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Barrier properties of the cuticle to water, solutes and pest and pathogen penetration in leaves grown in polluted atmospheres.

Conclusions

Whereas it was shown that cuticular water permeability of healthy broadleaf plants was neither affected by SO₂, O₃ or acid precipitation, this was not the case with respect to ion permeability. Acid precipitation applied to growing leaves from bud break can affect the cuticular barrier properties to ions positively or negatively. The question remains, however, whether effects of acid precipitation on ion transport properties of the cuticle are indeed significant in physiological terms, compared with effects on cell membrane permeability and composition of the leaf surface solution, respectively.

In contrast to findings with growing leaves, neither acid nor ozone caused effects when applied to isolated cuticles at very high concentrations for relatively short periods. This indicates that the interaction between air pollutants and plant cuticles is not mainly due to chemical reactions on the plant surface. It is rather mediated by plant metabolism. It is largely unknown, however, for how long the efficacy of the cuticle as a barrier is actively maintained by metabolic processes once a leaf has reached its final size. Riederer (1989) pointed out that the tubular wax crystals typically observed on conifer needles were inherently instable structures in terms of their energetic state, and thus the really intriguing fact was not the gradual change towards a morphologically amorphous appearance of epicuticular waxes, but the preservation of such physically labile structures for periods of months or even years by the plant. He concluded that conifers most probably continue to replace their waxes even in fully grown needles. Only when a needle can no longer sustain the metabolic effort necessary, aging of epicuticular structures becomes observable.

Pollutant stress, as well as any other biotic or abiotic stress, might well interact with the amount of resources allocated to the maintenance of cuticular structure. Perhaps it is a telling fact that the only effect of air pollutants on cuticular water permeability was observed on plants which were in a rather bad shape (see section 2.2). There is a wide open field for multiple-stress experiments.

There is also a wide spectrum of possible and probable effects of air pollutants on interactions between pests, pathogens and the plant surface on which research has hardly started yet. Potential effects might as well decrease or increase plant resistance to pathogens. To the author it appears that research in this area is most urgently needed.

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