Potassium recirculation in tomato plants in relation to potassium supply

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Abstract
The Ben Zioni ‐ Dijkshoorn hypothesis that NO3 uptake by roots is regulated by NO3 assimilation in the shoot was tested using the tomato plant. Plants were grown at three K levels and the fate of anion charge from NO3,‐ and SO4 2‐. assimilation followed in its distribution between organic acid anion accumulation and HCO3 ‐ efflux into the nutrient medium. For the high K treatment almost all of this charge was directed towards organic acid accumulation with HCO3 ‐ efflux accounting for only 3% of the total charge. On the other hand for plants supplied at the low K level, a substantial proportion of the anion charge was excreted as HCO3 ‐ (32%). Xylem sap analyses and NO3 reductase assay results indicated that in the tomato plant the upper plant parts constituted the major site for NO3 reduction. The quantitatively most important ionic constituents in the sap were K+, Ca2+ and NO3 ‐. Results have been presented that indicate that when K is in short supply in the nutrient medium, K recycling occurs within the plant to facilitate the upward transport of NO3 ‐ from root to shoot.