全 文 ：Vol. 30 , No. 4
pp. 321～328 　Apr. , 2004
作 　物 　学 　报
ACTA AGRONOMICA SINICA
第 30 卷 第 4 期
2004 年 4 月 　321～328 页
Amelioration of the Water Status and Improvement of the Anti2oxidant Enzyme Ac2
tivities by Exogenous Glycinebetaine in Water2stressed Wheat Seedlings
MA Qian2Quan , ZOU Qi , LI Yong2Hua , LI De2Quan , WANG Wei 3
( College of Life Sciences , Shandong Agricultural University , Tai’an 271018 , Shandong , China)
Abstract 　Two wheat ( Triticum aestivum L. ) cultivars with different drought tolerance (the drought2tolerant HF9703 and
the drought2sensitive SN215953) were used to examine the effect of exogenous glycinebetaine ( GB) applied to root system
on the drought tolerance of wheat seedlings. The results showed that GB treatment can ameliorate the water status of the
wheat seedlings under water stress. The GB2treated seedlings maintained higher water potential (ΨW) , turgor (ΨP) and
relative water content (RWC) than untreated seedlings , which were ascribed to the improved osmotic adjustment (OA)
caused by the enhanced accumulation of soluble sugars and free proline. Exogenous GB could improve the anti2oxidant en2
zyme activities in wheat seedlings under water stress. The GB2treated seedlings had higher SOD , APX and POD (but not
CAT) activities and lower MDA content than controls. It was concluded that the exogenous GB could improve the drought
tolerance of wheat seedlings by ameliorating the water status through improving the OA and anti2oxidant enzyme activities.
Our results also showed that the drought2tolerance cultivar maintained a better water status and higher anti2oxidative enzyme
activities than the drought2sensitive cultivar after water stress , moreover , exogenous GB had a more significant effect on
ameliorating the water status of drought2sensitive cultivar than that of the drought2tolerance cultivar while there was no dif2
ference in the effect of exogenous GB on the improvement of anti2oxidant enzyme activities between two cultivars.
Key words 　Glycinebetaine ; Triticum aestivum L. ; Water stress ; Water relations ; Anti2oxidant enzymes
根施甜菜碱对水分胁迫下小麦幼苗水分状况和抗氧化能力的改善作用
马千全　邹　琦　李永华　李德全　王　玮 3 Ξ
(山东农业大学生命科学学院 ,山东泰安 271018)
摘 　要 　研究了外源根施甜菜碱对抗旱性不同的两个冬小麦品种幼苗抗旱性的影响。结果表明 ,水分胁迫会诱导小麦
幼苗体内甜菜碱的积累 ,外源甜菜碱能提高体内甜菜碱含量 ,改善水分胁迫下两小麦品种的水分状况 ,提高其水势、膨压
和相对含水量 ,这可能是通过提高小麦幼苗体内一些有机渗透调节物质 (如可溶性糖和脯氨酸) 含量进而增强其渗透调
节能力而实现的。甜菜碱还能提高水分胁迫下幼苗体内 SOD、APX和 POD 活性 ,而对 CAT活性无影响。水分胁迫后 ,甜
菜碱预处理的幼苗叶片丙二醛含量低于未处理幼苗 ,表明外源甜菜碱能缓解水分胁迫引起的膜脂过氧化。结果还表明 ,
水分胁迫后 ,抗旱性品种的水分状况和抗氧化酶水平优于水分敏感型品种 ,外源甜菜碱在改善水分胁迫下的水分关系方
面对水分敏感型品种效应更大 ,而在提高抗氧化酶活性方面对两品种无明显差异。
关键词 　甜菜碱 ; 冬小麦 ;水分胁迫 ;水分关系 ;抗氧化酶
中图分类号 : S512
　　Drought is one of the major abiotic stresses that limit
crop productivity world2widely. In addition to its dehydra2
tion effect , which cause lose of water and turgor directly
and thus inhibit the normal growth of plants , water stress
can also cause many other damages , such as oxidative
stress. Closure of stomata as a result of water stress and a
consequent decrease in photosynthetic CO2 fixation due to
biochemical constrains in leaf mesophyll tissue result inΞFoundation items :Supported by the State Key Basic Research and Development Plan( G1998010100) and Fund of Shandong Agricultural University for post2
doctoral research(23051) 1
Biography :马千全 (1979 - ) ,男 ,山东潍坊人 ,山东农业大学植物学专业硕士 ,主要从事植物抗逆及其分子机制研究 ,现工作单位 :中国热带
农业科学院 (海南省儋州市) 。3 通讯作者 (Author for correspondence) :王玮 (WANG Wei) 。
Received(收稿日期) :2002209212 ,Accepted(接受日期) :2003204203.

