1966 WWW.CROPS.ORG CROP SCIENCE, VOL. 57, JULY– AUGUST 2017 RESEARCH T he specific gravity (SpGr) of potato (Solanum tuberosum L.) tubers is highly correlated with tuber dry matter and starch content (Vanasse et al., 1951), and since SpGr is easier to measure than these traits, it is widely used to estimate the culinary and processing quality of potatoes (Greenwood et al., 1952; Young et al., 1964). High-SpGr potatoes are better suited for frying, chip- ping, dehydration, baking, and mashing, and low-SpGr potatoes are usually used for boiling, canning, roasting, and potato salad. Specific gravity is typically measured on a sample of many tubers to estimate the mean value for a single source. Mean SpGr is Reliability of Measurement and Genotype ´ Environment Interaction for Potato Specific Gravity Yi Wang,* Lance B. Snodgrass, Paul C. Bethke, Alvin J. Bussan, David G. Holm, Richard G. Novy, Mark J. Pavek, Gregory A. Porter, Carl J. Rosen, Vidyasagar Sathuvalli, Asunta L. Thompson, Michael T. Thornton, and Jeffrey B. Endelman* ABSTRACT Specific gravity (SpGr) is often used to measure the processing quality of potato (Solanum tuberosum L.) tubers for French fries or potato chips because of its strong correlation with dry matter content and ease of measurement. For French fry processing genotypes, the desirable range for mean SpGr is typically 1.080 to 1.095, and a small variance around the mean is essential for product uniformity. Two multi-year, multi- location trials were conducted to investigate the genetics of SpGr in elite russet germplasm. Consistent with earlier studies, the mean SpGr was measured with high repeatability within each environment: the median plot-basis value was 0.83 for a national trial with six locations and 3 yr. In contrast, the median repeatability of the SD between tubers was only 0.21. Thus, multi-environment trials are needed to identify genotypes with a narrow SpGr distribution. Finlay–Wilkinson stability analysis of the mean SpGr established one genotype as an outlier: when best linear unbiased predictions were regressed on the environment means, this genotype had a regression coefficient of 2.1, compared with 0.4 to 1.4 for the others. The genetic correlation between environments showed a consistent regional pattern in mean SpGr over the years. There was a higher mean correlation between environments within the Pacific Northwest (0.97), Upper Midwest (0.91), and Northeast (0.85) than between environments from the different regions (0.35–0.78). Although breeding for national adaptation is an attractive idea, our results suggest that genetic gain may be easier to achieve at the regional level. Y. Wang, Kimberly Research and Extension Center, Univ. of Idaho, Kimberly, ID 83341; Y. Wang, L.B. Snodgrass, P.C. Bethke, and J.B. Endelman, Dep. Horticulture, Univ. of Wisconsin, Madison, WI 53706; P.C. Bethke, USDA–ARS Vegetable Crops Research Unit, Madison, WI 53706; A.J. Bussan, Wysocki Produce Farms, Plainfield, WI 54966; D.G. Holm, Dep. Horticulture and Landscape Architecture, San Luis Valley Research Center, Colorado State Univ., Center, CO 81125; R.G. Novy, USDA–ARS Small Grains and Potato Germplasm Research Unit, Aberdeen, ID 83210; M.J. Pavek, Dep. Horticulture, Washington State Univ., Pullman, WA 99164; G.A. Porter, School of Food and Agriculture, Univ. Maine, Orono, ME 04469; C.J. Rosen, Dep. Soil, Water, and Climate, Univ. Minnesota, St. Paul, MN 55108; V. Sathuvalli, Hermiston Agricultural Research and Extension Center, Oregon State Univ., Hermiston, OR 97838; A.L. Thompson, Dep. Plant Sciences, North Dakota State Univ., Fargo, ND 58108; M.T. Thornton, Parma Research and Extension Center, Univ. of Idaho, Parma, ID 83660. Received 3 Dec. 2016. Accepted 27 Feb. 2017. *Corresponding authors (wongyie@gmail. com, endelman@wisc.edu). Assigned to Associate Editor Duli Zhao. Abbreviations: BLUP, best linear unbiased prediction; FA2, second-order factor analytic; FW, Finlay–Wilkinson; h 2 , broad-sense heritability; HH, hollow heart; NAT, National Agronomic Trial; PNW, Pacific Northwest; SpGr, Specific gravity; WRT, Wisconsin Russet Trial. Published in Crop Sci. 57:1966–1972 (2017). doi: 10.2135/cropsci2016.12.0976 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. Published online August 28, 2017