Full text of DOI:10.1111/j.1744-7348.2003.tb00288.x

Ann. appl. Biol. (2003), 143:215-229
215
Printed in UK
Dry matter production and partitioning in potato plants subjected to
combined deficiencies of nitrogen, phosphorus and potassium
By P D JENKINS* and S MAHMOOD
Institute of Rural Sciences, University of Wales, Llanbadarn, Aberystwyth, Ceredigion SY23 3AL, Wales, UK
(Accepted 17 April 2003; Received 5 November 2002)
Summary
Three experiments examined effects on growth, dry matter partitioning and nutrient uptake in potato
plants grown in large pots under different combinations of adequate and deficient levels of nitrogen,
phosphorus and potassium. N supply affected the growth of all leaves, with low N reducing both the
size of individual leaves and the extent of branch growth. P and K availability affected the growth of
later formed leaves and only when both were deficient was branch growth substantially reduced. At
later stages of growth, total green leaf area was significantly reduced by deficiency of each of the
nutrients. Partitioning of dry matter to tubers was markedly reduced by K deficiency and increased in
one experiment by P deficiency. When both P and K were deficient, partitioning approximated that
under non-limiting conditions. Leaf weight ratio (LWR) was higher under K deficiency, but not when
P was also deficient, and was consistently higher when the ratio of K : P in dry matter was less than
approximately five. In these experiments, LWR was not consistently related to shoot N% and N supply
had relatively little effect on partitioning. There were large treatment effects on tuber dry matter
percentage, characterised by significant interactions especially between N and K. Deficiency of one
nutrient increased the concentration of others but uptake was highly regulated as crop content of all
three nutrients was reduced when the supply of any one was deficient. The results show that the
response of potatoes to single deficiencies may be influenced greatly by the levels of other nutrients.
Key words: Solanum tuberosum, potato, nutrient deficiency, nitrogen, phosphorus, potassium,
partitioning
Introduction
cultivars. For example, there is evidence that
nitrogen (N) supply has little effect on the pattern of
partitioning in determinate cultivars, in contrast to
the situation in more indeterminate types where
foliage growth is favoured at the expense of tubers
when high levels of fertiliser N are supplied (Harris,
1992). Much less is known about the effects of
phosphorus (P) and potassium (K) on partitioning
in potatoes. Studies undertaken in other crop plants
have shown the potential for marked effects, albeit
at low levels of supply (Fredeen et al., 1989; Cakmak
et al., 1994). The pattern of partitioning in potatoes
is influenced greatly by the presence of the tubers
which become very strong sinks for assimilates and
any factor that influences the normal pattern of
partitioning to tubers would be of considerable
significance for yield formation.
Crops grow by intercepting solar radiation and
utilising part of the absorbed energy in the process
of photosynthesis to create dry matter. Yield depends
not only on the total amount of dry matter produced
but also on how much is partitioned to the
economically useful plant parts. In both of these
yield-determining processes, mineral nutrients play
a key rôle. Crop cultivars have been selected to
optimise the distribution of dry matter in favour of
yield formation, and whereas this character shows a
high degree of genetic control, environmental
factors, including the influence of available mineral
nutrients, can perform a significant modulating
function (Cakmak & Engels, 1999).
Early-maturing potato cultivars partition a
significant amount of dry matter to tubers at
relatively low total plant dry weight, thereby
allowing crops to be harvested after comparatively
short periods of growth. These cultivars are highly
determinate in their growth characteristics with the
production of new leaves ceasing at a relatively early
stage (Allen & Scott, 1992). The ability of external
factors to modify partitioning may be limited in such
Whilst it is recognised that different nutrients may
interact in the way they influence crop growth and
yield (Marschner, 1995), studies on potatoes have
generally considered effects of single elements.
Where the potential for interactions has been tested,
the focus has been on yield responses in field trials
with no detailed measurements of growth (e.g.
Archer et al., 1976; Farrar & Boyd, 1976). This study
*Corresponding Author E-mail: pdj@aber.ac.uk
© 2003 Association of Applied Biologists

216
P D JENKINS & S MAHMOOD
was undertaken to examine how factorial
combinations of low and high levels of N, P and K
influence dry matter production and partitioning in
an early-maturing potato cultivar. The main objective
was to examine the extent to which interactions
between nutrients might influence these processes
and, in order to regulate nutrient supply precisely,
the experiments were conducted in sand culture in
the glasshouse. An additional objective of the study
was to determine whether imbalances in supply of
these nutrients might lead to the accumulation of
nitrate-N in tubers and other plant parts. These results
will be reported in a subsequent paper.
supplied equally to all pots, as solution mixtures
together with the N applications, at the following
concentrations: MgSO4 4.99 mM, CaCl2 2.52 mM,
H BO3 0.50 mM, MnSO4 0.30 mM, MoO3/H MoO
0.301 mM, CoCl 0.01 mM, ZnSO4 0.20 mM,2CaCl42
0.01 mM, CuCl2 0.20 mM, FeEDTA 0.20 mM. The
pots were water2ed on a regular basis when required
by application directly into the saucers.
Expts 1 and 2 comprised eight treatments,
representing all combinations of low and high levels
of N, P and K, arranged in a randomised complete
block design with six replications. A single
destructive harvest was taken 92 days after 50%
plant emergence in Expt 1 and 58 days after
emergence in Expt 2. In Expt 3, there were four
treatments comprising all combinations of low and
high N and K, each provided with the high level of
P. In each block of this experiment there were three
pots per treatment combination, one of which was
sampled at each of the three harvests, viz. 45, 57
and 79 days after 50% plant emergence. The design
was a randomised complete block with five
replications. In each experiment, the pots were
arranged within blocks at 40 cm centres and re-
randomised weekly. The treatments are indicated in
the text by combinations of H and L (signifying high
or low levels respectively of the particular nutrient)
and presented in the order N-P-K; e.g. HLH signifies
high N and K and low P.
Materials and Methods
Treatments
Three experiments conducted in consecutive years
(1995-1997) examined effects on growth, dry matter
distribution and nutrient uptake of factorial
combinations of two levels each of N, P and K. The
levels, generally designated high and low in the text,
were chosen on the basis of unpublished work to
allow unlimited growth at the high level and a
significant limitation to growth at the low level. The
potato plants used in the experiments were grown
in a fine sand medium in the glasshouse in 20 litre
plastic pots provided with deep saucers. Asingle pot
represented one treatment plot. The temperatures
were principally in the range 15-25°C and only
natural light was provided. Expt 1 was conducted
between May and September (107 days), Expt 2
between April and June (69 days), and Expt 3
between March and June (90 days).
