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.