Daily Light Integral Influences Production of Steviol Glycosides

The genus Stevia consists of approximately 220–230 species, with one species, S. rebaudiana (stevia), distinguished by its relatively high production of non-toxic, non-nutritive, ent-kaurene diterpenoid glycosides. Some of these glycosides are approximately 300 times sweeter than sucrose.  Steviol glycoside concentrations vary widely among stevia genotypes.  The influence of environmental factors, such as irradiance and photoperiod, on steviol glycoside synthesis is poorly understood.  To investigate the effect of total accumulated irradiance, or the daily light integral (DLI), on steviol glycoside production, two stevia genotypes, 10-43-41 and 11-464, were grown in a greenhouse at Michigan State University under a range of DLIs and a constant 16-hr photoperiod.  A total of six DLIs, ranging from 3.5 to 20.1 mol·m²·d^-1, were achieved by growing plants under shade cloths varying in light transmission, and by growing plants at different times of year.  Following two months of growth in treatments, steviol glycoside concentration was quantified from leaf material and plant height and leaf area were determined.  Plant height of both genotypes decreased as DLI increased.   Increasing the DLI reduced leaf size of 10-43-41 but not 11-464.  Total steviol glycoside (TSG) concentration increased as DLI increased from 3.5 to 8.4 mol·m²·d^-1, and was similar at DLIs of 8.4 mol·m²·d^-1 or greater.  In addition to affecting TSG, DLI also differentially influenced the concentrations and relative proportions of specific steviol glycosides. For example, stevioside concentration decreased as DLI increased, while rebaudioside A, B and C concentrations increased as DLI increased (up to max DLI observed).  These results indicate that the total amount of photosynthetic light received can affect both total steviol glycoside production and production of specific glycosides.