Measurements
From soon after plant emergence, the total number
of unfolded leaves and the length of selected leaves
were assessed once or twice weekly. Leaves were
numbered in ascending order from the base of the
above-ground stem upwards so that, for example,
leaf four on the main stem was the fourth visible
leaf to appear. Similarly, on apical branches, the
numbering proceeded in ascending order from the
first leaf to appear. Leaf areas were derived from
leaf lengths using relationships established at harvest
in Expt 2 where 20 leaves from each treatment were
sampled and their lengths regressed against their
areas measured with a leaf area meter (Delta-T Ltd,
Cambridge, UK) (Table 1). The chlorophyll content
of the terminal leaflets of selected leaves was
measured in Expts 1 and 2 using a calibrated SPAD
meter (Minolta Ltd, Osaka, Japan). The calibration
had been performed previously on potato leaves
following extraction of chlorophyll in the dark in
acetone with absorbance measured at 647 and 664
nm.
Pre-sprouted seed tubers (80 ± 10g) of cv. Rocket,
with longest sprouts of approximately 5 mm, were
planted singly into each pot at a depth of 10 cm below
the surface of the sand. At an early stage of growth,
stems were thinned to leave only three mainstems
per plant. P was provided in the form of
superphosphate (2.00 g P and 0.20 g P pot-1 at the
high and low level respectively) and K in the form
of muriate of potash (3.77 g K and 0.38 g K pot-1
respectively), both of which were mixed uniformly
into the sand below the level of the seed tuber
immediately prior to planting. N was provided in 1
litre solutions of ammonium nitrate applied to the
surface of the sand at concentrations of either 8.12
or 4.12 mM for the high and low levels respectively.
In Expt 1, applications were made at 15 day intervals
from 50% plant emergence for the first six wk and
subsequently at weekly intervals. In Expts 2 and 3,
applications were made from emergence at weekly
intervals. In each experiment, nutrient
supplementation ceased between 2 and 3 wk prior
to the final harvest. The total amount of N applied
at the two levels was, therefore, 1.82 g and 0.92 g
pot-1 in each experiment. Other nutrients were
At harvest, after removal from the pots, the roots
were extracted from the sand by washing, and the
plants were divided into green leaves, senesced
leaves, above-ground stems plus branches, below-
ground stems plus stolons, roots and tubers. Total
area of green leaves was assessed and all parts were

Dry matter distribution in nutrient-deficient potatoes
217
dried at 80°C for 48 h. The numbers, fresh weight
Results
and dry matter percentage of tubers were also
assessed. After weighing, the dried samples were
analysed for N, P and K content using standard
colorimetric procedures as described by Faithfull
(1971).
Leaf growth
The detailed leaf measurements made in Expt 1
are summarised in Tables 2 and 3 as mean values
per stem. The mean number of above-ground leaves
on the main stem axis, i.e. below the first flower,
was 13.9 (± 0.12) and was not influenced
significantly by treatments. However, other aspects
of leaf production showed considerable sensitivity
to nutrient supply. The growth of the earlier formed
leaves (e.g. numbers 4 and 6) was influenced
primarily by N supply, as indicated by the maximum
area achieved, with overall means for high and low
N of 144.3 and 107.3 for leaf 4 and the corresponding
values for leaf 6 were 173.0 and 133.6. The growth
of these leaves was not influenced significantly by
P or K supply. The higher N level also resulted in
consistently larger leaf areas of the later formed
leaves but, in addition, effects of K supply were
apparent on the growth of leaves 10 and 12. All
comparisons of high K with the corresponding low
K treatment indicated a restriction in the generation
of leaf area as a consequence of inadequate K. P
Table 1. Relationships used to derive leaf area (y,
cm2) from leaf length (x, cm) under different
combinations of low (L) and high (H) levels of N, P
and K
Treatment (N-P-K)
Equation
R2
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
y = 0.206x2.146
y = 0.145x2.234
y = 0.157x2.213
y = 0.169x2.156
y = 0.348x1.969
y = 0.082x2.429
y = 0.569x1.807
y = 0.239x2.063
0.96
0.93
0.95
0.98
0.96
0.97
0.88
0.93
Table 2. Effects of combinations of low (L) and high (H) levels of N, P and K on the maximum area (cm2) of leaves
on the main stem axis and on two leaves on the principal apical branch (Expt 1). Transformed data (ln (x+1)) in
parentheses
Treatment
combinations
(N-P-K)
Main stem leaf number
Apical branch leaf number
4
6
8
10
12
1
3
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
105.7
133.5
98.5
130.1
156.8
122.1
125.6
166.3
188.7
156.0
181.1
127.5
139.0
96.3
119.4
183.2
199.2
153.9
169.5
93.4
109.8
75.5
104.4
121.4
162.7
116.1
170.0
48.1
61.3
39.8
59.9
88.0
120.1
90.6
128.7
18.0 (1.4)
18.5 (2.2)
15.5 (1.6)
24.6 (2.7)
4.3 (0.7)
67.6 (4.2)
60.0 (4.1)
90.4 (4.4)
2.4 (0.7)
5.0 (1.4)
5.9 (1.2)
7.9 (1.7)
1.5 (0.5)
14.5 (2.3)
38.0 (3.6)
56.8 (3.9)
91.3
132.1
140.2
155.9
149.1
SE
13.05
13.28
15.78
12.26
11.01
(0.49)
(0.39)
(df = 35)
Treatment main
effects
low N
high N
107.3
144.3
133.6
173.0
120.5
176.5
95.8
142.5
52.3
106.8
19.2 (2.0)
55.6 (3.3)
5.3 (1.2)
27.7 (2.6)
low P
high P
127.9
123.7
160.5
146.2
162.2
134.8
121.8
116.5
79.4
79.8
27.1 (2.1)
47.6 (3.2)
5.9 (1.2)
27.2 (2.6)
low K
high K
123.0
128.5
143.6
163.1
140.2
156.8
101.6
136.7
66.6
92.5
24.4 (2.0)
50.3 (3.4)
12.0 (1.5)
21.1 (2.3)
SE
6.53
6.64
7.89
6.13
5.51
(0.24)
(0.20)
(df = 35)

218
P D JENKINS & S MAHMOOD
supply did not affect the area of leaves on the main
stem axis and neither were there any significant
interactions between nutrients in relation to the area
of these leaves.
individual size. Basal branches formed in the axils
of the first four above-ground leaves (Table 3).
Whilst very few appeared in LLL and LHL, it is
clear that the potential number of basal branches was
not achieved when any of the nutrients was present
at the lower level of supply. The total number of
leaves on basal branches varied considerably
between treatments but correlated closely with the
number of branches (r = 0.99, P < 0.001). Thus, in
the control (HHH), a substantial amount of leaf was
produced on the basal branches but this was limited
to varying degrees in all other treatments.
