The coordination and control of leaf senescence

Leaf senescence is a process that is evolutionary advantageous to plants as it allows the transfer of nutrients from no longer required processes to areas where development is occurring, such as seed development. The process is age dependent and involves genetic, molecular and metabolic processes. It is important to the plant that the senescence process is under fine control so that it occurs at a time that is beneficial to the plant and not at one that is disadvantageous.

What controls leaf senescence

Senescence is an age related process that is ultimately controlled by both internal and exogenous stimuli; the plant is able to integrate this information to analyse the most beneficial time to begin the senescence process. Some of the environmental factors that enable a plant to gather information about its age include abiotic factors such as drought, temperature, light, ozone and availability of nutrients. Biotic processes that lead to senescence include shade and pathogen detection.

The coordination of leaf senescence

There is a locality of cell death in leaf senescence that is dependent upon environmental stress stimuli. This leads to an apparent uneven spread of the senescence program. However when no environmental stress occurs it is seen that the process is coordinated so that senescence starts at the leaf tip and leaf margins, and makes its way towards the leaf base. This allows nutrients to be gathers from the furthest regions of the leaf, and to be passed out of the leaf. If senescence was to first occur at the leaf base then nutrients would not be able to pass from the outer reaches of the leaf to the rest of the plant.

The leaf senescence process is also developmentally coordinated, and is often controlled by the processed that require the nutrients contained in leaves such as seed development. The leaf senescence process may occur at the same time as the mortality of other organs or the whole plant (this occurs in monocarpic plants that only has one round of reproduction), or it may occur in a manner that removes the leaves but does not affect other organs.

A well known control process in senescence is called correlative control; this involves the stimulation of the leaf senescence by signals initiated in reproductive organs. Indeed it has been demonstrated that removing these stimuli in plants such as the pea converts the process of leaf senescence to that of juvenile leaf development.

Not all plants that research into leaf senescence occurs are under correlative control, indeed the monocarpic model organism Arabidopis displays whole plant senescence, but is still useful in the understanding of leaf senescence as its leaf has distinct juvenile, adult, and senescence phases.