American Journal of Plant Sciences, 2011, 2, 70-77
doi:10.4236/ajps.2011.21010 Published Online March 2011 (http://www.SciRP.org/journal/ajps)
Stem Reserve Mobilization and Sink Activity in
Wheat under Drought Conditions
Anil K. Gupta, Kamaljit Kaur, Narinder Kaur
Department of Biochemistry, Punjab Agricultural University, Ludhiana, India.
Received October 1st, 2010; revised November 18th, 2010; accepted November 24th, 2010.
ABSTRACT
The effect of water deficit on stem reserve mobilization and sink activity in wheat (Triticum aestivum L.) cu ltivars, viz.,
C306 (drought tolerant) and PBW343 (drough t sensitive) was studied. Drought was maintain ed in pot raised plants by
withholding irrigation at 95 days after sowing (DAS), i.e. just five days before the initiation of anthesis. Drought in-
duced a significan t reduction in mean biomass of all the in ternodes of sensitive cultivar as compared to those of toler-
ant one. Mobilized dry matter and mob ilization efficiency were observed to be higher in the internodes of tolerant cul-
tivar, both under control and stress conditions, which resulted in enh anced transloca tion of stem reserves to the grains.
Water soluble carbohydrates (WSC), which mainly occur as fructans, were observed to be higher in the internodes of
tolerant cultivar than those of sensitive one. When drought wa s applied, fructans were mob ilized more effectively from
the internodes of tolerant cultivar. A significantly higher sucrose synthase activity in the grains of tolerant cultivar,
under drought conditions, increased the sink strength by unloading the assimilates in the sink, thereby increasing fur-
ther mobilization of assimilates to the grains. Grains of sensitive cultivar attained maturity much earlier as compared
to the tolerant one, both und er control and stress conditions. The longer duration of grain maturatio n in tolerant culti-
var supported enhanced mobiliza tion of stem reserves, thus restricting heavy decrease in grain yield, under stress con-
ditions, as compared to the sensitive cultiva r. It may, therefore, be concluded that certa in characteristics viz., enhanced
capability of fructan storage, higher mo biliza tio n efficienc y, stronger sink ac tivity and longer du ratio n of grain matura-
tion might help the drought tolerant cu ltivar in coping the stress conditions.
Keywords: Acid Invertase, Fructans, Water Soluble Carbohydrate, Sink Activity, Stem Reserve Mobilization, Sucrose
Synthase, Triticum Aestivum
1. Introduction
In semi arid areas of the world with a Mediterranean
climate, wheat crop often enters the reproductive phase
when rainfall decreases and soil evaporation increases,
leading to adverse water deficit conditions [1,2]. Drought
limits growth and productivity of plants and plays a cen-
tral role in their geographical range [3]. Wheat yields
have been reported to reduce by 50-90 per cent of their
irrigated potential by drought in marginal rainfed envi-
ronments, on at least 60 million hectares in the develop-
ing countries [4,5]. Drought is, thus, reported to be a
major constraint, since it depends not only on severity
and duration of the stress event, but also on the plant
development stage and morphology [6].
Carbon requirements for grain filling in wheat are
mainly from current assimilation by photosynthesis and
remobilization of reserves from the stems [7]. Remobili-
zation of assimilates is an active process that involves
translocation of stored reserves from stems and sheaths
to grains [8]. Pre-anthesis assimilate reserves in stem and
sheath of wheat contribute 25 to 33% of the final grain
weight [9]. Extensive studies demonstrated that water
deficit conditions resulted in early senescence and more
remobilization of pre-anthesis stored assimilates to grains
in cereals [10, 11]. Small grains stems stored carbohy-
drates in the form of glucose, fructose, sucrose and starch,
but the main reserve was fructan [12]. Drought induced
earlier mobilization of non-structural reserve carbohy-
drates (largely, fructans) from stem and leaf sheaths,
which provided a greater proportion of kernel dry weight
at maturity. It can account for 70-92 per cent of grain dry
matter, under conditions of drought [13]. It is generally
accepted that stem reserve mobilization or percentage of

Stem Reserve Mobilization and Sink Activity in Wheat under Drought Conditions 71
stem reserves in grains is affected by sink size, environ-
ment and cultivar [1]. It has been proposed that wheat
genotypes capable of synthesizing and storing a high
concentration of soluble carbohydrates in the stems prior
to anthesis are more likely to exhibit improved grain
yield under conditions of water deficit [14]. Variation in
stem water soluble carbohydrates among wheat geno-
types is one of the genetic factors that influence grain
weight and yield under water limited environments [15].
