Potato Research 32 (1989) 17-32
Effects of water stress on photosynthesis and chlorophyll
fluorescence of five potato cultivars
A. H. C. M. S C H A P E N D O N K , C. J. T. SPITTERS and P. J. GROOT
Foundation for Agricultural Plant Breeding (SVP), P.O. Box 117, 6700 AC Wageningen, the
Netherlands
Accepted for publication 15 August 1988
Additional key words: drought tolerance, stomatal behaviour, breeding
Summary
Reduction of leaf photosynthesis due to water stress has been analyzed into various components
and genetic variation in these components has been evaluated. Five potato cultivars were grown
on nutrient solution in a conditioned glasshouse. Water stress was imposed by adding polyethylene glycol to the nutrient solution. Photosynthesis, transpiration and chlorophyll fluorescence
were measured on intact leaves during the stress period and after recovery from the stress.
Water stress reduced photosynthesis, initially as a consequence of stomatal closure, but after
3 days increasingly by inhibiting directly the photosynthetic capacity (mesophyll limitation).
Stomatal closure correlated with the reduction in photosynthesis, but it was not the sole cause
of this reduction because the internal CO 2 concentration in the leaves was not affected by water
stress, indicative of inhibitory factors other than stomatal ones. Chlorophyll fluorescence emission suggested that the Calvin cycle was inhibited, while quantum efficiency was not affected
at 17 ~ Increasing the temperature to 27 ~ reduced quantum efficiency but only in the stress
environment. The recovery of young leaves after relief of the stress was associated with a lower
stomatal conductance but a higher mesophyll conductance compared with the control, which
caused a low internal CO, concentration and probably invoked photo-inhibition and leaf damage.
Cultivar differences in photosynthetic rate were highly significant under both optimal and
stress conditions, and corresponded with differences in mesophyll conductance.
Introduction
W a t e r stress is a m a j o r c o n s t r a i n t to w o r l d p o t a t o p r o d u c t i o n . T h e effects o f water
stress on t u b e r yield d e p e n d on the a g g r e g a t e r e s p o n s e o f m o r p h o - p h y s i o l o g i c a l
processes, such as p h o t o s y n t h e s i s , l e a f area e x p a n s i o n , l e a f senescence, p a r t i t i o n i n g
o f a s s i m i l a t e s w i t h i n the p l a n t , t u b e r i n i t i a t i o n a n d b u l k i n g (reviews by Van L o o n , 1981,
1986). It d e p e n d s also on the t i m i n g o f the stress within the growth p e r i o d (Spitters
& S c h a p e n d o n k , 1988) a n d on c l i m a t i c a n d soil c o n d i t i o n s . Selection for d r o u g h t tolerance is t h e r e f o r e c o m p l i c a t e d by the m a n y processes involved a n d their i n t e r a c t i o n with
the e n v i r o n m e n t .
T h e present s t u d y is restricted to the effect o f water stress on the rate o f p h o t o s y n t h e sis u n d e r c o n t r o l l e d c o n d i t i o n s . R e d u c t i o n in p h o t o s y n t h e t i c rate is a n a l y z e d into various c o m p o n e n t s a n d genetic v a r i a t i o n in these c o m p o n e n t s is evaluated. This m a y cont r i b u t e to a m o r e c o n s c i o u s selection o f p a r e n t s for h y b r i d i z a t i o n a n d to the
d e v e l o p m e n t o f screening tests for key factors in d r o u g h t tolerance.
~lotato Research 32 (1989)
17
A. H. C. M. SCHAPENDONK, C. J. T. SPITTERS AND P..[. GROOT
In potato, reduced photosynthetic rate due to water stress has often been reported
(reviews by Bodlaender et al., 1986; Van Loon, 1986). However, attributing of these
effects to the underlying processes o f CO 2 transport from outside the leaf to the site
o f carboxylation, electron transport, and CO2 fixation in the Calvin cycle has
received little attention.
