Photorespiratory glycine metabolism in corn leaf discs

The total glycine pool in Zea mays L. Mo17×B73 leaf discs was measured after steady state photosynthesis in 50%, 21% and 1% O₂. The glycine pool was a function of O₂ concentration; it was largest in 50% O₂ and smallest in 1% O₂. Incubation of discs with methyl hydroxybutynoic acid in 21% O₂ in the light caused an accumulation of carbon in glycolate. This accumulation was O₂ sensitive, as subsequent photosynthetic periods in 50%, 21%, and 1% O₂ resulted in the largest glycolate pool in 50% O₂ and the smallest in 1% O₂. At the same time, the O₂-dependent increase in the glycine pool was eliminated. After untreated leaf discs reached steady state photosynthesis in 21% O₂, measurements made subsequently in darkness, or in 1% O₂ in the light, showed that the glycine pool decreased. On the basis of these results, we conclude that a major portion of the total glycine pool in corn is an intermediate in the photorespiratory glycolate pathway. Considering both the rate of decay of the glycine pool in the dark and the rate of decay of the glycine pool after changing from 21% to 1% O₂, we conclude that this glycine pool is turning over slowly.