Studies on relationship between water regime and flavour strength in watercress(Rorippa nasturtium-aquaticum(L) Hayek), cabbage (Brassica oleracea capitata) and onion(Allium cepa)
Growth of certain vegetables under conditions of water stress has been shown to lead to increased flavour strengths. Watercress was grown in sand-compost mixtures in a glasshouse with three different water regimes. One group was maintained at approximately pot capacity by addition of a nutrient solution, in a second the solution was withheld until the incipient wilting point was reached, and the third group was intermediate between the other two. Cabbages and onions were grown in the field under moveable transparent shelters with two different water regimes. Flavour was determined gas chromatographically and by sensory and other methods. As determined by means of βphenylethyl isothiocya-nate peak areas, the overall range of watercress flavour strength was 6.4-fold. The corresponding values for cabbages and onions were 3.8-fold and 3.9-fold, respectively, based on gas-chromatographic headspace total peak areas. These ranges were relatively small as compared with corresponding changes at extremes of sulphur nutrition, and a possible explanation of the observations lies in the inducement of a mild form of sulphur deficiency arising from liberal watering. An alternative hypothesis is suggested, depending upon the accumulation of low molecular metabolites in plants subject to stress conditions. Examples, from the literature, of such accumulations under conditions of nutrient deficiency, water stress and low temperatures are discussed. By analogy, accumulation of flavour precursors, glucosinolates in the case of the cruciferous plants and alkyl-L-cysteine sulphoxides in onion, offer a probable mechanism of the observed facts.