Plants in all treatments produced a certain amount
of branch growth, both apical and basal, after flower
appearance on the main axis. The extent of branch
growth varied considerably between treatments and
was restricted by deficiency of all three nutrients. A
deficiency of either P or K had a relatively small
effect on the number of apical branches, but when
both were deficient (i.e. HLL), apical branching was
severely restricted not only in terms of the number
of branches but also in terms of the number of leaves
on these branches and their size. Thus, the lower
level of either P or K limited the area of the apical
branch leaves but when both were deficient very little
expansion of these leaves occurred. There were also
large effects of N supply on all aspects of branch
growth with the higher level of supply increasing
significantly the number of apical branches, the
number of leaves on these branches and their
At the time plants were sampled in Expt 2 there
remained a substantial amount of green leaf and
senescence was limited. This was also the case at
the first two harvests in Expt 3, and therefore, in
both experiments, total green leaf area per plant was
determined. Single deficiencies of any one of the
nutrients resulted in large reductions in leaf area
(Table 4), the largest effect being shown by the low
N levels. There were significant interactions between
N and K in both experiments (P < 0.05) resulting
from the relatively larger effect of low K in
depressing leaf area under the conditions of higher
N supply. In Expt 3, the suppressive effect of low K
Table 3. Effects of combinations of low (L) and high
(H) levels of N, P and K on the mean number of
apical branches stem-1, total number of leaves on
apical branches, mean number of basal branches
stem-1, total number of leaves on basal branches
(Expt 1). Transformed data (ln(x+1)) in parentheses
Table 4. Effects of combinations of low (L) and high
(H) levels of N, P and K on the total green leaf area
(cm2 plant-1) in Expt 2 (58 days after emergence) and
in Expt 3 (45 days (S1) and 57 days (S2) after
emergence)
Treatment
combinations
(N-P-K)
Apical branches
Basal branches
Number Leaf number Number Leaf number
Treatment combinations
(N-P-K)
Expt 2
Expt 3
S 1
S 2
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
1.2 (0.7) 3.9 (1.4)
2.4 (1.2) 8.1 (2.2)
2.4 (1.2) 9.7 (2.3)
2.8 (1.3) 11.2 (2.5)
0.7 (0.4) 3.3 (0.8)
3.0 (1.4) 12.0 (2.5)
3.3 (1.5) 18.4 (2.9)
3.7 (1.5) 16.4 (2.8)
0.7 (0.5) 2.5 (1.0)
1.5 (0.9) 5.1 (1.7)
0.6 (0.4) 1.8 (0.8)
1.7 (0.9) 5.3 (1.8)
1.3 (0.8) 4.2 (1.6)
2.0 (1.1) 6.6 (2.0)
2.6 (1.2) 8.9 (2.2)
3.7 (1.5) 14.6 (2.7)
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
2636
3042
3233
3436
3688
4410
4084
5478
-
-
-
-
2653
2728
-
1390
1806
-
-
-
4350
5232
2395
3467
SE
(0.12)
(0.23)
(0.12)
(0.21)
SE (df = 35, Expt 2)
(df = 12, Expt 3)
218.7
187.9
139.5
(df = 35)
Treatment
main effects
Treatment main effects
low N
high N
2.2 (1.1) 8.2 (2.1)
2.7 (1.2) 12.5 (2.3)
1.1 (0.7) 3.7 (1.3)
2.4 (1.2) 8.6 (2.1)
low N
high N
3087
4415
2691
4791
1598
2931
low P
high P
1.8 (0.9) 6.8 (1.7)
3.0 (1.4) 13.9 (2.6)
1.4 (0.8) 4.6 (1.6)
2.1 (1.0) 7.6 (1.9)
low P
high P
3444
4058
low K
high K
1.9 (0.9) 8.8 (1.8)
3.0 (1.4) 11.9 (2.5)
1.3 (0.7) 4.4 (1.4)
2.2 (1.1) 7.9 (2.1)
low K
high K
3410
4091
3502
3980
1892
2636
SE
(0.06)
(0.12)
(0.06)
(0.10)
SE (df = 35, Expt 2)
(df = 12, Expt 3)
109.3
132.8
98.6
(df = 35)

Dry matter distribution in nutrient-deficient potatoes
219
on leaf area appeared to become greater in the twelve
days between harvests 1 and 2. For example, the
single deficiency of K reduced leaf area by 17% at
harvest 1 but by 31% at harvest 2 suggesting that
the leaf senescence, apparent in all treatments at this
time, occurred more rapidly under low K conditions
compared with the control (HHH). By contrast, low
N (LHH) reduced leaf area by 48% at both harvests.
Data were collected in Expts 1 and 2 on the
chlorophyll concentration of individual leaves, and
results for leaf 6 are presented to demonstrate the
principal trends (Fig. 1). For much of the period of
active growth of this leaf, the highest chlorophyll
concentrations were found in the treatments deficient
in K alone (HHL) or in both P and K (HLL).
Maximum area of leaf 6 in all treatments occurred
approximately 46 days after 50% plant emergence
in Expt 1 and after approximately 42 days in Expt 2,
and in most cases the chlorophyll content declined
rapidly after this time. The rate of decline at these
later stages of the leaf’s growth was least rapid in
HHH so that when the final measurements were
made, 60 days after plant emergence in Expt 1 and
after 54 days in Expt 2, HHH still retained a
substantial amount of chlorophyll.
Dry matter production
Different levels of dry matter were produced in
the three experiments, at least partly as a
consequence of different duration of growth from
emergence to harvest, but these differences were
apparent mainly in the treatment receiving adequate
amounts of all three nutrients, i.e. HHH (Figs 2, 3 &
4). Where one or more of the nutrients were provided
at a lower level, dry matter production was
significantly reduced (P < 0.01), nevertheless,
substantial growth was achieved in all treatments
together with full development and tuber formation.
Deficiency in P alone (HLH) reduced dry weight by
33% in Expt 1 and by 16% in Expt 2. For K, the
single deficiency reduced dry weight by 45% and
by 33% in Expts 1 and 2 respectively, while in Expt
3, the effect increased with delay in harvest from
21% at 45 days from emergence to 31% at 57 days
and 58% at final harvest, 79 days after emergence.
In HLL, the combined deficiencies of P and K were
clearly not additive, and in both Expts 1 and 2, dry
matter production in this treatment did not differ
significantly from HHL.
The lower level of N also resulted in dramatic
decreases in dry weight. For example, in LHH, dry
matter production was 49% less than in HHH in Expt
1 and a similar reduction occurred at final harvest in
Expt 3. The corresponding figure for Expt 2 was
29%. The treatments receiving the lower level of N
showed a much smaller range of total dry weights
than the corresponding high N treatments.