The significance of drought resistant genotypes which
synthesize more fructans than drought sensitive ones has
been reported in literature [16,17].
The mobilization of stem reserves to the grains can be
studied indirectly by measuring the post anthesis changes
in internodal dry matter, as in [18] and sink activity of
the grains or directly by measuring the changes in inter-
nodal water soluble carbohydrate content during grain
filling period, as in [19,20] or a combination of all the
parameters. The effect of water deficit on carbohydrate
status of two wheat cultivars, viz., PBW 343 (a double
dwarf cultivar sensitive to drought) and C306 (a tall cul-
tivar tolerant to drought) during early seedling growth
has already been studied [21]. With the intention to know
how these two cultivars behave (in terms of fructan mo-
bilization) during grain filling period. The aim of this
investigation was to study the effect of drought (which
can be successfully maintained in pot experiments) on
stem reserve mobilization in these two wheat cultivars.
2. Materials and Methods
2.1. Plant Materials and Growth Conditions
Wheat (Triticum aestivum L.) cultivars, viz., PBW 343
(drought sensitive) and C306 (drought tolerant) were
grown in 60 pots (30 for each cultivar). Each pot (25 cm
height and 20 cm diameter) was filled with 10 kg soil.
The soil was sandy loam with pH 7.3 to 7.4. The pots
were placed in field and sheltered from rain by a remov-
able, transparent polythene shelter. Six seeds were sown
in each pot. Plants were watered three times a week so as
to maintain a soil water content close to field capacity.
Drought was maintained in half of the pots by withhold-
ing irrigation at 95 days after sowing (DAS), i.e. just five
days before the initiation of anthesis.
2.2. Analysis of Biomass, Moisture Content,
Mobilized Dry Matter and Mobilization
Efficiency of Internodes
The stem internodes, viz., basal, middle, apical and pe-
duncle were oven dried at 60˚C up to a constant weight.
The moisture content of internodes was calculated by
measuring the fresh and dry weight of internodes. The
magnitude of mobilized dry matter of each internodes
was estimated as difference between post anthesis max-
imum and minimum biomass. Mobilization efficiency of
dry matter in each internodes was estimated by using the
proportion (%) of mobilized dry matter relative to post
anthesis maximum biomass of that internode [18].
2.3. Analysis of Moisture Content and Yield of
Grains
The percentage of moisture content was calculated by
measuring the fresh and dry weight of grains. Grain yield
was calculated as number of grains per spike.
2.4. Extraction and Determination of Enzyme
Activities from Grains
Sucrose synthase (SS) {EC 2.4.1.13} was extracted by
grinding the grains (0.5 g) in a cold chilled mortar with a
pestle with 3 ml of 100 mM HEPES buffer (pH 8.2),
containing 10 mM EDTA, 15 mM KCl, 5 mM MgCl2
2mM sodium diethyl dithiocarbomate and 5 mM β-mer-
captoethanol [22]. Insoluble polyvinylpolypyrrolidone
(100 mg per g tissue) was also added while extracting the
enzymes. The extract was centrifuged at 10,000 g for 15
min at 4˚C and supernatants were used for enzyme esti-
mation. The assay mixture of SS consisted of 50 μmoles
HEPES buffer (pH 6.5), 2 μmoles UDP and 50 μmoles of
sucrose in a total volume of 0.4 ml. Reaction was initi-
ated by adding 0.1 ml of enzyme at 30˚C and reaction
was stopped by adding 0.5 ml of alkaline copper tartrate
reagent and fructose released was estimated at 510 nm
[21].
