Journal of Experimental Botany, Vol. 48, No. 312, pp. 1461-1468, July 1997
Journal of
Experimental
Botany
The role of P; recycling processes during photosynthesis
in phosphate-deficient bean plants
Agnieszka Kondracka1 and Anna M. Rychter
Institute of Experimental Plant Biology, University of Warsaw, Pawinskiego 5a, 02-106 Warszawa, Poland
Received 18 September 1996; Accepted 10 March 1997
Abstract
in others photosynthesis is not affected (Foyer and
Spencer, 1986; Dietz and Helios, 1990; Crafts-Brandner,
Bean plants {Phaseolus vulgaris L. cv. Zlota Saxa) were
1992a). It may depend on the extent of nutrient defigrown on complete (control plants) and phosphateciency in the leaves, but also on the capability of plant
deficient (low-P plants) culture solutions for 17 d.
metabolism to cope with low internal phosphate supply.
Phosphate deficiency markedly reduced leaf growth,
During photosynthetic carbon assimilation orthobut only slightly decreased the photosynthesis rate.
phosphate (P;) derived from ATP is consumed in the
The intensity of reactions releasing inorganic orthochloroplast stroma according to the simplified equation:
phosphate during photosynthesis was examined. In the
leaves of low-P plants the pools of photorespiratory
metabolites (glycolate and glycine + serine) were markedly increased. At the same time synthesis of soluble
sugars from intermediates of glycolic acid cycle was
probably enhanced. In low-P leaves the phosphoenolpyruvate carboxylase activity and malate synthesis
were increased. Phosphoenolpyruvate and malate
were effectively used for amino acid synthesis. Both
aspartate and alanine accumulation was twice higher
in low-P leaves. It was found that no enhancement in
starch and sucrose synthesis rate takes place in
phosphate deficient bean leaves. Modifications of
photosynthetic metabolism observed under moderate
phosphate deficiency facilitate plants acclimation to
low-P conditions by enhancement of P, recirculation
during glycolic and phosphoenolpyruvate metabolism.
Key words: Phosphate deficiency, photosynthesis,
photorespiration, phosphoenolpyruvate carboxylase.
Introduction
It has been known for many years that phosphate deficiency affects plant growth and the rate of photosynthesis.
The growth limitations often do not correlate with a
decreased rate of photosynthesis. In some plants phosphate starvation limits carbon assimilation (Fredeen et al.,
1989; Foyer and Spencer, 1986; Jacob and Lawlor, 1993),
1
3CO2 + 6H2O + P; + hv = triose phosphate + 3H2O + 3O2
(Sivak and Walker, 1986). Triose phosphates generated
are then used to form the end, non-phosphorylated products of carbon assimilation and P; released can be re-used
for ATP synthesis. In photosynthetic carbon metabolism
P; is liberated during synthesis of carbohydrates, organic
acids, and amino acids and during photorespiration.
It was found in different plant species that under
phosphate deficiency some of the above-mentioned reactions may be enhanced and thereby may temporarily
serve as an additional orthophosphate source. In phosphate-starved plants, an increased synthesis of starch
(Foyer and Spencer, 1986; Fredeen et al., 1989; Rao et al.,
1993) or both starch and sucrose (Dietz and Helios,
1990) was reported. Also the enhanced activity of
phosphoenolpyruvate carboxylase was observed under
low phosphate levels (Champigny et al., 1983; Duff eJ al.,
1989). Furthermore, it was proposed, that photorespiration may protect plants against phosphate deficiency
(Heber et al., 1989; Harley and Sharkey, 1991; Peterson,
1991; Jacob and Lawlor, 1993).
The purpose of this study was to compare in one plant
the importance of all the above Pj releasing reactions for
protecting photosynthetic carbon assimilation against
phosphate deficiency. The effect of moderate phosphate
deficiency on photosynthetic carbon metabolism was
studied in bean plants (Phaseolus vulgaris) and the role
To whom correspondence should be addressed. E-mail: [email protected]
6 Oxford University Press 1997
1462
Kondracka and Rychter
extracted, separated and analysed as described by Kondracka
and Maleszewski (1986).
of glycolic acid cycle, phosphoenolpyruvate metabolism
and starch and sucrose synthesis for P ; recycling was
analysed.
