Excretion is usually omitted from orthodox accounts of plant physiology. It has been argued that the inactivity of plants renders excretion unnecessary . But, although it might be argued that the rate of metabolic activity in a non-motile autotroph produces a reduced excretory load, there is no reason to suggest it should be altogether absent. I postulate that leaf fall, abscission, is the mechanism of excretion in vascular plants. The traditional view of leaf fall centres on the occlusion of the vascular bundles by a seasonally produced cork layer, so that the leaf dies and is detached from the plant body.
However, the shedding of leaves depends on other determinants: thus, abscission may occur in plants where an abscission layer has not formed, and leaf fall may not take place in species where an abscission layer is present . Senescence is widely used as a synonym for yellowing, though it is hard to conceive of a six-month leaf as senescent when the (older) plant which produced it is in its prime. The changes that take place in the yellowing leaf are a coordinated sequence of events and though some vital indicators (including protein content and photosynthetic activity) fall during the process, others undergo an increase. Among these are levels of soluble carbohydrate and nitrogen (associated with translocation) and, most significantly, RNA content and respiration rate .
Supportive of my view that leaf yellowing has an anaboIic component is the observation that cycloheximide (which inhibits protein synthesis) arrests the process . The classical reason given for leaf fall is that it obviates damage to a plant during winter. The evergreen species demonstrate that this is not obligatory, and with deciduous conifers (such as Larix) and genera like Lonicera, which include both evergreen and deciduous species of outwardly similar appearance, the picture becomes less clear. If evergreen leaves are anatomically adapted to resist winter conditions they need not be shed, but in fact they are subject to leaf-fall - though it takes place continually, rather than on a seasonally-mediated basis. The shedding of all leaves at predetermined intervals suggests that functional constraints are fundamental. Non-laminar leafy structures (such as tendrils, which develop from petioles) are often retained, whilst leaf-like organs developed from stem progenitors (including phyllodes) are shed, even though they are not leaves.
It appears that these organs have two discrete functions. In addition to acting as the plants metabolic centre, the leaf is also the structure that at the end of its useful metabolic life is systematically stripped of its vital constituents and charged with metabolic waste materials. The anthocyanins and tannins which are synthesized, like the oxalates which accumulate in yellowing leaves, are in reality among the excretory products the plant needs to discard. The yellowed leaf becomes an excretophore and the shedding of the leaf may be seen as the plants excretory mechanism.
Brian J Ford