Depression of Nitrate and Ammonium Transport in Barley Plants with Diminished Sulphate Status. Evidence of Co-regulation of Nitrogen and Sulphate Intake

When barley plants were grown in a solution with nitrate as the sole N-source but deprived of sulphate ( —S plants) for 1 to 5 d, the capacity for sulphate transport by the roots increased very markedly; subsequent measurement of influx using 35S-labelled SO4"" showed increases of > 10-fold compared to plants continuously supplied with sulphate ( + S plants). There were only small effects on plant growth over a 5 d period and yet the influx of NO3, labelled with the short-lived tracer 13N, was diminished by approximately 30%. By contrast, the influx of phosphate was little affected by sulphate-deprivation. When a sulphate supply was restored to — S plants, the sulphate influx was quickly repressed over the subsequent 24 h and the nitrate influx was restored to >90% of the value in +S plants. When plants were grown in a solution with a mixed nitrate and ammonium supply and deprived of sulphate for 1 d or 5 d the depression of nitrate influx was more strongly marked (up to 55% depression). The influx of ammonium was also depressed after 5 d of sulphate-deprivation, but not at 1 d, nor where the concentration of ammonium in the uptake solution was lowered to 20mmol m-3 or less. Additional measurements with 15N-labelled nitrate and ammonium over longer periods were used to determine net uptake. Net uptake of nitrate was depressed to a similar extent to efflux, but net ammonium uptake was depressed only in unbuffered uptake solution where the pH decreased to pH 4-9 during the uptake period. The 15N-tracer experiments showed that the translocation of label to the shoot, from both nitrate and ammonium, was depressed to a greater extent than net uptake in — S plants. The depression of nitrate influx, caused by 5 d of sulphate deprivation, could be relieved almost completely by providing plants with 10mol m-3 L-methionine during the day prior to influx measurement. This treatment substantially decreased sulphate and potassium (86Rb-labelled) influx in both +S and — S plants, but greatly increased total S-status of the plants. This methionine treatment had no effect on ammonium influx or net uptake in — S plants but increased influx significantly in +S ones. When plants were grown with sulphate but deprived of nitrate for 4 d there was a marked depression of the sulphate influx (by 48-65%) but a smaller effect on phosphate influx (21-37% of +N). The results are discussed in relation to the effects of sulphate-deprivation on growth rate and the root:shoot weight ratio. It is concluded that the effects on influx and net uptake of nitrogen are more severe than could be accounted for by these factors. The decreased translocation of either nitrate, or the products of nitrate and ammonium assimilation from the roots, is suggested as a reason for the depression of influx. The restoration of nitrate influx and net uptake by methionine suggests that, for this ion at least, a shortage of S-amino acids within the plant may lead to the accumulation of inhibitory concentrations of non-S amino acids in the transport pool.