In other plant species, it has been demonstrated that water stress can reduce the photosynthetic rate indirectly by closure o f the stomata or directly by a reduction of the
photosynthetic capacity of the leaves. There is, however, no consensus about the
primary site of the reduction in photosynthesis (review by Kaiser, 1987). There is also
no consensus whether photoreactions in the thylakoid membranes or biochemical
reactions of the Calvin cycle are most affected (Keck & Boyer, 1974; Ogren & Oquist,
1985). Furthermore most experiments do not distinguish between water stress and heat
stress. These factors are positively correlated but it is desirable to measure their effects
separately (Ceccarelli, 1984).
Material and methods
Experimental design
Water stress was imposed as uniformly as possible over the root system of the various
plants by growing the plants on nutrient solution and adding polyethylene glycol to
establish a low matrix potential in the root environment. Five potato cultivars were
chosen for the experiment on the basis o f their differences in drought tolerance in the
field (Beschrijvende Rassenlijst voor Landbouwgewassen; Dutch List of cultivars) and
in pot experiments (Beekman & Bouma, 1986): Alpha, Bintje, Saturna, Kennebec and
Veenster. Sprouted eye pieces were planted on 20 April 1987 in small tubes filled with
rockwool. The tubes were pierced through the bottom of small boxes (35 x 35 z 25 cm),
filled with coarse sand, to allow for stolon growth and tuber formation. Each box contained four plants o f the same cultivar and there were three replicates for the control
and the drought treatment. The roots grew through the rockwool into containers of
8 1, through which a nutrient solution (Steiner solution half strength) was circulated
from two main reservoirs connected to the containers by a system of pipes. The rockwool was in contact with the nutrient solution, thus acting as a wick to keep the water
content in the stolon boxes at a constant level.
Plants were grown in a glasshouse with air temperatures at 17 ~ (day) and 12 ~
(night). The nutrient solution was kept at 18 ~ Fifty days after planting, on 9 June
(day 0) half o f the plants were transferred to containers with nutrient solution and 10 ~
polyethylene glycol (PEG, M = 20,000). The addition o f PEG reduced the water potential in the root zone to - 0.27 MPa (pF = 3.4) by lowering the matrix potential component (Steuter, 1981). The resulting viscosity of the nutrient solutions hampered circulation and therefore oxygen was supplied by a pump. Transpiration by the plants
increased the concentration o f PEG slightly. On day 6, water stress was enhanced by
doubling the percentage o f PEG, equivalent to a matrix potential o f -1.16 MPa
(pF=4.1). On day 8, the vapour pressure deficit of the ambient air was enhanced by
a temperature jump from 17 ~ to 27 ~ On day 10, the solutions were replaced by
water and the temperature was reset to 17 ~ to allow for recovery. Fig. 1 shows a diagram o f the daily reference evapotranspiration for a short grass cover as a function
o f global radiation and temperature, calculated by (a) the Makkink equation (De
Bruin, 1988) and (b) the water stress during the measuring period. A time table of the
18
Potato Research 32 (1989)
EFFECTS OF WATER STRESS
(mm/day)
Evspotranspiration
potential (M
Water
2.00
0.00 '
1.60
-0.30
1,20
-0.60
0.80
-0.90
0.40
- 1.20
Ps)
~
a
0.00
0
i
2
4
b
i
i
I
I
i
i
6
8
10
12
14
16
- 1.80
0
I
i
i
i
I
i
i
I
2
4
6
8
10
12
14
16
Days
Days
Fig. 1. Calculated reference evapotranspiration (a) and the matrix potential in the nutrient solution (b) as a function of time after onset of the stress.
Table 1. Time schedule of the measurements of photosynthesis (P), chlorophyll fluorescence
(F) and the osmotic value of cell sap (O) and the leaf number sampled (unfolded from the top
of the plant at day 0). On 9 June 16.00 h half of the plants were transferred to containers with
nutrient solution and l0 °70 polyethylene glycol.
Date
Day hr.
