Specific features of photorespiration in photosynthetically active organs of C₃ plants

Abstract

Photorespiration of photosynthetically active organs of C₃ plants (leaf, ear, stem, and leaf sheath) and C₄ plants (leaf, tassel, stem, leaf sheath, ear husk) grown under greenhouse and field conditions was studied. Photorespiration was measured using a PTM-48A high-technology monitor of photosynthesis (Bioinstruments S.R.L., Moldova). It is shown that photorespiration (CO₂ ejection after light turning off — apparent photorespiration) in C₃ plants is characteristic only for their leaves. In other photosynthesizing organs, photorespiration was absent, like in the photosynthesizing organs of C₄ plants. The absence of such after-light CO₂ outburst was observed for 31 genotypes: 18 cereal species belonging to four species (Triticum aestivum L., T. durum Desf., Secale cereale L., and Triticale); 6 grain legumes belonging to 2 species (Pisum sativum L. and Glycine max L.); 7 species of wild and rarely cultivated genotypes (T. boeoticum Boiss., T. dicoccoides Koern., T. dicoccum Schuebl., T. spelta L., T. compactum Host., T. monococcum L., and T. sphaerococcum Persiv.), and 2 genotypes of C₄ plants (Zea mays L. and Sorgum vulgaris L.). In all tested photosynthetically active genotypes, except of the C₃ plant leaves, apparent photorespiration was absent, but rather active glycolate cycle operated. The activity of this cycle was determined from the activity of the key enzyme of this cycle — glycolate oxidase. It was supposed that C₃ plants have two mechanisms of CO₂ assimilation: the first one — the mechanism of C₃ type localized in the leaves and the second one localized in other photosynthesizing organs, similar or with some elements of C₄ mechanism of CO₂ assimilation, limiting after-light CO₂ ejection during the metabolism of glycolate.