Protein Turnover in Isolated Barley Leaf Segments and the Effects of Stress

Leaf segments prepared from the first leaves of barley (Hordeumvulgare) exhibit a rapid loss of protein when given a matricstress with polyethylene glycol. Protein synthesis was reducedby the stress but a greater effect of stress was seen on proteindegradation. Growing leaves were exposed to ³H₂O for 4 d ormore to label total protein, and the half-life of protein 2-³H,in the isolated segments prepared from such leaves, was shownto be c. 140 h in the absence of stress. Stress reduced thisto c. 62 h. A short pulse with ³H₂O preferentially labels rapidlyturning-over protein and a 24 h pulse given to isolated leafsegments labelled proteins with a half-life of c. 64 h in thepresence or absence of stress. Degradation of the 24 h pulse-labelledproteins was inhibited by cycloheximide. Proline accumulationoccurred in the stressed segments and was inhibited by cycloheximide.The results are discussed in the light of current views concerningprotein degradation and possible relationships between proteolysisand proline accumulation.