Measurement
9
l0
12
16
17
18
24
0
l
3
7
8
9
15
P;
P;
P;
F
P;
P;
June
June
June
June
June
June
June
F
F
O
F
F
Leaf hr.
7;
'7;
7;
7
7,
7,
7
7
7
l; 7, I
l; 7, I
m e a s u r e m e n t s is depicted in Table 1.
Photosynthesis and transpiration
Gas exchange m e a s u r e m e n t s were carried out o n leaves attached to the plant with a
portable leaf c h a m b e r analyzer (LCA; Analytical Development Co. (ADC), UK). T h e
measuring leaves (2 top leaflets per pot) were the sixth or seventh unfolded leaves from
the top. These leaves were fully expanded and they will be referred to as 'old' leaves.
Potato Research 32 (1989)
19
A, H. C. M. SCHAPENDONK, C. J. T, SPITTERS AND P. J. GROOT
Young leaves, unfolded during the stress treatment and with a m i n i m u m length o f 5 cm
are denoted as 'young' leaves. Subsequent measurements were made on the same
leaves. Air was obtained from a gas cylinder to ensure constant composition. It was
led through a water bath to humidify it before entering the leaf chamber. All measurements were done at light saturation, provided by an incandescent lamp cooled by a
fan. Average conditions within the c h a m b e r were: 1700/~mol quanta m - 2 s J (340 W
m - 2 ) of photosynthetically active radiation ( 4 0 0 - 7 0 0 rim), temperature of 22.6 ~
vapour pressure deficit of 0.57 kPa, and CO2 concentration of 330 vpm. Rates of
photosynthesis and transpiration were calculated from the measured concentrations
o f CO 2 and vapour in the ingoing and outgoing air stream and the flow rate of the
stream by the procedure described by von C a e m m e r e r & Farquhar (1981). Resistances
and conductances were estimated on the basis of the following equations:
P
=
(c e -
T = (w e -
ci)/
(Fc, b +
Fc,s) =
wi) / ( r ~ , b + r,,,0
(c i -
Co)/
re. m
(1)
(2)
where P is the gross photosynthetic rate (gCO 2 m -2 h-~); T the transpiration rate
(gH20 m -2 h - I ) ; c the CO2 concentration (vpm) with indices e, i and o referring to
outside the leaf, inside the substomatal cavity, and at the place of carboxylation,
respectively; w the water vapour pressure (kPa) with indices e and i referring to outside
and inside the leaf; r the resistance (s m -t) with the first indices c and w referring to
CO 2 and water vapour, and the second indices b, s and m to b o u n d a r y layer, stomata
and mesophylI, respectively. Mesophyll resistance consists of a small transport c o m p o nent and a dominating carboxylation component. Conductances were calculated as
the reciprocals of the corresponding resistances (g= 1/r).
External concentrations c e and we were measured. The b o u n d a r y layer resistance
(rw,b) was estimated to be 18.6 s m -j from measurements with wet filter paper.
Vapour pressure inside the leaf (w 0 was assumed to equal the saturated vapour pressure at the leaf temperature. Leaf temperature was estimated from the energy balance
of the leaf according to the method described by G o u d r i a a n (1977, p. 78). Leaf temperature exceeded air temperature by, on the average, 2.5 ~ in the control and 3.5 ~ in
the stress environment. Vapour pressure deficit (VPD) across the interface between leaf
and air was therefore twice as high as the deficit above the leaf, which emphasizes the
importahce of accounting for difference in leaf and air temperature when estimating
the resistances.
