Tree Physiology 22, 1297–1303
© 2002 Heron Publishing—Victoria, Canada
Growth of young apple trees in relation to reserve nitrogen and
carbohydrates
LAILIANG CHENG1,2 and LESLIE H. FUCHIGAMI3
1
Department of Horticulture, Cornell University, Ithaca, NY 14853, USA
2
Author to whom correspondence should be addressed ([email protected])
3
Department of Horticulture, Oregon State University, Corvallis, OR 97331, USA
Received February 25, 2002; accepted May 5, 2002; published online November 15, 2002
Summary Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertilized with a nutrient solution (fertigation) containing 0, 2.5, 5, 7.5, 10, 15 or 20 mM nitrogen (N)
in a modified Hoagland’s solution from June 30 to September 1. In mid-October, half of the trees in each N treatment
were sprayed twice with 3% urea, 1 week apart. The remaining
trees served as controls. All trees were harvested after leaf fall
and stored at 2 °C over winter. One group of trees from each
treatment was destructively sampled before bud break to determine amounts of reserve N and total nonstructural carbohydrates (TNC); the remaining trees were transplanted to N-free
medium in the spring. These trees were supplied with Hoagland’s solution with or without 10 mM N (from 15N-depleted
NH 4NO3) for 60 days, starting from bud break. With increasing N supply from fertigation, tree N concentration increased,
whereas TNC concentration decreased. Foliar urea applications increased tree N concentration and decreased TNC concentration in each N fertigation treatment. There was a negative
linear relationship between tree N concentration and TNC concentration. Irrespective of whether N was provided the following spring, trees with high N reserves but low carbohydrate reserves produced a larger total leaf area at the end of the
regrowth period than trees with low N reserves but high carbohydrate reserves. The pooled data on reserve N used for new
growth showed that, regardless of the spring N supply, there
was a linear relationship between total N accumulated in the
tree during the previous season and the amount of reserve N
remobilized for new shoot and leaf growth. About 50% of tree
N content was remobilized to support new shoot and leaf
growth over the range of tree N status examined. We conclude
that the initial growth of young apple trees in the spring is determined mainly by reserve N, not reserve carbohydrates. The
amount of reserve N remobilized for new growth in spring was
proportional to tree N status and was unaffected by current N
supply.
Keywords: fertigation, foliar urea, growth, Malus domestica,
remobilization, total nonstructural carbohydrates (TNC).
Introduction
Deciduous fruit trees accumulate nitrogen (N) and carbohydrates by the end of the growing season and use these reserve
nutrients to support initial growth and development the following spring (Titus and Kang 1982, Tromp 1983, Oliveira
and Priestley 1988, Loescher et al. 1990). Total nonstructural
carbohydrates (TNC), including both starch and soluble sugars, make up 15 to 30% of the total dry matter of a dormant apple (Malus domestica Borkh.) tree. In contrast, total N accounts for less than 2% of total dry mass in winter. Both N and
carbohydrate reserves are essential for new growth because
they provide energy and building blocks before significant
root uptake of N and photosynthesis occur in spring. However,
the limitation that these nutrient reserves impose on tree
growth in the spring has not been quantified. An understanding of whether reserve N or reserve carbohydrate limits growth
in the spring has important practical implications for managing reserve N and reserve carbohydrates to improve apple tree
performance.
Manual defoliation has been used to study the relationship
between reserve carbohydrate concentration and tree development the following spring. Early defoliation of apple (Abusrewil and Larsen 1981, Tustin et al. 1997), cherry (Prunus
avium L.) (McCamant 1988, Loescher et al. 1990), pecan
(Carya illinoensis Wangeth) (Worley 1979) and pistachio
(Pistacia vera L.) (Nzima et al. 1999) trees in the fall results in
poor growth the following spring that is associated with decreased TNC concentrations. However, because early defoliation also decreases reserve N (Faby and Naumann 1986, Guak
et al. 2001), it remains unclear whether the reduced growth of
defoliated trees in spring is caused by low reserve carbohydrates, low reserve N, or both.
Nitrogen metabolism and carbohydrate metabolism are interrelated, because carbon assimilation depends on N metabolism to provide the photosynthetic machinery, and N assimilation requires carbohydrate input for the carbon skeleton and
energy supply. It is expected that, with increasing soil N supply during the growing season, tree growth and N reserves will
increase, but carbohydrate reserves may decrease. Foliar N applications in the fall may further increase N reserves and con-
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CHENG AND FUCHIGAMI
vert some of the carbohydrates to amino acids and proteins.