Acid invertase (AI) {EC 3.2.1.26} was extracted by
crushing the grains (0.5 g) in a cold chilled mortar with a
pestle with 3 ml of 20 mM sodium phosphate buffer (pH
7.0), as in [23]. The extract was centrifuged at 10,000 g
for 15 min at 4˚C and supernatants were used for enzyme
estimation. AI activity was determined by incubating 160
μmoles of sodium acetate buffer (pH 5.0), 100 μmoles of
sucrose and 0.1 ml of enzyme in a total volume of 1 ml at
30˚C for 1 h. The reaction was stopped by adding 1 ml of
alkaline copper tartrate solution and reducing sugars
formed were estimated at 510 nm using spectrophotome-
ter (Milton Roy) [24]. The protein content in the enzyme
extract was determined [25].
2.5. Extraction and Determination of Water
Soluble Carbohydrates (WSC) and
Fructans from Internodes
WSC were extracted by crushing 25 mg of tissue with 10
ml of distilled water and then incubated at 80˚C for 1 h.
The mixture was then centrifuged at 10,000 g for 15 min
and the supernatant was taken for estimation. WSC were
determined by using the reactions with phenol and sul-

Stem Reserve Mobilization and Sink Activity in Wheat under Drought Conditions 72
phuric acid [26]. Fructans were determined, after de-
stroying free fructose by adding 0.5 ml of 30% NaOH to
0.5 ml of sugar extract and keeping the mixture in boiling
water bath for 10 min. Thereafter fructans were estimated
by using resorcinol and HCl [27].
3. Results
The effect of drought on stem reserve mobilization of
PBW343 and C306 cultivars was studied by analyzing
the dry matter and carbohydrate composition of inter-
nodes along with sink activity of grains.
3.1. Effect of Drought on Biomass, Mobilized
Dry Matter and Mobilization Efficiency
of Inter Nodes
Drought induced a significant reduction in the biomass of
stem internodes of sensitive cultivar as compared to
those of tolerant one (Table 1). The mean biomass of all
the internodes of sensitive cultivar were reduced whereas
only the apical and peduncle internodes of tolerant culti-
var showed reduced mean biomass, under stress condi-
tions. The stressed plants of sensitive cultivar showed
higher reduction in the mean biomass of apical and pe-
duncle internodes (52% and 61% respectively) than those
of tolerant one, which showed only 33-36% decrease,
when compared to their respective controls (Table 1). In
addition, the mobilized dry matter of internodes and the
differences between post anthesis maximum and mini-
mum weight were higher in the tolerant cultivar as com-
pared to the sensitive one, both under control as well as
stress conditions. Mobilization efficiency of dry matter,
estimated as the ratio of mobilized dry matter to maxi-
mum weight was also higher in all the internodes of tol-
erant cultivar than that of sensitive one, under both con-
trol and drought conditions (Table 1). The peduncle in-
ternodes of tolerant cultivar showed the maximum mobi-
lization efficiency under drought conditions (Table 1).
3.2. Effect of Drought on Moisture Content of
Internodes
The data on the percentage of moisture content of inter-
nodes showed that drought caused in higher mobilization
of stem reserves, particularly from the upper internodes
(apical and peduncle) of tolerant cultivar as compared to
those of sensitive one (Table 2).
3.3. Effect of Drought on Moisture Content and
Yield of Grains
The analysis of grain moisture content showed that the
sensitive cultivar attained maturity much earlier com-
pared to the tolerant one, under both control and stress
conditions (Table 3). Mean grain yield of sensitive cul-
tivar was 32.5 ± 2.0 and 18.5 ± 2.1 for control and
stressed plants respectively. The tolerant cultivar had
25.7 ± 1.3 and 19.0 ± 1.7 grains per spike for control and
stressed plants respectively. Drought induced approxi-
mately 43% decrease in grain yield (number of grains per
spike) of sensitive cultivar and 26% decrease in grain
yield of tolerant one when compared each cultivar with
own respective control.
