Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T‐5)

Tomato growth was examined in solution culture under constant pH and low levels of NH4+ or NO3−. There were five nitrogen treatments: 20 mmoles m−3 NH4+, 50 mmoles m−3 NO3−, 100 mmoles m−3 NH4+ 200 mmoles m−3 NO3−, and 20 mmoles m−3 NH4++ 50 mmoles m−3 NO3−. The lower concentrations (20 mmoles m−3 NH4+ and 50 mmoles m−3 NO3−) were near the apparent Km for net NH4+ and NO3− uptake; the higher concentrations (100 mmoles m−3 NH4+ and 200 mmoles m−3 NO3−) were near levels at which the net uptake of NH4+ or NO3− saturate. Although organic nitrogen contents for the higher NO3− and the NH4++ NO3− treatments were 22.2–30.3% greater than those for the lower NO3− treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH4+ or NO3− concentrations are held constant over the root surface at near the apparent Km concentration. Relative growth rates for the two NH4+ treatments were much higher than have been previously reported for tomatoes growing with NH4+ as the sole nitrogen source. Initial growth rates under NH4+ nutrition did not differ significantly (P≥ 0.05) from those under NO3− or under combined NH4++ NO3−. Growth rates slowed after 10–15 d for the NH4+ treatments, whereas they remained more constant for the NO3− and mixed NH4++ NO3− treatments over the entire observation period of 24–33 d. The decline in growth rate under NH4+ nutrition may have resulted from a reduction in Ca2+, K+, and/or Mg2+ absorption.