Response of stored potato seed tubers from contrasting cultivars to accumulated day-degrees

In potato (Solanum tuberosum L.), the accumulated day-degrees (temperature sum, calculated by accumulating the daily temperatures) from dormancy break until seed tuber use has been suggested as an indicator of the physiological status of the seed. We tested whether similar temperature sums differing in timing of a short period of high temperatures gave similar seed performance. Four field experiments were performed in which seed was used that had been exposed to different storage temperature regimes, differing in total temperature sum or in timing or duration of a warm period. Emergence, number of stems, number of tubers, and early and mature tuber yield were assessed. During the storage period, the onset of sprouting was recorded. Cultivars with a high rate of physiological degeneration ("ageing") were usually sensitive to warm storage during the second part of the storage period, especially if the first 12 to 18 wk of storage had also been warm. This was reflected in reduced emergence (10%), low densities of stems (0.5 stems m¿2) and tubers (5 tubers m¿2), and low yields, especially with early harvesting (20 g m¿2). Specific phasing of the warm period could reduce yields to levels even below the yield of the seed tubers exposed to the highest accumulated temperature sum. A higher temperature sum after the end of dormancy advanced and accelerated the process of ageing of seed tubers. Cultivars with a high rate of ageing showed much greater difference between the same temperature sums built up over time in different ways than cultivars with a low rate of ageing. The resulting maximum differences in final fresh tuber yield between seed lots exposed to the same temperature sum could be 65 Mg ha¿1 for Astarte (a cultivar with a high rate of ageing) compared with nil for Désirée (a cultivar with a low rate of ageing)