Keeping cabbage on the market

Chlorophyll molecules are often damaged during photosynthesis, but receive some protection from other pigments known as carotenoids, which shield against the stress caused by excess light. Nevertheless, chlorophylls and other pigments need to be produced constantly to allow the process of photosynthesis to function undisturbed. At the same time, antioxidants such as vitamin C also protect the pigments from photo-oxidative damage. This is the dangerous combination of excess light and oxygen that induces the formation of highly reactive and consequently harmful free radicals which can destroy several vital components of the cells.

One of the first products of chlorophyll breakdown has a brownish colour, something we recognise when we're shopping and put back a tired-looking cabbage that has lost its fresh green colour. This has big implications for the retailer, for whom waste produce represents lost profit. It's clearly important to retailers that the produce has a good shelf-life, and this depends on the pigment contents and the changes that occur in them after harvest. The content of antioxidants such as vitamin C can also decrease during storage, especially if conditions are not ideal.

Studies directed by Professor Béla Böddi, in our laboratory at the Department of Plant Anatomy of Eötvös University in Budapest, focus on the molecular background of chlorophyll biosynthesis in a number of crop plants: wheat, sunflower, cabbage and pea. We analysed one key step of this reaction chain: the transformation of the pigment protochlorophyllide into chlorophyllide, which is the direct precursor of chlorophyll. This reaction is unique, because it requires light for activation.

In cooperation with biophysicists from Semmelweis University in Hungary, we have determined some of the kinetic properties of the enzyme responsible for this reaction; and with researchers from the Jagiellonian University, Poland and Göteborg University, Sweden, we have also analysed this process under various stress conditions including the presence of excess mercury and salt stress, which were both shown to impair chlorophyll synthesis when applied at high concentrations.

Our research group has demonstrated that the organization of these chlorophyll precursors is crucial for normal chlorophyll biosynthesis and greening, and if these pigments are not properly assembled and protected, they are quickly degraded upon illumination. Oxidative damage - sometimes even wilting and death of the plants - then occurs. This discovery can be useful to plant propagators, for instance in forestry and horticulture, where high mortality of seedlings may be related to similar phenomena and to poor growth conditions.

We have done a detailed analysis of the different leaves of white cabbage heads, using electron microscopy to examine the plastids, and spectroscopic methods to characterize the pigments. In these heads, the outermost green leaves contain chlorophylls and chloroplasts, but as the leaves age, chlorophyll breakdown, and sometimes wilting, occurs in them. Shopkeepers often remove these leaves from the heads. Interestingly, the outer leaves provide efficient shading to the leaves developing in the centre of the heads. In the absence of light, these inner leaves are not able to synthesize chlorophyll, but only low amounts of its precursor, protochlorophyllide, and they contain special plastids. This is why the innermost leaves of cabbage head are pale yellow or white in colour, rather than green. When the outer, green leaves are removed or when cabbage heads are halved, the inner leaves get exposed to light. This process leads to oxidative damage of pigments and also to a decrease in vitamin C content, especially when the heads are stored at low temperatures and under relatively high light intensities. Lettuce, artichoke, avocado, celery and dark-germinated seedlings used in the kitchen (due to their high vitamin content) are also adversely affected. To protect the level of antioxidants such as vitamin C, food plants should be shaded from harvest to kitchen.

Together with researchers at the Corvinus University in Budapest and at the University of Burgundy in Dijon, we aim to study these processes in different varieties of cabbage, lettuce and apricot under different postharvest conditions, and to develop storage systems under conditions of controlled atmosphere. Apart from leading to a better understanding of plant metabolism, our results could also lead to the development of new quality indices for the storage of produce, which could feed through into quality assurance systems in the agro-food industry, and the means to improve the shelf-life of fruit and vegetables. Using our understanding of basic science, we plan to develop cheap and simple measuring methods to determine ideal storage conditions and to minimize the costs of storage, and the loss of produce and profit.

It seems that our discoveries are relevant to everyone's daily life: cabbages really are helping people to think green.

Katalyn Solymosi, Eötvös Loránd University, Budapest. www.atomiumculture.eu