Therefore, manipulation of the N supply may differentially alter the amounts of reserve N and reserve carbohydrates, enabling us to determine which reserve limits tree growth the following spring.
The objectives of this study were to determine: (1) effects of
soil N supply during the growing season and foliar N applications after terminal bud set on tree N and TNC concentrations
and the relationship between the two; (2) initial growth in
spring in relation to reserve N and TNC; and (3) effects of N
supply in the spring on utilization of reserve N for the new
growth of young apple trees.
conditions. Starting at bud break on April 22, one group of
trees received 300 ml of 10 mM N from 15N-depleted NH 4NO3
(0.03% 15N abundance; ISOTEC, Miamisburg, OH) in Hoagland’s solution every 3 days for 60 days. The other group of
trees received no N but received all other mineral nutrients
from the Hoagland’s solution. All trees were harvested on
June 21, 1998, after 60 days of growth. At the time of harvest,
all lateral shoots and spurs had stopped growing, except the
terminal shoot. Each tree was divided into leaves, new shoots
(including spurs), 2-year-old stem, shank and roots. Total leaf
area was measured with an LI-3000 portable leaf area meter
(LI-COR, Lincoln, NE). All samples were oven-dried,
weighed and ground for total N and 15N measurements.
Materials and methods
Carbohydrate and nitrogen analysis
Plant culture and nitrogen treatments
Fuji apple (Malus domestica Borkh.) trees on M.26 rootstock
were bench-grafted in late March 1997. Each grafted tree was
potted in a 3.8-l container filled with a 1:2:1 (v/v) mixture of
peat moss, pumice and sandy loam soil. The trees were grown
in a lath house until early June. During this period, beginning
at bud break in early May, the trees were irrigated every
2 weeks with a solution of 150 mg N l –1 (fertigation), supplied
as Plantex® 20N-10P2O5-20K2O water-soluble fertilizer with
micronutrients (Plantex, Brampton, ON, Canada). When new
shoots were about 15 cm long, trees were selected for uniformity and moved outdoors in full sunlight where they were fertigated weekly with Plantex® for another 3 weeks. Beginning
on June 30, trees were randomly assigned to one of seven N
treatments (0, 2.5, 5, 7.5, 10, 15 or 20 mM N). There were 30
trees in each N treatment in a completely randomized design,
with a surrounding guard row. The trees were fertigated twice
weekly for 2 months with 300 ml of a modified Hoagland’s
solution per pot (Cheng and Fuchigami 2000). Thereafter,
trees received no soil-applied N or other nutrients. Trees were
subirrigated from saucers beneath the pots. All trees had
ceased growing by September 25.
Half of the trees in each N fertigation treatment were
sprayed with 3% urea on October 13 and again on October 21.
The remaining trees served as controls. Leaves of trees
fertigated with the lowest N concentration began to abscise on
November 2, which was about 5 to 10 days earlier than trees
fertigated with medium to high N concentrations. Foliar urea
applications advanced leaf abscission by about 3 days. By November 30, leaf abscission was complete in trees in all treatments, and all trees were bare-rooted and stored over winter at
2 °C with their roots buried in moist sawdust. Five trees from
each treatment were destructively sampled on March 1, 1998,
before bud break (referred to as dormant trees). Each tree was
divided into 1-year-old stem, rootstock shank, and roots. All
samples were frozen at –80 °C, freeze-dried and then ground
to pass a 40-mesh screen. A composite sample was made for
each tree to determine concentrations of N and TNC. The remaining trees in each treatment were divided into two groups
and transplanted to N-free medium (perlite:vermiculite 1:1,
v/v) on April 5. All trees were grown outside under natural
Fifty-mg composite samples, with xylitol added as an internal
standard, were extracted three times at 70 °C with 80% ethanol
(3 ml each, 30 min per extraction). Tissue suspensions were
centrifuged at 4000 g for 10 min after each extraction, and the
supernatants were combined. The extract was passed through
ion exchange columns consisting of 1 ml of Amberlite IRA-67
(acetate form; Sigma) and 1 ml of Dowex 50W (hydrogen
form; Sigma) to remove charged material. The extract was
then evaporated to dryness at 55 °C and dissolved in 10 ml of
water. After appropriate dilution, 25 µl was injected into a
Dionex DX-500 series chromatograph, equipped with a Carbopac PA-1 column, a pulsed amperometric detector and a
gold electrode (Dionex, Sunnyvale, CA). Carbohydrates were
eluted at a flow rate of 1.0 ml min –1 with 200 mM NaOH for
15 min. The concentration of individual soluble carbohydrates
was determined based on peak area and the calibration curve
derived from the corresponding standard authentic sugar. Total
soluble sugars include sorbitol, glucose, fructose and sucrose.