Table 1. Effect of withholding irrigation at 95 DAS stage on biomass (mg, mobilized dry matter (mg) and mobilization effi-
ciency (ME) of stem internodes of PBW 343 and C306 cultivars.
PBW343 C306
Days after
anthesis Basal Middle Apical Peduncle Basal Middle Apical Peduncle
0 115 ± 18
(95 ± 12)
124 ± 20
(72 ± 10)
125 ± 12
(63 ± 7)
122 ± 14
(52 ± 10)
150 ± 10
(132 ± 11)
111 ± 30
(133 ± 13)
153 ± 21
(113 ± 20)
123 ± 10
(115 ± 20)
7 112 ± 20
(94 ± 10)
142 ± 30
(80 ± 12)
183 ± 24
(75 ± 21)
135 ± 32
(50 ± 12)
162 ± 18
(127 ± 10)
163 ± 13
(125 ± 20)
184 ± 24
(90 ± 9)
200 ± 30
(100 ± 21)
14 109 ± 29
(90 ± 15)
130 ± 15
(77 ± 13)
145 ± 19
(73 ± 19)
127 ± 13
(48 ± 10)
92 ± 12
(123 ± 14)
112 ± 15
(117 ± 14)
133 ± 30
(85 ± 12)
132 ± 15
(89 ± 11)
28 95 ± 16
(85 ± 12)
110 ± 12
(68 ± 7)
128 ± 22
(67 ± 20)
112 ± 21
(43 ± 9)
80 ± 13
(108 ± 21)
84 ± 6
(98 ± 10)
115 ± 10
(77 ± 11)
105 ± 9
(68 ± 10)
42 81 ± 10
(82 ± 9)
101 ± 11
(63 ± 10)
120 ± 10
(60 ± 10)
100 ± 15
(40 ± 7)
70 ± 10
(101 ± 17)
62 ± 12
(72 ± 10)
91 ± 11
(70 ± 13)
80 ± 9
(53 ± 13)
Mean
Biomass
100
(88)
118
(71)
137
(67)
117
(45)
102
(116)
100
(105)
129
(84)
121
(80)
Mobilized dry
matter
34
(13)
41
(17)
63
(15)
35
(12)
92
(31)
101
(61)
93
(43)
120
(62)
ME (%) 29.6
(13.7)
28.9
(21.2)
34.4
(20.0)
25.9
(23.1)
56.8
(23.5)
62.0
(45.9)
50.5
(38.0)
60.0
(53.0)
Data represent mean ± S.D. of four replicates. Values without parentheses are for control plants and those with parentheses are for water deficit plants.

Stem Reserve Mobilization and Sink Activity in Wheat under Drought Conditions 73
Table 2. Effect of withholding irrigation at 95 DAS stage on moisture content (%) of stem internodes of PBW 343 and C306
cultivars.
