Journal of Experimental Botany, Vol. 49, No. 327, pp. 1715–1721, October 1998
Comparative effect of water, heat and light stresses on
photosynthetic reactions in Sorghum bicolor (L.) Moench
Vinita Jagtap1, Sujata Bhargava1,3, Peter Streb2 and Jürgen Feierabend2
1 Department of Botany, University of Pune, Pune-411007, India
2 Botanisches Institut, J.W. Goethe Universität, D-60054 Frankfurt, Germany
Received 16 January 1998; Accepted 8 June 1998
Abstract
Introduction
Five varieties of Sorghum bicolor (L.) Moench., differing in their drought tolerance under field conditions
have been used to study the effect of individual components of drought stress, namely high light intensity
stress, heat stress and water stress, on their photosynthetic performance. Chlorophyll content, chlorophyll
fluorescence, ribulose-1,5-bisphosphate carboxylase
(Rubisco, EC 4.1.1.39) content, phosphoenolpyruvate
carboxylase (PEPcase, EC 4.1.1.31) activity and photosynthetic oxygen evolution were used as key parameters to assess photosynthetic performance. The
results indicated that photochemical efficiency of
photosystem II (PSII) was severely reduced by all
three stress components, whereas PEPcase activity
was more specifically reduced by water stress.
Degradation of Rubisco and chlorophyll loss occurred
under high light and water stress conditions. Of the
four drought-tolerant varieties, E 36-1 showed higher
PEPcase activity, Rubisco content and photochemical
efficiency of PSII, and was able to sustain a higher
maximal rate of photosynthetic oxygen evolution under
each stress condition as compared to the other varieties. A high stability to stress-induced damage, or acclimation of photosynthesis to the individual components
of drought stress may contribute to the high yields of
E 36-1 under drought conditions. In the E 36-1 variety
markedly higher levels of the chloroplastic chaperonin
60 (cpn 60) were observed under all stress conditions
than in the susceptible variety CSV 5.
Light, water and temperature are among the major determinants of plant growth. Combination of high light
intensities, high temperatures and water deficits, which
arise during drought stress are known to severely inhibit
photosynthesis, and hence crop productivity (Ludlow
et al., 1990). High light intensity causes photoinhibition,
which leads to a reduction in the quantum yield of
photosynthesis (Powles, 1984; Kyle, 1987). High temperature and water deficit predispose plants to photoinhibition (Powles, 1984; Greer et al., 1986; Feierabend et al.,
1992), besides affecting photosynthetic efficiency directly
(Havaux, 1992).
Sorghum is a C plant commonly cultivated in the
4
drought-prone areas of the semi-arid tropics. Several
drought-tolerant varieties of Sorghum have been identified
on the basis of their ability to give good yields under
drought conditions in the field. Since photosynthesis has
long-term consequences on crop yield, adaptation of the
photosynthetic apparatus to withstand drought stress
would constitute an important stress-tolerance mechanism. However, studies on the effect of drought are complicated by several factors like timing and severity of
drought stress ( Fukai and Cooper, 1995), ability of plants
to recover from drought-induced damage (Greer et al.,
1986) and, finally, the complex nature of drought stress
in the field (Ludlow et al., 1990). This makes it difficult
to ascertain the mechanism by which a particular variety
exhibits drought tolerance.
In an effort to assess the photosynthetic damage caused
to plants exposed to drought stress and to understand
the mechanism by which plants cope with this stress, two
strategies have been used.
(a) Plants have been exposed to individual components
of drought stress, namely high light intensity, high temperature and low water availability.
Key words: Chlorophyll fluorescence, drought stress,
oxygen evolution, phosphoenolpyruvate carboxylase,
Sorghum.
3 To whom correspondence should be addressed. Fax: +91 212 353899. E-mail: [email protected]
© Oxford University Press 1998
1716
Jagtap et al.
(b) The stress factor has been applied continuously so
as to eliminate the possibility of recovery, and hence
enable a better assessment of the damage caused.
