Respiratory characteristics of wheat (Triticum aestivum L. cvs Gabo and WW15), mung bean (Vigna radiata L. Wilczek cv. Celera) and sunflower (Helianthus annuus L. cv. Sunfola) were studied in plants grown under a normal CO2 concentration and in air containing an additional 340 (or 250) μl l−1 CO2. Such an increase in global atmospheric CO2 concentration has been forecast for about the middle of the next century. The aim was to measure the effect of high CO2 on respiration and its components. Polarographic and, with wheat, CO2 exchange techniques were used. The capacity of the alternative pathway of respiration in roots was determined polarographically in the presence of 0.1 mM KCN. The actual rate of alternative pathway respiration was assessed by reduction in oxygen consumption caused by 10 mM salicylhydroxamic acid.
Each species responded differently. In wheat, growth in high atmospheric CO2 was associated with up to 45% reduction in respiration by both roots and whole plants. Use of respiratory inhibitors in polarographic measurements on wheat roots implicated reduction in the degree of engagement of the alternative pathway as a major contributor to this reduced respiratory activity of high-CO2 plants. No change was found in the total sugar content per unit wheat root dry weight as a result of high CO2. In none of the species was there an increase in the absolute, or relative, contribution by the alternative pathway to total respiration of the root systems. Thus the improved photosynthetic assimilate supply of plants grown in high CO2 did not lead to increased diversion of carbon through the non-phosphorylating alternative pathway of respiration in the root. On the contrary, in wheat grown in high CO2 the reduced loss of carbon through that route must have contributed to their larger dry weight.