PBW343 C306
Days after
anthesis Basal Middle Apical Peduncle Basal Middle Apical Peduncle
0 52.4 ± 3.2
(41.1 ± 2.7)
50.3 ± 4.5
(40.4 ± 4.6)
52.2 ± 8.3
(46.4 ± 6.4)
48.6 ± 3.8
(49.5 ± 5.7)
58.5 ± 0.9
(51.9 ± 2.1)
63.2 ± 3.3
(55.6 ± 2.1)
70.0 ± 2.5
(61.0 ± 2.9)
74.2 ± 0.8
(65.5 ± 2.5)
7 55.1 ± 2.7
(29.4 ± 4.7)
51.5 ± 2.2
(27.1 ± 1.2)
53.3 ± 1.2
(33.1 ± 3.3)
54.1 ± 1.3
(19.5 ± 2.9)
54.3 ± 1.6
(38.7 ± 3.9)
53.6 ± 1.8
(36.6 ± 2.3)
51.5 ± 1.8
(33.9 ± 3.4)
55.3 ± 1.2
(30.2 ± 2.5)
14 58.0 ± 1.5
(24.5 ± 0.4)
59.2 ± 1.2
(25.5 ± 3.2)
59.2 ± 1.2
(28.0 ± 4.1)
58.5 ± 0.9
(16.7 ± 1.7)
51.0 ± 4.0
(29.7 ± 5.3)
51.3 ± 3.9
(27.2 ± 2.8)
51.4 ± 2.3
(28.5 ± 7.4)
55.2 ± 0.6
(22.9 ± 14.4)
28 55.2 ± 9.0
(5.4 ± 0.1)
56.2 ± 8.2
(9.8 ± 1.9)
51.0 ± 5.7
(7.5 ± 2.2)
46.3 ± 10
(6.7 ± 1.0)
49.5 ± 4.1
(18.1 ± 1.5)
50.4 ± 3.3
(19.4 ± 0.4)
50.3 ± 6.3
(13.5 ± 4.5)
50.5 ± 1.3
(10.3 ± 0.9)
42 1.2 ± 0.3
(0)
1.8 ± 0.9
(0)
1.1 ± 0.4
(0)
1.8 ± 0.1
(0)
10.1 ± 0.5
(11.7 ± 3.0)
11.6 ± 1.6
(4.6 ± 1.1)
6.7 ± 1.2
(2.2 ± 0.6)
5.6 ± 1.1
(2.0 ± 0.8)
Data represent mean ± S.D. of four replicates. Values without parentheses are for control plants and those with parentheses are for water deficit plants.
Table 3. Effect of withholding irrigation at 95 DAS stage on moisture content (%) of grains of PBW343 and C306 cultivars.
Cultivar Days after anthesis
7 14 21 28 35 42
PBW 343 69.7 ± 1.5
(60.9 ± 4.0)
51.9 ± 2.9
(48.5 ± 0.6)
45.5 ± 0.9
(7.7 ± 2.4)
29.2 ± 1.8
(5.5 ± 0.6)
5.7 ± 0.3
(5.0 ± 0.8)
1.5 ± 0.3
(1.0 ± 0.1)
C306 70.8 ± 0.4
(65.5 ± 0.9)
64.5 ± 1.5
(51.4 ± 4.8)
49.7 ± 0.7
(29.9 ± 2.0)
43.9 ± 2.4
(9.6 ± 0.8)
24.1 ± 2.4
(6.6 ± 1.5)
4.5 ± 0.9
(2.4 ± 0.6)
Data represent mean ±S.D. of four replicates. Values without parentheses are for control plants and those with parentheses are for water deficit plants.
3.4. Effect of Drought on Sucrose Synthase and
Acid Invertase Activity of Grains
The grains of sensitive cultivar showed the maximum of
SS at 14 DAA, then it declined reaching negligible val-
ues at 35DAA stage. The tolerant cultivar had the maxi-
mum SS activity at 7 DAA, then it declined gradually
reaching negligible values at 35DAA (Table 4). When
drought was applied, SS activity increased in the grains
of both the cultivars, but the levels were significantly
higher in the tolerant one (Table 4). The sensitive culti-
var had very low SS activity at 21 DAA which then de-
clined reaching a negligible value at 28 DAA (Table 4).
In other words, SS activity was higher in the grains of
tolerant cultivar than in that of sensitive one, both under
control and stress conditions (Table 4).
AI activity of grains was observed to be comparable in
control plants of both the cultivars at 7 and 14 DAA and
then declined, reaching negligible values at 21 DAA
(Table 4). Under stress conditions, acid invertase activity
was observed to be a little higher in the grains of sensi-
tive cultivar at 7 DAA (Table 4).
3.5. Effect of Drought on Carbohydrate Status of
the Two Cultivars
The content of water soluble carbohydrate (WSC) in the
internodes of control plants of tolerant cultivar was higher
than those of sensitive one, at all the growth stages ex-
amined (Table 5). When drought was applied by with-
holding irrigation, the mobilized WSC content remained
higher in the internodes of tolerant cultivar compared to
the sensitive one (Table 5). Fructan content in the inter-
nodes of tolerant cultivar was also higher than the sensi-
tive one, under both control and stress conditions (Table
6). The content of mobilized fructan was higher in all the
internodes of the control and drought stressed plants of
tolerant cultivar compared to the sensitive one (Table 6).
