Effect of Methionine Sulfoximine on the Accumulation of Ammonia in C(3) and C(4) Leaves : The Relationship between NH(3) Accumulation and Photorespiratory Activity.

Additions of methionine sulfoximine (MSX), an inhibitor of glutamine synthetase (GS), result in an increase in NH(3) in seedling leaves of C(3) (wheat [Triticum aestivum cv. Kolibri] and barley [Hordeum vulgare var Perth]) and C(4) (corn [Zea mays W6A x W182E] and sorghum [Sorghum Vulgare var MK300]) plants. NH(3) accumulation is higher in C(3) (about 17.8 micromoles per gram fresh weight per hour) than in C(4) (about 4.7 micromoles) leaves. Under ideal conditions, when photosynthesis is not yet inhibited by the accumulation of NH(3), the rate of NH(3) accumulation is about 16% of the apparent rate of photosynthesis. A maximum accumulation of NH(3) was elicited by 2.5 millimolar MSX and was essentially independent of the addition of NO(3) (-) during either the growth or experimental period. When O(2) levels in the air were reduced to 2%, MSX resulted in some accumulation of NH(3) (6.0 micromoles per gram fresh weight per hour). At these levels of NH(3), there was no significant inhibition of rates of CO(2) fixation. There was also a minor, but significant, accumulation of NH(3) 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 NH(3) in both C(3) and C(4) leaves. These results support the hypothesis that GS is important in the assimilation of NH(3) in leaves and that the glycine-serine conversion is a major source of that NH(3).