Stomatal resistance for water vapour transport (rw.s) was then estimated from
measured transpiration rate from Equation 2. Stomatal resistance for CO2 (re.s)
amounts to 1.6 times that for water vapour (r,,.0, and b o u n d a r y layer resistance for
CO 2 (re.b) is 1.37 times that for water vapour (r,,..b) (Von C a e m m e r e r & Farquhar,
1981). The CO2 concentration at the site of carboxylation is equal to the compensation point, which was supposed to be 45 vpm. Subsequently, c i and re.m were estimated from gross photosynthesis by Equation 1. Since leaves were exposed to high light
for only a short period, gross photosynthesis was derived from the measured rate of
net photosynthesis supposing 5 ~ dark respiration. Water use efficiency is defined here
as CO 2 uptake per unit o f transpiration ( W U E = P/T, g CO 2 g - l H20)" From Equations 1 and 2 and the ratio o f the diffusion coefficients of CO2 and HzO, water use
efficiency, adjusted for the vapour pressure deficit (VPD) gradient (we-wi), is de20
Potato Research 32 (1989)
EFFECTS OF WATER STRESS
rived to approximate:
VPD-WUE
= VPD.P/T
= (c e - c i ) / 1.6
(3)
which is constant when ci and c~ are constant.
Osmotic potential
Osmotic potentials were determined on companion leaves that were harvested on day
7, at three time intervals: 9.00 am, 12.00 am, 15.00 pm. Companion leaves of three
plants per pot (seventh unfolded leaves from the top) were placed on water to restore
the potential turgescence and subsequently frozen at liquid nitrogen temperature. Osmotic potentials were determined upon thawing with a Wescor vapour pressure osmometer.
Chlorophyll fluorescence
Chlorophyll fluorescence was measured to discriminate between various components
of photosynthesis in relation to water stress (Havaux & Lannoye, 1985; Schreiber &
Bilger, 1985). Chlorophyll fluorescence was measured on attached leaves by a weak
measuring light beam (not photosynthetically active) pulsed with a high frequency
(100 kHz), at different wavelengths to the chlorophyll fluorescence. The photodetection system was locked to the frequency of the measuring light beam, thus preventing
interference of the measurements by light scattered from the photosynthetically active
light (Schreiber et al., 1985).
Intact leaves o f both stress and control plants were dark adapted by folding aluminum foil around the leaves, 20 minutes before the measurement. Fluorescence induction curves were recorded using a modulation fluorometer (PAM 101 Chlorophyll
Fluorometer, H. Walz, Effeltrich, FRG). The leaf was clamped in a small cuvette,
flushed with air.
The fluorescence signals curves were related to the efficiency of energy transfer in
the chloroplasts and to the mesophyll conductance. The latter comprises the rate of
electron transport from photosystem II to photosystem I and the Calvin cycle activity.
When photosystem I1 is fully oxidized, fluorescence is low (i.e. quenched). A saturating light flash was fired to determine the maximal fluorescence (Fro). The ratio between
Fm and the initial fluorescence after dark adaptation (Fo) is an indication of the efficiency of energy transfer from antennae pigments to photosystem lI. Synchronously,
the red light (100 ~mol m -2 s -~) was switched on to activate photosynthesis. Repetitive saturating light flashes (10 000 ;tmol m -z s -t) were applied at a frequency of
0.25 Hz to induce transients, (Fv)~, superimposed on the fluorescence evoked by the
red light (Fv). A detailed discussion of the fluorescence quenching analysis is given
by Schreiber & Bilger (1985). The ratio of the fluorescence signals just before and during the flashes gives information about the amount of oxidized photosystem II acceptors at that moment. Adopting the proposition of Krause et al. (1982) and Bradbury
& Baker (1983), this ratio increases in the light due to the activation of the electron
transport chain which causes a re-oxidation of the acceptor site of photosystem II and
thus a decrease of the fluorescence (Q-quenching).
Additional information, regarding the energy status of the chloroplasts can now be
derived from the differences between the maximum fluorescence in the first flash (Fm)
and the lower responses in the subsequent flashes (F,)~. The pH-gradient increases
Potato Research 32 (1989)
21
A. H. C. M. SCHAPENDONK,
C . J. T. S P I T T E R S
AND
P. J. G R O O T
I,D
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~
.
:o:
¢,1
o
Potato Research 32 (1989)
EFFECTS OF WATER STRESS
initially in the light and is subsequently used for the formation of ATP, which in turn
feeds the Calvin cycle with energy. This is reflected by the slow decrease o f the Equenching, when the Calvin cycle is activated.