Metabolite and enzyme activity determinations
Leaves (500-600 mg) were sampled after 5-6 h of the light
period. Glucose, sucrose, and starch were extracted and assayed
according to Fredeen et al. (1989). Glycolate was extracted and
assayed according to Creach and Steward (1982). Glycolate
Plant growth conditions
Bean plants (Phaseolus vulgaris L. cv. Zlota Saxa) were cultured oxidase activity was assayed according to Kerr and Groves
(1975) and phosphoenolpyruvate carboxylase activity according
hydroponically in a growth chamber, essentially as described
to
Nato and Mathieu (1978).
by Ciereszko et al. (1996). Seedlings were germinated in
darkness for 4 d and then transferred to containers filled with
Leaf phosphorus determinations
nutrient medium containing 1.7 mmol I"1 P, (control plants) or
medium without P, (low-P plants). The cotyledons were
Leaves (200 mg) were ground in 0.5 ml 10% perchloric acid,
removed when the seedlings were 7-d-old. Plants were grown
homogenates were diluted with 7 ml 5% perchloric acid and
under 16 h photoperiod at quantum flux density of 150/xmol
centrifuged after 30 min. Inorganic orthophosphate and organic
2
1
soluble phosphates were assayed in supernatant by Fiskephotosynthetically active radiation (400quanta m
700 nm) provided by fluorescent tubes. Fully expanded primary
Subbarow method (1925). For total phosphorus determination,
leaves of 15-17 d plants were taken for study.
60 mg leaf segments were solubilized with 2 ml mixture of
concentrated HNO3 and perchloric acid (3:1 by vol) at 70 °C
Gas exchange measurements
for 3 d, neutralized with concentrated NH4OH and assayed
as above.
Net photosynthesis of attached primary leaves was measured
by infrared CO2 analysis (ADC, England) in a closed gas
exchange system, under light and temperature conditions as in
Results
the growth chamber. In experiments where the leaves were
exposed to changed gas conditions (lowered O2 or enhanced
Plant growth and primary leaf composition
CO2 concentration), plants were adapted to the changed
atmosphere for 20 min before gas exchange measurements.
Removal of phosphate from the nutrient solution signi14
ficantly affected growth of bean plants after 14 d from
CO2 assimilation and analysis of "C-products
the beginning of imbibition. After 18 d, phosphate defiThe primary leaves were detached, weighted and placed in a
ciency already led to the marked inhibition of growth
plexiglas chamber in a closed circulating system of a total
volume of 6 I in light conditions of 150fxmol quanta m~2 s"1.
(data not shown). Analyses performed on randomly
The CO2 concentration was continuously monitored by infrared
selected control and low-P plants demonstrated that,
14
gas analysis. At a CO2 concentration of 0.035%, CO2 was
between 15 and 17 d, low-P plants showed marked
introduced (0.6 or 1 MBq for an assimilation period of
morphological changes and decreased rate of growth
20 s-2 min or 2-6 min, respectively) to an appropriate assimilation volume (0.89 1 or 2.45 1 for assimilation periods 20 s-2 min (Table 1). At this stage of phosphate deficiency, total
and 2-6 min, respectively). Photosynthetic products were
plant mass was reduced by 10%. Shoot mass was
Materials and methods
Table 1. The characteristics of 15-17-d-old bean plants, grown on phosphate-sufficient (control) and phosphate-deficient (low-P)
medium (for growth and leaf composition measurements n = 36, for gas exchange measurements n = 18, for radioisotopic photosynthesis
determination n = 9)
Parameters
Plant mass (g)
Shoot mass (g)
Root mass (g)
Shoot: root ratio
Total leaf mass (g)
Total leaf area (cm2)
Primary leaf
mass (g)
area (cm2)
specific area (m2 kg"')
total phosphorus (^g g" 1 FW)