the inactivation of electron transfer reactions , thus cause
an accumulation of reducing power (NADPH) and a de2
crease in NADP+ content . Under such conditions , oxygen
acts as an alternate accepter of electrons , resulting in the
formation of the superoxide radical (O -·2 ) and hydrogen
peroxide (H2O2 ) , which is a reduction product of O -·2 ,
and the hydroxyl radical (OH·) produced by the Haber2
Weiss reaction. These reactive oxygen species (ROS) can
cause lipid peroxidation and consequently membrane inju2
ry , protein degradation , inactivation of SH2containing
enzymes , pigment bleaching and disruption of DNA str2
ands[1 ,2 ] . Plants are endowed with an array of enzymes
such as superoxide dismutase ( SOD) , ascorbate peroxi2
dase (APX) , guiacol2type peroxidase ( POD) and cata2
lase (CAT) and small ROS scavenging molecules such as
glutathione , ascorbic acid ,α2tocopherol and carotenoids
to cope with ROS[1 ] . Physiological and genetic evidence
clearly indicates that the ROS scavenging systems of
plants are an important component of the stress protective
mechanism[3 ] . During water stress , these defense mecha2
nisms may be induced to maintain the equilibrium between
the formation and the detoxification of ROS , while
numerous experiments indicated that the enzymes usually
could not match the increasing production of the ROS be2
cause of the decrease or unchange in the enzymes activi2
ties under water stress[4 ,5 ] .
Glycinebetaine ( GB) is regarded as a particularly
effective compatible solute that occurs naturally in a wide
variety of plants to acclimate to water stress. The action of
GB is not confined to osmoregulation ; numerous experi2
ments have indicated that it acted as an osmoprotectant by
stabilizing both the quaternary structure of proteins and
the highly ordered structure of membranes against the ad2
verse effect of various unfavorable environmental stresses.
Although GB was ineffective in scavenging hydroxyl ra2
dicals[6 ] , studies on transgenic tobacco[7 ] and Arabidop2
sis [8 ] showed that the anti2oxidant enzyme activities in
transgenes increased significantly , however , whether
these increases were caused by the increased GB level in
transgenic plants was not known. As far as we know ,
there is still no report on the effect of GB on the structures
and functions of anti2oxidant enzymes. Considering its
enzyme2protective character under stress conditions , we
proposed that GB might have a role in the protection of
anti2oxidant enzymes. In the present experiment , we
examined the effect of exogenous GB applied through root
on the water statues and anti2oxidant enzyme activities in
wheat seedlings under water stress.
1 　Material and Methods
1. 1 　Plant material and treatments
　　Seeds of two winter wheat ( Triticum aestivum L. )
cultivars with different drought tolerance ( the drought
tolerant HF9703 and the drought sensitive SN215953)
were germinated on filter papers moistened with water for
24 h after being sterilized with 0. 2 % sodium hypochlo2
rite , then the germinating seeds were placed orderly on
nylon gauze with appropriate density and cultured in Ho2
agland solution in white trays (25 cm ×18 cm ×5 cm) .