The tissue residue after soluble sugar extraction was dried and
digested with amyloglucosidase at 55 °C overnight to convert
starch to glucose. The concentration of glucose was quantified
with the Dionex chromatograph. Total nonstructural carbohydrates were the sum of starch and soluble sugars, and are also
referred to as reserve carbohydrates throughout this paper.
Tissue N concentration was determined colorimetrically
with an autoanalyzer after micro-Kjeldahl digestion (Schuman et al. 1973). Total N in a dormant tree comprises N that
can be remobilized to support new growth the following spring
and N that cannot be remobilized. Strictly speaking, only the
former is reserve N; however, unlike the case of carbohydrates, no chemical method has been developed to measure total reserve N directly. Therefore, we used total N to indicate
reserve N status of a dormant apple tree, assuming that reserve
N was proportional to total N (see Figure 7).
The isotopic ratios (15N/14N) of the samples harvested at the
end of the regrowth period of 1998 were measured by Isotope
Services in Los Alamos, NM, USA. These ratios were converted to atom% 15N abundance based on a natural abundance
value of 0.3663% (Junk and Svec 1958). The percentage of N
derived from the labeled fertilizer (NDFF%) was calculated
as (0.3663 – tissue atom% 15N)100/(0.3663 – fertilizer atom%
15
N). For trees that received no N during the regrowth period,
TREE PHYSIOLOGY VOLUME 22, 2002
APPLE TREE GROWTH, RESERVE NITROGEN AND CARBOHYDRATES
total N present in the new shoots and leaves was taken as the
amount of reserve nitrogen remobilized from the N storage
pool for new growth. We assumed that other sources of N, such
as air and water, provided negligible amounts of N to tree
growth. The total tree N accumulated during the previous year
was the sum of N in the new growth (leaves and shoots) and
that in the 2-year-old stem, shank and roots. For trees that received 10 mM N from the 15N-depleted NH 4NO3 during the
regrowth period, the amount of reserve N remobilized for new
shoot and leaf growth was calculated as the difference between
the total amount of N present in the new shoots and leaves and
that derived from the labeled fertilizer. Similarly, total tree N
accumulated during the previous year was calculated as the
sum of N, excluding that from the labeled fertilizer, in new
shoots and leaves, 2-year-old stem, shank and roots.
Statistical analysis
For concentrations and contents of reserve N and carbohydrates, analysis of variance (ANOVA) for a 7 (N fertigation) ×
2 (foliar N) factorial design was used. For total leaf area and
leaf N status at the end of the regrowth period, ANOVA for a
7 (N fertigation) × 2 (foliar urea) × 2 (spring N supply) factorial design was used. Linear regression analysis was used to
evaluate relationships between tree N concentration and TNC
concentration and between total N accumulated during the
previous year and reserve N remobilized for new shoot and
leaf growth. All statistical analyses were performed with SAS
software (SAS Institute, Cary, NC).
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showed a curvilinear response to N supply from fertigation, increasing almost linearly up to 10 mM N, and then leveling off
with further increases in N supply.
Concentrations and contents of nitrogen and carbohydrates
in dormant Fuji/M.26 trees
Tree N concentration increased linearly as the N supply from
fertigation increased (Figure 2A). Foliar urea applications significantly increased tree N concentration across all seven N
fertigation treatments. However, this increase was dependent
on tree N status, with low-N trees being more responsive than
high-N trees (Figure 2A).
The TNC concentration, which included both starch and
soluble sugars, showed a curvilinear response to N supply
from fertigation (Figure 2B). It was highest in trees fertigated
with low N, declined with increasing N supply, and then
reached the lowest concentration in trees fertigated with
20 mM N. Foliar urea applications significantly decreased
TNC concentration, depending on tree N status, with low-N
trees being more responsive than high-N trees (Figure 2B).
