The relationship between the post-illumination CO₂ burst and glycine metabolism in leaves of C₃ and C₃-C₄ intermediate species of Moricandia

Abstract

The free-pool sizes of amino acids involved in photorespiratory metabolism have been determined in leaves of Moricandia species during the post-illumination CO₂ burst. The kinetics of the burst and the time to attainment of steady-state rates of dark respiration were much slower in the C₃-C₄ intermediate species Moricandia arvensis (L.) DC than in the C₃ species Moricandia moricandioides (Boiss.) Heywood. When plants were equilibrated at a high photon flux density (PFD; 1200 μmol · m⁻² · s⁻¹ PAR) the glycine and serine pool sizes in leaves of M. arvensis were 1.9 and 1.4 μmol · mg⁻¹ phaeophytin, respectively, values which were twice those in leaves of M. moricandioides. Amounts of glycine and serine were smaller at a lower PFD (150 μmol · m⁻² · s⁻¹) but were still twice as large in M. arvensis. Amounts of other amino acids involved in photorespiration or background cell metabolism (glutamate/glutamine, alanine, valine and threonine) were comparable in both species and did not respond to irradiance or change markedly during the dark burst. In contrast, during the first minute of the post-illumination burst the glycine pool in the leaves of both species had declined by at least 60%. It continued to decline, reaching 6–7 % of the level in the light by the time steady-state rates of dark respiration had been established. The rate of disappearance of glycine was comparable in both species and therefore depletion to steady-state dark levels took longer in M. arvensis than in M. moricandioides (8.4 and 4.6 min, respectively). These data indicate that almost all of the glycine pool in the leaves of C₃ and C₃-C₄ Moricandia species is a consequence of photorespiratory metabolism. The significance of a large but readily metabolised pool of glycine in the leaves of M. arvensis is discussed.