Tomato plants ( Lycopersicon esculentum Mill. cv. San Marzano), grown in dilute nutrient solutions containing (in meq ˙ 1 ‐1 ) 0.5 NaNO 3 , 0.5 NH 4 NO 3 or 0.25 (NH 4 ) 2 SO 4 as the nitrogen source, were detopped for collection of xylem sap and measurement of trans‐root electrical potentials. The plant parts and the xylem exudate were subsequently analysed for mineral content. The commonly observed effects of NH 4 + were noted, including reduction of calcium concentration in the xylem sap, and of calcium content in stems and leaves, compared with NO 3 ‐ fed plants. This effect was attributed principally to the less negative trans‐root electrical potential measured in NH 4 + ‐fed plants, and the resultant reduction of inward driving force on passively moving divalent cations.
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Macklon et al. (1980) studied this question.
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