When all data for tree N and TNC concentrations were
pooled, a negative linear relationship was found between N
Results
Dry mass of dormant Fuji/M.26 trees
Because total dry mass of dormant Fuji/M.26 trees did not differ significantly between trees treated with foliar urea and control trees, the data were pooled (Figure 1). Tree dry mass
Figure 1. Dry mass of dormant Fuji/M.26 trees in response to nitrogen
(N) fertigation during the 1997 growing season and foliar urea application in the fall of 1997. Dry mass data of control and urea-treated
trees (five each) were combined at each N fertigation concentration
because no significant difference was detected. Trees were destructively harvested on March 1, 1998. Each value is the mean with standard error (SE).
Figure 2. Tree nitrogen (N) concentration (A) and concentration of total nonstructural carbohydrates (TNC) (B) of dormant Fuji/M.26 trees
in response to N fertigation during the 1997 growing season and to foliar urea applications in the fall of 1997. Trees were destructively harvested on March 1, 1998. Each value is the mean with SE of five
replicates. Closed circles denote trees that were fertigated with one of
seven N concentrations during the growing season, whereas open circles denote fertigated trees that also received fall applications of foliar
urea. Values of P were < 0.0001 for N fertigation, foliar urea, and the
interaction between the two.
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CHENG AND FUCHIGAMI
concentration and TNC concentration (Figure 3). As tree N
concentration was increased through N fertigation or foliar
urea application, TNC concentration decreased.
On a whole-tree basis, the total amount of N increased linearly with increasing N supply from fertigation (Figure 4A).
Total reserve carbohydrates increased with increasing N supply up to 7.5 mM, then decreased with further increases in N
supply (Figure 4B). Foliar urea applications increased total N
reserves, but caused a decrease in total carbohydrate reserves
in trees in each N fertigation treatment (Figure 4).
Relationship between spring growth and reserve N and
reserve carbohydrates
Independently of whether N was provided in the spring, tree
total leaf area at the end of the regrowth period increased
curvilinearly with increases in previous-year N supply from
fertigation (Figure 5). Trees sprayed with foliar urea in the previous fall produced a larger total leaf area the following spring
at each given N fertigation concentration. That is, when tree
size was similar, trees with higher N reserves but lower carbohydrate reserves had more new leaf growth than trees with
lower N reserves but higher carbohydrate reserves. Current N
supply in the spring increased tree total leaf area only about
10%, although the increase was statistically significant.
In addition to new leaf growth, leaf N concentration was related to tree reserve N status. Regardless of the current N supply in the spring, trees sprayed with foliar urea in the previous
fall maintained a higher leaf N concentration at the end of the
regrowth period than control trees (Figure 6). When no N was
provided in the spring, leaf N concentration (Figure 6A) corresponded well with tree N concentration before regrowth (Figure 2A). Providing trees with an adequate supply of N during
the regrowth period improved leaf N status, but mainly in trees
previously fertigated with low N concentrations (Figure 6B).
Figure 3. Relationship between tree nitrogen (N) concentration and
total nonstructural carbohydrate (TNC) concentration in dormant
Fuji/M.26 trees. Trees were destructively harvested on March 1,
1998. Closed circles denote trees that were fertigated with one of
seven N concentrations during the 1997 growing season and open circles denote fertigated trees that also received fall applications of foliar
urea. Regression equation: y = 135.92 – 4.84x (r 2 = 0.908, P <
0.0001).
Reserve N utilization for new growth in relation to total tree
N accumulated during the previous year and the current N
supply in spring
When all data for reserve N utilization for new shoot and leaf
growth were pooled, there was a single linear relationship between total N accumulated in the tree during the previous
growing season and the amount of reserve N remobilized for
new shoot and leaf growth in the spring, irrespective of the current N supply during the regrowth period (Figure 7).
For trees supplied with 10 mM N during spring regrowth,
the percent contribution of reserve N to new shoot and leaf
growth increased curvilinearly from 43 to 90% as the amount
of N accumulated in the tree in the previous year increased
from 0.15 to 1.21 g tree –1 (Figure 8). For all of the trees that received foliar urea applications the previous fall, the percent
contribution was maintained at 85–90%.
Discussion
We found a negative relationship between tree N concentra-
Figure 4. Total nitrogen (N) content (A) and total nonstructural carbohydrate content (B) of dormant Fuji/M.26 trees as affected by nitrogen fertigation during the 1997 growing season and foliar urea
applications in the fall of 1997. Trees were destructively harvested on
March 1, 1998. Each value is the mean with SE of five replicates.
Closed circles denote trees that were fertigated with one of seven N
concentrations during the growing season and open circles denote
fertigated trees that also received foliar urea applications. For total
tree N, P < 0.0001 for both N fertigation and foliar urea application,
with no significant interaction between the two. For total tree carbohydrates, P < 0.0001 for both N fertigation and foliar urea application,
and P = 0.0002 for the interaction between the two.
