Effect of NO3- supply on N metabolism of potato plants (Solanum tuberosum L.) with special focus on the tubers

The response of the tubers to NO₃^– was studied in comparison to the other organs of Solanum tuberosum var. Sava, with special focus on: (a) whether tubers are capable of primary N assimilation; (b) whether N assimilation is stimulated by NO₃^–; and (c) whether primary N assimilation in tubers is important for tuber growth. NO₃^– reduction via nitrate reductase (NR; EC 1.6.6.1) and NH₄⁺ assimilation via glutamine synthetase (GS; EC 6.3.1.2) occurred predominantly in the shoots, but up to 20% was contributed by the tubers under low‐NO₃^– conditions. NR activation was highest in tubers (up to 90%) and declined in all organs with increasing NO₃^– supply. NR and GS activity responded with a decline in tubers and roots as opposed to an increase in the shoots. This corresponded to relative organ growth: growth of tubers and roots was stimulated relative to that of shoots at a limiting NO₃^– supply. Absolute growth of all organs was stimulated by NO₃^–, whereas tuber number declined. The concentration of N compounds increased with NO₃^– supply in all organs: NO₃^– increased most dramatically in the shoots (81‐fold), free amino acids most markedly in the tubers (three‐fold). The amount of patatin and of the 22 kDa protein complex in the tuber reached a minimum when the amount of Rubisco in the shoot reached maximum as a response to NO₃^– supply. Tuber sucrose and starch increased by 40%, whereas glucose and fructose declined two‐fold as plant N status increased. It is concluded that tubers are potentially N autotroph organs with capacity for de novo synthesis of amino acids. Primary N assimilation in tubers, however, declines with increasing NO₃^– supply and is not of major importance for tuber growth.