Studies on 18O2-Uptake in the Light by Entire Plants of Different Tobacco Mutants

Abstract

Photorespiratory activity was measured in entire plants of five tobacco variants. These tobacco variants are: the green type N. tabacum var. John William’s Broadleaf (su/su Aur/aur or su/su Aur/Aur) the chlorophyll-deficient tobacco mutant Su/su (Su/su Aur/Aur) and the chlorophyll- deficient mutant Su/su var. Aurea (Su/su Aur/aur). Furthermore, two recently characterized phenotypes originating from N. tabacum var. Consolation namely “consolation green” (Aa Bb) and “consolation yellow-green” (aa bb). In entire plants of these phenotypes photorespiration was measured as 18O2-uptake in the light. This uptake was compared with the enhancement of CO2- fixation in the Warburg effect i. e. when the oxygen partial pressure is lowered from 21% Oz to 3% O2. The principal conclusion from these measurements is firstly that under the assay conditions which are identical for all 5 phenotypes (330 ppm CO2, 14000 lux white light and 25 °C) all five phenotypes yield considerable differences in photorespiratory activity. Furthermore, we were able to show that in the different phenotypes the global O2-uptake in the light is repartitioned to different degrees among different metabolic pathways. Thus, in JWB which is under the assay conditions the only fast growing species, only half of the measured 18O2-uptake belongs to glycolate metabolism or photorespiration proper, the other half belongs to a Mehler type reaction in which excess reducing power is eliminated apparently already at the level of photosynthetic electron transport. In the chlorophyll-deficient mutant Su/su, however, the observed 18O2-uptake in the light belongs under the assay conditions exclusively to glycolate metabolism (no Mehler type reaction). The chlorophyll-deficient mutant Su/su var. Aurea behaves more like JWB, that is, part of its 18O2-uptake is due to a Mehler type reaction and only the remainder is involved in CO2- metabolism, which has been already found out previously by genetic analysis. In addition photorespiration depends in Su/su more on the temperature than in the other phenotypes tested. One of the implications of our results could be that it makes a difference to the plant whether excess reducing power is disposed of at the level of the photosynthetic electron transport chain (via a Mehler type reaction) or at the level of CO2-metabolism.