soluble organic phosphates (^g g~l FW)
inorganic orthophosphate (/ig g" 1 FW)
chlorophyll content (mg g" 1 FW)
net photosynthesis rate measured as:
CO2 assimilation (^mol CO2 g" 1 FW min" 1 )
14
C incorporation (kBq g" 1 FW min" 1 )
Control
4.67±041
3.50±0.34
1.17±0.26
2.9
Low-P
Low-P
(% of control)
4.18±0.38
2.65±0.31
1.53±0.25
90
76
131
59
76
78
1.7
2.13±0.31
112.6±I3.5
1.62 ±0.29
87.9±11.8
1.73±022
92.1 ±12.2
5.34 ±0.07
747 ±86
265 ±24
125±11
2.76±031
1.43 ±0.26
77.7± 10.9
5.43 ±0.08
118 ± 25
3.84±0.42
83
84
102
16
37
10
139
1.69±0.18
14.6 ±0.91
1.51 ±0.21
13.9±0.82
89
95
97±8
12±2
P, recycling in photosynthesis
Net photosynthesis in ambient and low-photorespiratory
conditions
The net photosynthesis rate was measured by infrared
gas analysis in light, temperature, and air conditions
similar to those in the growth chamber. The differences
between low-P and control plants in net photosynthetic
rate of primary leaves were small and statistically insignificant (Table 1). Similarly, the rate of net CO2 assimilation calculated as the intensity of 14C incorporation to
photosynthetic products after reaching the steady-state
14
CO2 exchange rate (after 3 min of 14CO2 feeding) was
the same in phosphate-sufficient and -deficient leaves.
Lowering of O2 concentration (from 21% to 2%)
resulted in the enhancement of net photosynthesis rate in
control leaves by 34%, but in low-P leaves by 10% only.
Also the increase of CO2 concentration (from 0.035% to
0.09%) resulted in the enhancement of net photosynthesis
rate by 220% in control leaves and by 83% only in low-P
leaves (Fig. 1). Therefore, in phosphate deficient leaves,
low-photorespiratory conditions caused significant limitations of the net photosynthesis. These limitations were
Photorespiratory metabolism in phosphate-deficient leaves
In control bean leaves the level of glycolate was
358 + 36 nmol g" 1 FW and phosphate starvation caused
the enhancement of the glycolate level to 472 +
41 nmol g" 1 FW, by about 30% as compared to control
(avg + SD, n = 8). Accumulation of newly assimilated 14C
in the photorespiratory intermediates was also enhanced
in low-P leaves: the glycolate pool was enlarged to 145%
of the control and glycine + serine pool size was almost
twice higher (Fig. 2). Under the same conditions, the
activity of glycolate oxidase in crude extracts was
enhanced by 24% and 47% as expressed on fresh
weight and extractable proteins, respectively (Table 2).
These results may indicate the higher production of
glycolic acid and other photorespiratory metabolites in
P-deficient leaves.
The decrease in photorespiration rate caused by
lowering the O2 concentration (from 21% to 2%) increased
low-P
control
JO
O
20
o
B. glycine + serine
30
26
total
21% O2, 0.035% CO,
2% O,, 0.035% CO,
21% 02,0.09% CO,
not present in ambient gas conditions, when photorespiratory metabolism is favoured.
fixed
decreased by 24% and root mass increased by 31% what
resulted in lowering by 41% the shoot/root ratio.
Primary leaf surface area and mass were decreased in
phosphate-deficient plants by 16-17%. Specific leaf area
was not affected until the 17th day of growth. Total
phosphorus, soluble organic phosphates, and inorganic
orthophosphate contents were decreased in primary leaves
of low-P plants by 6, 3 and 10 times, respectively
(Table 1). The leaf chlorophyll content was increased in
low-P plants by nearly 40%.
1463
r,
n
°
a
low-P
16
•
10
control
6
0-
0
!ow-P
Fig. 1. Net photosynthesis in primary leaves of 15-17-d-old bean
plants, grown on phosphate-sufficient (control) and phosphate-deficient
(low-P) medium, measured by gasometric method in ambient conditions
(n=12) and after lowering the oxygen concentration or increasing
carbon dioxide concentration (n = 6).