When the third leaf was fully expanded , the seedlings
were transplanted to Hoagland solution containing 1. 5
mmolΠL GB (CK no containing GB) and cultured for 3 d
(the culture solution was changed everyday) . The GB
concentration was chosen according to the preliminary ex2
periment which indicated that 1. 5 mmolΠL GB was the ex2
act concentration with the maximum effect and could not
cause salt stress. Then the culture solution was replaced
by Hoagland solution containing 15 % PEG 6000 (osmotic
potential is about - 1. 0 MPa) . The controls were cul2
tured in Hoagland solution without PEG (osmotic potential
is about - 0. 15 MPa) . The third leaves were taken for
measurements at 48 h after water stress.
1. 2 　Methods
1. 2. 1 　Extraction and quantification of GB
GB was extracted and quantified according to Zuniga
and Corcuera. [9 ] In general , dried plant material (0. 5 g)
was shaken with 20 mL deionized water for 24 h at 25 ℃.
The extracts were diluted 1∶1 with 1 molΠL H2 SO4 . Al2
iquots of 0. 5 mL were cooled in ice water for 1 h and then
0. 2 mL of cold KI2I2 reagent was added. The tubes were
stored at 0 —4 ℃for 16 h and then centrifuged at 5 000 ×
g for 30 min at 0 ℃. The supernant was discarded and
the periodide crystals were dissolved in 5 mL 1 ,22dichlo2
roethane. The absorbance was measured at 365 nm (ε365
= 2 244) after 2. 5 h and quantified according to the
standard curve.
223 　　　作 　　物 　　学 　　报 30 卷 　

1. 2. 2 　Leaf water status estimation
Leaf relative water content ( RWC) was estimated
using the formula : RWC = ( FW - DW) / ( TW - DW) ×
100 , where FW = fresh weight , TW = weight after rehy2
dration for 24 h at 4 ℃ in the dark and DW = dry
weight , at 80 ℃for 48 h[10 ] . Water potential (Ψw ) was
measured with a HR233Tdew point micro voltmeter (Wes2
cor InC. , Logan , UT , USA) after equilibration in the
chamber for 2 h. Osmotic potential (ΨS) of the cell sap
was measured with a vapor pressure osmometer (Wescor
InC. , Logan , UT , USA) after the cell sap was collected
according to Bajji et al . [10 ] . Turgor pressure (ΨP ) was
calculated as ΨP = Ψw - ΨS .
1. 2. 3 　Determination of osmotic adjustment (OA) and
organic solutes
The osmotic potential at full turgor (Ψ100S ) was eval2
uated by placing detached leaves in test tubes containing
deionized water , leaves were hydrated to attain full tur2
gor , then frozen at - 20 ℃and the cell sap was obtained
and determined according to the method as osmotic poten2
tial determination (see Section 1. 2. 2) . Osmotic adjust2
ment (OA) was evaluated as the difference between the
Ψ100S values estimated in control and stressed leaves.
Total soluble sugar concentration in cell sap was de2
termined using the method of Yu et al . [11 ] . Free proline
content was measured according to Hou[12 ] . Soluble pro2
teins were determined according to Bensadoun and
Weinstein[13 ] , using bovine serum albumin as standard.
1. 2. 4 　Anti2oxidant enzyme activities determination
All operations were carried out at 0 —4 ℃. The
leaves were excised , weighted rapidly (1 g FW) , and
ground with a pestle in an ice2cold mortar with 8 mL 0. 05
molΠL Na2 HPO4ΠNaH2 PO4 (pH 7. 8) buffer. The homoge2
nates were filtered through four layers of cheesecloth and
then centrifuged at 15 000 ×g at 4 ℃for 15 min. The
supernatants were collected for the assays of enzyme activ2
ities. Determination of SOD , CAT , POD and APX activi2
ties were carried out following the procedure previously re2
ported[14 ] .
MDA content was determined according to Zhao et
al . [15 ]
Data of effect of exogenous GB on the well watered
seedlings were omitted for preliminary experiment showing
that the exogenous GB had no obvious effect on the
physiological and biochemical parameters examined in the
present experiment in the well watered seedlings.
1. 3 　Statistics
Measurements on the water status were made at least
three repeats and all the other measurements were made at
least five repeats. Differences between the treatments were
subjected to analysis of Paired t2test (SigmaPlot 2 000) .