35
(a)
30
25
20
15
10
5
Although terminated after different periods of
180
160
140
120
100
80
0
10
20
30
40
50
60
40
35
(b)
30
25
20
15
10
5
60
40
20
0
LLL LLH LHL LHH HLL HLH HHL HHH
0
Fig. 2. Effect of combinations of low (L) and high
(H) levels of N, P and K on the dry weights (g
plant-1) of tubers, senesced leaves, green leaves,
below-ground stems, above-ground stems plus
branches, and roots (Expt 1). SE for total dry
weight plant-1 = 2.904 (df = 35).
10
20
30
40
50
60
Days after 50% plant emergence
Fig. 1. Effect of combinations of low (L) and high
(H) levels of N, P and K on the chlorophyll
concentration (µg cm-2) of leaf number 6 ((a) Expt
1, (b) Expt 2). Bar = SE (df = 35).
Black = tubers; stripes = dead leaves; dark grey =
green leaves; mid-grey = below-ground stem;
light grey = above-ground stem; unshaded = roots.
LLL = !; LLH = "; LHL = ; LHH = ; HLL
+
(
= !; HLH = "; HHL = #; HHH = $.

220
P D JENKINS & S MAHMOOD
growth, tubers constituted at least 68% of total dry
weight by the time of final harvest in each
experiment. In Expt 1, leaf senescence was well
advanced by this time, especially in LLL, LHL and
HLL where very little green leaf was present. By
contrast in Expt 2, a substantial amount of green
leaf was still present at harvest, with HHL and HHH
giving very similar amounts. Although there was
little leaf senescence in Expt 3 at the first two
harvests, by the final harvest very little green leaf
remained in LHL, LHH and HHL.
level the resulting HI was also very similar to that
of the control.
Leaf weight ratio (LWR)
This is defined here as leaf weight expressed as a
proportion of total plant dry weight. The most
pronounced effect in each experiment was again the
different pattern of partitioning shown by HHL in
which significantly more dry matter was retained in
leaves compared with all other treatments (P < 0.01;
Table 5). In Expt 3, this effect was apparent by the
time of the first harvest (data not presented). A
similar trend was seen in LHL but the effect was
much smaller. In Expt 2, the combined deficiency
of P and K in HLL produced a pattern of partitioning
to leaves identical to that of the control and therefore
markedly different from where only K was deficient.
This was not apparent to the same extent in Expt 1
although, even in this experiment, LWR in HLL was
Dry matter distribution
Harvest index (HI)
This is defined here as the tuber dry weight
expressed as a proportion of total plant dry weight.
The general trend was for deficiency of N and P to
increase HI while it was reduced by low K levels
(Table 5). However, the largest effect, seen in each
experiment, was produced by the single deficiency
of K which gave a markedly lower HI in comparison
with all other nutrient combinations, indicating a
substantial reduction in the amount of dry matter
partitioned to tubers in this treatment. This was
apparent at both low and high levels of N supply
but the effect was relatively greater in the latter. In
Expt 2, the single deficiency of P resulted in a
significant increase in HI compared with HHH (P <
0.01) but when both P and K were deficient (i.e. in
treatment HLL), the pattern of partitioning to tubers
reverted to that demonstrated by HHH. Similarly, in
Expt 1, HLL did not differ significantly from HHH.
When all three nutrients were supplied at the lower
80
(a)
60
40
20
0
140
(b)
120
100
80
60
40
20
0
160
140
120
100
80
240
(c)
220
200
180
160
140
120
100
80
60
40
60
40
20
20
0
0
LLL LLH LHL LHH HLL HLH HHL HHH
LHL
LHH
HHL
HHH
Fig. 3. Effect of combinations of low (L) and high
(H) levels of N, P and K on the dry weights (g
plant-1) of tubers, leaves, below-ground stems,
above-ground stems plus branches, and roots
(Expt 2). SE for total dry weight plant-1 = 4.369
(df = 35).
Fig. 4. Effect of combinations of low (L) and high
(H) levels of N, P and K on the dry weights (g
plant-1) of tubers, senesced leaves, green leaves,
stems (above and below-ground), and roots
sampled 45 (a), 57 (b) and 79 (c) days after 50%
emergence (Expt 3). SEs for total dry weight plant-1
= (a) 3.055, (b) 5.507, (c) 3.772 (df = 12).
Black = tubers; dark grey = green leaves; mid-
grey = below-ground stem; light grey = above-
ground stem; unshaded = roots.
Black = tubers; stripes = dead leaves; dark grey =
green leaves; light grey = stem; unshaded = roots.

Dry matter distribution in nutrient-deficient potatoes
221
markedly lower than that of HHL. N supplied at the
higher level also increased LWR significantly in each
experiment (P < 0.001) although the degree of
response depended on the level of the other nutrients.
The positive response to N was greater at the high
level of P and at the low level of K. Most of the
interactions involving K were influenced greatly,
however, by the very marked response in HHL noted
above.
weight (Table 5). The measurements are assumed
to indicate the relative thickness of leaves, i.e. higher
values suggesting thinner leaves, but differences in
tissue density could also be involved. Only green
leaves were used in calculations. Low levels of K
reduced SLA significantly in both experiments (P <
0.001) while the overall effect of low P in Expt 2
was to increase SLA significantly (P < 0.01). In Expt
2, the combined deficiency of P and K (HLL)
resulted in a SLA intermediate in value between that
of the single deficiencies and not significantly
different from that of the control. N level had no
significant effect on SLA in these experiments.
Root dry weight fraction (RDWF)
By final harvest, root weight accounted for less
than approx. 3% of total plant dry weight (Table 5).
Despite the relatively high SEs significant treatment
differences occurred, although the effects were not
consistent across all experiments. In Expt 1, the
general trend was towards lower RDWF where K
was deficient but, in Expt 2, this was only apparent
in HHL. In contrast, the main effect in Expt 3 was a
higher RDWF in the low N treatments and K supply
showed no significant effect.
Tuber production
Total tuber numbers
The number of tubers produced varied
considerably between experiments (Table 6). The
relatively low numbers in Expt 1 were associated
with an absence of any significant treatment effects
but such effects were apparent in Expt 2 where
almost double the number of tubers were recorded
and both N and K increased numbers when applied
at the higher level. In this experiment, the lowest
numbers were produced when K alone was deficient.