1
2
3
4
5
6
time (min)
Fig. 2. Radioactivity incorporated into glycolate (A) and
glycine + serine (B) as a percentage of the total radioactivity fixed
during 20 s-6 min of I4CO2 assimilation in primary leaves of
15-17-d-old bean plants, grown on phosphate-sufficient (control) and
phosphate-deficient (low-P) medium.
1464
Kondracka and Rychter
Table 2. Activities of glycolate oxidase and phosphoenolpyruvate
carboxylase in primary leaves of 15-17-d-old bean plants,
grown on phosphate-sufficient (control) and phosphate-deficient
(low-P) medium
differently in phosphate-deficient and in phosphatesufficient leaves.
Activity of phosphoenolpyruvate carboxylase
Enzyme activities were expressed on the leaf fresh weight or on protein
content in the extract. The solubilities of proteins from control and
low-P leaves in extraction buffers were respectively: for glycolate oxidase
assay 7.01 ±0.39 and 6.18±0.24 and for PEP carboxylase assay
7.71+0.43 and 4 7 8 ± 0 88mg proteins g" 1 FW (n=16 for glycolate
oxidase determination, n = & for phosphoenolpyruvate carboxylase
determination).
Enzyme
Activity of PEP carboxylase was significantly enhanced
in crude extracts of low-P leaves as compared to control
leaves (Table 2). The increase was by 26% as expressed
on fresh weight and more than twice when expressed
on mg of extractable proteins. It was the result of the
markedly diminished solubility of proteins from low-P
leaves in extraction buffer.
Control
Low-P
Low-P
(% of control)
1.19±0.19
1.48 ±0.07
124
Synthesis and accumulation of malate and amino acids
170±27
250± 18
147
167±87
211 ± 10
126
18.5±0.6
38.1 ±9.2
206
Oxaloacetate, the product of PEP carboxylase, can be
converted to malate or aspartate. In control leaves incorporation of 14C into malate was increasing with time,
indicating significant accumulation of newly produced
malate (Fig. 4A). The pool saturation was observed 3 min
after 14CO2 introduction. In phosphate-deficient leaves,
incorporation of 14C into malate during the first 60 s of
14
CO2 assimilation was higher, suggesting its enhanced
synthesis as compared to control leaves. In low-P leaves
14
C initially incorporated into malate was relocated to
other metabolites, probably aspartate. Malate pool size
measured as radioactivity of 14C-saturated malate was
about five times lower in low-P than in control leaves.
Nevertheless, the initial contribution of 14C-aspartate in
total 14C-photosynthetic products was lower in phosphate-deficient leaves as compared to control, the pool
size of aspartate was twice as high in low-P leaves
(Fig.4B).
Pyruvate, the product of PEP or malate metabolism,
can be transaminated to alanine. Alanine pool became
14
C-saturated as quickly as malate and aspartate (2.5 min
after 14C introduction). In phosphate-deficient leaves the
Glycolate oxidase
limo\O2g-' FW
min"1
nmol O 2 mg~' protein
min"1
PEP carboxylase
nmol COjg" 1 FW
nun*1
nmol CO 2 mg~'protein
min" 1
the 14C incorporation into soluble sugars in control leaves.
However, in low-P leaves the pattern of 14C incorporation
was different. At the beginning (up to 1 min after 14CO2
introduction) the incorporation to soluble sugars in lowphotorespiratory conditions was lower than in ambient
O2 concentration, but after 2 min become higher then in
photorespiratory conditions, similarly as in control plants
(Fig. 3). In low-photorespiratory conditions lowering of
soluble sugars production might be the result of decreased
P, recycling. However, the observed delay in soluble
sugars production may suggest also the changes in the
paths of their synthesis depending on presence or absence
of photorespiration. It seems that glycolic acid cycle
intermediates may contribute in soluble sugars synthesis
40
40
A. control
1
30
3o
--
°
21%
20
a»
2%O,
30
a
o
O
B. low-P
2%C
20
°2
21% 0 ,
•
10
10
3
4
time (min)
L
3
4
time (min)
Fig. 3. Radioactivity incorporated into sucrose in ambient (21%) and low (2%) oxygen concentration during 20s-6min of 14CO2 assimilation in
primary leaves of 15 17-d-old bean plants, grown on phosphate-sufficient (control) and phosphate-deficient (low-P) medium.
