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

Abstract

The effect of the ammonium-nitrogen ( -N) to nitrate-nitrogen ( -N) ratio on 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 to -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 toxicity.Leaf and sulfate ( ) contents sharply decreased with increasing PMR-N.Six GSLs including glucoraphanin, 4-(-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 toxicity.Total GSL content in the leaves ranged from 9 to 13 mol 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 mol kg -1 DW, with the lowest value occurring at PMR-Ns of 50 and 100.The four GSLs glucoraphanin, 4-(-D-glucopyranosyldisulfanyl)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.