Studies on Protein Synthesis by Senescing and Kinetin‐treated Barley Leaves

Using sterile conditions, changes in total protein synthesis were followed. over an 8 day incubation period, in detached first seedling leaves of barley from 8 day old plants during senescence and after kinetin treatment. In senescing leaves, total 14 C‐alanine incorporation was enhanced by nearly 20% within 6 h of leaf detachment and by about 30 % after 24 h. Kinetin treatment stimulated protein synthesis even more, for total incorporation was promoted ca. 50 % after 6 h and by ca. 60 % after 24 h incubation. The leaf supernatant (30,000 × g for 30 min) proteins were separated on DEAE‐Sephadex (A‐50) columns into approximately 14 fractions and changes in 14 C labelling of these fractions were studied following leaf detachment and on incubation on water or kinetin for 6 days. In senescing leaves, 14 C‐incorporation into supernatant proteins was sustained, even as protein levels declined rapidly The varied stabilities of the different leaf proteins was suggested by the characteristically changing specific activities of the different protein fractions. Although kinetin greatly promoted incorporation into all protein fractions, no evidence was surmised of specific effects on individual leaf proteins. Studies of changes in total protein synthesis in attached senescing first seedling leaves taken from plants aged 7 to 27 days revealed a relatively small increase in 14 C‐incorporation. However, incorporation could be greatly increased in leaves up to 15 days old by detaching and preincubating such leaves for up to 2 days on water, prior to measurement. The promotion of 14 C‐incorporation into protcins follwing leaf excision could result from early changes in permeability and precursor pool size.