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

Abstract

ABSTRACT Tomato growth was examined in solution culture under constant pH and low levels of NH 4 + or NO 3 − . There were five nitrogen treatments: 20 mmoles m −3 NH 4 + , 50 mmoles m −3 NO 3 − , 100 mmoles m −3 NH 4 + 200 mmoles m −3 NO 3 − , and 20 mmoles m −3 NH 4+ + 50 mmoles m −3 NO 3 − . The lower concentrations (20 mmoles m −3 NH 4 + and 50 mmoles m −3 NO 3 − ) were near the apparent K m for net NH 4 + and NO 3 − uptake; the higher concentrations (100 mmoles m −3 NH 4 + and 200 mmoles m −3 NO 3 − ) were near levels at which the net uptake of NH 4 + or NO 3 − saturate. Although organic nitrogen contents for the higher NO 3 − and the NH 4 + + NO 3 − treatments were 22.2–30.3% greater than those for the lower NO 3 − 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 NH 4 + or NO 3 − concentrations are held constant over the root surface at near the apparent K m concentration. Relative growth rates for the two NH 4 + treatments were much higher than have been previously reported for tomatoes growing with NH 4 + as the sole nitrogen source. Initial growth rates under NH 4 + nutrition did not differ significantly ( P ≥ 0.05) from those under NO 3 − or under combined NH 4 + + NO 3 − . Growth rates slowed after 10–15 d for the NH 4 + treatments, whereas they remained more constant for the NO 3 − and mixed NH 4 + + NO 3 − treatments over the entire observation period of 24–33 d. The decline in growth rate under NH 4 + nutrition may have resulted from a reduction in Ca 2+ , K + , and/or Mg 2+ absorption.