To emphasize the effect of exogenous GB , we only mark
the difference between the GB and non2GB treated seed2
lings under water stress with different letters. The differ2
ence between the control and the water stress as well as
the difference between the two cultivars were omitted from
the table and figures and list in the article when neces2
sary.
2 　Results
2. 1 　Effect of exogenous GB on water status of wheat
seedlings under water stress
　　Obvious water loss was observed after being treated
with 15 % PEG6000 for 2 days in both cultivars , which
could be reflected by the decrease in Ψw and RWC and
the loss of turgor ( Table 1) . However , the drought2tol2
erant HF9703 exhibited a higher ability to maintain better
water status under water stress than the drought2sensitive
SN215953. After water stress , the water potential of
HF9703 was significant higher ( t = 4. 513a ) than that of
SN215953 , and the turgor and RWC in the former were
very higher than those of the later ( t = 6. 025b and t =
4. 656b , respectively) .
Water stress induced the accumulation of the GB in
vivo in both cultivars , the drought2tolerant HF9703 accu2
mulated about 1152time GB of the drought2sensitive
SN215953 under water stress ( Table 1) . Application of
GB from roots could improve the endogenous GB content
in both cultivars , while SN215953 absorbed more exoge2
nous GB than HF9703. It can be seen from Table 1 that
the GB content increased 93. 6 % in SN215953 while it
was only 39. 5 % in HF9703 due to GB treatment under
water stress. And the absolute increase of GB in
SN215953 (26. 5 μmolΠg DW) was significantly higher
than that in HF9703 (17. 1μmolΠg DW) ( t = 8. 16b ) .
323　4 期 MA Qian2Quan et al . : Amelioration of the Water Status and Improvement of the Anti2oxidant Enzyme Activities111 　　　

Table 1 Water status of two wheat cultivars with different drought tolerance subjected to water stress
after being pretreated with GB ( values are means of at least three replicates ± S E)
Parameter
HF9703
CK WS WS + GB
SN215953
CK WS WS + GB
GB (μmolΠg DW) 　6. 80 ±0. 98 　43. 3 ±5. 86 　60. 4 ±4. 50 b 　5. 6 ±0. 67 　28. 3 ±3. 12 　54. 8 ±5. 33 c
Ψw 　(MPa) - 0. 30 ±0. 02 - 1. 21 ±0. 03 - 1. 08 ±0. 03 a - 0. 21 ±0. 02 - 1. 34 ±0. 02 - 1. 12 ±0. 03 b
ΨS 　(MPa) - 1. 19 ±0. 05 - 1. 50 ±0. 03 - 1. 61 ±0. 05 a - 1. 12 ±0. 03 - 1. 44 ±0. 02 - 1. 59 ±0. 02 c
ΨP 　(MPa) 　0. 89 ±0. 03 　0. 29 ±0. 04 　0. 53 ±0. 05 c 　 　0. 91 ±0. 03 　0. 10 ±0. 03 　0. 47 ±0. 04 c
RWC ( %) 　95. 4 ±1. 30 　79. 6 ±2. 92 　86. 0 ±3. 40 a 　96. 1 ±1. 75 　72. 2 ±4. 10 　81. 4 ±2. 21 a
Note : 1. The letter a , b and c followed the data means that the GB2treated seedlings is significantly different at the P = 0. 05 , 0. 01 and 0. 001 level respectively
from the non2GB treated seedlings under water stress.
