Effect of Selenium Fertilization and Methyl Jasmonate Treatment on Glucosinolate Accumulation in Broccoli Florets

Broccoli (Brassica oleracea ssp. italica) is a rich source of glucosinolates (GSs), phytochemicals that are hydrolyzed into isothiocyanates with known human anticarcinogenic bioactivity. Increasing dietary intake of the element selenium (Se) can also reduce the risk of cancer. Previous research reported that Sc fertilization at high concentrations reduces the concentration of GSs in brassicaceous plants. This research was conducted to determine the effect of Se fertilization on accumulation of different types of GSs in broccoli floret tissues in five genotypes. Methyl jasmonate (MeJA), an elicitor known to stimulate biosynthesis of indolyl GSs, was used to analyze changes in biosynthetic capability of indolyl GSs in broccoli floret tissue under Se-enriched conditions. Five broccoli genotypes were subjected to root fertilization with low and high levels of Na2SeO4 solutions (0.17 and 5.2 mm), MeJA sprays to aerial portions of the plants (250 mu M), and the combined treatment of 5.2 mm Se with 250 mu M McJA, respectively. The effect of Se fertilization on GS accumulation varied among genotypes and the level of Se fertilization. Variation in the level of Se fertilization resulted in a dose-dependent decrease in glucoraphanin concentrations with no significant effect on indolyl CS accumulation in broccoli florets across the five genotypes. MeJA treatment increased indolyl and aromatic GS accumulation in floret tissues. MeJA-mediated increases in these GSs were inhibited in the high Se fertilization treatment, but the increase in neoglucobrassicin concentrations was less affected than other GSs in florets across the five genotypes. An experiment conducted with 6-week-old broccoli plants under the high Se treatment demonstrated greater accumulation of Se with depressed accumulation of sulfur and complete inhibition of MeJA-mediated indolyl GS accumulation compared with those changes in florets of mature broccoli plants. These results suggest that GS accumulation under Se fertilization may be influenced by not only the level of Se fertilization, but also the differences in sizes of available pools of resources (sulfur and sulfur-containing amino acids) required for GS biosynthesis and accumulation in broccoli plants. Partitioning of the variance indicated that the existence of substantial variability in GS concentrations was primarily attributed to differences in genotype response across different treatments. Results suggest that cultivar selection and breeding of broccoli can be used to develop broccoli germplasm with enhanced capacity for Se uptake and stability of GS biosynthesis with varying Se fertilization.