TREE PHYSIOLOGY VOLUME 22, 2002
APPLE TREE GROWTH, RESERVE NITROGEN AND CARBOHYDRATES
Figure 5. Total leaf area of Fuji/M.26 trees at the end of the regrowth
period in relation to previous nitrogen (N) fertigation and foliar urea
treatments (1997) with no N supply (A) or 10 mM N supply (B) during the spring regrowth period in 1998. Trees were destructively harvested on June 21, 1998. Each value is the mean with SE of five
replicates. Closed circles denote trees that were fertigated with one of
seven N concentrations during the 1997 growing season and open circles denote fertigated trees that also received foliar urea applications
in the fall. The P values are < 0.0001 for previous N fertigation and foliar urea application, and < 0.002 for spring N supply, with no significant interactions among the three.
tion and TNC concentration (Figure 3). This relationship was
predicted because little N exists as ammonium or nitrate in apple tissues, and N assimilation consumes carbohydrates for the
carbon skeleton and for energy. However, previous experiments failed to detect changes in TNC concentration, either
short- or long-term, after soil N fertilization, even though soluble N increased (Priestly 1972, Priestley and Catlin 1974,
Catlin and Priestley 1976). In these studies, utilization of carbohydrates for N assimilation may have been compensated for
by supplies from other parts of the tree during the growing season. Alternatively, the magnitude of changes in carbohydrate
concentration caused by N assimilation might not have been
apparent against the background of a large pool of carbohydrates (Priestley and Catlin 1974). In our experiment, tree N
concentration was dramatically increased by N fertigation or
by foliar urea applications (Figure 2). This may have made the
corresponding decrease in carbohydrates large enough to be
detected. Both utilization of the carbon skeleton and respiratory carbon loss during the process of N assimilation contributed to the decrease in carbohydrate concentration after foliar
urea application, but we did not determine the ratio between
1301
Figure 6. Leaf nitrogen (N) concentration of Fuji/M.26 trees at the
end of the regrowth period, in relation to previous N fertigation and
foliar urea treatments (1997) with no N supply (A) or 10 mM N supply (B) during the spring regrowth period in 1998. Trees were destructively harvested on June 21, 1998. Each value is the mean with SE of
five replicates. Closed circles denote trees that were fertigated with
one of seven N concentrations during the 1997 growing season and
open circles denote fertigated trees that also received fall applications
of foliar urea. The P values are < 0.0001 for previous N fertigation, foliar urea application, spring N supply and the interaction between
previous N supply and foliar urea application, and < 0.0005 for the interaction between previous N fertigation and spring N supply, with no
other significant interactions.
Figure 7. Reserve nitrogen (N) remobilized for new shoot and leaf
growth at the end of the regrowth period in relation to total amount of
N accumulated in the tree during the previous growing season (1997).
Closed circles denote trees that received no nitrogen and open circles
denote trees that received 10 mM N during the spring regrowth period
in 1998. Regression equation: y = –0.0244 + 0.5346x (r 2 = 0.96, P <
0.0001).
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CHENG AND FUCHIGAMI
Figure 8. Contribution of reserve nitrogen (N) to new shoot and leaf
growth of Fuji/M.26 trees supplied with 10 mM N during the
regrowth period in 1998 in relation to total amount of N accumulated
in the tree during the previous growing season (1997). Each value is
the mean with SE of five replicates. Closed circles denote trees that
were fertigated with one of seven N concentrations during the 1997
growing season and open circles denote fertigated trees that also received foliar urea applications in the fall of 1997.
the two. Presumably, a large proportion of the carbon in the
carbohydrates used during N assimilation may still exist in the
form of amino acids and proteins.
The differential responses of tree N and carbohydrates to N
supply (Figure 4) made it possible to determine spring growth
in relation to reserve N and carbohydrates. Total leaf area at
the end of the regrowth period increased with increasing N
supply from fertigation in the previous year (Figure 5). When
trees of similar size were compared, trees with higher N reserves but lower carbohydrate reserves produced a larger total
leaf area than trees with lower N reserves but higher carbohydrate reserves. These findings provide strong evidence that the
initial growth of young apple trees in the spring is determined
mainly by reserve N, not by reserve carbohydrates. This contrasts with the conventional view that growth of apple trees in
spring is largely limited by reserve carbohydrate supply.
