Physiological age index: a new, simple and reliable index to assess the physiological age of seed potato tubers based on haulm killing date and length of the incubation period

Chronological and physiological age of seed tubers have major impacts on po tato yields. This paper presents a new, simple and reliable physiological a ge index (PAI) that considers and reconciles the effects of chronological a nd physiological age. PAI calculation is based on the haulm killing date of the seed crop (T-0) and the end of the incubation period of seed tubers, m easured under standardized conditions. The PAI formula is T-1/T-2, where T- 1 is the time from haulm killing date (T-0) to possible planting date and T -2 the time from T-0 to the end of the incubation period. The PAI expresses physiological ageing of seed potato tubers within a range from 0 (for phys iologically young) to 1 (old) tubers. T-0 test the PAI existing data were r eevaluated and re-elaborated and specific experiments regarding seed origin and storage conditions for different cultivars were performed during 1994- 1999. The PAI proved useful in assessing differences due to differences in growing conditions, cultivar, haulm killing, seed origin and storage system , and pre-planting treatments. For example, for cv. Spunta 6 days after hau lm killing the PAI was 0.025 and after 100-storage days the PAI was 0.56, 0 .52 and 0.49 for seed tubers stored in heaps in the field, at relatively hi gh temperatures, natural diffuse light and a cold (4 degreesC) and ventilat ed store, respectively. The PAI is related to ground cover duration and yie ld of the future crop. For a PAI of 0.55 tuber yield was 55 t ha(-1), while for a PAI of 0.80 tuber yield was 40 t ha(-1). The PAI is easy to measure, non-invasive, objective, reproducible and reliable and could be used for m odelling purposes to describe performance of seed tubers. (C) 2001 Elsevier Science B.V. All rights reserved.