2. CK means well watered seedlings ; WS means water stressed seedlings ; WS + GB means water2stressed seedlings which were pretreated with 1. 5 mmolΠ
L GB. (See Section 1)
　　The increased GB in wheat seedlings due to GB
treatment had a positive effect on the maintenance of water
status under water stress. Table 1 showed that the water
status in GB2treated seedlings was better than in non2GB
treated seedlings under water stress in both cultivars. The
Ψw increased very significantly and the RWC increased
significantly , while the ΨS had a little but significant de2
crease in HF9703 ( t = 3. 422a ) and had a very significant
decrease in SN215953 ( t = 16. 931c ) , causing a signifi2
cant increase in ΨP in both cultivars. As to its effect on
ameliorating the water status under water stress , it seemed
that exogenous GB had a relative larger influence on the
drought2sensitive cultivar than on the drought2tolerant
one. For example , the increasing of ΨP in SN215953
(0. 37 MPa) was very significant than that in HF9703
(0. 24 MPa) ( t = 9. 886b) . Similar trends could also be
observed in Ψw ( t = 7. 451b ) and ΨS ( t = 3. 544a ) .
2. 2 　Effect of exogenous GB on the OA of two wheat
cultivars
　　Our experiment showed that the drought2tolerant
HF9703 had a significantly bigger OA (0. 097 MPa) than
the drought2sensitive SN215973 ( 0. 056 MPa ) under
water stress ( t = 7. 981b ) ( Fig. 12A) . GB treatment
could improve the OA in both cultivars significantly. The
OA of GB2treated seedlings of both cultivars reached at
about 0115 MPa , which indicated that exogenous GB had
a higher positive effect on the improvement of OA of
drought2sensitive cultivar than on that in drought2tolerant
cultivar.
Fig. 1 Effect of glycinebetaine on the osmotic adjustment( OA) ,soluble sugars and
free proline content in wheat seedlings under water stress
See note of Table 1 for the meanings of the letters above the bars and the abbreviations in the symbol box.
　　Water stress could induce sharp accumulations of
soluble sugars and free proline. As shown in Fig. 12B and C , the soluble sugar contents of both HF9703 andSN215953 were about twice of the controls after water
423 　　　作 　　物 　　学 　　报 30 卷 　

stress , and the free proline content increased about 182
fold in HF9703 and 142fold in SN215953. Fig. 12B and C
also showed that exogenous GB could enhance the accu2
mulation of organic solutes. The soluble sugar content in
GB2treated seedlings was 0. 62fold higher than that in
non2GB treated seedlings in both cultivars , and that of
free proline content was 0. 62fold and 1. 12fold in HF9703
and SN215953 respectively.
2. 3 　Effect of exogenous GB on the anti2oxidant en2
zyme activities
　　All the anti2oxidant enzyme activities in both culti2
vars except the CAT in SN215953 were increased after
water stress . As shown in Fig. 2 , the SOD , APX and
POD activities in HF9703 increased about 22. 3 % ( t =
5. 011b ) , 21. 6 % ( t = 5. 815b ) and 40 % ( t = 8. 531c )
respectively , while those in SN215953 increased 21. 0 %
( t = 4. 096a ) , 15. 4 % ( t = 4. 235a ) and 42 % ( t =
61918c ) respectively. The CAT activity increased 38. 3 %
( t = 7. 531c ) in HF9703 but it decreased 30 % ( t =
51886b) in SN215953. It also can be seen from Fig. 2 that the activities of anti2oxidant enzymes in HF9703 werealways higher than those in SN215953 under water stress.GB treatment could improve the activities of SOD ,APX and POD in both cultivars. The SOD , APX andPOD activities increased 18. 3 % ( t = 3. 156a ) , 30. 8 %( t = 6. 238b ) and 37. 5 ( t = 6. 336b ) respectively in GB2treated HF9703 after pretreated with GB under waterstress , while the SOD , APX and POD activities in GB2treated seedlings were 20. 6 % ( t = 5. 674b ) , 36. 7 % ( t= 6. 017b ) and 32. 6 % ( t = 3. 365a ) higher respectivelythan those in non2GB treated seedlings (Fig. 22A , C andD) . But GB treatment had no effect on CAT activities inboth cultivars (Fig. 22B) .The MDA content was significantly lower in GBtreated plants than in untreated plants ( t = 6. 729c inHF9703 and t = 5. 130b in SN215953) after water stress( Fig. 22E) and the MDA content in water stressedHF9703 was significantly lower than that in water stressedSN215953 ( t = 8. 235b ) .