4. Discussion
The two wheat cultivars showed significantly variable
agronomical and biochemical parameters which attrib-
uted to their differential response to drought conditions.
PBW 343, the sensitive double dwarf cultivar showed
relatively little tolerance against drought as compared to
C306, the tall, tolerant cultivar. It could be inferred that
when drought was applied by withholding irrigation at 95
DAS stage, i.e., just five days before the initiation of
anthesis, the dry matter accumulation in the internodes of
sensitive cultivar was much more affected than those of
tolerant one, leading to a decrease in mean biomass of
the internodes of sensitive cultivar. Under optimal grow-
ing conditions, carbon assimilation rates are high and a

Stem Reserve Mobilization and Sink Activity in Wheat under Drought Conditions 74
Table 4. Effect of withholding irrigation at 95 DAS stage on specific activity of sucrose synthase and acid invertase (nmole
sucrose hydrolysed mg-1 protein min-1) in grains of PBW343 and C306 cultivars.
Days after anthesis
Enzyme
7 14 21 28
Sucrose synthase
PBW343 16.6 ± 0.05
(24.5 ± 0.09)
40.2 ± 1.0
(61.9 ± 7.4)
31.4 ± 4.7
(5.4 ± 0.8)
5.0 ± 0.1
(negligible)
C306 52.8 ± 3.8
(89.7 ± 3.6)
37.4 ± 3.9
(23.5 ± 4.0)
32.8 ± 2.4
(19.7 ± 1.7)
8.1 ± 0.01
(1.2 ± 0.02)
Acid Invertase
PBW343 0.96 ± 0.02
(9.8 ± 0.08)
1.26 ± 0.05
(1.89 ± 0.03)
Negligible
(Negligible)
Negligible
(Negligible)
C306 1.55 ± 0.04
(2.0 ± 0.05)
1.69 ± 0.06
(1.88 ± 0.42)
Negligible
(Negligible)
Negligible
(Negligible)
Data represent mean ± S.D. of three replicates. Values without parantheses are for control plants and those with parantheses are for water deficit plants. Activ-
ity was negligible at 35 and 42 DAA.
Table 5. Effect of withholding irrigation at 95 DAS stage on water soluble carbohydrate (WSC) content (mg/g dry wt.) of
stem internodes of PBW 343 and C306 cultivars.
PBW343 C306
Days after
anthesis Basal Middle Apical Peduncle Basal Middle Apical Peduncle
0 310 ± 83
(186 ± 32)
287 ± 19
(152 ± 27)
272 ± 43
(162 ± 37)
170 ± 18
(137 ± 12)
310 ± 10
(315 ± 35)
307 ± 30
(263 ± 14)
170 ± 24
(250 ± 22)
170 ± 14
(270 ± 24)
7 210 ± 41
(128 ± 24)
273 ± 24
(105 ± 15)
317 ± 18
(115 ± 04)
210 ± 19
(130 ± 10)
425 ± 96
(251 ± 25)
415 ± 22
(183 ± 19)
380 ± 28
(125 ± 05)
260 ± 18
(133 ± 20)
14 72 ± 14
(113 ± 11)
78 ± 04
(92 ± 06)
108 ± 24
(142 ± 14)
67 ± 02
(115 ± 36)
347 ± 51
(168 ± 37)
391 ± 46
(152 ± 12)
379 ± 13
(123 ± 37)
321 ± 28
(129 ± 10)
28 24 ± 03
(106 ± 28)
27 ± 05
(75 ± 14)
21 ± 02
(80 ± 09)
33 ± 06
(96 ± 13)
102 ± 22
(127 ± 17)
123 ± 25
(137 ± 27)
72 ± 20
(110 ± 15)
60 ± 02
(121 ± 20)
42 20 ± 03
(89 ± 21)
25 ± 02
(60 ± 12)
17 ± 03
(48 ± 12)
24 ± 04
(57 ± 11)
68 ± 10
(100 ± 23)
56 ± 02
(99 ± 24)
32 ± 04
(89 ± 16)
34 ± 07
(90 ± 16)
Mobilized
WSC (mg/g
dry Wt.)