Five varieties of Sorghum from Africa and India,
differing in their ability to tolerate drought under field
conditions, have been subjected to stress conditions, as
mentioned above. Seedlings of the varieties have been
used to study the effect of each component of drought
stress on various photosynthetic parameters, such as
chlorophyll content, chlorophyll fluorescence (indicator
of photochemical efficiency of PSII ), activity of PEPcase
and Rubisco content (key reactions determining CO
2
assimilation), and the rates of photosynthetic oxygen
evolution (reflecting the overall capacity of photosynthesis). In addition, expression of the major chloroplastic
chaperone cpn 60 was compared under the different
experimental conditions as a representative of potential
stress proteins that might play a role in protecting proteins
against denaturation under stress conditions (Nover,
1991; Schmitz et al., 1996). The objective of these experiments was to study whether the impact of components of
drought stress on plant productivity could be identified
already at a young stage of vegetative growth, in terms
of differential effects on the stability or efficiency of
photosynthetic reactions, in the five varieties of Sorghum
differing in their drought tolerance.
Materials and methods
Growth of plants and stress application
The drought-tolerant and susceptible varieties were selections
from the germplasm collection of Sorghum bicolor (L.) Moench
at ICRISAT (International Crop Research Institute for the
Semi-Arid Tropics), India, and were based on the yields under
drought stress imposed at different stages of their growth (N
Seetharama, personal communication). The drought-tolerant
varieties were PA 59 from Cameroon, E 36-1 from Ethiopia, IS
22380 from Sudan, M 35-1 from India and the droughtsusceptible variety CSV 5 from the National Sorghum
Program, India.
The seeds were surface-sterilized in a 3% (w/v) calcium
hypochlorite-chloride solution. They were placed under vacuum
for 10 min, and at normal pressure for 30 min. The calcium
hypochlorite-chloride solution was decanted and the seeds
immersed in deionized water for 90 min followed by several
rinses with deionized water. The seeds were placed in vermiculite
in plastic boxes and fertilized with modified Knop’s solution
(Feierabend and Schrader-Reichhardt, 1976). They were grown
under a light intensity of 150 mmol m−2 s−1 PAR (photosynthetically active radiation), for a 12 h photoperiod. Temperature
was maintained at 25 °C and relative humidity at 75%. The
plant age for stress treatment was selected on the basis of
similar fresh weight and chlorophyll content per shoot, so that
the different varieties were considered to be approximately of
the same physiological age and had developed three leaves
when exposed to stress conditions for 24 or 48 h. The variety E
36-1 was exposed to stress conditions at the age of 10 d, the
varieties PA 59, IS 22380 and M 35-1 at the age of 13 d, and
CSV 5 at the age of 14 d. At this stage, the seedlings were
transferred to plastic pots containing baked clay granules
topped with 0.5 cm layer of vermiculite, and 25 ml of Knop’s
solution was added per pot. Subsequently, these plants were
subjected to stress. Light stress was given by exposing the
seedlings to a continuous light intensity of 710 mmol m−2 s−1
PAR. Temperature stress was given by exposing plants to a
constant temperature of 40 °C. Water stress was given by
including 7.5% PEG 6000 (polyethylene glycol 6000, corresponding to an osmotic stress of approximately −0.2 MPa)
in the Knop’s solution added to the pots. The other environmental regimes were maintained as for controls. Control plants
were maintained for 24 or 48 h at a light intensity of
150 mmol m−2 s−1 PAR at 25 °C and relative humidity of 75%.
All parameters were measured from at least three independent
experiments, except for the experiments on oxygen evolution,
which were carried out twice. Measurements were performed
with 3 cm long segments of the second leaves.
Chlorophyll estimation
Total chlorophyll content was measured in 80% (v/v) acetone
extracts by the method of Arnon (1949).
Chlorophyll fluorescence
Chlorophyll fluorescence was measured in a Hansatech Ltd.