Results
There are two conflicting views on the m a j o r sites for the inhibition of photosynthesis
due to water stress; stomatal and mesophyll limitation. The experimental results of
the gas exchange measurements wiI[ be interpreted along the lines of these theories.
(1) Stomatal limitation. According to the classical view, water stress induces closure
of the stomata, either due to a lowered leaf water content or due to some other signal
transduced to the leaves. The greater resistance of the stomata for CO, diffusion
results in a reduction of the C O , concentration inside the leaf and so in a lowered rate
o f photosynthesis (reviews by Bradford & Hsiao, 1982; Ceccarelli, 1984). This mechanism of stomatal limitation is reflected in a reduced internal CO, concentration
(smaller value of the ci/c e ratio) and a greater share of the gas phase resistance (r b + rs)
to the total resistance for CO2 (r b + r~ + rm). Photosynthesis is less inhibited than transpiration because the gradient for CO 2 is increased, while that for water vapour remains the same (Equations 1 and 2). Water use efficiency, at given vapour pressure
deficit, is therefore enhanced by water stress (Equation 3).
(2) Mesophyll limitation. In addition to stomatal closure, water stress can reduce the
photosynthetic capacity directly, either by inhibiting the Calvin cycle or the rate of
electron transport over the chloroplast membranes (review by Kaiser, 1987). Stomatal
aperture is also reduced, but in such a way that the internal CO: concentration remains unaffected (e.g. Wong et al., 1979, 1985). Thus stomatal aperture adapts to the
mesophyll limitation and both show a correlated response to water stress, probably
mediated by ABA (Schulze, 1986). The mechanism of mesophyll limitation is expressed
in that the q / c e ratio and the share of gas phase resistance to total resistance remain
unchanged when water stress occurs. Photosynthesis and transpiration are equally
reduced and water use efficiency remains the same.
Primary effects o f water stress
Measured rates of photosynthesis and transpiration and derived components are
presented averaged over the five cultivars (Fig. 2, Table 2) and for each of the cultivars
individually (Fig. 3) as a function of time after exposure to water stress. As expected,
both the rates of photosynthesis and transpiration dropped after exposure to water
stress (Fig. 2a, b). The CO_, concentration in the Ieaf was, however, only reduced at
Fig. 2. Time courses of gas exchange parameters for 'old' leaves (open triangles) and 'young'
leaves (closed triangles) in the stress treatment expressed relative to the control, averaged over
the five cultivars. Presented are (a) rate of photosynthesis, (b) transpiration rate, (c) product
of water use efficiency and leaf vapour pressure deficit, (d) mesophyll conductance, (e) ratio
between internal and external CO, concentration, (O share of gas phase resistance to total resistance of CO 2 transport. Bars represent the standard error of difference.
Potato Research 32 (1989)
23
A. H. C. M. S C H A P E N D O N K , C. J. T. S P I T T E R S A N D P. J. G R O O T
.~,~
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Potato Research 32 (1989)
EFFECTS OF WATER STRESS
Photosynthesis
C02inlCO2ex
1.00
1.50
I
..4•
0.50
0.00
/
/
\
1.00
I
I
I
I
I
I
I
2
4
6
8
10
12
14
j
0.50
16
0
J
A
A
~
,
~
~
,
2
4
6
8
10
12
14
16
Days
Fig. 3. Time courses of (a) photosynthesis and (b) the ratio of internal and external CO: concentrations in the stress treatment, expressed relative to the control treatment. Cuhivars: Alpha
(+), Veenster ( • ), Bintje ( A ), Saturna (+), Kennebec ( o ).
day I and increased to the control level after 3 days (Fig. 2e). This indicates that initially the rate of photosynthesis reduced immediately by stomatal closure but within a
few days it was not the only cause of the reduction any longer and the conductances
for CO2 in the gas phase and in the mesophyll were equally reduced (Fig. 2f). The
mesophyll conductance is determined by the rate o f electron transport (Q-quenching)
from photosystem 11 to photosystem 1 over the thylakoid membranes and by the rate
o f CO2-assimilation by the Calvin cycle (E-quenching).
