A Physiological Purpose for Abscission

ABSTRACT

Those of us who work with microscopical life know that animal and plant cells share comparable mechanisms of excretion. There is no difficulty in following this through to higher animals, for the organs of excretion are well known, but what of higher plants? An excretory mechanism in vascular plant life is unknown. The nearest we come to the phenomenon, according to the standard texts, is in the deposition of oxalate crystals in the leaves of aquatic genera such as Nuphar, familiar to microscopists. Yet, although excretion is one of the fundamental characteristics of life (along with nutrition, respiration, motility and the rest), this term is usually absent from the text- books on plant physiology.

Parallel to this is the unexplained purpose of leaf fall, abscission. Nutritional competition has been given as one reason for the loss of leaves, though this can de disproved by the administration of nutriment to plants with moribund leaves, and is incompatible with seasonal leaf fall in deciduous species. The phenomenon is clearly important to plants, since even evergreen species shed their leaves (continually) and the phenomenon is carefully regulated by the plant.

Here I wish to advance a theory that solves both puzzles: it is that leaf fall is the vascular plant’s excretory mechanism, This conceptual step makes plant physiology clearer. Any metabolically active organism must excrete, and it seems this elusive mechanism in vascular plants lies in the fall phenomenon. My observations of plants have taken me from arctic forests to the tropical rain forests of Africa, the Pacific, Australasia and the Far East, and reveal that plants with high growth rates share similar leaf fall behaviour, even if other constraints — including light levels, temperature or water supply — are very different. I have related this to research indicating that leaves do not became “senescent” when they yellow. This would entail a progressive lowering of metabolism and of physiological activity. In contrast to this expected observation, it has been shown that leaves may show an increase in certain forms of metabolism. Thus respiration, RNA content show a secondary peak near the end of the yellowing process. At the same time there is a rise in both soluble nitrogen and soluble carbohydrate levels as translocation phenomena become established.

It should be noted that abscission (leaf detachment) is frequently taught as dependent upon cork-layer formation in the petiole. However, it has been shown that:

a) abscission is frequently observed in species where no abscission layer is present;

b) abscission need not occur even when an abscission layer has become developed;

c) leaves which ordinarily fall when an abscission layer is present, can be induced to abscise when the layer has yet to form.

Many of the constituents of leaves during yellowing are excretory products. Thus, oxalates increase in leaf cells prior to leaf fall, and these are already known to be excretory products in higher plants.

There are other speculative questions to answer, one such being this: could pigmented petals represent a modified excretory organ through which are shed the metabolic wastes peculiar to the metabolic processes of sexual reproduction in angiosperms? The repercussions in education, research and even in agricultural practice might be considerable, and reactions to this view are invited.

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