Biphasic superoxide generation in potato tubers. A self-amplifying response to stress.

Potato (Solanum tuberosum) cultivars differ quantitatively in their responses to mechanical stress including the ability to synthesize melanin pigments in tuber tissues. Investigations into the cellular events induced by mechanical stress on tuber tissues have shown that an early cellular response is a significant and rapid synthesis of superoxide radicals. This burst of radical production distinctively displays a reproducible biphasic pattern over time with peaks of generation at 2 and 5 h. A concomitant consequence of the generation of these free radicals is elevated levels of oxidatively modified tuber proteins. Both radical generation and protein modification vary between cultivars but both are directly proportional to the amount of melanin pigments produced. Cell-free extracts of mechanically stressed tissues, pectic fragments, and scission products generated from cell walls are able to induce superoxide generation in non-stressed tissues, indicating the participation of a biologically active factor that induces a further a phase of radical synthesis.