In the autumn, many leaves go through a decaying process: they turn from green to yellow, maybe other colours to brown and eventually fall off. Is this process reversible at any stage, i.e. consider a leaf that has started to get yellow on a plant in a controlled environment. Would it be possible to make it all-green again by properly adjusting heat/light/water supply/nutrients?

Joakim,

This sounds like an excellent project to undertake yourself! The quick answer is I don't know and I don't know if this has been undertaken experimentally. Any one out there??

On many broadleaved deciduous trees, the shedding of leaves is regulated by the availability of day length. During winter periods, when daylight is at a premium, to have large floppy leaves using up lots of energy is not in the best 'interests' of the tree. The trees thus create a 'plug' of cells at the base of the leaf that gradually closes off the phloem (the fluid conducing tubes within a plant). This means that the sugars trapped within the cell will degrade and promote the production on anthocyanins (the reddy pigments to cranberries, strawberries etc.). At the same time, the production of chlorophyll (giving the leaf its green colour) stops.

Once the phloem are fully plugged, the leaves become dominated by the decaying sugars, anthocyanins and caretenoids (yellow and orangey colour producers for bananas, carrots etc.), thus changing colour. The changes in relative amount of these three causes the changes in leaf colour through the period until leaf drop.

Thus, if you could take a tree / shrub, induce the first stages of phloem closure and then control light levels such that the phloem either don't close fully or re-open, thereby not restricting sugar access and egress and staying the cessation of chlorophyll production, you may well be able to revert the leaf to its chlorophyll colouration.

Let us know how you get on, if you try this experimentally!

Dave.

Joakim

A very interesting question indeed.
The processes in the leaves in autumn can be separated in two parts: the reversible, called "senescence" and the irreversible, the "programmed cell death". However, the use of both terms can be overlapping. A "re-greening" of a leaf can only happen in the very early stages of yellowing. Only few cases are reported. In 1974 WR Krul noted that removal of the epicotyl of soybean can reverse visible senescence. There is a detailed discussion about it by Wouter G. van Doorn and Ernst J. Woltering in the Journal of Experimental Botany, Vol. 55, No. 406, pp. 2147–2153, October 2004, called "Senescence and programmed cell death: substance or semantics?"

Tanja