Experiments on the hormonal regulation of protein synthesis and senescence in detached leaves of Prunus serrulata are described.
Senescence is associated with a fall in the content of total protein in the leaf, a loss of chlorophyll and an increase in the soluble nitrogenous fractions. Auxins such as n-butyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D) effectively retard these changes when supplied to leaves at particular stages of growth and development. 2,4-D is found to stimulate oxygen uptake in young and mature leaves, compared with controls, but has little effect upon oxygen uptake of autumn leaves, while protein levels are maintained by the auxin in freshly detached autumn leaves but not in younger leaves, unless they have already been detached for precise periods of time and considerable protein loss has already taken place.
2,4-D prevents protein loss in autumn leaves by maintaining protein synthesis. The incorporation of [14C] leucine into protein falls by as much as 60% in 7 days in control halves of leaves, but in halves of the same leaves which have been supplied with 2,4-D, the incorporation of [14C] leucine into protein is sustained at, or above, the original level.
It is proposed that auxins participate in the balance of hormonal factors which regulate the metabolism of protein. They would appear to be essential for maintaining the incorporation of amino acids into protein, and at certain stages of leaf development, they would appear to be limiting for the proper functioning of the processes of protein synthesis.
The results are discussed in connection with known effects of environment on the auxin content of leaves, and with effects of auxins and other plant growth regulators on protein and nucleic acid metabolism.