Seed glucosinolate, oil and protein contents of field-grown rape (Brassica napus L) affected by soil drying and evaporative demand

The effect of soil drying on seed yield, oil, protein, and glucosinolate contents was studied in rape (Brassica napus. L) grown in sandy and loamy soils in lysimeters in the field. By controlling irrigation, the plants were exposed to early drought (ED) during the vegetative and the flowering stage or late drought (LD) during the pod filling stage. Under low evaporative demands (2-4 mm day(-1)) in 1991, seed and oil yields were not significantly influenced by soil drying. Under high evaporative demands (4-5 mm day(-1)) in 1992, the ED and LD treatments on sand decreased the seed yield by 8% and 17% of the fully irrigated (FI) treatment, respectively; oil yield was significantly decreased (17% in both ED and LD treatments) on sand, only; protein yield was not significantly reduced by drought. In the fully irrigated treatment, the glucosinolate content was 9.7 mu mol/g dry matter (d.m.) in 1991 and 13.7 in 1992. Both ED and LD treatments increased glucosinolate content to between 11.7 and 24 mu mol/g d.m. in the two years. The results reveal that glucosinolate synthesis was increased when leaf or pod midday water potential was less than - 1.4 MPa for extended periods. Below this potential, the tissue turgor pressure of leaves and pods was low or zero. When turgor was low and the number of stress days (SD) exceeded 6.4, the glucosinolate content increased linearly with the number of stress days by 1.49 mu mol (glucosinolate) g(-1) (d.m.) SD-1. Water stress occurring during vegetative growth also increased seed glucosinolate content. It is proposed that glucosinolates are produced as secondary metabolites in droughted tissue at low turgor and that under these conditions glucosinolate precursors are produced for later use. In 1992 under severe stress, the glucosinolate content also correlated with seed size.