Synthetic and Maintenance Respiratory Losses of 14CO2 in Uniculm Barley and Maize

Abstract

The respiratory losses of ¹⁴CO₂ from whole plants of uniculm barley and maize were measured following exposure of the whole plant or a single leaf to air containing a uniform specific activity of ¹⁴CO₂ for 30–60 min during normal photosynthesis. The total respiratory efflux of ¹⁴CO₂ could be described in terms of two main components: an intense efflux characterized by a half-life of 4–8 h, which was identified with the biosynthesis of new tissue in meristems; and a much less intense efflux characterized by a half-life of 26–120 h, which was primarily identified with the maintenance of metabolic activity. The (bio)synthetic efflux of ¹⁴CO₂ totalled between 25 and 35 per cent of the labelled assimilate (¹⁴C: ¹⁴C), and was generally insensitive to temperature and light intensity. The maintenance efflux totalled between 12 and 27 per cent of the labelled assimilate or its derivatives: the total generally increased with high temperature and low light intensity. The rate of the maintenance efflux showed a normal temperature response (Q10≏2). It is concluded that the efficiency of conversion of assimilate into new growth is unlikely to exceed 65 per cent in the long term, and will generally be less.