Supplemental light before harvest increases phenethyl isothiocyanate in watercress under 8-hour photoperiod

Watercress (Nasturtium officinale R.Br.) plants were grown in growth chambers at 15 degrees C or 25 degrees C and either 8- or 12-h photoperiod (PP). The photosynthetic photon flux (PPF) was 265 mu mol.m(-2).s(-1) in all chambers, but beginning 1 week before harvest, half of the plants in each chamber were subjected to a higher PPF (435 mu mol.m(-2).s(-1)). At harvest, watercress leaves and stems were analyzed for phenethyl isothiocyanate (PEITC) concentration. Without supplemental PPF, watercress grown at 25 degrees C and 12-h PP produced higher PEITC concentration in leaves and stems than plants grown at 15 degrees C and 12-h PP, or plants grown at 8-h PP and either temperature. With one week of supplemental PPF before harvest, plants grown at 15 or 25 degrees.C and the 8-h PP produced PEITC concentrations as high as plants exposed to 12-h PP and similar temperatures. However, a week of supplemental PPF did not alter PEITC concentrations in plants grown at the 12-h PP, regardless of temperature. At 25 degrees C, plants grown under the low PPF and the 12-h PP produced 62% greater dry mass than plants exposed to a week of high PPF and the 8-h PP, but did not differ in PEITC content. Thus, the effect of one week of high PPF on PEITC concentration depended on photoperiod.