Specific leaf area (SLA)
Data were available from Expts 2 and 3 (harvest
2) to calculate SLA, i.e. leaf area per unit leaf dry
Table 5. Effects of combinations of low (L) and high (H) levels of N, P and K on harvest index (HI), leaf weight
ratio (LWR), root dry weight fraction (root dry weight/total dry weight) and specific leaf area (cm2 g-1) assessed
92 (Expt 1), 58 (Expt 2) and 79 (Expt 3) days after emergence
Treatment
combinations
(N-P-K)
HI
LWR
Root dry wt fraction
Specific leaf area
Expt 3†
Expt 1 Expt 2 Expt 3
Expt 1 Expt 2 Expt 3
Expt 1 Expt 2 Expt 3
Expt 2
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
0.78
0.79
0.76
0.78
0.78
0.78
0.69
0.80
0.80
0.83
0.77
0.83
0.80
0.83
0.72
0.80
-
-
0.16
0.14
0.18
0.14
0.17
0.15
0.24
0.13
0.14
0.12
0.17
0.12
0.15
0.12
0.23
0.15
-
-
0.016
0.021
0.021
0.020
0.015
0.021
0.016
0.020
0.029
0.026
0.026
0.031
0.030
0.024
0.022
0.029
-
-
236.8
282.8
220.2
296.8
251.9
299.4
190.1
267.4
-
-
0.75
0.83
-
0.18
0.11
-
0.022
0.020
-
124.9
159.8
-
-
-
-
-
0.68
0.83
0.24
0.12
0.016
0.014
129.3
159.1
SE
0.010
0.007
0.011
0.006
0.006
0.008
0.0019 0.0025 0.0020
13.04
5.81
(df = 35, Expts 1,2; df = 12, Expt 3)
Treatment
HI
Expt 1 Expt 2 Expt 3
LWR
Root dry wt fraction
Specific leaf area
Expt 3†
142.4
main effects
Expt 1 Expt 2 Expt 3
Expt 1 Expt 2 Expt 3
Expt 2
low N
high N
low P
high P
low K
high K
0.78
0.76
0.78
0.76
0.75
0.79
0.81
0.79
0.81
0.78
0.77
0.82
0.79
0.75
0.15
0.17
0.16
0.17
0.19
0.14
0.14
0.16
0.13
0.17
0.17
0.13
0.14
0.18
0.020
0.018
0.018
0.019
0.017
0.021
0.028
0.026
0.027
0.027
0.027
0.027
0.021
0.015
259.2
252.2 144. 2
267.7
243.6
0.71
0.83
0.21
0.11
0.019
0.017
224.8
286.6
127.1
159.5
SE
0.005
0.004
0.008
0.003
0.003
0.006
0.0009 0.0013 0.0014
6.52
4.11
(df = 35, Expts 1,2; df = 12, Expt 3)
† Assessed 57 days after emergence

222
P D JENKINS & S MAHMOOD
However, when both K and P were deficient (HLL),
significantly more tubers were produced (P < 0.01),
resulting in total number not significantly different
from the control. In this experiment, the beneficial
effect of the higher level of either N or K on numbers
was greater when the other nutrient was also applied
at the higher level, resulting in a significant N × K
interaction (P < 0.05). A similar interaction was also
apparent in Expt 3 at the final harvest (Table 7). The
level of P had no overall effect. The successive
harvests in Expt 3 enabled treatment effects to be
monitored over time. In the control (HHH), numbers
remained relatively constant in the period of 34 days
between the three harvests, whereas in other
treatments, numbers declined markedly. Although
treatment differences were not apparent at the first
two harvests, large effects were seen at the final
harvest with the deficiencies of N and K, both singly
and in combination, producing similar numbers of
tubers and substantially fewer than the control. From
the first two harvests, it appears that initiation of
tubers was not greatly influenced by nutrient level,
but reabsorption of some tubers was a significant
feature under conditions of inadequate nutrient
Table 6. Effects of combinations of low (L) and high (H) levels of N, P and K on total tuber numbers plant-1
sampled 92 days (Expt 1), 58 days (Expt 2) and 45, 57, and 79 days (S 1-3 respectively; Expt 3), after emergence
Treatment
combinations (N-P-K)
Expt 1
Expt 2
Expt 3
S2
S1
S3
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
8.8
9.5
9.7
11.5
9.5
11.0
10.8
11.2
21.3
21.0
20.2
21.3
25.2
27.3
17.8
27.5
-
-
-
-
-
-
15.8
16.2
-
15.4
16.0
-
8.6
10.2
-
-
-
-
16.8
18.4
16.6
18.0
8.2
20.6
SE
1.12
1.59
1.32
2.18
1.00
(df = 35, Expts 1,2; df = 12, Expt 3)
Treatment main effects
low N
high N
9.9
10.6
21.0
24.5
16.0
17.6
15.7
17.3
9.4
14.4
low P
high P
9.7
10.8
23.7
21.7
low K
high K
9.7
10.8
21.1
24.3
16.3
17.3
16.0
17.0
8.4
15.4
SE
0.56
0.79
0.93
1.54
0.71
(df = 35, Expts 1,2; df = 12, Expt 3)
Table 7. Significance of main effects and interactions for total tuber number and tuber dry matter percentage
Total tuber number Tuber dry matter %
Expt 2 Expt 3 Expt 2 Expt 3
Expt 1
Expt 1
S1
S2
S3
S1
S2
S3
N
P
K
N×P
N×K
P×K
ns
ns
ns
ns
ns
ns
P < 0.01
ns
P < 0.01
ns
P < 0.05
ns
ns
-
ns
-
ns
-
ns
-
ns
-
ns
-
P < 0.001
ns
P < 0.01
P < 0.001
ns
P < 0.001
ns
ns
P < 0.05
ns
-
P < 0.01
P < 0.05
ns
-
ns
-
-
ns
-
ns
-
P < 0.001
-
-
ns
-
-
P < 0.05
-
P < 0.001
P < 0.05 P < 0.001
-
ns
P < 0.001
ns
ns
-
-
-
ns
ns
-
-
-
N×P×K
ns = not significant

Dry matter distribution in nutrient-deficient potatoes
223
supply.