The concept of carbohydrate-driven regrowth may have
been based on the correlations observed between changes in
reserve carbohydrates and tree growth in the following two
situations. First, as new growth resumes in spring, reserve carbohydrate concentrations decrease (Priestley 1960, 1963).
Second, when trees are manually defoliated to alter reserve
carbohydrate concentrations, reduced growth is associated
with decreases in reserve carbohydrate concentrations (Worley 1979, Abusrewil and Larsen 1981, Loescher et al. 1990).
However, causal relationships cannot be established in either
case. In the former, N and other phloem mobile nutrients in the
storage tissues all decrease because of remobilization when
new growth occurs in the spring (Mason and Whitfield 1960,
Taylor 1967, Tromp 1983). In the latter, N mobilization from
foliage to storage tissues was prevented by defoliation. In addition, root uptake of mineral nutrients may also have been decreased by early defoliation through indirect effects on new
root growth. As a result, the concentration of N in storage tis-
sues was reduced by early defoliation (Faby and Naumann
1986, Guak et al. 2001).
We did not determine the contribution of reserve carbohydrates to new growth because reserve carbohydrates could not
be distinguished from current photosynthates. However, by
using 14CO2, Hansen (1971) estimated that until flowers begin
to show color on spurs and during the development of the first
five to six leaves for extension shoots, about one-half to twothirds of the building materials come from reserves. Based on
a carbon balance model for growing shoots of “Jonamac” apple trees, it takes 15 days for a spur or 19 days for an extension
shoot to become a net carbon exporter, and only about 20% of
the carbon required for new growth during this period comes
from reserves (Johnson and Lakso 1986). Regardless of the actual contribution of reserve carbohydrates to new growth, it
appears that they were not limiting for new growth in our experiment. Although higher concentrations of reserve carbohydrates accumulated in the trees that received no foliar urea
applications, these trees could not make the best use of their
reserve carbohydrates for new growth unless some of the carbohydrates were used to assimilate N from the foliar N applications (Figures 2 and 5). Thus, nitrogen plays a pivotal role in
the utilization of reserve carbohydrates for new growth.
The single, linear relationship between the total N accumulated in the tree during the previous year and the amount of reserve N remobilized for new shoot and leaf growth (Figure 7)
shows that remobilization of reserve N for new growth is proportional to tree N status, and is unaffected by the current N
supply. The dependence of reserve N utilization on tree N status has been shown in other studies, but most studies compared
only two tree N contents (Millard and Proe 1991, 1993,
Millard 1996). This is the first time that such a relationship has
been demonstrated over a wide range of tree N status. About
50% of tree N content was remobilized to support new shoot
and leaf growth over the range of tree N status examined. This
percentage is similar to that obtained with field-grown mature
walnut (Juglans regia L.) trees (Weinbaum and van Kessel
1998).
The percent contribution of reserve N to new growth in the
spring depends on the current N supply and on the amount of
reserve N remobilized for new growth. When no N was provided during regrowth, all of the N for new growth came from
reserves. When adequate N was supplied during regrowth,
remobilization provided about 43% of the total N for new
growth in trees with low N status; this percentage increased to
85–90% in trees with a medium to high N status (Figure 8).
Thus, reserve N contributes significantly to the N economy of
new growth even under adequate N supply conditions in the
spring. Similar findings with apple and several other tree species have been reported by Millard (1996). Our results also indicate that, irrespective of whether N was provided in the
spring, the amount of reserve N largely sets the potential for
initial growth (Figures 4 and 5). The current supply of N in the
spring only slightly increased total leaf area, its main effect being to improve or maintain the N status of the new growth (Figure 6).
TREE PHYSIOLOGY VOLUME 22, 2002
APPLE TREE GROWTH, RESERVE NITROGEN AND CARBOHYDRATES
We conclude that there is a negative relationship between N
concentration and the concentration of nonstructural carbohydrates in dormant young apple trees. Initial growth in the
spring is determined by the amount of reserve N and is not limited by reserve carbohydrates. The amount of reserve N remobilized for new growth is proportional to tree N status and
is unaffected by the current N supply.
Acknowledgments
This research was supported, in part, by the Washington Tree Fruit
Research Commission and the Pacific Northwest Nursery Improvement Institute. We gratefully acknowledge the help of Drs. Damayanthi Ranwala, Anil Ranwala and Bill Miller with carbohydrate
analysis, critical reading of the manuscript by Drs. Ian Merwin and
Marvin Pritts, and the editorial assistance of Priscilla Licht.
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