Development of photorespiration during chloroplast biogenesis in wheat leaves

Tobin, A. K., Sumar, N., Patel, M., Moore, A. L. and Stewart, G. R. 1988. Development of photorespiration during chloroplast biogenesis in wheat leaves.—J. exp. Bot. 39: 833–843. The rate of light-dependent ammonia accumulation in L-methionine sulphoximine (MSO: glutamine synthetase inhibitor)-treated wheat (Triticum aestivum L. cv. Maris Huntsman) primary leaf sections increased with mesophyll cell maturity. Ammonia production in the more mature sections (beyond 2.0 cm from the basal meristem) was inhibited by elevated CO2 concentrations and by incubation with 10 mol m−3 pyrid-2-yl hydroxymethane sulphonate (HPMS). In contrast, the low levels of ammonia which accumulated in the immature sections (0 to 2.0 cm from the base) were unaffected by such treatments. This indicates that the ammonia produced in mature wheat leaf sections is of photorespiratory origin and that the capacity of this pathway increases with mesophyll cell and chloroplast development. Rates of CO2-dependent oxygen evolution by leaf sections (under saturating CO2) increased in parallel with ammonia production. Levels of endogenous nitrate were relatively high and increased from 5.15 mol × 10−13 mesophyll cell−1 in meristematic cells to 6.6 mol × 10−12 mesophyll cell−1 in mature tissue. There was no significant change in leaf nitrate level during 30 min light incubation of the wheat leaf sections, indicating that the majority of the nitrate was metabolically inactive and stored in the vacuole. Activities of key enzymes of photorespiration (glutamine synthetase, glycollate oxidase), nitrogen metabolism (nitrate reductase, glutamate dehydrogenase, glutamine synthetase) and mitochondrial respiration (cytochrome oxidase), showed specific and distinct patterns of development during leaf growth. Chloroplast glutamine synthetase (GS2) and peroxisomal glycollate oxidase developed in apparent synchrony with the major increase in activity occurring in regions beyond4.0 cm from the leaf base, i.e. where photorespiration was developing. Cytosolic glutamine synthetase (GS1) and nitrate reductase (in vivo) activities were identical throughout leaf growth, reaching maximum rates at 4.0 cm from the base and then remaining constant. Activities of the mitochondrial enzymes glutamate dehydrogenase (GDH) and cytochrome oxidase were high in meristematic cells and increased in parallel, attaining a maximum towards the leaf tip. This indicated a respiratory, as opposed to a photorespiratory, role for GDH in whea