P, recycling in photosynthesis
1465
14
A.
malate
•
•
><
•
Table 3. Rates of C accumulation in pools of photosynthetic
metabolites (MBq g~' FW min'1) during 14CO2 fixation in
primary leaves of15-17'-d-old bean plants, grown on phosphatesufficient (control) and phosphate-deficient (low-P) medium
control
u
Metabolites
Control
Low-P
Low-P
(% of control)
Starch
Soluble sugars
Amino acids
3.63 ±0.42
3.45 ±0.60
3.06±0.59
3.07 ±0.58
2.90 ±0.47
5.83 ±0.66
85
84
190
r
o
\
low-P
•S 21
n
Q
a
Synthesis and accumulation of soluble sugars and starch in
leaves
The low phosphorus level in the leaves resulted in a great
increase in leaf carbohydrate content, both during the
light and the dark period (Fig. 5). Glucose, sucrose and
starch levels were higher in low-P leaves both at the end
of the dark period and after 6 h of light. In low-P leaves,
the increase of sucrose content due to 6 h of continuous
light was much lower than in control leaves. The elevation
of starch content after 6 h of light was similar in leaves
of both groups of plants (3.3 and 3.9 mgg" 1 FW,
respectively, in control and low-P leaves).
The rates of soluble sugars and starch synthesis were
measured as the velocity of 14C incorporation during
14
CO2 assimilation. Constant rates of soluble sugars and
starch synthesis were achieved 3 min after 14CO2 introduction. The rates of synthesis of both soluble and insoluble
carbohydrates were slightly decreased in low-P leaves
(Table 3). The level of sugars in the leaf is the result of
equilibrium between its synthesis and both translocation
and consumption. In bean plants, the increase in the
carbohydrate content during phosphate deficiency is not
the result of its enhanced synthesis, but rather the result
of changes in their redistribution and mobilization.
C. alanine
low-P
control
0
1
2
3
4
5
6
time (min)
Fig. 4. Radioactivity incorporated into malate (A), aspartate (B) and
alanine (C) during 20 s-6 min of 14 CO 2 assimilation in primary leaves
of 15-17-d-old bean plants, grown on phosphate-sufficient (control)
and phosphate-deficient (low-P) medium.
rate of alanine synthesis was enhanced and its accumulation was twice as high as compared to the control
(Fig.4C).
After 6 min of 14CO2 assimilation, above 20% and 35%
of newly assimilated carbon in control and low-P leaves,
respectively, was in the carbon skeletons of amino acids.
The rate of synthesis of amino acid carbon skeletons was
almost twice as high in low-P as in control leaves
(Table 3). Glycine, serine, aspartate, and alanine constituted more than 90% of this fraction.
Discussion
In the present study, during the early period of phosphate
deficiency in bean plants typical changes of growth parameters were observed: shoot and leaf mass was decreased
and root mass was increased. It was found that at this
stage of phosphate deficiency, no profound inhibition of
photosynthesis was observed (Table 1). However, in conditions favouring the enhancement of carbon assimilation
(low O2 or high CO2 concentration) the increase of the
photosynthesis rate was much higher in phosphatesufficient then in phosphate-deficient plants (Fig. 1). In
moderate phosphate deficiency, the lack of the inhibition
of photosynthesis in ambient conditions may be the result
of the increased intensity of inorganic orthophosphate
liberating reactions during photosynthetic metabolism. In
the conditions supporting intensive carbon assimilation
P, recycling becomes insufficient and photosynthesis does
1466
Kondracka and Rychter
0.26
D 8 h of dark
H 6 h of light
A.