LD 2 leaf chamber equipped with a LD 1 light emitting diode,
fluorescence detector and a TR1 transient recorder (Delieu and
Walker, 1983). Five 3 cm pieces from the central portion of the
second leaf were dark-adapted for 10 min. F (minimal fluoreso
cence in the dark-adapted state) was measured during a 60 ms
flash at an intensity of 440 mmol m−2 s−1. For F (maximal
m
chlorophyll fluorescence) measurements, the leaf pieces were
dark-adapted for a further 5 min and the fluorescence was
recorded during 1 min illumination at the same light intensity.
F (variable fluorescence) was calculated as the difference
v
between F and F .
m
o
Oxygen evolution
Oxygen evolution was measured from five leaf segments (3 cm
long) of the central portion of the second leaf, with a Hansatech
leaf disc electrode in the presence of approximately 5% CO
2
(Delieu and Walker, 1981). P
(maximal rate of overall
max
photosynthesis) values were measured at saturating light
intensities of 1200 (mol m−2 s−1 PAR and expressed as mmol
O evolved mg−1 chlorophyll h−1.
2
PEPcase activity
PEPcase activity was measured according to the method of
LeVan Quy et al. (1991) with slight modifications. Five 3 cm
pieces from the central portion of the second leaf were ground
in 3 ml extraction buffer, consisting of 50 mM HEPES-KOH,
pH 7.4, 12 mM MgCl , 1 mM EGTA, 1 mM EDTA, 1 mM
2
DTE, and 10% (v/v) glycerol. The extract was centrifuged at
13 000 g (Hermle ZK 401) for 15 min. The assay mixture
contained 50 mM TRIS-HCl, pH 7.6, 20 mM NaHCO , 130 mM
3
NADH, 10 mM MgCl , 5 mM DTE, 1 unit malate dehydro2
genase, and 50 ml extract in a final volume of 3 ml. The reaction
was initiated by the addition of PEP (phospoenolpyruvate) to
give a final concentration of 3.25 mM.
Immunoblotting of Rubisco and cpn 60
For immunoblotting of Rubisco, the protein extracts prepared
for the PEPcase assay were used. For immunoblotting of cpn
60, more concentrated extracts were prepared as described by
Schmitz et al. (1996). Electrophoresis of proteins was performed
Photosynthetic adaptation to drought 1717
in the presence of 0.4% (w/v) SDS on polyacrylamide gels.
Separation gels consisted of a 10 to 15% polyacrylamide
gradient, and the stacking gel was made up of 6% polyacrylamide. The buffer system of Laemmli (1970) was used. Proteins
were loaded on the basis of equal fresh weight of the leaf tissue
as determined before the stress exposures. For immunoblotting,
the proteins were transferred from the gel to a nitrocellulose
membrane (Schleicher and Schuell BA 85, 0.45 mm pore size)
by electroblotting. The membrane was then developed with
antibodies against Rubisco or the beta-subunit of cpn 60 from
rye leaves and the antigen bands visualized by peroxidaseconjugated goat antirabbit antibodies using o-dianisidine and
H O as the peroxidase substrates, according to Schmitz
2 2
et al. (1996).
Nucleic acid extraction and Northern hybridization
Total RNA was extracted, electrophoretically separated on
agarose gels, and, after transfer to nitrocellulose membranes
hybridized with 32P-labelled DNA probes of a rye leaf cpn 60b cDNA, exactly as described by Schmitz et al. (1996).
Results
Young plants of four Sorghum varieties which are
drought-tolerant under field conditions, designated as E
36-1, IS 22380, PA 59, and M 35-1, and of one susceptible
variety, CSV 5, were raised under controlled and initially
equal growth conditions at 25 °C and a low light intensity.
Plants that had reached a comparable stage of vegetative
development with three leaves were then exposed to either
high light, high temperature or to water stress.
