Abstract
Additions of methionine sulfoximine (MSX), an inhibitor of glutamine synthetase (GS), result in an increase in NH3 in seedling leaves of C3 (wheat [Triticum aestivum cv. Kolibri] and barley [Hordeum vulgare var Perth]) and C4 (corn [Zea mays W6A × W182E] and sorghum [Sorghum Vulgare var MK300]) plants. NH3 accumulation is higher in C3 (about 17.8 micromoles per gram fresh weight per hour) than in C4 (about 4.7 micromoles) leaves. Under ideal conditions, when photosynthesis is not yet inhibited by the accumulation of NH3, the rate of NH3 accumulation is about 16% of the apparent rate of photosynthesis. A maximum accumulation of NH3 was elicited by 2.5 millimolar MSX and was essentially independent of the addition of NO3− during either the growth or experimental period. When O2 levels in the air were reduced to 2%, MSX resulted in some accumulation of NH3 (6.0 micromoles per gram fresh weight per hour). At these levels of NH3, there was no significant inhibition of rates of CO2 fixation. There was also a minor, but significant, accumulation of NH3 in corn roots treated with MSX. Inhibitors of photorespiration (isonicotinic hydrazide, 70 millimolar; 2-pyridylhydroxymethanesulfonic acid, 20 millimolar) or transaminase reactions (aminooxyacetate, 1 millimolar) inhibited the accumulation of NH3 in both C3 and C4 leaves. These results support the hypothesis that GS is important in the assimilation of NH3 in leaves and that the glycine-serine conversion is a major source of that NH3.
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