Effect of ammonium:: nitrate nutrient ratio on nitrate and glucosinolate contents of hydroponically-grown rocket salad (Eruca sativa Mill.)

The effect of the ammonium-nitrogen (NH4+-N) to nitrate-nitrogen (NO3--N) ratio on NO3- and glucosinolate (GSL) contents in rocket salad (Eruca sativa Mill.) was investigated. Rocket salad plants were provided with five nutrient solutions with the same total nitrogen (N) level (10 mmol L-1), but with different (0, 25, 50, 75 and 100) percent mole ratios of NH4+ to NO3--nitrogen (PMR-N). Rocket growth (height and dry weight [DW] of the leaves and roots) was severely inhibited at a PMR-N of 100. The leaves were withered and showed chlorotic and necrotic phenomena from NH4+ toxicity. Leaf NO3- and sulfate (SO42-) contents sharply decreased with increasing PMR-N. Six GSLs including glucoraphanin, 4-(beta-D-glucopyranosyldisulfanyl) butyl GSL, glucoerucin, glucobrassicin, dimeric 4-mercaptobutyl GSL and 4-methoxyglucobrassicin were identified from rocket salad by their retention times using high-pressure liquid chromatography and confirmed using electrospray ionization mass spectrometry (ESI-MS). An unknown compound (m/z 414 as desulfo-GSL) only appeared at a PMR-N of 100. This compound together with the other GSLs appears to be involved in detoxification of NH4+ toxicity. Total GSL content in the leaves ranged from 9 to 13 pmol kg(-1) DW, with the highest content occurring at a PMR-N of 50 and the lowest value at a PMR-N of 100. In contrast, total GSL content in the roots ranged from 31 to 48 pmol kg(-1) DW, with the lowest value occurring at PMR-Ns of 50 and 100. The four GSLs. glucoraphanin, 4-(beta-D-glucopyranosyldisuffanyl)butyl GSL, glucoerucin and dimeric 4-mercaptobutyl GSL were the major compounds in the leaves, whereas glucoerucin was, found in great quantities in the roots.