important determinant of tuber quality and, therefore,
of considerable practical significance. The effects
of the different nutrient regimes were to produce
considerable variability in TDM% suggesting
complex interactions between individual nutrients
and levels of supply (Tables 7 and 9). In Expt 1, the
low level of P produced a lower TDM% overall but
interactions involving P were not significant. In Expt
2, there was a general trend for the low level of N to
produce a higher TDM% but only at the high level
of P and at the low level of K, the latter effect also
being found in Expt 3. Effects of K were quite
variable across experiments. In Expt 1, the higher
level of K decreased TDM% overall but not when
both N and P were provided at the higher level. In
Expt 2, although there was no significant effect of
K overall, a significant interaction occurred with P
(P < 0.001) whereby the higher K level reduced
TDM% at low P level but not at high P (Table 7). It
is clear from Expt 3, however, that the relative
differences between treatments changed at
successive harvests. Thus, at the first harvest, HHL
and LHL showed significantly greater values than
HHH and LHH respectively (P < 0.05), whereas at
the second the corresponding differences were not
significant. At the final harvest, the limited supply
of K in HHL gave the lowest TDM% but, when both
Tuber fresh weight
Tuber yield was generally much greater when all
three nutrients were supplied at the higher level
(Table 8). A single deficiency of K (HHL) reduced
yield by 52% and 34% in Expts 1 and 2 respectively
and by 63% at final harvest in Expt 3. These
reductions were thus slightly greater than for total
dry weight. The corresponding figures for a single
N deficiency (LHH) showed reductions of 47%, 26%
and 44% for Expts 1 to 3 respectively, slightly
smaller than those for total dry weight. Effects of P
deficiency were smaller but still substantial in Expt
1 (30% yield reduction in HLH) but were non-
significant in Expt 2. When seen in combination,
the effects of nutrient deficiency were clearly non-
additive, nevertheless, the smallest yields were
obtained when both N and K were deficient. In Expt
3, substantial yield increases occurred in the control
between successive harvests, whereas the other
treatments showed much slower rates of tuber
growth and in HHL no yield increase occurred after
57 days post-emergence.
Tuber dry matter percentage (TDM%)
This measure of dry matter concentration is an
Table 8. Effects of combinations of low (L) and high (H) levels of N, P and K on total tuber fresh weight (g plant-1)
sampled 92 days (Expt 1), 58 days (Expt 2) and 45, 57, and 79 days (S 1-3 respectively; Expt 3), after emergence
Treatment combinations
(N-P-K)
Expt 1
Expt 2
Expt 3
S2
S1
S3
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
282.2
351.6
275.8
370.2
348.5
489.9
338.1
701.2
349.0
448.4
350.7
465.0
448.8
586.9
417.8
629.0
-
-
-
-
-
-
158.8
185.4
-
222.0
331.2
-
270.4
477.9
-
-
-
-
187.4
282.8
318.0
501.7
317.2
859.4
SE
14.48
17.76
14.06
21.79
19.45
(df = 35, Expts 1,2; df =1 2, Expt 3)
Treatment main effects
low N
high N
320.0
469.4
403.3
520.6
172.1
235.1
276.6
409.8
374.2
588.3
low P
high P
368.1
421.3
458.3
465.6
low K
high K
311.1
478.2
391.6
532.3
173.1
234.1
270.0
416.5
293.8
668.7
SE
7.24
8.88
9.94
15.40
13.76
(df = 35, Expts 1,2; df = 12, Expt 3)

224
P D JENKINS & S MAHMOOD
N and K were deficient (i.e. LHL), the TDM% was
controlling uptake of nutrients.
significantly higher (P < 0.05).
The treatment combinations produced wide
variations in the ratios of N:P:K in total dry matter.
In view of the interactive effects of P and K supply
found on dry matter partitioning in these
experiments, relationships were examined between
K:P ratios (derived from K and P percentages in total
dry matter) and measurements of partitioning. Fig.
5 illustrates the relationship between LWR and K:P
ratio. The highest values for LWR were associated
with K:P ratios of < 5, whereas at higher values there
was no consistent relationship between the two
variables. Correlation analysis was also employed
to test the association between LWR and N% in
above-ground (i.e. haulm) dry matter. There was a
significant positive correlation in Expt 1 (r = 0.89,
P < 0.01) but the relationship was influenced greatly
by the values for HHL so that when this treatment
was removed from the analysis the correlation was
no longer significant. A significant correlation was
found between these two variables at the final harvest
of Expt 3 (r = 0.99, P < 0.01) but not at the first two
harvests or in Expt 2.
Plant nutrient content
As expected, there was a general trend for the
higher supply level of a nutrient to result in a higher
tissue concentration for that nutrient and a higher
uptake level (Table 10). The only exceptions were
seen in Expts 1 and 3 in the comparison between
HHH and LHH in which N% was very similar. There
was also a general trend for deficiency in one nutrient
to lead to a higher concentration of the other two
nutrients. However, effects on nutrient uptake were
more complicated (Table 11). For example, a
deficiency of P or K reduced N uptake but only when
N was supplied at the higher level. Similarly, at high
N and P levels, P uptake was markedly reduced when
K was deficient but at low levels of N and/or P, K
deficiency had little effect on P uptake, at least in
Expts 1 and 2. The general response, therefore, was
for uptake of nutrient x to be reduced by deficiency
of nutrient y but only when x was provided at the
higher level. A growth restriction imposed by the
deficiency of any one nutrient limited the uptake of
not only that nutrient but also both other nutrients if
they were being provided at the higher supply level.
The results demonstrate a highly regulated system
Discussion
Growth of crop plants can be analysed in terms of
Table 9. Effects of combinations of low (L) and high (H) levels of N, P and K on tuber dry matter percentage 92
days (Expt 1), 58 days (Expt 2) and 45, 57, and 79 days (S 1-3 respectively; Expt 3), after emergence
Treatment combinations
(N-P-K)
Expt 1
Expt 2
Expt 3
S2
S1
S3
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
18.0
16.3
19.0
17.4
18.1
17.0
17.8
18.2
18.2
16.6
18.6
17.8
17.4
16.8
16.1
17.9
-
-
-
-
-
-
16.4
15.3
-
17.9
17.1
-
18.6
17.7
-
-
-
-
16.0
14.8
16.8
16.3
17.2
18.4
SE
0.39
0.28
0.29
0.34
0.38
(df = 35, Expts 1,2; df = 12, Expt 3)
Treatment main effects
low N
high N
17.7
17.8
17.8
17.1
15.8
15.4
17.5
16.6
18.1
17.8
low P
high P
17.4
18.1
17.2
17.6
low K
high K
18.2
17.2
17.6
17.3
16.2
15.1
17.3
16.7
17.9
18.1
SE
0.19
0.14
0.20
0.24
0.27
(df = 35, Expts 1,2; df = 12, Expt 3)

Dry matter distribution in nutrient-deficient potatoes
225
their capacity to intercept solar radiation, their
efficiency in converting intercepted radiation into
dry matter and the distribution of the latter between
the range of competing plant sinks. In potatoes,
additional factors such as the number of tubers
produced and tuber dry matter percentage need also
to be considered. The way these factors respond to
nutrient supply, especially N, has been the subject
of numerous investigations but the effect of
interactions between nutrients, the subject of this
article, has not been reported extensively. Effects
on light use efficiency were not included in this study
and deserve investigation.
concentrations of chlorophyll were found under
conditions of low P and K supply. Rao & Terry
(1989) noted that under P deficiency sugar beet
leaves were markedly greener, the explanation being
that chloroplast and chlorophyll production are
influenced less than leaf expansion under these
conditions. The limited data on chlorophyll
concentration also point to a more rapid loss of
chlorophyll under conditions of P and K deficiency,
which would result in more rapid senescence of
green leaves and lower total green leaf area at later
stages of growth. The recycling of P and K from
older leaves to sustain the growth of younger leaves
would have lead to a premature loss of physiological
functioning and early senescence (Marschner, 1995).
