The Effect of Field Methyl Jasmonate Treatments On Glucosinolate and Phenolic Concentrations In Broccoli Florets

Glucosinolates and phenolic compounds found in the edible portions of brassica vegetables are reported to provide putative health-promoting bioactivity. This experiment focuses on the effect of MeJA treatment on glucosinolate and phenolic concentrations in broccoli head tissue under field conditions.  The effect of MeJA on glucosinolates and total phenol and flavonoid content in the florets of the commercial broccoli hybrids ‘Pirate’, ‘Expo’, ‘Green Magic’, ‘Imperial’, and ‘Gypsy’ were assayed in replicated field plantings in 2009 and 2010. 250 micromoles of MeJA was sprayed on aerial plant tissues four days prior to harvest at commercial maturity. Replicate samples were harvested, freeze-dried, and later assayed for glucosinolates and total flavonoids (quercetin and kaempferol) by HPLC and for total phenols by spectrophotometry. The MeJA treatment significantly increased glucoiberin (16%), glucoraphanin (10%), gluconasturtiin (59%), and neoglucobrassicin (345%) concentrations but decreased gluconapin (37%) among the genotypes across the two years. The year effect was significant on glucosinolate concentrations in control and treated groups except for gluconasturtiin and neoglucobrassicin in the MeJA treatment. There was a significant treatment by genotype interaction where MeJA increased neoglucobrassicin and gluconasturtiin in four of the genotypes with exception of ‘Pirate’. In contrast, total phenol and flavonoid concentrations were not influenced by MeJA across the genotypes. There was, however, a significant year effect, where the total phenol content of hybrids grown in 2010 was 81% higher than the same genotypes grown in 2009 for both the control and treatment groups. Quercetin and kaempferol concentration of hybrids grown in 2010 was 192% higher than the total flavonoid content in 2009. Interestingly, MeJA treatment increased total flavonoid content in 2009 but had no effect in 2010, suggesting there was a interaction between MeJA treatment and the different environment conditions in 2009 and 2010. Partitioning of variance indicated that the aliphatic glucosinolates (glucoiberin, progoitrin, and glucoraphanin) concentrations were primarily influenced by the genotype (over 40% of total variance). Concentrations of the indolyl glucosinolate neoglucobrassicin were largely influenced by MeJA treatment (61%). Both year (26%) and genotype (28%) described much of the variance for glucobrassicin. In contrast, total phenol (85%) and total flavonoid contents (53%) were strongly affected by the different environmental conditions in 2009 and 2010 and to a lesser extent by the genotype (3% and 8% respectively).