NET UPTAKE OF SULFATE AND ITS TRANSPORT TO THE SHOOT IN SPINACH PLANTS FUMIGATED WITH H2S OR SO2 - DOES ATMOSPHERIC SULFUR AFFECT THE INTER-ORGAN REGULATION OF SULFUR NUTRITION

Spinach plants (Spinacea oleracea L. cv. Estivato) were grown an nutri ent solutions under deficient, normal and excess sulfate supply. In bo th young and mature plants net uptake of sulfate and its transport to the shoot increased with increasing sulfate supply, but bath processes proceeded at a higher rate in young as compared to mature plants. The relative sulfate transport, i.e. the relative amount of the sulfate t aken up that is transported to the shoot, decreased with increasing su lfate supply. Apparently, net uptake of sulfate is not strictly contro lled by the sulfur demand of the shoot, but xylem loading appears to c ounteract excess transport of sulfate to the shoot. Fumigation with H2 S or SO2 reduced net uptake of sulfate by the roots in sulfur-deficien t plants and absolute as well as relative sulfate transport to the sho ot independent of the three sulfate levels supplied to the plant. At t he same time thiol contents of the shoot and the root were enhanced by fumigation with H2S and SO2. These findings are consistent with the i dea that thiols produced in the leaves can mediate demand-driven contr ol of sulfate uptake by the roots and its transport to the shoot.