Tree Physiology 19, 535--540
© 1999 Heron Publishing----Victoria, Canada
Magnesium nutrition and photosynthesis in Pinus radiata: clonal
variation and influence of potassium
OSBERT J. SUN1 and TIM W. PAYN2
1
New Zealand Forest Research Institute Limited, P.O. Box 29 237, Christchurch, New Zealand
2
New Zealand Forest Research Institute Limited, Private Bag 3020, Rotorua, New Zealand
Received August 5, 1998
Summary Magnesium (Mg) nutrition and photosynthesis
were studied in clones of Pinus radiata D. Don grown in sand
culture for 21 weeks at four Mg concentrations (0.008, 0.04,
0.2 and 0.4 mM) and three potassium (K) concentrations (0.25,
0.5 and 2.5 mM). We found significant clonal variation in Mg
nutrition of P. radiata. Plants grown at 0.04 mM [Mg] or less
showed pronounced visible symptoms of foliar Mg deficiency.
Net photosynthetic rate and leaf conductance were closely
related to shoot Mg concentrations below a concentration of
0.6 mg Mg gDW−1. Potassium enhanced the development of
visible symptoms of foliar Mg deficiency. At the lowest Mg
concentration tested (0.008 mM), the severity of needle chlorosis and necrosis increased with increasing K concentration
in the culture solution. With increasing Mg concentration,
2.5 mM [K] in the culture solution markedly increased root Mg
concentration, but decreased shoot Mg concentration, suggesting that excessive K inhibited Mg mobilization from roots to
shoots. Rates of growth and photosynthesis were both severely
inhibited at 0.008 mM [Mg].
potassium (K) was probably the major cause of UMCY in
P. radiata (Beets et al. 1993, Beets and Jokela 1994).
Magnesium has an active role in the action of some enzymes
and in maintaining the integrity of plant ribosomes. In addition, it is a key constituent of chlorophyll. Severe Mg deficiency in trees causes reductions in rates of photosynthesis and
carbohydrate export from the source tissues (Mehne-Jakobs
1995, 1996), changes in ultrastructure (Puech and Mehne-Jakobs 1997), and damage to vascular tissues (Hannick et al.
1993). Apart from the direct effect of supply limitation, Mg
uptake is influenced by the status of other cations in the growth
medium (Diem and Godbold 1993, Puech and Mehane-Jakobs
1997).
Little is known about the effects of variations in Mg nutrition
on P. radiata, or about the role of excessive K supply in
inducing Mg deficiency. Therefore, we examined variability in
Mg nutrition among several clones of P. radiata, and tested the
hypothesis that an excessive K supply interferes with Mg
uptake and mobilization from roots to shoots.
Keywords: antagonism, chlorosis, crown dieback, Monterey
pine, necrosis, UMCY, upper mid-crown yellowing.
Materials and methods
Clonal materials and growth conditions
Introduction
The wide occurrence of upper mid-crown yellowing (UMCY),
a disorder of needle yellowing and crown dieback in the upper
mid-portion of tree crowns, is of major concern to the forestry
industry in New Zealand because of potential loss of productivity of Pinus radiata D. Don plantations. The disorder is
typically associated with low needle retention and a reduced
foliar magnesium (Mg) concentration. Trees with severe
UMCY symptoms show needle chlorosis and necrosis and
thinning of the upper mid-crowns, leading to a marked reduction in photosynthetic leaf area. Although trees with UMCY
symptoms predominantly occur on pumice soils that are low in
Mg, stands containing trees with UMCY symptoms are found
throughout New Zealand. Beets and Jokela (1994) suggested
that UMCY was under both environmental and genetic control,
and that a nutritional imbalance involving primarily low concentrations of Mg and proportionately high concentrations of
Ten micropropagated clones of P. radiata were obtained from
the tissue culture laboratory of Fletcher Challenge Forests
Limited at Te Teko, New Zealand. The history of the clones
was unknown because of the Company’s confidentiality on its
selection program. The plantlets, which were growing on agar
in plastic containers when air-freighted to Christchurch, were
transplanted to seedling trays containing perlite and placed in
a thermostatically controlled mist greenhouse to induce foliar
hardiness. After three weeks in the mist house, plants were
transferred to 4-l pots containing silica sand and placed in a
greenhouse. All plants were supplied with a low concentration
(1.78 mM of N) of Ingestad’s (1971) complete nutrient solution for two weeks before receiving the experimental treatments. Temperature in the greenhouse was thermostatically
controlled, but fluctuated between 25 °C (day) and 15 °C
(night), and maximum photosynthetically active irradiance
was slightly above 1000 µmol m −2 s −1 at midday.