290
(97)
262
(92)
300
(114)
187
(80)
357
(215)
359
(164)
348
(161)
287
(180)
Data represent mean ± S.D. of four replicates. Values without parentheses are for control plants and those with parentheses are for water deficit plants.
proportion of assimilates is allocated to stem storage.
When carbon assimilation is reduced by stress, storage in
stems also gets affected [28].
The stress tolerance efficiency of cereals was depend-
ent not only on the assimilation of stem reserves but also
on the effective partitioning of these reserves to the
grains [29,30]. Higher mobilization efficiency resulted in
better utilization of pre-anthesis stored reserves from the
internodes of tolerant cultivar. Water deficit induced
reallocation of 75-92% of preanthesis 14C stored in stem
and sheath to grains has been reported in literature [10].
Reserve storage mainly occur as water soluble carbo-
hydrates (WSC) which can accumulate in wheat stems to
more than 40% of total stem dry weight [31]. WSCs in
wheat stems are mainly composed of fructans, sucrose,
glucose and fructose, with fructans being the major
component [32]. WSC content and fructan content,
which were manifold higher in the internodes of drought
tolerant cultivar than those of sensitive one could be due
to greater storage capacity of the taller cultivar. The ca-
pacity for maintaining large storage in stems appeared to
be a genetically controlled constitutive trait [15,33]. Taller
cultivars have been reported to have greater stem storage
[1]. The Rht-B1b and Rht-D1b dwarfing genes of wheat
were found to reduce reserve storage (fructans) by 35%
and 39% respectively, as a consequence of 21% reduc-
tion in stem length [34]. WSC levels were reported to be
positively correlated with mRNA expression levels of
fructan synthesizing enzymes, e.g., Suc: Suc 1-fructosy-
ltransferase (1-SST) and Suc: fructan 6- fructosyltrans-

Stem Reserve Mobilization and Sink Activity in Wheat under Drought Conditions 75
Table 6 . Effect of withholding irrigation at 95 DAS stage on fructan content (mg/g dry wt.) of stem internodes of PBW 343
and C306 cultivars.
PBW343 C306
Days after
anthesis Basal Middle Apical Peduncle Basal Middle Apical Peduncle
0 200 ± 08
(71 ± 5)
160 ± 16
(71 ± 13)
224 ± 16
(87 ± 10)
155 ± 27
(88 ± 16)
216 ± 56
(310 ± 37)
277 ± 17
(262 ± 40)
110 ± 22
(232±56)
99 ± 14
(123 ± 40)
7 131 ± 23
(120 ± 7)
171 ± 32
(92 ± 14)
261 ± 20
(99 ± 7)
142 ± 19
(103 ± 8)
405 ± 38
(196 ± 52)
408 ± 36
(144 ± 16)
336 ± 51
(113 ± 23)
224 ± 25
(122 ± 22)
14 16 ± 01
(55 ± 08)
24 ± 01
(59 ± 16)
44 ± 15
(92 ± 22)
24 ± 01
(80 ± 02)
301 ± 51
(149 ± 31)
362 ± 46
(128 ± 16)
352 ± 13
(109 ± 14)
308 ± 28
(118 ± 8)
28 10 ± 01
(28 ± 03)
10 ± 01
(24 ± 06)
09 ± 01
(43 ± 04)
12 ± 2
(48 ± 3)
30 ± 4
(100 ± 9)
48 ± 11
(100 ± 17)
27 ± 7
(99 ± 4)
40 ± 9
(112 ± 10)
42 5 ± 0.4
(17 ± 1)
5 ± 0.9
(16 ± 2)
5 ± 0.3
(40 ± 10)
4 ± 0.1
(37 ± 9)
18 ± 4
(36 ± 8)
14±5
(43±5)
13 ± 4
(52 ± 5)
11 ± 2
(34 ± 4)
Mobilized
fructan
content
195
(103)
166
(76)
256
(59)
151
(66)
387
(274)
394
(219)
339
(180)
297
(89)
Data represent mean ± S.D. of four replicates. Values without parentheses are for control plants and those with parentheses are for water deficit plants.
