POSTHARVEST CHANGES IN ENDOGENOUS ABA LEVELS AND ABA METABOLISM IN RELATION TO DORMANCY IN POTATO-TUBERS

The effects of postharvest storage duration, and storage and incubation temperatures on the loss of innate tuber dormancy, endogenous levels of free end conjugated abscisic acid (ABA) and the metabolism of (+)-[H-3]ABA were examined over three growing seasons in tubers of potato (Solanum tuberosum L. cv. Russet Burbank). The storage and incubation temperatures used were chosen in order to differentiate between events relating solely to dormancy status vs those relating to the resumption of sprout growth following the termination of dormancy. In tubers stored at 20 degrees C, dormancy was lost and sprout growth began after 35 to 50 days of storage. In contrast, dormancy was lost after 50 to 80 days of storage at 3 degrees C but subsequent sprout growth was initiated only after transfer to 20 degrees C. After 35 days of storage, endogenous levels of free ABA were highest (860 +/- 35 pmol g(-1) fresh weight) in tubers held at 3 degrees C, intermediate (659 +/- 78 pmol g(-1) fresh weight) in tubers transferred from 3 to 20 degrees C 7 days prior to analysis and lowest (471 +/- 18 pmol g(-1) fresh weight) in tubers stored at 20 degrees C. Regardless of temperature, free ABA levels declined with increasing duration of storage. The onset of sprouting was not associated with any threshold level of free ABA. Base-labile, conjugated ABA was present in lower and more variable amounts than free ABA. The decline in free ABA was not accompanied by a commensurate increase in conjugated ABA. (+)-[H-3]ABA was readily taken up and metabolized by both dormant and nondormant tubers. In short-term (less than or equal to 7 days) studies, (+)-[H-3]ABA was metabolized to phaseic acid and dihydrophaseic acid. In tubers held at 20 degrees C, dihydrophaseic acid was the major metabolite while in tubers held at 3 degrees C there was a transient accumulation of phaseic acid with dihydrophaseic acid predominating only after 7 days. During postharvest storage, there was no change in the rate of ABA metabolism regardless of the dormancy status and/or growth potential of the tubers. These results demonstrate that ABA is readily metabolized by potato tubers and that the principal route of catabolism consists of the oxidative metabolism of ABA to phaseic and dihydrophaseic acids with minimal esterification to conjugated ABA. These results also suggest that a decline in endogenous ABA below a threshold level is not a prerequisite for the loss of potato tuber dormancy and the onset of sprout growth.