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SUN AND PAYN
Experimental design and treatments
Assessment of needle chlorosis and necrosis
The experimental design was a factorial with ten clones, four
Mg concentrations (0.008, 0.04, 0.2, and 0.4 mM), and three K
concentrations (0.25, 0.5, and 2.5 mM) with three single-treeplot (pot) replications. Nitrogen (N) concentration in all the
treatments was maintained at 7.14 mM (equivalent to 100 mg
l −1), with other essential elements held constant relative to N
as defined by Ingestad (1971). The ratio of NO −3 -N to NH 4+ -N
in the treatment solutions varied between 44:56 in the 0.008
mM [Mg] + 0.25 mM [K] treatment and 67:33 in the 0.4 mM
[Mg] + 2.5 mM [K] treatment, with pH ranging from 5.23 to
5.78. Table 1 lists the chemical composition of the stock
solutions.
Treatments began two weeks after plants were transplanted
to the 4-l pots. Applications of nutrient solution were made
with increasing frequency from an initial rate of once every
two weeks to twice a week after three months up to 21 weeks.
At each application, 500 ml of solution was added to each pot
from the top and drained freely at the bottom. Lids made of
10-mm polystyrene were placed on top of the sand to minimize
evaporation and growth of algae. All pots were placed randomly, and rearranged following each nutrient application.
The severity of foliar Mg deficiency was visually assessed
before plants were harvested for biomass and nutrient analysis.
Needle chlorosis and necrosis were categorized into six classes
based on percentage of leaf areas displaying Mg-deficiency
symptoms: 0 = less than 5% chlorosis; 1 = chlorosis only,
between 5 and 20%; 2 = predominantly chlorosis, between 20
and 40%; 3 = predominantly chlorosis associated with slight
necrosis, between 40 and 60%; 4 = showing both necrosis and
chlorosis, between 60 and 80%; and 5 = showing severe chlorosis and necrosis, greater than 80%.
Biomass and plant tissue nutrient analysis
Plants were harvested after 21 weeks in the nutrient treatments
for determination of shoot and root biomass and tissue nutrient
concentrations. Tissue nutrient analyses were conducted by the
Forest Nutrition Laboratory of the New Zealand Forest Research Institute Limited, Rotorua, based on the procedures of
Nicholson (1984). Uptake rates of Mg (U[Mg] ) per unit root dry
weight (DWRt) were calculated by the modified formula of
Ingestad and Ågren (1988):
U [Mg] =
10 6 C[Mg] RGR
DW Pl
(e --1)
,
M [Mg]
DW Rt
(1)
Measurements of photosynthesis and leaf conductance
Net photosynthetic rate (A), stomatal conductance to diffusion
of water vapor (gsw), and intercellular CO2 concentration (Ci )
were measured in the greenhouse with a portable photosynthesis system (LI-6200, Li-Cor, Inc., Lincoln, NE) one week
before the scheduled harvesting took place. The measurements
were made on six fully expanded needles in the upper-mid part
of the main stem.
Leaf conductance was partitioned into stomatal (gsc) and
nonstomatal or residual conductance (grc) to diffusion of CO2
according to Farquhar and Sharkey (1982).
where C[Mg] is the weighted Mg concentration of the plant,
M[Mg] is the molecular weight of Mg, DWPl is total plant dry
weight at harvesting, and RGR is mean relative growth rate,
which was calculated as:
RGR =
ln DWPl -- ln DW 0
,
∆T
(2)
where DW0 is initial total plant dry weight, estimated by
destructively harvesting six plants of similar size at the start of
the treatment, and T is the duration of the experiment.
Table 1. Chemical composition of stock solutions (modified from Ingestad (1971)).
Compound
NH4NO3
Mg(NO3)2.6H2O
KNO3
K2HPO4
K2SO4
(NH4)2SO4
(NH4)2HPO4
HNO3
Ca(NO3)2.4H2O
Fe2(SO4)3
MnSO4.4H2O
H3BO3
CuCl2.2H2O
ZnSO4.7H2O
Na2MoO4.2H2O
Solution B
Solution [Mg] (g l −1)
Solution [K] (g l −1)
(g l−1)
[Mg0.008mM]
[Mg0.041mM]
[Mg0.205mM]
[Mg0.410mM]
[K0.256mM]
[K0.512mM]
[K2.560mM]
−
−
−
−
−
−
−
1.6
20.6
1.25
0.81
0.57
0.036
0.066
0.008
16.5
1.07
−
−
−
−
−
−
−
−
−
−
−
−
−
15.1
5.34
−
−
−
−
−
−
−
−
−
−
−
−
−
8.4
26.7
−
−
−
−
−
−
−
−
−
−
−
−
−
−
53.4
−
−
−
−
−
−
−
−
−
−
−
−
−
86.1
−
12.9
−
−
16.8
27.7
−
−
−
−
−
−
−
−
81.0
−
25.8
−
−
16.8
27.7
−
−
−
−
−
−
−
−
93.5
−
61.4
36.6
22.2
−
−
−
−
−
−
−
−
−
−
TREE PHYSIOLOGY VOLUME 19, 1999
Mg NUTRITION IN PINUS RADIATA
Data analysis
Data were evaluated by analysis of variance. Where significant
interactions between factors were absent, means of the main
effects were compared by Duncan’s multiple-range test with a
confidence level of P ≤ 0.05.