The different photosynthetic parameters were studied
after 24 h and 48 h exposure to stress conditions and
compared to control plants kept for the same time under
the original growing conditions. For some parameters
like chlorophyll content, variable fluorescence, PEPcase
activity, and Rubisco content, significant alterations were
seen to occur after 48 h and hence only these results are
presented. Changes in oxygen evolution under waterstressed conditions occurred more rapidly and for these
experiments the results obtained after 24 h are presented.
Under high light or high temperature only minor changes
in fresh weight occurred. More marked losses of fresh
weight occurred in the presence of PEG.
Chlorophyll content and photochemical efficiency
When chlorophyll content was measured in terms of fresh
weight (fw) of leaf tissue (mg Chl g−1 fw), CSV 5 showed
maximum 3.8±0.08 mg Chl g−1 fw and E 36-1 showed
the minimum chlorophyll content 2.4±0.27 mg Chl g−1
fw. Chlorophyll content of the other varieties were for M
35-1, 3.0±0.02 mg Chl g−1 fw, PA 59, 3.7±0.2 mg
Chl g−1 fw, and IS 22380, 3.5±0.07 mg Chl g−1 fw. When
subjected to stress conditions, some loss of chlorophyll
occurred in all varieties, relative to unstressed controls
( Fig. 1a). In order to demonstrate changes in absolute
amounts of chlorophyll, and not those arising from loss
of water content, comparisons were made on the basis of
leaf segments instead of fresh weight basis. The most
marked decline in chlorophyll content occurred due to
light stress in all varieties, except for E 36-1. PA 59
showed large decreases under heat stress. Under water
stress about 20–30% chlorophyll losses were observed in
all five varieties. E 36-1 was least affected by heat or high
light stress.
The ratio of variable to maximum fluorescence was
used to estimate photochemical efficiency of PSII. As seen
in Fig. 1b, all varieties, except E 36-1, showed about
20–40% decreases in the F /F ratios under high light
v m
stress. When exposed to high temperature stress, inhibition in the photochemical efficiency was negligible in E
36-1, 10% in M 35-1, and about 20–50% in the other
varieties. Water stress brought about maximum decreases
in the F /F ratios, and E 36-1 and M 35-1 once again
v m
showed better photochemical efficiency than the other
varieties.
Enzymes of CO fixation
2
As key reactions of CO fixation, changes in the activity
2
of PEPcase and the content of Rubisco were compared
after exposure to high light, heat or water stress. PEPcase
is the primary CO -fixing enzyme in the C plant Sorghum
2
4
and thus might determine its photosynthetic efficiency.
Rubisco content was estimated by immunoblotting and
the large subunit content of Rubisco was compared in
the five varieties. The activity of PEPcase was, on the
basis of fresh weight, greatly enhanced after exposure of
the plants to high light intensity and, to a lesser extent
also at high temperature (Fig. 1c). Only in PA 59 the
activity of this enzyme was lowered by 30% during
exposure to 40 °C. Water stress, however, led to a marked
decline of up to 70% in the PEPcase activity, except for
the varieties E 36-1 and M 35-1, which maintained as
high levels of PEPcase activity as the untreated controls.
Since the fresh weight declined under water stress, the
absolute losses of PEPcase activity per leaf segment were
even more striking in the varieties CSV 5, IS 22380 and
PA 59.
The content of the large subunit of Rubisco did not
vary greatly among the unstressed controls. While
Rubisco was relatively unaffected by heat stress, significant decreases in Rubisco content were seen after exposure
to water stress and high light ( Fig. 2a). A conspicuous
exception was the behaviour of E 36-1. In this variety the
Rubisco content in high light was comparable to that in
untreated controls, indicating a much higher stability
than in the other varieties ( Fig. 2a). In M 35-1, Rubisco
appeared to be more stable under water stress than in the
other varieties, which parallels the particularly high
PEPcase activity of this variety under similar conditions.
1718
Jagtap et al.