Fig. 2 Effect of GB on the anti2oxidant enzyme activities and
malondialdehyde( MDA) content in wheat seedlings under water stress
See note of Table 1 for the meanings of the letters above the bars and the abbreviations in the symbol box
523　4 期 MA Qian2Quan et al . : Amelioration of the Water Status and Improvement of the Anti2oxidant Enzyme Activities111 　　　

3 　Discussion
3. 1 　Effect of GB on the amelioration of water status
　　Declines in water status were observed in both culti2
vars after water stress. However , the drought2tolerant
HF9703 maintained a relative favorable water status com2
pared with SN215953 , this might be ascribed to its high
osmotic adjustment (OA) under water stress ( Fig. 12A) .
Water stress induced the accumulation of endogenous GB
and root applied GB could improve the level of GB in vivo
and the increased GB level in vivo could offset the de2
crease in water status caused by water stress in both culti2
vars. After water stress , the water potential (ΨW ) and
relative water content ( RWC) in GB2treated plants de2
clined slowly than that in untreated plants in both culti2
vars (Table 1) , thus the water status of GB2treated plants
was better than that of the untreated plants , indicating
that the deleterious effect of water stress on the wheat
seedlings could be compensated or delayed by the GB
treatment. Our results were in agreement with the results
of Xing et al . [16 ] , but somewhat differ from the results of
Ma¨kela¨ et al . [17 ] , who found that the foliar applied GB
had no effect on the RWC of tomato leaves under salt2 and
drought2 stress. Perhaps there was an intrinsic difference
between wheat and tomato ,the former was a GB nature2
accumulator while the later could not accumulate GB it2
self . On the other hand , the ability of roots to absorb ex2
ogenous GB might be better than leaves. It might be also
because of the different compartmentation of exogenously
applied GB in the two crops , which was important for its
action. It is not known whether exogenously applied GB is
accumulated in the same cells or cellular compartment just
like endogenously synthesized GB.
The amelioration of water status under water stress by
exogenous GB might be ascribed to the improvement in the
osmotic adjustment (OA) . Numerous experiments have
suggested that OA plays an important role in the adapta2
tion of plants under drought[10 ] and OA is considered to be
an important component of drought tolerance mechanism
in plants. After GB treatment , the OA of the two wheat
cultivars had an obvious increase ( Fig. 12A) and the in2
crease is caused , at least partly , by the enhanced accu2
mulation of soluble sugars and free proline ( Fig. 12B and
C) in cell sap due to GB treatment . Usually , OA is de2
fined as a decrease in the cell sap osmotic potential (ΨS)
resulting from a net increase in the intracellular solutes
rather than from a loss of cell water. The enhanced accu2
mulation of these organic solutes in GB2treated seedlings
caused a more significant decrease in the ΨS in cell sap
than in non2GB treated seedlings and the decreasing ΨS
facilitates the maintenance of turgor (ΨP ) ( Table 1) ,
which was necessary for the growth and survive of plants
under stress conditions. It should be mentioned that the
significant increase in OA could not be caused solely by
the accumulation of the two types of compatible solutes ,
while the exogenous GB had no effect on the Na + and K+
content , which were considered the main inorganic com2
patible solutes in vacuoles under stress conditions ( data
not shown) . Further research is needed to elucidate the
mechanism of how the exogenous GB improves the osmotic
adjustment .
Our results showed that the exogenous GB seemed to
have a more effective influence on ameliorating the water
status of the drought2sensitive cultivar under water stress
(Table 1) , it might be ascribed to the more absorbed ex2
ogenous GB by this cultivar , and indicated that the low
ability of synthesizing GB of SN215953 (28. 3μmolΠg DW
vs. 43. 3μmolΠg DW in HF9703 , t = 12. 56c ) under
water stress might take part responsibility for its drought2
sensitivity.
