Effects of the supply of K, Ca, and Mg on the absorption and assimilation of ammonium- and nitrate-nitrogen in tomato plants

Forty-two-day-old tomato (Lycopersicon esculentum Mill. cv. Fukuju Nigo) plants were treated with complete, K-free, Ca-limited, and Mg-free nutrient solutions for 10 days using 2 mM NH4NO3 as the N source, which was replaced with 4 niM 15NH4Cl or Na15NO3 for the following 2 days to examine the absorption, translocation, and assimilation of inorganic N in relation to the supply of K, Ca, and Mg. The changes in the leaf nitrate reductase activity (NRA) were examined in tomato plants grown in nutrient solutions in which the form of N source and the supply of K and Ca were varied. Leaf contents of K, Ca, and Mg decreased to 38, 45, and 67% of those of the control plants in the low-K plants, the low-Ca plants and the low-Mg plants, respectively. Tomato plants absorbed NO3-N more efficiently than NH4-N. A larger amount of labelled N from NO3-N than from NH4-N was translocated to shoot. Removal of K from the nutrient solution slightly enhanced the absorption of NH4-N, but suppressed the absorption, translocation, and assimilation of NO3-N with lowered leaf NRA. Limited supply of Ca did not affect the absorption and assimilation of NH4-N, but caused a lower absorption of NO3-N without altering the distribution of labelled N. In the low-Ca plants the leaf NRA level was low and the incorporation of labelled N from NO3-N into protein was reduced. Removal of Mg from the nutrient solution did not affect the absorption and assimilation of NH4-N and NO3-N in tomato plants. These results indicate that tomato plants require an adequate supply of K and Ca for absorbing N03-N and maintaining high levels of NRA compared with the assimilation of NH4-N.