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

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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.