Fig. 2. Comparison of the effects of exposure to high light intensity
(HL; 710 mmol m−2 s−1 PAR), heat (HT; 40 °C ) and water stress ( WS;
−0.2 MPa) on: (a) the content of the large subunit of Rubisco, (b) the
content of b-subunit of cpn 60 and its mRNA transcript in leaves of
different varieties of Sorghum. Proteins were detected by immunoblotting
of electrophoretic separations of extracts obtained from 3 cm segments
from the middle of the second leaves after a 2 d exposure to stress
conditions. Proteins were loaded on the basis of equal fresh weight of
the leaf tissue before the stress exposures. Transcripts (mRNA) for the
b-subunit of cpn 60 were detected by Northern hybridization (with a
cDNA probe) of separations of total RNA prepared after 1 d exposure
to stress conditions. Controls (Con) of non-stressed plants were kept at
25 °C, 150 mmol m−2 s−1 PAR and 75% relative humidity. The Sorghum
varieties were: C, CSV 5; E, E 36-1; I, IS 22380; P, PA 59; M, M 35-1.
Photosynthetic oxygen evolution
The effects of light, heat and water stress on the maximal
rate of overall photosynthesis at light saturation (P )
max
are summarized in Fig. 1d. The maximal capacity of
photosynthetic oxygen evolution remained unaffected
when Sorghum seedlings were subjected to a continuous
Fig. 1. Effect of 2 d exposure to high light intensity (710 mmol m−2 s−1
PAR), heat (40 °C ) and water stress (−0.2 MPa) on (a) chlorophyll
content, (b) ratio of variable to maximum fluorescence (F /F ), (c)
v m
PEPcase activity, and (d) photosynthetic oxygen evolution, in five
varieties of Sorghum. Results for photosynthetic oxygen evolution under
water stress are after 1 d exposure. Control plants were kept for
identical time periods at 150 mmol m−2 s−1 PAR, 25 °C, and 75%
relative humidity. Values are means of three experiments for (a), (b)
and (c), and two experiments for (d). Error bars represent standard
errors of the mean.
Photosynthetic adaptation to drought 1719
high light intensity of 710 mmol m−2 s−1 for up to 48 h.
High temperature stress of 40 °C brought about no inhibition of oxygen evolution in most of the varieties indicating
that they could adapt to the high temperature. E 36-1
even showed a doubling of photosynthetic oxygen evolution after high temperature exposures. In PA 59 lightsaturated photosynthesis was, however, diminished by
about 40% at high temperature, in spite of it being a
drought-tolerant variety. Water stress exerted most severe
effects on all varieties already within 24 h, except for E
36-1, where photosynthetic oxygen evolution was not
impaired by water stress. The results indicated that overall
photosynthesis could adapt quite well to high light intensity or heat, and, in case of E 36-1, to water stress too.
Expression of chaperonin 60
The marked difference observed among stress-exposed
Sorghum varieties with respect to the stability of photosynthetic constituents raised the question of whether the
high stability observed in variety E 36-1 might be related
to a higher content of stress proteins acting as chaperones.
Chaperones are known to protect proteins from stressinduced denaturation and thus help to avoid damage and
subsequent degradation (Nover, 1990, 1991). Therefore,
as an example, the expression of cpn 60, which is the
predominant chaperone of chloroplasts in the tolerant
variety E 36-1 and in the susceptible variety CSV 5, were
compared under control and stress conditions. Expression
of the cpn 60 protein was analysed by immunoblotting
with an antibody against the b-subunit of cpn 60 from
rye leaves, and its transcript levels were compared by
Northern hybridization with a cDNA probe for cpn 60b from rye. Under control conditions equal amounts of
cpn 60-b were observed in leaves of the CSV 5 and E
36-1 varieties (Fig. 2b). However, under all stress conditions the amounts of cpn 60-b was lower in CSV 5 than
in E 36-1. The most conspicuous loss of cpn 60-b occurred
in high light and under water stress. In contrast to the
marked changes observed in CSV 5, the cpn 60-b content
of E 36-1 was unaffected by high light intensity, increased
under high temperature stress and was only moderately
reduced by exposure to water stress. Transcript levels for
cpn 60-b increased under high light and high temperature,
particularly in the susceptible variety CSV 5. Under water
stress conditions the cpn 60-b transcript was strongly
diminished in both varieties. However, again, its level was
still significantly higher in the susceptible variety (CSV 5)
than in the tolerant variety ( E 36-1). Although CSV 5
contained higher amounts of cpn 60-b mRNA under all
stress conditions, this obviously did not allow sufficiently
enhanced new synthesis of the protein in order to compensate for the greater losses of cpn 60-b in CSV 5, as
compared to E 36-1.
