1. Summary A mass field screening for heat tolerance in a large set of sweetpotato accessions (1973) was carried out in the coastal desert of Northern Peru. Yield parameters and chlorophyll content (Normalized Difference Vegetation Index = NDVI) were determined. Great variation of storage root yield was found between genotypes: the number of clones that produced >40 t ha -1 decreased from 203 to 21 out of 1973 under heat stress while the number of clones that did not produce storage roots increased from 59 to 415. Mean storage root yields were 26.8% lower in the heat stress than in the no heat stress treatment. There was a very low positive correlation between NDVI and storage root production (r = 0.21). This study identified a group of 146 accessions that performed well under heat stress (>20 t ha -1 ) and thus are promising candidates for potential selection of high yielding, early bulking and heat tolerant varieties. However, further multi-locational heat stress trials are necessary to corroborate trait stability in different environments. Introduction Sweet potato is a robust crop with a wide range of adaptation to agro- ecological conditions, high yield potential, low input cultivation requirements, effective vegetative propagation and high nutritive value but storage root initiation and growth is adversely affected by temperature. Improved sweet potato varieties with increased tolerance to heat could improve productivity and facilitate the use of more marginal heat prone production areas. Sweet potato landraces and unimproved genotypes represent a valuable resource for heat tolerance in breeding programs. The International Potato Center holds the largest in vitro collection worldwide of sweet potato germplasm of which only a fraction has been evaluated for heat tolerance. This vast pool for future adaptive breeding has remained fairly untapped due to the lack of necessary evaluation data. The objective of this study was therefore to screen a representative collection of sweet potato clones for heat tolerance. Materials and Methods In total 1973 unique accessions were propagated in vitro, multiplied, and subsequently planted at the experimental site in Piura, Northern Peru (hot and arid coastal region) (Figure 1). Between 15 and 66 additional test clones were distributed across the experimental site to control field heterogeneity amounting to a total number of 2039 and 1988 clones planted during heat and no heat exposure, respectively. Alpha Lattice statistical design (Patterson and Williams, 1976) was used with two replications and two treatments in two different seasons: winter 2013 and summer 2014. The effects of abiotic stress on plant performance are evaluated by means of chlorophyll content (NDVI index, Rouse et al. 1974), and yield parameters (Table 1). Screenings were carried out during the decisive stages of physiological crop development: storage root initiation (60 d.a.p.), maximum root bulking (90 d.a.p.), and the final period of tuber growth (110 d.a.p.; Figure 1). Harvest was scheduled 120 d.a.p. Data were analyzed using R (2014). Table 1: Mean and range of selected agronomic traits in a large set of sweetpotato clones screened during heat stress exposure and no heat stress exposure. Figure 2.:Sweetpotato yields (t ha -1 ) of a large set of germplasm accessions tested under heat stress and no heat stress. Conclusions There is a number of at least 21 accession that showed high yields and early-bulking under heat stress conditions and thus are interesting candidates for further selection and breeding efforts. Additional information on nutritional quality of high yielding clones will complement data required for identification of clones useful for future breeding programs. Chlorophyll content (NDVI) was only weakly correlated to storage roots yields and thus require further analysis to validate usefulness of remote optical methods to fast screen germplasm. Bibliography Patterson H.D. and Williams E.R. 1976. Biometrika 63:83-92 Rouse, J. W., Haas, R. H., Schell, J. A., Deering, D. W., and Harlan, J. C. 1974. Monitoring the vernal advancement and retrogradiation (greenwave effect) of natural vegetation. NASA/GSFC Type III final report, Greenbelt, Md., 371 pp. R Core Team (2014). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL http://www.R-project.org/. Bettina Heider 1 Elisa Romero 1 Raul Eyzaguirre 1 Wolfgang Grüneberg 1 Stef de Haan 1 International Potato Center, P.O. Box 1558, Lima 12, Peru tolerance in sweetpotato Mass field screening for variation of heat Resilient Nutritious Sweetpotato Figure 1: Variety of sweet potato clones and experimental site in Piura, Northern Peru. Results Mean storage root yields were reduced by 26.8% in the heat stress treatment compared to no heat stress. Harvest index decreased by 28.1% (Table 1). Under heat stress exposure a total of 21 out of 1973 clones yielded >40 t ha -1 as opposed to 203 clones that reached the same yield or more without heat stress. 146 clones under heat stress and 1071 clones without heat stress passed the 20 t ha -1 yield level. Fifty-nine and 415 clones did not produce storage roots under no heat stress and heat stress exposure, respectively. The yield variation in tested clones under heat and no heat stress are displayed in Figure 2. The third quartile contains 25% (N = 509 clones) with a minimum yield of 7.9 t ha -1. Table 2: The 0.75, 0.80, and 0.85 quantiles for root yield under heat and no heat stress treatments and corresponding number of clones above them. NDVIs varied between 0.38 and 0.91 (Table 2). A positive though very weak correlation (r = 0.21) between NDVI and storage root yield and a moderate correlation between biomass yield and NDVI (r = 0.49) could be observed (Table 3 and Figure 3). Figure 3: Scatterplot matrix showing association of chlorophyll content (NDVI), root yield (TRY) and total biomass yield (biom) in t ha -1. Table 3: Pearson correlation coefficients between selected yield traits and chlorophyll content (NDVI).