Energy metabolism of Rumex leaf tissue in the presence of senescence-regulating hormones and sucrose

Hormones which inhibit senescence of Rumex leaf tissue in the dark include gibberellin A3, and the cytokinins 6-benzylamino purine and zeatin. These hormones inhibit respiratory metabolism in this tissue, but do not change the pattern or total amount of oxygen consumption during senescence. Abscisic acid, a senescence accelerator, correspondingly stimulates oxygen consumption. This correlation of senescence rate and respiration rate holds with regard to the hormone concentrations effective and the continued activity of the hormones when added after the lag phase of chlorophyll breakdown. Transfer experiments show that the respiratory inhibition due to gibberellin A3 and the promotion due to abscisic acid become established within 3 hours of hormone addition. When gibberellin A3 and zeatin were rapidly added to narrow strips of tissue, no inhibitions of oxygen uptake were observed in the first 12 minutes. Senescence-inhibiting concentrations of sucrose strongly stimulate respiratory meabolism, raise the respiratory quotient, and cause inhibition of chlorophyll and protein breakdown which is distinct from the effect of gibberellins or cytokinins. It is proposed that the gibberellins and cytokinins may act by inhibiting a rate-limiting process in the metabolism of starving leaf tissue. This rate-limitation hypothesis envisions a possibly subtle action of the hormone which retards but does not alter the pattern of macromolecular changes during cell autolysis.