Oxygen-17 appears only in protein in water-stressed soybean leaves labeled by ¹⁷O₂

We have used a rotational-echo adiabatic-passage double-resonance ¹³C{¹⁷O} solid-state NMR experiment to prove that the glycine produced in the oxygenase reaction of ribulose bisphosphate carboxylase-oxygenase is incorporated exclusively into protein (or protein precursors) of intact, water-stressed soybean leaves exposed to ¹³CO₂ and ¹⁷O₂. The water stress increased stomatal resistance and decreased gas exchange so that the Calvin cycle in the leaf chloroplasts was no more than 35% ¹³C isotopically enriched. Labeled O₂ levels were sufficient, however, to increase the ¹⁷O isotopic concentration of oxygenase products 20-fold over the natural-abundance level of 0.04%. The observed direct incorporation of glycine into protein shows that water stress suppresses photorespiration in soybean leaves.