'&o
so
e
0)
0.16
0.1
CO
o
o
5
B.
1-
1
I
'60
0.8-
i
T
60
e
4)
00
O
O
3
1
0.6
0.4
T
1
0.2
n
10
c.
eo
60
00
2-
m
control
low-P
Fig. 5. Glucose (A) sucrose (B) and starch (C) content in primary
leaves of 15—17-d-old bean plants grown on phosphate-sufficient
(control) and phosphate-deficient (low-P) medium. Leaves of plants at
the end of dark period or after 6 h of light were taken to study (n = 6-8)
not increase. It is proposed that the most important factor
in Pj recycling during photosynthesis in phosphatedeficient bean leaves is, probably, the glycolate pathway
and reactions utilizing phosphoenolpyruvate, mainly reaction catalysed by PEP carboxylase.
At the early stage of photorespiratory metabolism,
2-phosphoglycolate is hydrolysed in the chloroplast
stroma and P; is released. It may be, to some extent, the
additional source of P; during its shortage. The important
role of photorespiration for supporting photosynthesis
when isolated chloroplasts were incubated at low P, level
was shown by Usuda and Edwards (1982). It was proposed, that photorespiration substantially increases P;
availability for photosynthesis in the leaves of spinach,
cotton and tobacco (Heber et cil., 1989; Harley and
Sharkey, 1991; Peterson, 1991). Sharkey and Vanderveer
(1989) showed that in bean leaves in conditions of
diminished photorespiration the stromal P, concentration
decreases.
In this study it was demonstrated that, in low-P leaves,
the increased accumulation of glycolic acid cycle intermediates occurs (Fig. 2), in spite of their enhanced utilization (Table 2). The photorespiratory metabolites in
phosphate-deficient bean leaves seem to be more important substrates for soluble sugars synthesis, than in phosphate-sufficient leaves (Fig. 3). The enhanced soluble
sugars synthesis supported by glycolic acid intermediates
in low-P leaves may be the result of the limitations in
returning the glycerate to the Calvin cycle caused by
decreased ATP level in the chloroplast stroma and the
low activity of chloroplast glycerate kinase (Harley and
Sharkey, 1991).
The accumulation of glycolic acid cycle intermediates
could be the result of their enhanced synthesis. In low-P
leaves the possibility of stimulation of photorespiration
in phosphate-deficient plants was posed (Jacob and
Lawlor, 1993; Lauer et al., 1989). The increase of the
compensation point in phosphate-starved plants (Lauer
et al., 1989) and the enhanced excretion of glycolate by
Chlorella cells growing in phosphate-deficient culture
(Kozlowska and Maleszewski, 1994) may support this
suggestion. Moreover, Jacob and Lawlor (1993) found,
that the in vivo CO2/O2 specifity factor of ribulose-1,5bisphosphate carboxylase/oxygenase was lowered under
extreme phosphate deficiency in sunflower. The results
presented in this paper also indicate the possibility of the
enhanced glycolate production. However, the problem of
the possible stimulation of photorespiration under phosphate deficiency needs careful study. The measurement
of the real intensity of glycolate synthesis and its consumption in photosynthetic carbon oxidation pathway is
difficult (Sharkey, 1988). The non-fixed stoichiometry of
photorespiratory CO2 loss additionally complicates this
problem (Hanson and Peterson, 1987; Zelitch, 1992).
Moreover, it is not clear whether sources of glycolate
other than ribulose-l,5-bisphosphate oxygenation are
present in the plant cell (Zelitch, 1988).
During the carboxylation of phosphoenolpyruvate,
catalysed by PEP carboxylase, P, is liberated in the cytosol
and the chloroplast stroma in the leaves of C3 plants.
