|Wang, Yi -|
|Bussan, Alvin -|
Submitted to: Proceedings Wisconsin Annual Potato Meetings
Publication Type: Proceedings
Publication Acceptance Date: January 21, 2011
Publication Date: February 1, 2011
Citation: Bethke, P.C., Wang, Y., Bussan, A.J. 2011. Stem-end chip defect formation under controlled environment conditions and in the field. Proceedings Wisconsin Annual Potato Meetings. 24:17-19. Technical Abstract: Stem-end chip defect is a serious tuber quality defect that affects chipping potatoes. This defect results in dark colored vascular tissues and adjacent cortical tissues in potato chips after frying. Chips with stem-end defects are unappealing to consumers and may cause raw product to be rejected at processing plants. Defect incidence varies between years with mild defects appearing in many years, and severe, widespread defect incidence in some years. A multi-year research project has investigated the potential causes for stem-end chip defect. Progress to date is reported here for trials in controlled environment greenhouses and field trials. Methods Plants of potato cultivar FL 1897 were grown in the UW Biotron in 2008 and 2009. Plants were selected for uniformity after emergence and early growth, and 96 plants were randomized across treatments. Control plants were grown with day/night temperatures of 72 °F/64.4 °F and a 12 h light period with sodium vapor lights supplementing natural daylight as needed. Plants were irrigated with ¼-strength Hoagland’s nutrient solution through two drip lines attached to an automated pumping system. Soil water potential was maintained at >-20 kPa for well-watered controls. Stress treatments included heat stress (86 °F daytime temperature), drought stress (-20 to -60 kPa soil water potential) or heat and drought stress for two weeks at either early tuber bulking or late tuber bulking. After the second stress period, plant maturity was varied by continuing to water some plants with nutrient solution to delay maturity while other plants were watered with reverse osmosis (RO) water to encourage maturation and senescence. Tubers were harvested at four times. Harvests were made at the end of the two stress periods. A third harvest occurred when plants watered with nutrient solution were green and plants watered with RO water had extensive leaf and stem senescence. The fourth harvest occurred after natural senescence of all vines. Tubers were either stored at 55 °F for 2 months or processed one day after harvest for sugar and enzyme analysis. For sugar analysis, tissues were removed from the tuber stem and bud ends. Additional samples of bud and stem end tissue were frozen and used for acid invertase activity assays. Center slices from three tubers per plant were fried and stem-end defects were scored on a scale of 0 to 5 with 0 being no defect and 5 being a severe defect with color development extending more than ½ inch in from the stem end of the tuber. Results. A total of 576 chips from 2008 and 2009 were scored for stem-end chip defect. Of these, 72% had defects with 17% having moderate to severe defect scores of 3, 4 and 5 and 55% having mild defect scores of 1 and 2. Defects with scores of 4 or 5 were rare in both years and occurred in less than 6% of all chips. Water stress, heat stress and maturity treatments had little affect on the percentage of severe stem-end chip defects. In all cases, defect percentages in tubers from treated plants were comparable to those in tubers from plants that had not experienced the stress or maturity treatment. Heat stress was the only treatment that gave an indication that it increased the incidence of stem end chip defect. Maturity at harvest had a large affect on tuber bud-end and stem-end sucrose contents at harvest, with sucrose being significantly greater (p <0.01) in tubers harvested from immature rather than mature plants. Vine maturity at harvest, however, changed the percentage of tubers that had severe defects by 2%. The activity of acid invertase at the stem and bud ends of tubers without defects, tubers with mild defects and tubers with severe defects was quantified. A weak correlation was observed where tubers with more severe stem-end defects had higher rates of acid invertase activity at the stem end than tubers without defects or tubers with mild defects. Tuber-to-tuber variability in invertase activity was large, however, perhaps because of the localized, non-uniform nature of the defect within tuber tissues. The incidence and severity of stem-end chip defect was monitored in field plots in 2010 in Hancock, WI. Defects were a common in these trials, as they were throughout the upper Midwest during that year. Defect incidence differed between cultivars and for some cultivars, defect severity increased during storage. In other cases defect incidence decreased and tuber quality improved significantly by mid-November.