Genotype, nitrogen fertility and sulphur availability interact to affect flavour in garlic (Allium sativum L)

Experiments were carried out to investigate the influence of combined nitrogen (N) and sulphur (S) fertility on the organosulphur compound content of garlic bulbs, especially alliin, as a specific flavour quality trait related to the health-value of garlic. Three cultivars, 'Printanor', 'MorasoF and 'Messidrôme', were grown in the greenhouse and in vitro. Sulphur was increased in the macronutrients within the range 0 -144 mg l-1. Numerous combinations of both S and N were tested in vitro. In the greenhouse, increasing N levels, from 184.8 mg l-1 to 856.8 mg l-1, were tested with a single high level of S (128 mg l-1). Increasing S alone did not affect the growth or bulb weight of any garlic cultivar, but slightly increased the alliin content of bulbs. No symptoms of S deficiency were observed under greenhouse conditions, although the same garlic cultivars grown in vitro at 0 mg S l-1 clearly suffered, their size being greatly reduced. This suggests an important role for S contamination, such as atmospheric S, in the process of S absorption by garlic plants, which is discussed. Increasing N levels significantly accelerated garlic maturity and decreased bulb yield. The effect of N on the organosulphur compound content of bulbs was cultivar-dependent, but alliin accumulation was not enhanced. 'Printanor' and 'Morasol' had the highest alliin levels and were negatively influenced by increasing N, while a slight synergistic effect of S and N was observed on 'Messidrôme'. 'Messidrôme' also had the highest flavour potential when including precursors, and was more efficient in accumulating S into organosulphur compounds. It is therefore advisable to choose the most appropriate variety, and to adapt the N × S fertilisation regime according to the environment, in order to produce garlic with the highest health-value.