Very few studies on potatoes have examined in detail
the effect of P or K supply on leaf growth although
those of Watson & Wilson (1956) and Dyson &
Watson (1971) suggest a marked sensitivity of leaf
growth to supply of these nutrients. Jenkins & Ali
(1999) demonstrated the potential for large
reductions in total leaf area at low P levels in field
grown plants but the response was affected
substantially by the cultivar type with earlier-
maturing and more determinate types showing the
In the work reported here, the capacity of plants
to intercept solar radiation would have been limited
to varying degrees in all treatments where at least
one of the nutrients was present at the lower level.
Expt 1 showed that the area of all leaves was reduced
by low N supply, as noted by Vos & Biemond (1992),
but only the later formed leaves, including those on
branches, were affected by K and Psupply. However,
the data on total green leaf area recorded in Expts 2
and 3 indicated the potential of all nutrients to reduce
substantially the photosynthetic leaf area when
provided at sub-optimal levels. Higher
Table 10. Effects of combinations of low (L) and high (H) levels of N, P and K on the concentration of nutrients in
total dry matter
Treatment
Combinations
(N-P-K)
N%
P%
K%
Expt 1
Expt 2
Expt 3†
Expt 1
Expt 2
Expt 3†
Expt 1
Expt 2
Expt 3†
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
1.45
1.34
1.48
1.32
1.74
1.50
1.93
1.34
1.29
1.17
1.39
1.16
1.71
1.39
1.85
1.37
-
-
0.15
0.13
0.33
0.28
0.13
0.10
0.34
0.26
0.17
0.14
0.37
0.30
0.15
0.11
0.36
0.29
-
-
1.59
2.94
1.52
2.88
1.48
2.91
1.27
2.18
1.44
3.32
1.45
2.94
1.33
3.15
1.24
2.70
-
-
1.47
1.06
-
0.40
0.34
-
1.50
2.96
-
-
-
-
1.94
1.06
0.42
0.32
1.21
2.18
SE
0.045
0.039
0.028
0.006
0.006
0.016
0.098
0.093
0.100
(df = 35, Expts 1,2; df = 12, Expt 3)
Treatment
main effects
low N
high N
1.40
1.63
1.25
1.58
1.26
1.50
0.22
0.21
0.25
0.23
0.37
0.37
2.23
1.96
2.29
2.11
2.23
1.69
low P
high P
1.51
1.52
1.39
1.44
0.13
0.30
0.14
0.33
2.23
1.96
2.31
2.08
low K
high K
1.65
1.38
1.56
1.27
1.71
1.06
0.24
0.19
0.26
0.21
0.41
0.33
1.46
2.73
1.37
3.03
1.36
2.57
SE
0.023
0.019
0.020
0.003
0.003
0.011
0.049
0.047
0.071
(df = 35, Expts 1,2; df = 12, Expt 3)
† At final harvest

226
P D JENKINS & S MAHMOOD
largest reductions in leaf canopy size. Work on other
crops has also concluded that leaf growth is very
sensitive to P supply (e.g. Mollier & Pellerin, 1999).
Some of the largest interactions between
treatments were for branch growth. Branch growth
was markedly affected by N supply, as seen in many
other studies (Millard & MacKerron, 1986; Vos &
Biemond, 1992), but the single deficiency of P or K
had only a small effect on the number of apical
branches. However, when both P and K were
deficient, growth of apical branches was largely
inhibited. This was not simply related to N uptake
as the treatments receiving low N took up less N
than HLL but still produced more branches. The
inherent capacity to continue leaf growth on
branches is limited in an early-maturing cultivar such
as that used in this study. Later-maturing types with
greater potential for the continuation of leaf
production on branches may respond differently.
In potatoes, most studies on the effect of nutrients
on dry-matter partitioning have focused on nitrogen,
the increased supply of which may, depending on
maturity type, produce preferential enhancement of
foliage growth at the temporary expense of tubers
(Harris, 1992). This emphasis on nitrogen is
understandable as this is the nutrient that has the
largest effect on growth and is the one most likely
to be deficient. In the work reported here, however,
0.30
0.25
0.20
0.15
0.10
0.05
0
0
10
20
K:P ratio
30
40
Fig. 5. Relationship between leaf weight ratio
(LWR) and K:P ratio in Expts 1, 2 and 3 (final
harvest).
Expt 1 = $; Expt 2 = "; Expt 3 = #.
Table 11. Effects of combinations of low (L) and high (H) levels of N, P and K on total nutrient uptake (g plant-1)
Treatment
Combinations
(N-P-K)
N
P
K
Expt 1
Expt 2
Expt 3†
Expt 1
Expt 2
Expt 3†
Expt 1
Expt 2
Expt 3†
L L L
L L H
L H L
L H H
H L L
H L H
H H L
H H H
0.94
0.96
1.01
1.08
1.42
1.61
1.69
2.14
1.02
1.04
1.17
1.16
1.68
1.64
1.73
1.92
-
-
0.10
0.09
0.22
0.23
0.10
0.11
0.30
0.42
0.14
0.13
0.31
0.30
0.15
0.13
0.34
0.41
-
-
1.03
2.13
1.04
2.35
1.21
3.12
1.11
3.49
1.14
2.96
1.23
2.92
1.31
3.73
1.17
3.78
-
-
0.99
1.08
-
0.26
0.36
-
0.99
3.12
-
-
-
-
1.57
2.05
0.34
0.61
0.98
4.15
SE
0.066
0.043
0.036
0.008
0.009
0.023
0.115
0.093
0.116
(df = 35, Expts 1,2; df = 12, Expt 3)
Treatment
main effects
low N
high N
1.00
1.72
1.10
1.74
1.04
1.81
0.16
0.23
0.22
0.26
0.31
0.48
1.64
2.23
2.06
2.50
2.06
2.56
low P
high P
1.23
1.48
1.35
1.49
0.10
0.29
0.14
0.34
1.87
2.00
2.29
2.27
low K
high K
1.27
1.45
1.40
1.44
1.28
1.57
0.18
0.21
0.23
0.24
0.30
0.48
1.10
2.77
1.21
3.35
0.98
3.63
SE
0.033
0.022
0.026
0.004
0.005
0.016
0.057
0.046
0.082
(df = 35, Expts 1,2; df = 12, Expt 3)
† At final harvest

Dry matter distribution in nutrient-deficient potatoes
227
the pattern of distribution of dry-matter was affected
to only a small degree by N supply. This may be
because the higher N rate was too low to produce
significant responses but also may be a consequence
of the early-maturing characteristics of the cultivar
which, as a determinate type, may have been less
liable to perturbations in dry-matter partitioning than
more indeterminate cultivars (Harris, 1992). The
principal effect on partitioning was seen at low K
levels and resulted in a substantial reduction in
allocation of dry-matter to tubers (i.e. lower harvest
index) with relatively more retained within the
foliage. This was seen consistently in each
experiment. Pronounced effects of K deficiency on
dry matter partitioning have been reported for other
species, primarily in studies employing nutrient
solutions. The ratio of shoot to root dry matter (S:R)
was generally increased under K deficiency, the
suggested mechanism being the failure of effective
phloem loading of sucrose leading to the retention
of carbohydrate in leaves (Cakmak et al., 1994;
McDonald et al., 1996). However, Andrews et al.