ferase (6-SFT) and these enzymes were differentially
expressed among genotypes differing in WSC concentra-
tions [15]. High fructan depolymerization in the inter-
nodes of tolerant cultivar, under drought conditions sug-
gested a degree of osmotic adjustment leading to a possi-
ble mechanism of protection against dehydration. It
might, therefore, be proposed that the effectively higher
mobilization of fructans, under drought conditions, helped
the tolerant cultivar in coping stress conditions.
While the size of storage is pre-eminent, the size of
sink and its capacity to utilize the imported carbon is also
important for allowing grain filling from stem reserves.
In drought conditions, the enhanced mobilization of stem
reserves, is directly linked to the increased demand of
these reserves by the ear. Kernels are very strong sinks
for carbohydrates, during grain filling period [35]. High
levels of enzymes involved in the unloading of assimi-
lates in the sink through cleavage of sucrose would in-
crease the sink capacity by lowering the local concentra-
tion of sucrose, thereby generating a gradient that allows
further unloading of sucrose from phloem [36]. The en-
hanced SS activity in the grains of drought stressed
plants of tolerant cultivar as compared to those of sensi-
tive one increased the sink strength and attributed to-
wards faster remobilization of assimilates to the grains.
Sucrose synthase activity is therefore regarded as bio-
logical marker of sink strength [37]. Higher activity of
SS in wheat grains subjected to water deficit conditions
has been reported earlier [38,39]. AI activity was rela-
tively little higher in the grains of sensitive cultivar as
compared to those of tolerant one, under stress conditions
and thus could not contribute towards effectively in-
creasing the sink strength of sensitive cultivar. The high-
er sink activity of tolerant cultivar, under drought condi-
tions, thus induced higher mobilization of stem reserves
to the grains, thereby restricting a heavy decrease in
grain yield (in terms of number of grains per spike) as
was observed for sensitive cultivar. Yield is the most
important economic trait, and grain production is the
main selection criterion for drought resistance of wheat
[40]. The current findings in relation to water stress and
the earlier observations [14] tend to support the sugges-
tion that drought tolerant wheat cultivars are capable of
synthesizing, storing and mobilizing high concentration
of water soluble carbohydrates from their stems and have
stronger sink activity as compared to drought sensitive
cultivars, under water stress conditions. This would en-
able the tolerant cultivar to exhibit improved grain yield
even during stress conditions. It was suggested that under
stress conditions the tolerant genotypes activated their
protection mechanisms faster and more efficiently than
the sensitive ones [3].
Water stress induced shortening of grain filling dura-
tion with smaller kernels at maturity was earlier reported
[41]. Grains of sensitive cultivar attained maturity much
earlier as compared to tolerant one, both under control
and drought conditions. It was reported that during
drought conditions, the rate at which stem reserves were
mobilized to the grains was not sufficiently high to com-
pensate for the reduction in grain filling period [33].
Therefore, genotypes with longer grain maturation period
appeared to have an advantage in this respect. It is pro-
posed that longer duration of grain maturation in case of
drought tolerant cultivar allows greater utilization of
stem reserves as compared to sensitive one, under stress
conditions.
We can conclude that the higher capability of the tall
wheat cultivar, C306 to tolerate drought stress is due to

Stem Reserve Mobilization and Sink Activity in Wheat under Drought Conditions 76
its ability of synthesizing and storing larger concentration
of fructans in its stems coupled with higher mobilization
efficiency, stronger sink activity and longer duration of
grain maturation under water deficit conditions, suggest-
ing its use in breeding program or in drought area where
other sensitive wheat cultivars may be severely affected.
5. Acknowledgements
Thanks are due to University Grants Commission, New
Delhi for providing grant for this work.
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