Results
Visible symptoms of foliar Mg deficiency
Visible symptoms of foliar Mg deficiency, such as chlorosis
and necrosis, developed mainly in trees growing at 0.008 or
0.04 mM [Mg]. In trees growing at 0.04 mM [Mg], the symptoms were typically chlorosis in the older needles, whereas in
the 0.008 mM [Mg] treatment, chlorosis and necrosis developed in needles of all ages. The needle chlorosis rating varied
significantly (P ≤ 0.0001) among the ten clones and with the
Mg concentration of the culture solution (Figure 1a). We observed significant (P ≤ 0.005) clonal variation in the response
of needle chlorosis rating to [Mg], with clear differentiation
between Clones S and Z and Clones U and V.
537
The effect of K concentration ([K]) on the visible injury
symptoms of the foliage was highly (P ≤ 0.0005) dependent on
[Mg]. Needle chlorosis rating increased with increasing [K]
only when [Mg] was held at 0.008 mM (Figure 1b).
Leaf photosynthesis and conductance
Values of A, gsc and grc were closely related to shoot Mg
concentration in the range 0.2 to 0.6 mg Mg gDW−1 (Figure 2).
The change in A was coupled more closely with grc than with
gsc (Figure 3); however, the relationships of A, gsc and grc with
shoot Mg concentration, as well as between A and leaf conductance (gsc and grc), were similar for all clones.
Shoot biomass and root to shoot ratio
Shoot biomass and root to shoot ratio differed significantly
among clones (P ≤ 0.0001), and shoot biomass differed significantly among clones in response to [Mg] (P ≤ 0.005) after 21
weeks of treatment (Figure 4). The responses of root to shoot
ratio to [Mg] were inconsistent among the ten clones in the
range between 0.04 and 0.4 mM [Mg]; however, plants grown
at 0.008 mM [Mg] showed a marked reduction in root to shoot
ratio (Figure 4b).
The effect of [Mg] on shoot biomass was also significantly
(P ≤ 0.001) influenced by [K]. Shoot biomass was more susceptible to decreasing [Mg] when [K] was held at 0.25 mM
(Figure 5). In all four Mg treatments, both increasing and
reducing [K] from 0.5 mM resulted in significant (P ≤ 0.05)
reductions in shoot biomass.
Tissue Mg concentration and uptake rate
Shoot and root Mg concentrations were closely related to the
supply of Mg, with the relationship being further affected by
[K] (Figure 6). The effect of [K] on the response of root Mg
concentration to [Mg] differed from that of shoots. With increasing Mg concentration, 2.5 mM [K] in the culture solution
markedly increased root Mg concentration, but decreased
shoot Mg concentration.
The rate of uptake of Mg was significantly affected by both
clone and [Mg] (P ≤ 0.0001), and also by an interaction
between [Mg] and [K] (P ≤ 0.005); it increased with increasing
[Mg], and varied among clones (Figure 7a). The uptake rate of
Mg at 0.008 mM [Mg] increased with increasing [K] (Figure 7b).
Discussion
Figure 1. Clonal variation in, and effects of Mg and K supply on,
needle chlorosis rating in greenhouse-grown P. radiata. Vertical bars
indicate the standard errors of the means.
We observed large clonal variations in growth and Mg nutrition in P. radiata. Some clones were more tolerant to low Mg
supply than others independently of their performance with
non-limiting Mg supply. Based on the severity of needle chlorosis and necrosis, and the reduction in growth, the supply of
Mg became limiting at concentrations below 0.04 mM for the
young P. radiata plants.
Net photosynthetic rate and stomatal and non-stomatal conductance declined sharply with decreasing shoot Mg concentrations below 0.6 mg Mg gDW−1. At shoot Mg concentration
greater than 0.6 mg Mg gDW−1, photosynthetic rate was not
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SUN AND PAYN
Figure 4. Shoot biomass and root to shoot ratio of greenhouse-grown,
21-week-old P. radiata clones at four Mg supply rates. Vertical bars
indicate the standard errors of the means.