From the time course of the Q-quenching (Fig. 4a), it may be concluded that the
redox-state o f photosystem I1 was not affected by water stress, except for day 9, after
the temperature increase. This suggests that the electron transport rate and the quantum efficiency were not affected by water stress solely but effectively by a combination
o f water stress and high temperature.
The fluorescence signals related to the energy state o f the chloroplasts, i.e. the calculated energy quenching (Fig. 4b), show that the energy state o f the chloroplasts increased as a consequence o f water stress. Young leaves apparently suffered more than
old leaves. The recovery after alleviation o f stress, however, was also faster in young
leaves than in old leaves. The E-quenching on day 9 was not analyzed because the electron transport rate was severely impaired due to the temperature treatment. This would
bias the result because the inhibited electron flow itself will lead to a slow establishment
of a pH-gradient. Fig. 4c shows the exponentially fitted relation between the calculated
mesophyll conductances and the energy quenching. The increased E-quenching is
caused by an increase of the proton gradient due to a decrease of the ATP consumption
Potato Research 32 (1989)
25
A. H. C. M. SCHAPENDONK, C. ,l. T. SPITTERS A N D P. J. GROOT
Q-quenching
E-quenching
15 o
150
b
100
100
I
!
050
i
2
4
6
8
10
12
14
16
2
4
6
8
10
12
14
16
Days
Daya
E-quenching
Fm/Fo
0I 07600 :x&
C
110
d
100
A
O90
LAA
\4
tx
I
020
000
L
010
I
020
Mesophy[I
i
030
=
040
conductance
D
050
070
0
t
t
t
~
h
t
2
4
6
8
10
12
i
14
16
D~lys
Fig. 4. Time courses of fluorescence parameters for 'old' leaves (open triangles) and 'young'
leaves (closed triangles), (a) Q-quenching (electron transport rate), (b) E-quenching (energy state
of the leaves), (c) Relation between the mesophyll conductance (cm s ~) and the energy
quenching, (d) the ratio between the maximum and the dark fluorescence (Fm/Fo) standing
for membrane integrity.
26
Potato Research 32 (1989)
EFFECTS OFWATERSTRESS
in the Calvin cycle (Bradbury & Baker, 1983). The inhibited Calvin cycle apparently
relates to an increase of the mesophyll conductance. However quantitative conclusions
are difficult to draw because the fluorescence measurements were performed at relatively low red light intensities (100 p.mol) and thus electron transport and ATP production are expected to be rate limiting for carboxylation. The apparent accumulation of
energy even under these low light conditions therefore indicates that the Calvin cycle
was the main rate limiting factor at higher light intensities.
The ratio F m a x / F o (Fig. 4d) is a measure of the integrity o f the system that directs
the energy from the antennae pigments to photosystem II. The data indicate that the
energy transfer was inhibited especially under severe drought stress (at 20 % PEG).
In conclusion, water stress reduced the rate of photosynthesis at high light due to the
inhibition o f the Calvin cycle. Photosynthesis at low light was probably not affected
because no effect on quantum efficiency was detected.
Variations around the principal trend
Consideration of the results in more detail reveals some deviations from the primary
trend described above. During the first days after imposition of the stress, internal
CO2 concentration was lowered to some extent and the share of the gas phase resistance was increased (Fig. 2e, f), especially in the cultivars Veenster and Alpha (Fig. 3b).
On the first day, when this effect was greatest, the drop in internal CO z concentration
was on the average 11%, indicating that stomatal limitation was operative shortly after
exposure to water stress. Stomatal control on constant internal COz concentration
was restored after a few days (Fig. 2e, 3b). The decline of stomatal limitation did increase internal CO: concentration, which effect was responsible for the slight recovery in the rate o f photosynthesis during the first days of the stress period, especially
in the cultivars Veenster and Alpha (Fig. 3a).
