The leaf senescence pattern was examined in water-stressed male sterile and fertile lines of wheat (Triticum aestivum) and sorghum (Sorghum vulgare). The study was conducted at the seedling stage and during grain development. The loss of leaf area and chlorophyll content induced by water stress was similar in the male sterile and fertile lines of wheat at the seedling stage. At the grain filling stage, leaf senescence occurred at a faster rate in the fertile lines as compared to sterile lines of both wheat and sorghum. The study indicates that a reproductive sink accentuates drought-induced leaf senescence

Khanna-Chopra, R.

Sinha, S. K.

English

Publications of the IAS Fellows

Annals of Botany 61, 649-653, 1988 649
Enhancement of Drought-Induced Senescence by the
Reproductive Sink in Fertile Lines of Wheat and
Sorghum
R. KHANNA-CHOPRA and S. K. SINHA
Water Technology Centre, Indian Agricultural Research Institute, New Delhi- 110012, India
Accepted: 4 December 1987
ABSTRACT
The leaf senescence pattern was examined in water-stressed male sterile and fertile lines of wheat {Triticum
aestivum) and sorghum (Sorghum vulgare). The study was conducted at the seedling stage and during grain
development. The loss of leaf area and chlorophyll content induced by water stress was similar in the male
sterile and fertile lines of wheat at the seedling stage. At the grain filling stage, leaf senescence occurred at
a faster rate in the fertile lines as compared to sterile lines of both wheat and sorghum. The study indicates
that a reproductive sink accentuates drought-induced leaf senescence.
Key words: Drought resistance, leaf senescence, male sterile, sink.
INTRODUCTION
Breeding for drought resistance in crop plants is a
major objective in crop improvement programmes
around the world. Therefore, several efforts have
been made to develop suitable selection criteria for
drought resistance in crop plants (Chang et al.,
1982). Most of the criteria are based on responses
of seedlings to water stress (IRRI, 1982). However,
many studies have shown that the sensitivity of
crop plants to drought varies during growth and
development, the reproductive phase being the
most sensitive in most crops (Eastin et al., 1983).
Consequently, the criteria based on seedling
responses are not always successful under field
conditions (Chang et al., 1982; Reyniers et al.,
1982). Varieties assessed as drought resistant on
the basis of stability or drought index are usually
poor yielders (Fisher and Maurer, 1978; IRRI,
1982; Sinha et al, 1986), suggesting that a strong
reproductive sink is not compatible with drought
resistance. This suggests the possibility that the
strength of the reproductive ' sink' may alter the
response of the plant to drought or water stress. In
order to investigate this question, the effects of
drought on normal and male sterile lines of wheat
and sorghum were compared. Leaf senescence,
both in terms of area and loss of chlorophyll
content, was used as the criterion of drought
effects in these studies.
MATERIALS AND METHODS
Normal (FP-83-44, FP-83-33) and genetically
male sterile (FP-83-40-2, FP-82-32) lines of wheat
(T. aestivum) were obtained from the Genetics
Division of the Indian Agricultural Research
Institute, New Delhi, India. The plants (six per
pot) were grown in pots containing 20 kg soil in a
glasshouse and given adequate fertilizer to main-
tain normal growth.
One-month-old wheat plants were water stressed
by withholding the water supply and sampled after
0, 5, 7 and 10 d without water. Water potential of
the youngest fully expanded leaf was measured at
1000 h with a pressure chamber (Mode 3005, Soil
Moisture Equipment Corp.) according to Scholan-
der et al. (1964). Two plants constituted a replicate
and there were three replicates per treatment.
Green leaf area was recorded using an automatic
leaf-area meter (Model AAM-7, Hayashi Denkoh
Co. Ltd, Japan). Six plants constituted a replicate
and there were three replicates per treatment.
Chlorophyll content of leaves was determined
according to Arnon (1949).
A separate batch of wheat plants was subjected
to four stress treatments by adding 250, 500, 750,
1000 ml water per pot per d from anthesis.
Measurements of green leaf area and leaf chloro-
phyll content were done either on all the leaves of
the main shoot (lines FP-83-44 and FP-83-40-2) or
0305-7364/88/060649 + 05 $03.00/0 1988 Annals of Botany Company
BOT 61
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650 Khanna-Chopra and Sinha—Drought-Induced Senescence
only on the flag leaf (lines FP-83-33 and FP-83-32)
