The biosynthesis of steviol glycosides is affected by both genetic and environmental factors. To evaluate the influence of total daily solar radiation or daily light integral (DLI) under long-day conditions on steviol glycoside concentration, we grew Stevia rebaudiana under ambient irradiance or varying levels of shading at different times of the year in both greenhouse and field environments, resulting in DLIs ranging from 3.55 to 20.31 mol·m−2·d−1 in the greenhouse and 10.32 to 39.7 mol·m−2·d−1 in the field. Total steviol glycoside concentration of selected leaves from greenhouse-grown plants increased as DLI increased up to ca. 10 mol·m−2·d−1, remaining constant with further increases in DLI, and was similar across the range of DLIs evaluated in the field. DLI influenced both the concentration and the relative proportions of specific steviol glycosides. Rebaudioside A concentration increased as DLI increased from 3.55 to 8.53 mol·m−2·d−1, remaining similar with further increases in DLI. Rebaudioside D and stevioside concentration of selected leaves from field-grown plants decreased by 22% and 13%, respectively, as DLI increased from 10.32 to 39.7 mol·m−2·d−1, while rebaudioside A and M concentrations remained similar across this DLI range. Collectively, these results indicate that the greatest influence of DLI on steviol glycoside concentration occurs under relatively low DLIs (<10 mol·m−2·d−1). However, higher DLIs can significantly affect the synthesis of minor glycosides of increasing commercial importance including rebaudioside D.
Jennifer M. Evans, Veronica A. Vallejo, Randolph M. Beaudry, and Ryan M. Warner
Travis L. Stegmeir, Chad E. Finn, Ryan M. Warner, and James F. Hancock
Fragaria ×ananassa Duchesne ex Rozier, or the cultivated strawberry, resulted from the hybridization of two wild species, F. chiloensis (L.) Miller and F. virginiana Miller. In an attempt to recreate the cultivated strawberry, elite clones of F. chiloensis and F. virginiana were crossed within species and then hybridized to produce 26 reconstructed populations. Of these populations, FVC11 [(Frederick 9 × LH 50-4) × (Scotts Creek × 2 MAR 1A)] had unusually large fruit size and was selected for further analysis. In the summer of 2008, 78 individuals of this population were evaluated for their seasonal flowering patterns, inflorescence number, inflorescence height, crown production, flower number, fruit size, yield, internal color, soluble solids, fruit firmness, and plant vigor. Progeny means were compared with those of the parental means and most traits exhibited transgressive segregation, most notably yield and fruit weight. Significant positive correlations were found between many of the production traits, although there were significant negative correlations between fruit firmness and flower number per inflorescence, fruit firmness and soluble solids, and yield per plant and soluble solids. Overall performance scores were assigned to each genotype by summing their relative performance for each trait in the population. Individuals were identified that combined high values for fruit weight and yield with higher than average values for fruit color, firmness, and soluble solids. Use of this population in breeding programs could help expand the genetic base of the cultivated strawberry with limited linkage drag.