Discussion
Yields of crop plants growing under unfavourable climatic
conditions will substantially depend on adaptive mechanisms allowing them to maintain high photosynthetic production even under severe stress conditions. Our present
results show that Sorghum varieties which exhibited tolerance to drought under field conditions were not generally
distinguished from a sensitive variety by the maintenance
of higher photosynthetic capacities when subjected to
individual components of drought, namely high light,
heat or water deficit, at a young development stage. This
is unexpected, in as much as the tolerant varieties suffered
less lipid peroxidation damage and contained higher
activities of antioxidant enzymes than the susceptible
variety CSV 5 according to previous results (Jagtap and
Bhargava, 1995). Another remarkable aspect is that the
behaviour of the different drought-tolerant Sorghum varieties was not uniform. Photosynthetic components were
quite differently affected by the stress conditions applied
to the five Sorghum varieties.
Exposure to a higher light intensity induced chlorophyll
loss, led to a marked decline in the photochemical efficiency of PSII and caused considerable degradation of
Rubisco in both the susceptible and the tolerant varieties,
except for E 36-1. While such effects may arise due to
light stress-induced damage to chloroplasts ( Kyle, 1987;
Barber and Andersson, 1992), plants are also known to
show a protective response to light stress, which involves
structural alterations in the photosynthetic apparatus
(Styring and Jegerschöld, 1994; Demmig-Adams and
Adams, 1996; Havaux and Tardy, 1997). The fact that
maximum capacity of overall photosynthesis, as assayed
by O evolution at light saturation and a high CO
2
2
concentration, was not diminished after the high light
exposures, indicated that high light stress did not have
detrimental effects on photosynthetic activity per unit
chlorophyll. Reduction of F /F and Rubisco content
v m
may, therefore, also represent responses of acclimation of
the Sorghum varieties to a higher light intensity. However,
in E 36-1, there was neither a reduction of the photochemical efficiency and Rubisco content, nor was there a loss
of overall photosynthesis, indicating that the former was
not required as protective response to light stress. As a
favourable effect, PEPcase activity was stimulated at high
light intensity, as has also been reported by Hague and
Sims (1980), who demonstrated that new synthesis of the
enzyme was induced.
While under short-term conditions light may also protect against heat injury (Havaux et al., 1991), heat is
usually known to damage PSII in light ( Kyle, 1987) and
upon prolonged irradiation, damage to reaction centres
is usually followed by chlorophyll loss ( Krause, 1988).
In these experiments, heat treatment at 40 °C caused a
more or less marked decline in PSII activity, as indicated
1720
Jagtap et al.
by the F /F ratio, and in the chlorophyll content of
v m
most, but not all Sorghum varieties, while the Rubisco
content of the varieties was little affected. As in high
light, the PEPcase activity was also increased at high
temperature, except for one variety (PA 59). Except for
PA 59, maximum photosynthesis per unit chlorophyll at
light saturation was not impaired by the heat exposure
but even almost doubled in the variety E 36-1. Thus, in
general, heat of 40 °C did not represent a severe damaging
stress component for photosynthesis, and the observed
decreases in F /F ratio and chlorophyll content could
v m
be part of a protective acclimation, similar to that
observed during light stress. In the variety PA 59, all
photosynthetic components that were assayed appeared
to be adversely affected by high temperature, indicating
the inability of its photosynthetic apparatus to adapt to
heat stress, at least at this young developmental stage.
