Effect of Kinetin on Protein & Nucleic Acid Metabolism in Xanthium Leaves During Senescence.

The chemical changes that occur in leaves as they grow old have been well characterized for many species. The endogenous factors which control and regulate these changes in plant cells remain, to a great extent, obscure, and the problem of why the cell eventually dies is yet unsolved. A normal feature of the ageing leaf blade is a continuous decline in protein level (1). The most rapid fall occurs during senescence and is associated with irreversible yellowing, loss of chlorophyll, and the eventual death of the organ. When a mature leaf is excised from the plant and the petiole kept in water, these same symptoms of senescence occur and, provided the petiole does not form roots, the protein content of the blade may fall to less than half the original value within a few days. If, however, the petiole forms roots, the behavior of the excised leaf is quite different. It will remain green and photosynthetically active, increasing in dry weight and total protein for periods extending, in some cases, to several years (2), much longer, in fact, than the leaf would survive in situ. The factors responsible for senescence and the decline of total protein in the leaf are not fully understood. Experiments with excised leaves have shown that the decrease in protein content of the blade is not necessarily due to a lack of carbohydrate, nitrogen, or other nutrients (14, 4) or to an inability of the cells themselves to synthesize amino acids (13) but is due, rather, to a failing ability to incorporate these amino acids into protein (11). Provided the leaf has a growing root system, the incorporation of amino acids proceeds normally, and it seems logical to assume that the roots metabolize and supply the blade with certain factors necessary for the continued synthesis of protein. During the last 4 years, several chemical substances have been shown to retard senescence of leaf blades, and in this respect the compounds would atppear to substitute either directly or indirectly for the unknown so-called root factors. In 1957, Person, Samborski, and Forsyth (10) demonstrated that both chlorophyll degradation and protein loss in detached wheat leaves are retarded if the blades are floated on solutions of benzimidazole at 50 mg/liter. In the same year, Richmond and Lang (12) showed that similar effects could be obtained if excised leaves of Xanthium are kept with their petioles dipping into solutions of kinetin at 5 mg/liter. Mothes and Engelbrecht (5) sprayed solutions of kinetin directly onto leaves of Nicotiana and reported (1959) that the retention of chlorophyll is localized to the areas of the blade to which kinetin is supplied. They found that labelled amino acids migrate to, and accumulate in, the treated parts of tobacco leaves, and they suggest that kinetin retards leaf senescence by causing the treated areas to act as loci for the accumulation of metabolites (5, 6). Extensive investigations have shown that both protein synthesis and ribonucleic acid synthesis is stimulated in kinetin-treated parts of tobacco leaves. (15). Mothes and Engelbrecht (7) conclude, however, that "accumulation, . . . is not the consequence of synthesis, but mass synthesis of protein, for example, is the consequence of an accumulation of amino acids." In 1959, (8) it was demonstrated that a number of auxins are markedly effective in retarding the senescence of attached or detached autumn leaves of Prunus serrulata-senriko. Where ethanolic solutions of esters of 2,4-dichlorophenoxyacetic acid are applied to the blades, the cells retain their green color for some 10 to 20 days longer than those in the surrounding untreated parts of the blade, or in the control leaves. The retention of green color in the treated areas is associated with an actively photosynthetic chlorophyll and, a maintenance of the initial protein level (9). Attempts to retard senescence by various treatments with kinetin proved unsuccessful in this species. Precise information is still meagre concerning the part played by either auxins or kinins in the retardation of the biochemical changes associated with senescence but it is clear that they must exert a control through some fundamental cellular processes. In this paper are described some effects of one kinin, kinetin, upon the metabolism of protein and nucleic acids during the senescence of excised leaves of Xanthium pennsylvanicum. The results provide evidence that kinetin may control senescence in these 1 Received Jan. 15, 1962. 2 Present address: Agricultural Research Council Unit of Experimental Agronomy, Department of Agriculture, Oxford.