There was no active osmotic adjustment. The decline in osmotic potential, measured
7 days after onset of the stress, from - 0 . 8 0 MPa in the control to -0.83 MPa in the
stress environment was fully explained by the decrease of 5 % in relative leaf-water
content due to the stress.
The sudden increase of temperature after day 8 resulted in a dramatic decrease of
the Q-quenching, but only in the drought treatment. This reduction was completely
reversed when the temperature was lowered again to 17 ~ concomitant with alleviation
of the water stress (Fig. 4a). A combined water stress and heat stress seems to cause
a blockage of the electron transport after photosystem II. Bilger et al. (1985) observed
that heat treatment alone blocked the electron transport proportional to the measured
CO2 assimilation.
From the photosynthesis measurements no effects of the temperature jump would
be expected (Fig. 2a). However it should be noted that these measurements were done
at light saturation, where the effect of quantum efficiency is of less importance. The
data in Fig. 4a suggest a reduction of the quantum efficiency by 30 % due the temperature rise from 17 ~ to 27 ~ under the stress condition.
Recovery after relief of the stress
After relief of the stress, photosynthesis recovered rapidly. In old leaves, the degree
of recovery was on the average 60 ~ The incomplete recovery was probably due to
an acceleration of leaf senescence induced by the stress: at the end of the stress period
these leaves were visually scored to be green for 62 ~ on the average, while comparable
Potato Research 32 (1989)
27
A. H. C. M. SCHAPENDONK, C. J. T. SPITTERS A N D P. J. GROOT
caca
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=~
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Potato Research 32 (1989)
EFFECTS OF WATER STRESS
leaves in the control were still fully green (Table 3). In the young leaves, formed during
the stress period recovery o f photosynthesis was complete. Only in the cult ivar Veenster
some leaves were damaged too severely to recover fully, which reduced the average value
of this cultivar.
Mesophyll conductance was greater in the young leaves recovered from stress than
in those in the control treatment (3.3 versus 2.6 mm s-~). Conductance of the gas
phase was, however, smaller (3.3 versus 5.3 m m s -~) causing stomatal limitation, and
so a reduced internal CO 2 concentration and increased water use efficiency (Fig. 2c,
e). Apparently, photosynthetic capacity recovered faster than the regulation mechanism of the stomata.
The ratio F m / F o is a measure o f the energy transfer from the light harvesting pigments to photosystem I1. Drought treatment inhibited this process for a short period
but this was reversible even during the stress treatment. However, the decrease of the
ratio in the recovery stage affirms that recovery from water stress is accompanied by
structural damage. Alleviation o f the drought stress caused more damage than the
stress condition itself. A possible explanation may be found in the sudden decrease
of the internal CO2 concentration during the recovery stage. The resulting shortage
of CO 2 thus leads to a relative surplus of reducing power and this might lead to the
reduction of oxygen and subsequent damage of photosystem II by free radicals of oxygen. Damage upon watering young potato plants after a drought period is known in
practice and might be caused by this mechanism. Kaiser (1987) concluded from his
results that rehydration of membranes caused a greater damage than partial dehydration.
Cultivar differences in the control
Differences between cultivars remained relatively stable over time (Fig. 3) and therefore values averaged over time are given for each cultivar (Table 2). The cuttivar differences in photosynthesis were highly significant (P<0.01) and corresponded with
differences in mesophyll conductance. There was hardly any variation in internal CO.
concentration (P = 0.10). Due to the regulation of the stomata on constancy of internal
CO, concentration, cultivars with a higher photosynthetic rate showed both a greater
mesophyll conductance and a greater conductance in the gas phase, but the ratio of
both conductances showed hardly any variation (P=0.10). There were no significant
differences in water use efficiency (P>0.10).
Averaged over time, the cultivars Alpha and Veenster had the highest rate of photosynthesis and Kennebec and Saturna the lowest. Photosynthesis of the cultivar Bintje
showed an interaction with treatment duration.