Water stress caused the most severe damage to photosynthesis, as compared to heat and high light stress. Most
of the key reactions determining photosynthesis (chlorophyll, PSII, Rubisco) were reduced in CSV 5, IS 22380
and PA 59. E 36-1 and M 35-1 were able to perform
better, because they showed higher F /F ratios and also
v m
no loss of PEPcase activity when subjected to water stress.
There have been reports of drought stress-induced damage
to the photosynthetic apparatus ( Kaiser, 1987; Van
Rensburg and Krueger, 1993), and this damage was
thought to depend on other additional factors, like temperature and light intensity, which become inhibitory to
photosynthesis under conditions of water stress. Van
Rensburg and Krueger (1993) reported that the drought
stress-induced damage to the photosynthetic apparatus
was more severe and occurred earlier in droughtsusceptible, relative to drought-tolerant cultivars of
tobacco. In Sorghum, however, both drought-susceptible
and tolerant varieties showed similar decline in the F /F
v m
ratios as well as in maximum O evolution, except for E
2
36-1, where the damage was much less. Decreases in
PEPcase activity due to water stress correlated best with
the low rate of overall photosynthesis seen in all varieties,
except for E 36-1, and could be one of the major factors
limiting the acquisition of CO and photosynthetic per2
formance of Sorghum under drought stress conditions.
Various deficiencies observed under drought conditions
are thought to represent secondary consequences resulting
from the shortage of CO (Cornic and Massacci, 1996).
2
In M 35-1, however, there was a reduction in overall
photosynthetic performance in spite of an undiminished
PEPcase activity.
In conclusion it can be said that the Sorghum varieties
which exhibited drought-tolerance in the field, did not
necessarily show photosynthetic adaptation to the various
components of drought stress under controlled laboratory
conditions and at a young developmental stage. The only
variety showing excellent photosynthetic adaptation to
high light, as well as to heat and water stress was E 36-1.
The conspicuous stability of photosynthetic components
in E 36-1 correlated well with the significantly higher
contents of a chloroplastic chaperone, cpn 60, in E 36-1
than in the susceptible variety CSV 5, under all the stress
conditions that were applied. Though this is not yet
sufficient evidence to prove a causal relationship, chaperones may serve as favourable factors protecting proteins
from denaturation induced by various stress conditions,
thus maintaining them in a stable functional state (Nover,
1990, 1991; Schmitz et al., 1996). Variety M 35-1 also
seemed to be well adapted to heat and water stress.
However, PA 59 showed marked sensitivity to heat stress
and also performed poorly under water stress conditions,
in spite of its ability to withstand field drought
conditions better than the drought-susceptible CSV 5 (N
Seetharama, personal communication).
Monitoring young plants by assaying key parameters
like variable fluorescence, PEPcase activity or photosynthetic oxygen evolution thus allows the detection of
drought tolerance that is related to stable photosynthesis,
as shown for variety E 36-1 in the present work. For
screening for tolerant lines, water stress appears to be
most appropriate, as it represents the most critical drought
factor. However, these results show that such screening
cannot detect all varieties with improved drought tolerance. Under field conditions, unpredictable, either beneficial or detrimental, interactions of high light, heat and
water deficit can occur and may differentially affect
different varieties. In addition, other mechanisms, besides
photosynthetic adaptation, play important roles for
drought-tolerance (Fukai and Cooper, 1995; Bohnert and
Jensen, 1996) and could contribute to the drought tolerance exhibited, for instance, by PA 59. Besides biochemical and molecular mechanisms, differences in the
morphology of the varieties or in the time-course of
development may also constitute essential strategies for
differential drought-tolerance under field conditions.
During germination, water will usually be more readily
available. During further development, however, the
appearance of stages with lower or higher productivity,
or with changing drought-sensitivity may be more or less
favourably tuned to the course of the climatic changes
under field conditions.
Acknowledgements
This work was supported by the DAAD Sandwich Model
Fellowship (Germany) awarded to Vinita Jagtap. Valuable
experimental support by Gabriele Schmitz is greatly appreciated.
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