Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5) |
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Authors: | D R SMART A J BLOOM |
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Institution: | Department of Vegetable Crops, University of California, Davis, CA 95616, USA |
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Abstract: | 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. |
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Keywords: | tomato growth kinetics ammonium nitrate absorption |
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