Cultivar differences in response to water stress. In the stressed situation, genetic variation was established for the same components as in the control (Table 3, Fig. 3). There
were highly significant differences between cultivars in the rate of photosynthesis
(P < 0.01) and, associated with that, significant differences in mesophyll conductance,
stomatal conductance and transpiration. There were no significant differences in internal CO 2 concentration and thus neither in water use efficiency nor in the share of gas
phase resistance in total resistance.
The rate o f photosynthesis of the cultivar Veenster was reduced most, but the other
cultivars varied on the whole little in their reduction of photosynthesis to water stress
(Fig. 3, Table 3). The differential reaction of the cultivars Veenster and Alpha with
Potato Research 32 (1989)
29
A. H. C. M. S C H A P E N D O N K , C. J. 1". S P I T F E R S A N D P, J. G R O O T
respect to stomatal behaviour just after exposure to water stress has already been discussed.
Discussion
Mesophyll limitation has frequently been found to be the dominant effect of drought
stress (Cornic et al., 1983; Wong et al., 1985; Kaiser, 1987). Inhibition of the Calvin
cycle activity by water stress has been reported by Huber et al. (1984), Prange (1986),
and Ogren & Oquist (1985). There are, however, also reports pointing to stomatal limitation as the dominant mechanism (Vos & Oyarzun, 1987). In potato plants grown in
large soil bins, they observed that a decrease of the leaf water potential from -0. 5
to - 0 . 9 MPa, reduced photosynthesis by 58% and it decreased the internal CO2 concentration by 29 ~ In the present study, stomatal limitation and, as a consequence,
reduced internal CO2 concentration occurred during a short period just after exposure to stress but also during recovery after release from the stress, causing visible
signs of damage due to photo-inhibition.
Our experiments were carried out under temperate conditions in the glasshouse and
the water stress was imposed rather abruptly. In the field, however, water stress develops more slowly and it is mostly accompanied by higher levels of irradiance, temperature and evaporative demand. To conclude whether stomatal limitation becomes
more important under those conditions needs further research. The potentially toxic
effect of PEG, once taken up, was a matter of concern. Potato roots are quite fragile
and great caution was taken to prevent any fractures through which PEG could be taken
up. The full recovery of young leaves after alleviation of the water stress justifies the
thought that the effects were indeed due to water stress.
The observed reduction in photosynthetic capacity by water stress was primarily due
to inhibition of the Calvin cycle, while the rate of electron transport was not affected.
Thus, photosynthesis was reduced at light saturation, but probably not at low light
as quantum yield was not affected. That agrees with the findings of Wong et al. (1985).
On the other hand, Prange (1986) reported also a reduction in quantum efficiency. In
our research a reduction in quantum efficiency due to water stress was only found after
increasing the temperature from 17 ~ to 27 ~ This argument is supported by observations of Prange (1986), who only detected changes in chlorophyll fluorescence when
plants were kept in the light. Though not specified this may be due to a temperature
effect. Similar observations were reported for grasses (Schapendonk, 1986).
In the present experiment, cv. Veenster appeared to be very sensitive to water stress,
which is in agreement with its score for drought tolerance as given in the Dutch cultivar
list (Table 3). The other cultivars varied little in their response to water stress, which
is not in accordance with the differences given in the cultivar list. This emphasizes that
gas exchange measurements on leaves during a certain development stage of the crop
can explain, at best, only part of the variation in drought tolerance encountered in
the field (Spitters & Schapendonk, 1988). The variation in tuber yield reduction due
to drought is also determined by the effects of the stress on expansion and senescence
of the foliage, distribution of assimilates, tuber initiation and tuber formation.
30
Potato Research 32 (1989)
EFFECTS
OF WATER
STRESS
Acknowledgements
We are grateful to A b de Vos for his a s s i s t a n c e in d e v e l o p i n g the e x p e r i m e n t a l set up
a n d to E r i k Toussaint for his a s s i s t a n c e in the gas exchange m e a s u r e m e n t s .
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32
Potato Research 32 (1989)