Nature (1968), 217, 195
by Rupert Sheldrake

In senescent leaves proteins are hydrolysed to amino-acids and peptides, which might be expected to release protein-bound auxin and also to provide considerable amounts of trypotophan which can be converted by many plant tissues to the auxin indolyl-3-acetic acid (IAA). We have therefore investigated the concentrations of auxin in senescent leaves.

Mature trifoliate leaves from plant of Phaseolus vulgaris and leaves from young plants (2-3 weeks old) of Avena sativa were detached and placed with their petioles or bases in distilled water in the dark at 25° C. In these conditions, the leaves become senescent and turn yellow. Samples were taken at various times (at intervals of 1 or 2 days), weighed and stored in the deep freeze until they were extracted with peroxide-free ether for 3 h at 0° C. The ether extract was partitioned and the acidic fraction was run on paper chromatograms with isopropanol : ammonia : water (8:2:1 v/v). The zone corresponding to IAA was eluted and the auxin was estimated using an Avena coleoptile straight growth bioassay. The amounts of auxin extracted from the leaves at various times are shown in Figs. 1 and 2.

It can be seen that in both cases there is a large increase in the amount of auxin present over a period of 6 days. The amounts measured represent the resultant of auxin production and auxin destruction: in the case of Avena, after about the fourth day the rate of destruction exceeds the rate of production. The fall in total auxin was observed in each of six experiments.

The level of auxin in leaves and petioles is involved in the control of abscission so the production of auxin by senescent leaves, if it is a general phenomenon, may be an important factor which so far has been overlooked.