|Abstract or Summary
- Stevia (Stevia rebaudiana) is a perennial herbaceous plant native to Paraguay and used traditionally by the native Guarani peoples for centuries to sweeten medicinal teas. Stevia has only been commercially cultivated over the past 50 years and cultural practice information suitable for implementation by growers is limited. Currently stevia’s commercial value is found in its steviol glycoside content, especially rebaudioside A. Growers seek information to maximize productivity of rebaudioside A through improved cultural practices. Irrigation management is one of several important agronomic techniques growers can use to obtain maximum yield while also pursuing water use efficiency. The current trial investigated the effect that soil water tension as an irrigation onset criteria had on stevia dry leaf yields, steviol glycoside content and yield, and steviol glycoside ratios. The experiment was conducted in a field of Owyhee silt loam (coarse-silty, mixed, mesic Xerollic Camborthid) at the Oregon State University, Malheur Experiment Station, Ontario, Oregon, N 43° 58' 53.273'' W 117° 1' 26.396''. Two stevia varieties, SW109 and SW129 (S&W Seed Company Inc., Fresno, California), were subjected to soil water tension irrigation criteria of 10, 20, 40, 60, 80, kPa over a 57-day trial period. Harvested plant material was measured for dry leaf yield and several steviol glycosides. Analysis of variance showed significant variety differences in leaf yield, leaf stevioside and rebaudioside A content and yield, and steviol glycoside ratios (P=0.05). Examining the data by analysis of variance, dry leaf yield, stevioside%, rebaudioside C %, total steviol glycoside (TSG), rebaudioside C yield, and TSG % yield had significant higher results among the wetter compared to the drier soil water tension criteria treatments, yet a preferred treatment regime was not was not clearly evident. When comparing varieties by regression analysis, results showed a highly significant decrease in stevioside % content, and an increase in the ratio of rebaudioside A to stevioside from drier to wetter irrigation onset criteria, whereas, rebaudioside A % content did not show any significant trend at differing irrigation criteria. The results indicate that maximizing dry leaf productivity directly affects the total rebaudioside A yield, which in turn can provide increased crop value to the grower. Irrigating near 10kPa produced higher dry leaf yield and total rebaudioside A yield, than irrigation at drier criteria. Irrigating stevia at 10 kPa at full canopy, the crop coefficient, k[subscript c], was equivalent to 0.97 ET₀, averaging 6.4 mm·d⁻¹ for the days measured.