(1999) found no effect on S:R in K-deficient beans
(Phaseolus vulgaris), while in wheat, S:R was
significantly lower under these conditions.
levels of other nutrients which may explain some of
the apparent inconsistencies in the literature over
effects of K deficiency on partitioning.
Studies on N supply have consistently shown
positive correlations between S:R and shoot N %
which were independent of nutrient effects on overall
growth (e.g. Caloin et al., 1980; Andrews, 1993;
Paponov et al., 1999). Andrews et al. (1999, 2001)
proposed that shoot protein concentration may have
a key role in determining S:R under a wide range of
environmental conditions and could provide the basis
of a mechanism for the effect of a range of different
nutrient deficiencies. Shoot protein concentrations
were not measured in these experiments but evidence
for shoot N concentration influencing the pattern of
partitioning between above and below-ground parts
is equivocal. The positive correlation in Expt 1
between LWR and shoot N% was largely due to the
contribution of HHL which consistently showed an
elevated LWR and N% in comparison with other
treatments. Although K deficiency consistently
increased N%, P% was also increased under these
conditions, as the general trend was for deficiency
of one nutrient to increase the concentrations of
others. No correlation was found in Expt 2 and, in
Expt 3, the positive correlation between LWR and
shoot N% occurred only at the final harvest whereas
a significantly higher LWR was detected under K
deficiency even at earlier harvests when shoot N%
and LWR were not significantly correlated. As also
shown by Andrews et al. (1999, 2001), it seems
unlikely that shoot N% had a controlling influence
on dry matter distribution. However, if the single
deficiencies of K and P had increased and decreased
shoot protein concentration, respectively, the
possibility of which can be inferred from the studies
of Andrews et al. (1999, 2001), then a possible
explanation emerges for the patterns of dry matter
partitioning observed in these experiments.
In the experiments reported here, reduced P supply
had smaller effects on partitioning, although in Expt
2, low P resulted in a significantly higher harvest
index than the control, indicative of relatively more
dry-matter being allocated to tubers. Studies on other
species have shown that under low P supply
proportionally more dry-matter was allocated to
below-ground plant parts (Fredeen et al., 1989; Rufty
et al., 1993; Cakmak et al. 1994; Andrews et al.
1999). The likely mechanism in this case is the
greater sink strength of below-ground parts under
conditions of low P supply which has a very negative
effect on leaf growth (Mollier & Pellerin, 1999).
Surprisingly, the marked effects of low K on
partitioning seen in these potato experiments were
not observed when P was also deficient, so that in
treatment HLL the harvest index, leaf weight ratio,
root dry weight fraction and specific leaf area were
generally very similar to the control treatment where
each nutrient was supplied at an adequate level.
Clearly, the deficiency of either P or K alone
produced the opposite response to each other and in
combination their individual effects may be
cancelled out. However, the ratio of the two may
also be important as more normal patterns of
partitioning were obtained whenever the ratio of K:P
exceeded approximately five. This occurred
whenever K was supplied at the higher level or at
the lower when P was also deficient. The balance
between anions and cations may, therefore, be
important in controlling the pattern of allocation.
Whatever the mechanism, it is clear that the
responses to nutrient deficiency are affected by the
Treatment interactions were also found for tuber
dry matter percentage, although the effects were not
always consistent between experiments. The higher
N level produced a significantly lower TDM% in
Expt 2 but interacted with K supply in all three
experiments. Numerous studies have demonstrated
a significant effect of N and K on dry matter
concentration. In most cases, increasing N levels
decreased the concentration (Yungen et al., 1958;
Painter & Augustin, 1976; MacKerron & Davies,
1986), although some investigations failed to
demonstrate a response (Millard & Marshall, 1986)
or indeed reported an increase following N
application (Birch et al., 1967; Gray, 1974).
Similarly, increasing K supply is often associated
with a depression in TDM% (Kunkel & Holstad,
1972) although the form in which it is applied may
influence the response (Dickens et al., 1962; Birch
et al., 1967). In most cases, P supply has not been

228
P D JENKINS & S MAHMOOD
shown to have a significant effect. Nutrient supply
may influence TDM% in two different ways, through
effects on tuber growth and the degree of tuber
hydration. Jenkins & Nelson (1992) showed that over
successive harvests TDM% increased linearly in
relation to mean tuber size but the regression
coefficient was decreased with increasing N supply.
If growth continued significantly longer as a
consequence of the extra available N, the final dry
matter concentration at harvest could be greater than
in those treatments receiving a lower level of N.
Therefore, if tuber growth is significantly reduced
by an inadequate supply of a particular nutrient the
effect at harvest will be reduced dry matter
concentration. However, both N and K also influence
the water content of cells (Marschner, 1995) so that
lower tissue concentrations of these nutrients may
be expected to result in lower water content, i.e.
higher dry matter percentage, the final outcome thus
depending on the interaction of these two opposite
effects. The potential exists, therefore, for quite
complex interactions where different nutrients are
supplied at a range of different levels. For example,
in Expt 1, the low K level produced a higher TDM%
but not when N and P where applied at the higher
level. In Expt 2 and Expt 3, low N gave a higher
TDM% at low K but not at high K. Schippers (1968)
also provided some evidence for an interaction
between N and K supply although less marked than
that reported here.
responses found would be replicated by later-
maturing cultivars with a more extended period of
leaf growth and greater propensity for shifts in
partitioning needs to be investigated. These studies
need to be extended to the field and should test for
interactions at the higher levels of overall nutrient
supply that are more usual under normal cultivation
practice.
Acknowledgement
The authors acknowledge with gratitude the award
of a post-graduate studentship from the Government
of Pakistan to Sajjad Mahmood during the tenure of
which this work was carried out.
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