3. 2 　Effect of GB on improving the anti2oxidant en2
zyme activities
　　Oxidative stress is considered to be the secondary
stress of varieties of stresses , such as salt , drought , cold
and ABA , and the oxidative injury at the cellular level is
a major cause of crop damage. It is important to maintain
a high ROS scavenging ability under stress conditions to
avoid or delay the oxidative damage. Our results showed
that the water stress could lead to an increasing activity of
anti2oxidant enzymes , which is in agreement with the ob2
servation of other researchers’[1 , 2 ] , but differ from the re2
sults of Bartoli et al . [5 ] , who found that the anti2oxidant
enzyme activities were not affected by the water stress and
the results of Zhang et al . [4 ] who reported that the anti2
oxidant enzymes activities were suppressed by water
623 　　　作 　　物 　　学 　　报 30 卷 　

stress. These discrepancies might occur depending on
species or genotype , growth conditions , stress period , age
of plants and all the above , on stress intensity , as sug2
gested by Sghern et al . [18 ] . Our results also suggested
that the anti2oxidant enzyme activities were always higher
in HF9703 than in SN215953 after water stress , implying
a more effective suppression of the production of ROS in
the drought2tolerant cultivar than in the drought2sensitive
cultivar , which is consistent with the findings of Sairam
and Saxena[2 ] who reported that the drought2tolerance of a
wheat genotype was related to its higher anti2oxidant en2
zyme activity.
Despite the increase in anti2oxidant enzyme activi2
ties , there is still oxidative stress occurring in wheat seed2
lings after water stress , which can be reflected by the in2
crease in MDA content ( Fig. 22E) , indicating that the
anti2oxidant enzymes in droughted wheat seedlings could
not completely scavenge the ROS. GB treatment can in2
crease the anti2oxidant enzyme activities , which lead to a
more effective scavenging of ROS , trying to maintain the
balance between the formation and detoxification , and
thus increase the tolerance to oxidative stress caused by
water deficit , ultimately improve the tolerance of wheat
seedlings to water stress. The lower MDA content in the
GB2treated seedlings than in untreated seedlings ( Fig. 22
E) also proved the protective effect of GB against oxida2
tive stress caused by water stress. We propose this is how
the exogenously applied GB improves the tolerance of
wheat seedlings to water stress in addition to its ameliora2
tion effect on water status. However , the exact mechanism
about how the exogenous GB increases the anti2oxidant
enzyme activities remains unclear. We infer it might be
the case that the increased GB in vivo could enhance the
hydration of the protein surface by dissolving in the hydro2
phobic region in the hydration shell of the protein or dis2
solving in the free water around the protein molecular , as
reviewed by Hou et al . [19 ] , thus precluding the water loss
and maintaining the conformation of the enzyme , ultimate2
ly keeping the function of the protein under water stress.
It is also probable that the GB interacted directly with the
enzymes and kept their activities through a mechanism re2
maining unclear. Moreover , GB might induce the gene
expression of the anti2oxidant enzymes under water stress
and thus increase their activities.
Differing from its effect on the water status , the exo2
genous GB exhibited no difference as to it effect on im2
proving the anti2oxidant enzyme activities in two cultivars.
For its effective action in stress2tolerance , genetic
engineering of the biosynthesis of GB has been the focus
of considerable attention as a potential strategy for in2
creasing stress tolerance of plants , and varieties of trans2
genic plants have been obtained. Physiology experiment
showed that their stress tolerance was enhanced though at
different levels[20 ,21 ] . However , the GB level in most of
the transgenic plants could not match that in nature2accu2
mulators. Moreover , the complexity and difficulty of ge2
netic engineering and the limitation of GB synthesis in
transgenes by endogenous choline , a precursor of GB ,
make it impossible to be utilized widely at present . Thus
the application of exogenous GB , whether through root or
leave , has been another approach to increase the GB level
of plants. It has been suggested that plants are able to ab2
sorb exogenous GB and transport it to developing organs ;
moreover , the exogenous GB remained unmetabolized in
plants for a long time[22 ] . So application of exogenous GB
could be a simple but effective approach to improve the
stress tolerance of plants , which was supported by our re2
search.
References
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Photochemical response and oxidative stress in two clones of Coffea Cane2
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