Bernhard Kräutler and Thomas Müller at the University of Innsbruck in Austria explain why the changing colour of autumn leaves could be good news for your health

How green plant pigments disappear in the autumn, when the colours of the leaves of deciduous trees and shrubs change from green to red and yellow, has been a longstanding puzzle. While chlorophyll biosynthesis has been well studied, how chlorophyll breaks down remained a fascinating enigma until about 17 years ago. This lack of basic knowledge is all the more surprising, as chlorophyll metabolism is probably the most visible manifestation of life on Earth. In fact, it is even seen from outer space, and the total annual turnover of chlorophyll has been estimated to involve more than 1000 million tons. 

In 1991 a colorless chlorophyll catabolite from senescent (aging) plant leaves was identified as a linear tetrapyrrole, which turned out to be distantly related to bilirubin and phytobilins, products of heme breakdown.¹ Since then, chlorophyll breakdown products have been identified in a variety of plant leaves, and their structural features revealed. According to these studies, chlorophyll breakdown in higher plants leads first to coloured compounds - as transient, enzyme-bound intermediates only. In a later stage, fluorescent catabolites occur merely fleetingly, and colourless, nonfluorescent tetrapyrrolic catabolites are formed rapidly. The latter accumulate in de-greening leaves and are considered the final products of chlorophyll breakdown, which apparently is a well controlled program for rapid detoxification of the photoactive green plant pigment.

Catabolites formed as fruit and leaves change colour may be valuable nutritional components

But chlorophyll breakdown is not merely a detoxification process for the plant. It has also been associated with recycling of important nutrients, of reduced nitrogen, in particular. In the case of the catabolites, the four chlorophyll nitrogen atoms remain in the known tetrapyrrolic breakdown products and are thus not available for the plant to re-use. Nevertheless, they will eventually become part of more global recycling, possibly involving lower organisms. 

Recently, we addressed the puzzle of chlorophyll breakdown in ripening fruit. In freshly ripe apples and pears the very same linear tetrapyrroles were detected as chlorophyll catabolites. In fact, these breakdown products were also identical to the ones found in the de-greening leaves of the pear tree. Accordingly, chlorophyll breakdown appears to take a common pathway in fruit ripening and leaf senescence. As senescence is considered to accompany programmed cell death - yet ripening, commonly, is not - this finding is remarkable indeed.

In fruit, the tetrapyrrolic chlorophyll catabolites become a part of traditional food and may be a positive component in our diet as they are effective antioxidants. Exploratory studies have pointed to their beneficial health effects in mammals. Thus, the occurrence of such chlorophyll remains in apple peels may give a new twist to the old Welsh saying 'An apple a day, keeps the doctor away'.

140 years after Gregor Mendel used de-greening in peas as part of his experiments to establish the laws of inheritance, the basis for his observation is now known to be genetic control of chlorophyll breakdown. While the process may no longer be a total enigma, however, its wide-reaching benefits remain a field ripe for investigation.²

Read more in Bernhard Kräutler's perspective 'Chlorophyll breakdown and chlorophyll catabolites in leaves and fruit' in Photochemical and Photobiological Sciences.