Environmental stress affects the glucosinolate-myrosinase system in brassica species

Glucosinolates are secondary metabolites present in the order Capparales which includes the economically important Brassicaceae family. Although glucosinolates are generally not bioactive, enzymatic hydrolysis by myrosinase produces breakdown products with fungicidal, bactericidal , nematocidal, and allelopathic activity. Especially relevant for human health are reports that glucosinolate degradation products have cancer chemoprotective properties.

Glucosinolate levels and myrosinase activity are known to vary by species, plant organ, and stage of development. Plants grown in seasons characterized by high temperatures, high levels of photosynthetic photon flux (PPF) , and long photoperiods tend to contain high levels of glucosinolates, but there are few reports that attempt to correlate glucosinolate levels with specific climatic factors. The effects of season and associated climatic conditions on myrosinase activity has been completely neglected.

The objectives of this research were: 1) to measure glucosinolate concentration in six Brassica cultivars to determine how it correlates to the isothiocyanate profile upon hydrolysis by myrosinase, 2) to determine how temperature, PPF, and photoperiod affect glucosinolate levels and myrosinase activity in leaves, stems, and roots of a rapid-cycling base population of Brassica oleracea grown in a controlled environment, and 3) to determine how temperature, PPF, and photoperiod affect glucosinolate levels and myrosinase activity in ten Brassica cultivars grown during two fall and two spring seasons.

In this study, conversion of glucosinolates to isothiocyanates varied by cultivar and glucosinolate content alone was not a complete predictor of bioactivity resulting from glucosinolate breakdown products. In the controlled environment and field experiments, genotype, temperature, PPF, and photoperiod significantly affected glucosinolate levels, and genotype, temperature, and PPF significantly affected myrosinase activity. Therefore, selection of cultivar, botanical group, and planting time must be considered when glucosinolate levels and myrosinase activity in cruciferous crops are of interest.