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Cheryl A. Parris, Clinton C. Shock, and Michael Qian

Stevia (Stevia rebaudiana Bertoni) is a perennial herbaceous plant native to Paraguay, where it was used by the native Guarani peoples for centuries. Although steviol glycosides from stevia are powerful natural noncaloric sweeteners, stevia has been cultivated and commercially available only for the past 50 years. Cultural practices are still in development, and productivity potential in the United States is largely unknown. Currently commercial growers and processors worldwide are seeking to maximize the productivity of rebaudioside A, a steviol glycoside. The trials reported here examined the effects of location, harvest strategy, and cultivar on stevia dry leaf yield, steviol glycoside content, and steviol glycoside yield. Six or seven stevia cultivars were grown for ≈5 months at four western U.S. locations, with an irrigation criterion of 20 kPa. Stevia at every location was subjected to two harvest strategies: either one harvest at the end of the season or two harvests, one midseason and another at the end of the season. The main plots at each location were the stevia cultivars, and the split plots were the harvest strategies in a randomized complete block, split-plot design with four replicates. Dry leaf yield, leaf steviol glycoside content, and leaf steviol glycoside yield varied by cultivar, location, and cultivar by location, but not by harvest strategy or interactions of harvest strategy with location or cultivar. Dry leaf yield averaged 4.12 Mg·ha−1 with significant differences by cultivar and location. One of the steviol glycosides, rebaudioside A yield averaged 300 kg·ha−1 with significant differences by cultivar and by interactions of location with cultivar. Leaf productivity was greater at Ontario, OR, than at Hanford, CA, Indio, CA, or Yuma, AZ. Dry leaf yield greater than 7 Mg·ha−1 and rebaudioside A yields greater than 500 kg·ha−1 were observed at Ontario. Stevia perenniated at Hanford and Indio, providing the option of multiyear harvests from a single planting.

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Cheryl A. Parris, Clinton C. Shock, and Michael Qian

Stevia (Stevia rebaudiana Bertoni) is of interest for the production of steviol glycosides due to their noncaloric sweetening properties. Commercial stevia leaf production to date has focused on rebaudioside A and stevioside. Relatively little is known about the cultural practices necessary for the efficient production of stevia leaf and steviol glycosides. Irrigation management is an important agronomic technique that growers can use to obtain high yield while also pursuing water use efficiency (WUE). This trial investigated the effect that irrigation onset criteria measured as soil water tension (SWT) had on dry stevia leaf yields, steviol glycoside content and yield, and steviol glycoside ratios. Two stevia cultivars, SW 107 and SW 129 (S&W Seed Company Inc., Fresno, CA), were subjected to SWT irrigation criteria of 10, 20, 40, 60, and 80 kPa over a 57-day trial period at the Oregon State University (OSU) Malheur Experiment Station, Ontario, OR. Harvested plant material was evaluated for dry leaf yield and content of several steviol glycosides. Analysis of variance (ANOVA) showed significant cultivar differences in leaf yield, leaf stevioside and rebaudioside A content and yield, and steviol glycoside ratios (P = 0.05). Examining the data by ANOVA, dry leaf yield, stevioside percent, rebaudioside C percent, total steviol glycoside (TSG) percent, rebaudioside C yield, and TSG yield were significantly greater among the wetter (closer to 10 kPa) compared with the drier (closer to 80 kPa) SWT criteria treatments, yet a preferred treatment regime was not clearly evident. When comparing cultivars by regression analysis, there was a highly significant decrease in the stevioside content, and an increase in the ratio of rebaudioside A to stevioside from wetter to drier irrigation onset criteria (due to lower stevioside), whereas the rebaudioside A content did not respond significantly to differing irrigation criteria. Increasing dry leaf productivity with wetter irrigation criteria (closer to 10 kPa), directly increased the total rebaudioside A yield also, which in turn could provide increased crop value to the grower. Irrigating near 10 kPa produced higher dry leaf yield and total rebaudioside A yield, than irrigating at drier criteria.

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Chad E. Finn, Brian M. Yorgey, Bernadine C. Strik, Robert R. Martin, and Michael Qian

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Chad E. Finn, Bernadine C. Strik, Brian Yorgey, Michael Qian, Robert R. Martin, and Mary Peterson

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Chad E. Finn, Brian M. Yorgey, Bernadine C. Strik, Harvey K. Hall, Robert R. Martin, and Michael Qian