of plants with or without water stress. Leaf water
potential was measured using a pressure chamber
(as described earlier) on the flag leaf only. For leaf-
area measurement, three plants constituted a
replicate and there were three replicates in each
treatment. For chlorophyll estimation all green
leaves from the two main shoots were pooled in
each replicate of a treatment. The leaves were
chopped into very small pieces and 200 mg leaf
samples were randomly drawn for chlorophyll
extraction and measurement following Arnon
(1949). Three replicates were maintained for each
treatment. Flag-leaf water potential was measured
at 1000 h using two flag leaves as replicate and
maintaining three replicates for each treatment.
Sorghum {Sorghum vulgare) cv. 2077A (male
sterile) and 2077B (fertile) were grown in the field
in 5 m x 5 m plots in the rainy season in a
randomized block design at a population of 18
plants m~2 following recommended agronomic
practices. The plants were given urea fertilizer at
the rate 40 kg N ha"1 and irrigated whenever
necessary. Half the plots of each genotype were
given irrigation (6 cm) at anthesis and 15 d after
anthesis while the other half were kept unirrigated.
There was no rain after anthesis. Water stress
developed slowly in the unirrigated plants.
Leaf \jrw and leaf area were measured at anthesis,
and 15, 30 and 35 d after anthesis. Water potential
of the uppermost leaf was measured at 10.00 h
with a pressure chamber (as described earlier).
Two plants constituted a replicate and three
replicates were maintained for each treatment for
water potential measurement. For leaf area meas-
urement, only green leaves were selected. Ten
plants constituted a replicate and three replicates
were maintained for each treatment. Leaf area was
measured by automatic leaf area meter.
RESULTS AND DISCUSSION
A range of leaf \j/w from — 1-0 MPa to —3-5 MPa
was created as a result of withholding water for
different durations from seedlings of normal and
male sterile lines of wheat. Both the normal and
male sterile lines exhibited a similar reduction of
green leaf area, as well as in chlorophyll content
per plant, in response to decreasing leaf i/rw (Fig.
1). However, when a range of leaf ijrw was created
by applying different amounts of water at anthesis,
the fertile line showed a sharper decline in green
leaf area and chlorophyll content compared to the
sterile line with the decline in leaf \[rw (Figs 2
and 3).
In sorghum, A and B lines which are male sterile
and normal, respectively, the leaf \]rw was
—1-8+0-2 MPa and 1-6 + 0-3 MPa at the time of
anthesis. Both lines showed a decline in leaf area
per plant after anthesis (Fig. 4). Fifteen days after
anthesis the leaf area decreased in the normal time
by 34% in irrigated plants and 57% in non-
irrigated plants whilst in the 2077A line the
decrease in leaf area was 22% (irrigated) and 32%
(non-irrigated). By 35 d from anthesis the male-
sterile 2077A line had retained higher leaf area
100
80
6 0
Z 40
s
20
- FP-83-44
FP-83-40-2
•FP-83-33
=-83-32
- I - 2 - 3 - 4
f FP-83-44
FP-83-40-
- I - 2 - 3 - 4
Leaf <// (MPa)
FIG. 1. Effect of water stress on leaf area and chlorophyll content per plant in male fertile lines FP-83-44 (O O)FP-
83-33 ( • • ) , male sterile lines FP-83-40-2 (x x) and FP-83-32 (Q Q) of wheat at the seedling stage.
Values+ s.e. for leaf \^w were as follows: FP-83-44: 1-2 + 0-06, 1-7±009, 2-6 + 0-1 and 3-5±0-2 MPa, respectively. FP-
83-40-2: 1 -1 ± 0-05, 1-5 + 0-03, 2-3 + 007 and 3-4+005 MPa, respectively. FP-83-33: 115±O-O5, 1-4 + 0-1, 2-2 + 0-1,
4-0±0-2 MPa, respectively. FP-83-32: 115 + 005, 1-6 + 0-3, 2-0 + 0-2 and 3-7 + 0-4 MPa, respectively.
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Khanna-Chopra and Sinha—Drought-Induced Senescence 651
120 -
CM 1 0 0 -
ho
o
en
o
•5 2
20 -
-1-0
FP-83-40-2
-1-0 -2-0 -3-0 -4-0
(MPa)
FIG. 2. Effect of water stress on leaf area and chlorophyll content of main shoot in male fertile line FP-83-44
(O O) and male sterile line FP-83-40-2 ( # 0 ) of wheat during grain development. Values ±s.e. of leaf fw
were as follows: FP-83-44: l-6±0-2, 21 ±0-2, 2-6±0-3, 4-5 + 0-5 MPa, respectively. FP-83-40-2: 1-6 + 0-2, 2-35±0-2,
3-2 + 0-3 and 4-5 + 0-4 MPa, respectively.
S l 6
o
u>
£ 12
*oD
9
-
° 8
le
al
c
0)S 4
^ ^ N ^ FP-83-32
FP-83-33 N.
2 0
1-5
1-0
0-5
\
\FP-83-33 >
-
r
•\FP-83-32
-2-0 - 3 0 - 4 0 -2-0
Flag leaf ty (MPa)
-3-0 -4-0
FIG. 3. Effect of water stress on leaf area and chlorophyll content of flag leaf in male fertile line FP-83-33 (O O)
and male sterile line FP-83-32 ( • • ) of wheat during grain development. Values ±s.e. of leaf fw were as follows:
FP-83-33: 2-3±0-3,2-9 + 0-2, 3-6±0-4 MPa, respectively. FP-83-32:2-3 + 0-4, 2-9 + 0-3 and 3-7±0-4 MPa, respectively.
than the normal 2077B line, irrespective of the
irrigation treatment.
Fifteen days after anthesis the chlorophyll
content in the normal, B line decreased by 38%
(irrigated) and 70% (non-irrigated) compared to
the anthesis value (Fig. 4). In the male sterile A
line, the comparable decrease was 23 and 49%,
respectively. By 35 d after anthesis, the A line
maintained higher chlorophyll content compared
to the B line, irrespective of the irrigation
treatment.
The normal and male sterile lines are isogenic,
but for the lack of seed set in the latter. They are
expected to exhibit similar morphological and
physiological characteristics. Indeed, when seed-
lings of normal and male sterile sorghum and
wheat were subjected to water stress, they exhibited
a similar response in terms of the loss in chlorophyll
26-2
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652 Khanna-Chopra and Sinha—Drought-Induced Senescence
30 •
2-0
a
"a.
e
= 160
.c
§
I 120
Le
a
80
40
" $ BUN
20 30 35 A
Days after anthesis
15 35
FIG. 4. Leaf area and chlorophyll content per plant during grain development in sorghum under irrigated and non-
irrigated conditions in male sterile line 2077A ( • • , A-I, AUN) and male fertile line 2077B (O O, BI,
BUN), respectively.
content and leaf area. However, when normal and
male sterile plants were exposed to water stress in
the post-anthesis phase, the normal genotype
showed a faster rate of senescence. Therefore, on
the basis of the senescence pattern of leaf area, the
male sterile lines could be described as more
drought resistant. This implies that a genotype
failing to set seed would appear to be more
tolerant to drought than one having reproductive
sink. Indeed, most studies have concluded that
poor yielding varieties, or wild relatives of many
crops, are more drought resistant (Stone, 1974;
IRRI, 1982).
A major change during the transition from
seedling to reproductive plant is a shift in the site
of the ' sink' from roots to the developing grains.
In seedlings the roots serve as a major sink where
the metabolites mobilized during senescence of
leaves in response to drought could be stored
(Sharp and Davies, 1979; Aggarwal and Sinha,
1983; Reddy, 1986). However, in the reproductive
phase, the leaves subtending the developing
grains/fruit translocate nitrogenous compounds
to the developing grains (Waters et al., 1980).
There is a loss of soluble protein predominantly as
RuBP-Case from the leaves and this is coupled
with loss of chloroplast integrity (Thomas and
Stoddart, 1980). Therefore, a larger reproductive
sink possibly hastens the senescence of leaves.
Differences in leaf area between normal and
sterile lines were induced in response to water
stress during the reproductive phase. However, the
chlorophyll content of the remaining leaves did
not differ substantially because leaves did not
show a parallel loss of chlorophyll. The loss was in
acropetal succession leading to a complete sene-
scence of lower leaves. The topmost leaves main-
tained almost the same amount of chlorophyll, or
sometimes even more as compared to the leaves in
a similar position in control plants. Therefore, in
the whole plant system, the effects of drought are
most apparent on the overall chlorophyll content
of the plant.
The differences between normal and male sterile
lines could be due to difference in leaf turgor. The
male sterile lines might preferentially accumulate
solutes under water stress in the absence of
developing seeds. Leaf osmotic potentials have
been shown to be related to the source-sink
balance in cotton (Radin et al., 1986). The present
study thus shows that the size of the reproductive
sink could alter the response of a whole plant to
water stress. This could have an important bearing
on the degree of drought resistance of a genotype
based on grain yield.
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Khanna-Chopra and Sinha—Drought-Induced Senescence 653
ACKNOWLEDGEMENTS
This work was supported by the Indian Council of
Agricultural Research, New Delhi through the
Scheme of Professor of Eminence.
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Enhancement of drought-induced senescence by the reproductive sink in fertile lines of wheat and Sorghum

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Enhancement of drought-induced senescence by the reproductive sink in fertile lines of wheat and Sorghum

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