Sulphate uptake and xylem loading of young pea (Pisum sativum L.) seedlings

Sulphate uptake and xylem loading was analysed in young pea (Pisum sativum) seedlings. The rate of sulphate uptake into intact 8-days-old pea seedlings (determined by a 1 h exposure to radiolabelled sulphate in the nutrient solution) was 585 nmol sulphate g^–1 root fresh weight h^–1. When the cotyledons were removed on day 6 the 8-days-old seedlings took up only 7% of the controls. Interruption of the phloem transport by steam girdling of the stem or the root (1 h before incubation with radiolabelled sulphate) diminished sulphate uptake by approximately 50%. The addition of sucrose to the nutrient solution during incubation did not restore sulphate uptake rates indicating that the decrease was not due to a lack of energy. Apparently, a signal from the shoot and/or the cotyledons is necessary to stimulate sulphate uptake into the roots of pea seedlings. Glutathione fed to the roots for 3 h prior to incubation with radiolabelled sulphate diminished sulphate uptake by approximately 50%. The relative proportion of the sulphate taken up that was loaded into the xylem remained unchanged (between 7 and 9% of total uptake), even when the stem was girdled above the cotyledons or when the seedlings were pre-exposed to glutathione. Only removal of the cotyledons or girdling of the root below the cotyledons increased the proportion of sulphate loaded into the xylem to 13–15% of total uptake upon exposure to glutathione. Apparently, a signal from the cotyledons represses xylem loading to some extent.