitations to CO₂ assimilation on exposure of leaves of two <i>Brassica napus</i> cultivars to UV‐B

Abstract

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 CO 2 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 CO 2 ‐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 CO 2 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 CO 2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO 2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO 2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV‐B inhibition of CO 2 assimilation.