© 2004 Heron Publishing—Victoria, Canada
Nitrogen storage and its interaction with carbohydrates of young apple trees in response to nitrogen supply
Lailiang Cheng (1, 2), Fengwang Ma (1) and Damayanthi Ranwala (1)
1. Department of Horticulture, Cornell University, Ithaca, NY 14853, USA / 2. Author to whom correspondence should be addressed (LC89@Cornell.edu) / Received March 17, 2003; accepted June 20, 2003; published online December 1, 2003
Summary
Bench-grafted ‘Fuji/M.26’ apple (Malus domestica Borkh.) trees received a constant nitrogen (N) supply (10.7 mM) from bud break to the end of June, and were then fertigated
with 0, 5, 10, 15 or 20 mM N in a modified Hoagland’s solution for 2 months during the summer. In mid-October, half of the
trees fertigated at each N concentration were sprayed twice with 3% urea, whereas the remaining trees served as controls.
All trees were harvested after natural leaf fall and were stored at 2 °C. Five trees from each of the N treatment combinations
were destructively sampled during dormancy to determine the composition of N and total nonstructural carbohydrates (TNC).
As the N supply from fertigation increased, amounts of N in both free amino acids and proteins increased, whereas C/N ratios
decreased. Foliar urea applications in the fall significantly increased amounts of N in both free amino acids and proteins,
but decreased their C/N ratios. Arginine, the most abundant amino acid in both free amino acids and in proteins, accounted
for an increasing proportion of N in free amino acids and proteins with increasing N supply from fertigation or foliar urea
application. The ratio of protein N to free amino acid N decreased from about 27.1 to 3.2 as N supply from fertigation increased
from 0 to 20 mM, and decreased further to 3.0 in response to foliar urea applications in the fall. Concentrations of glucose,
fructose, sucrose and TNC decreased as the N supply from fertigation increased, whereas concentrations of sorbitol and starch
remained relatively unchanged. Foliar urea applications decreased the concentration of each TNC component and the TNC concentration
in each N fertigation treatment. A negative linear relationship was found between carbon in TNC and N in proteins and free
amino acids. The sum of carbon in TNC, proteins and free amino acids remained constant in response to N supply from fertigation.
However, foliar urea applications decreased the sum of carbon in proteins, free amino acids and TNC because about 21% of the
decrease in TNC carbon was not recovered in free amino acids or proteins. Young apple trees store N and carbon dynamically
in response to N supply. As N supply increases, an increasing proportion of N is found in the form of free amino acids, which
have a low carbon cost, although proteins remain the main form of N storage. Furthermore, part of the carbon from TNC is incorporated
into amino acids and proteins, decreasing the carbon stored as TNC and increasing the carbon stored as amino acids and proteins.
Keywords:
C/N ratio, foliar urea, free amino acids, Malus domestica, nitrogen fertigation, proteins, total nonstructural carbohydrates (TNC).
