Effect of environment and time of planting on the production and quality of yacon (Smallanthus sonchifolius) storage roots

The tuberous root yields of yacon (Smallanthus sonchifolius) were compared from field trials planted in September, October, and November at four sites (Pukekohe, Hamilton, Lincoln, Mosgiel) across New Zealand. The September plantings gave the highest yields at all sites, with the October plantings giving 17% less yield and the November plantings 61% less. The September planting at Hamilton produced the highest yield of 90 t/ha, Pukekohe 64 t/ha, Lincoln 22 t/ha, and Mosgiel 15 t/ha. Regression analysis established a strong positive relationship between dry root yield and accumulated growing degree days (GDD) below 10 degrees C calculated for base temperatures 0, 6, and 8 degrees C (R-2 = 0.87-0.88). Regression analysis R2 values declined above 10 degrees C. Dry root yields were high where heat accumulation was at least 1800 GDD(8) for the growing season with c. I t/ha of dried roots being produced for every additional 100 GDD8 above 1200 degree days. This relationship helps to identify suitable production environments for yacon. Best root production also needs an undefined level of cool temperatures as at the warmest site crown production was favoured. Total fructo-oligosaccharide (FOS) content of the yacon roots varied from 48.2% to 67.5%. November yacon had the lowest FOS content and conversely the highest content of non-FOS carbohydrates.