Figure 2. Relationships of net photosynthesis (A), stomatal conductance (gsc) and residual conductance (grc) to diffusion of CO2 with
shoot Mg concentration in greenhouse-grown P. radiata clones. Each
point represents the mean of three measurements.
much affected by increases in [Mg], suggesting that 0.6 mg Mg
gDW−1 is a critical concentration of shoot Mg for photosynthetic activity in P. radiata.
Reductions in photosynthetic rate and leaf conductance
were closely correlated with the severity of needle chlorosis
and necrosis. Mehne-Jakobs (1996) suggested that the yellowing of needles in conifers caused by light-dependent reduction
of pigments including chlorophyll, reflects a secondary effect
of Mg deficiency. Research by Mehne-Jakobs (1995, 1996)
showed that, in Picea abies (L.) Karst. (Norway spruce), Mg
deficiency was associated with foliar starch accumulation and
increased sugar content, and that Mg deficiency affected carboxylation efficiency rather than light use efficiency. Magnesium deficiency has been found to cause damage to vascular
tissues (Hannick et al. 1993). The reduced stomatal conductance of severely Mg-deficient P. radiata trees in the present
study may be an artifact associated with physical damage to
needles, because net photosynthetic rate was more closely
Figure 3. Relationships of net photosynthesis (A) with stomatal conductance (gsc) and residual conductance
(g rc) to diffusion of CO2 in greenhouse-grown P. radiata clones.
Each point represents the mean of
three measurements.
TREE PHYSIOLOGY VOLUME 19, 1999
Mg NUTRITION IN PINUS RADIATA
539
Figure 5. Shoot biomass of greenhouse-grown, 21-week-old P. radiata at four Mg supply rates and three K supply rates. Vertical bars
indicate the standard errors of the means.
correlated with non-stomatal factors than with stomatal activity in the Mg-deficient trees.
Although significant clonal variation in response to external
Mg supply was exhibited in both growth and Mg uptake, a
Figure 7. Clonal variation in, and effects of Mg and K supply on, Mg
uptake rate of greenhouse-grown P. radiata clones. Vertical bars
indicate the standard errors of the means.
Figure 6. Shoot and root Mg concentrations of greenhouse-grown
P. radiata at four Mg supply rates and three K supply rates. Vertical
bars indicate the standard errors of the means.
direct linkage between clonal performance and Mg nutrition
was not apparent. Furthermore, the photosynthetic response to
shoot Mg concentration did not seem to be clonal. Although
the conclusion might be drawn that Mg nutrition in P. radiata
is under a degree of genotypic control, the morphological
susceptibility to Mg deficiency may not be associated with a
similar pattern of growth response. Our results showed that the
most severe Mg deficiency (expressed as visible symptoms
and tissue Mg concentration) was found among the fastest
growing clones, indicating that a strong demand-driven supply
limitation also contributes to Mg deficiency. Nevertheless,
tolerance to severe Mg supply limitation was found to differ
among P. radiata clones.
We observed that K interfered with Mg nutrition. At severe
Mg supply limitation, the severity of needle chlorosis and
necrosis was enhanced by increasing K supply, but plant
growth was improved and Mg uptake increased. With increasing rate of Mg supply, increasing K supply markedly increased
root Mg concentration, but decreased shoot Mg concentration,
and decreased Mg uptake rate. An excess of K could inhibit the
mobilization of Mg from roots to shoots at high Mg concentrations. A recent study by Schell (1997) suggests that malic acid
might be involved in influencing the mobilization and translo-
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540
SUN AND PAYN
cation of Mg in sapwood of Fagus sylvatica L. by forming a
malate--magnesium complex. Potassium might play a role
either in such chemical bonding, or in the production of cationbinding chemical compounds.
One approach to minimize the potential loss of growth in
P. radiata plantations caused by Mg deficiency would be to
select and breed trees based on Mg nutrition characteristics, in
addition to other selection criteria. Our results suggest that
selection of genotypes capable of performing well on marginally Mg-deficient sites is feasible. Criteria for the selection of
such genotypes should include consideration of root systems
because they play important roles in Mg nutrition of P. radiata,
including uptake processes and root to shoot translocation.
Acknowledgments
This research was funded by the Upper Mid-Crown Yellowing Research Group, a consortium of New Zealand Forest Companies.
Fletcher Challenge Forests Limited supplied plant materials for the
study. The greenhouse experiment was carried out at Lincoln University with the assistance of Ross J. Mitchell, and plant analysis was
undertaken by Paul Gunn and Phyllis Lyle at the Forest Nutrition
Laboratory, NZ FRI, Rotorua. We gratefully acknowledge comments
on the manuscript by William Laing, Roger Sands, Tony Shelbourne
and Alison Low.
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TREE PHYSIOLOGY VOLUME 19, 1999