PEP carboxylase activity was greatly enhanced in phosphate-deficient bean leaves (Table 2). A similar increase
P, recycling in photosynthesis
was previously observed in leaves of wheat (Champigny
et al., 1983) and in black mustard cell suspensions (Duff
et al., 1989). In this study, in low-P leaves the increased
rate of malate synthesis and enhanced accumulation of
aspartate and alanine, the products of PEP metabolism,
were observed (Fig. 4). It seems, that at an early stage of
phosphate deficiency in bean leaves, the increased activity
of PEP carboxylase and the utilization of PEP to amino
acids synthesis might be the most important reactions for
orthophosphate recycling during photosynthesis.
In low-P bean leaves the rate of synthesis of amino
acids carbon skeletons was almost doubled (Table 3).
The dual role of enhanced amino acids synthesis in
phosphate-deficient plants was postulated: orthophosphate liberation (Harley and Sharkey, 1991) and the utilization of ammonia (Rufty et al., 1993). The second
possibility was supported by Rychter and coworkers
(unpublished results), showing that after 18 d of growth
phosphate deficiency had increased the level of ammonium ions in bean leaves by 25%.
Starch synthesis in the chloroplast stroma and sucrose
synthesis in the cytosol are the sources of P; liberated in
these compartments during photosynthetic reactions. The
importance of starch synthesis for P, recirculation in
phosphate-deficient plants was postulated (Foyer and
Spencer, 1986; Fredeen et al., 1989, Rao et al., 1993) as
well as the crucial role of the sucrose synthesis (Dietz
and Helios, 1990). However, a few reports showed that
low-P level does not necessarily result in changes in starch
and/or sucrose accumulation and it depends on diverse
capacity of different plant species to form starch and
sucrose (Foyer and Spencer, 1986; Crafts-Brandner,
19926; Rao et al., 1993).
The data presented clearly show that neither starch nor
sucrose synthesis is the extra source of P, under moderate
phosphate deficiency in bean leaves because the rates of
starch and soluble sugars synthesis were slightly decreased
in low-P leaves (Table 3). However, the leaf carbohydrate
content (glucose, sucrose, and starch) was increased in
phosphate deficient leaves both during a light and a dark
period (Fig. 5). Starch accumulation observed in leaves
of low-P bean plants may be the result of decreasing dark
(and probably light) starch mobilization. Depressed
starch hydrolysis must be the direct result of low P, level
in chloroplast stroma. In low-P leaves the light-promoted
accumulation of sucrose was lower than in control leaves.
It is consistent with results of Ciereszko et al. (1996)
showing, that in phosphate-deficient bean plants, soluble
sugars translocation to the roots was enhanced. The
changes of sucrose content reflect its role in whole plant
regulation rather than in photosynthetic metabolism
under moderate phosphate deficiency.
In natural environmental conditions low phosphorus fertility is a common, temporal limiting factor
(Theodorou and Plaxton, 1995). Plants have the ability
1467
to acclimate, within species-dependent limits, to changes
in phosphorus nutrition. Growth and metabolic changes
observed in plants grown in low phosphate culture can
be described in terms of acclimation to low phosphate
supply. Acclimation of growth and morphology consists
in preserving the phosphate content in leaves by lowering
shoot growth and facilitating phosphate uptake by enhancing root growth. Modifications of some metabolic routes
probably allow the plants to survive periods of lowered
phosphorus supply. Under conditions of phosphate
deficiency, the metabolic routes reducing Pj demand are
turned on during glycolysis (Theodorou and Plaxton,
1993; 1995) and respiration (Rychter and Mikulska, 1990;
Rychter et al., 1992). Enhanced remobilization of P,
occurs during the synthesis of aromatic amino acids and
secondary metabolites (Theodorou and Plaxton, 1995).
Results presented above strongly confirmed, that photosynthetic metabolism may be also the important source
of endogenous P;.
The data presented here prove the crucial role of the
PEP carboxylase reaction and PEP metabolism for P,
recirculation during photosynthesis in phosphate-deficient
bean leaves. Enhanced amino acids synthesis is probably
also the additional source of orthophosphate. The role of
photorespiratory metabolism for P, balance in chloroplasts seems to be significant, but needs further studies.
Acknowledgement
This work was partially supported by the Polish Committee for
Scientific Research (KBN) grant No. 6 P204 037 05.
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