全 文 ：豌豆种子吸胀过程中脱水耐性变化的时间模式 ?
宋松泉1 , Patricia BERJAK2 , Norman W . PAMMENTER2
(1 中国科学院植物研究所 , 北京 100093 ; 2 School of Biological and Conservation Sciences,
University of KwaZulu-Natal , Durban 4041 , South Africa)
摘要 : 研究了豌豆种子吸胀过程中脱水耐性的变化模式。种子在吸胀初期迅速吸收水分 , 然后缓慢吸收直
到平台期。电解质渗漏速率在吸胀初期增加直到 11 h, 然后随着吸胀下降。在吸胀过程中 , 种子的萌发率
逐渐增加 , 种子和胚轴的脱水耐性逐渐丧失 , 10%和 50%的种子和胚轴被脱水致死的含水量明显增加。赤
霉素和脱落酸处理改变豌豆种子的萌发特性 , 提高胚轴的脱水耐性。研究结果表明 , 吸胀的豌豆种子脱水
耐性的丧失是一种数量性状 , 正常性种子吸胀后脱水耐性的变化能够作为种子顽拗性研究的模式系统。
关键词 : 豌豆 ; 脱落酸 ; 脱水 ; 脱水耐性 ; 吸胀作用 ; 萌发 ; 正常性 ; 顽拗性
中图分类号 : Q 945 文献标识码 : A 文章编号 : 0253 - 2700 (2009) 03 - 239 - 08
Temporal Pattern of Changes in Desiccation Tolerance
during Imbibition of PisumsativumSeeds
SONG Song-Quan1 , Patricia BERJAK2 , Norman W . PAMMENTER2
( 1 Institute of Botany, Chinese Academy of Sciences, Beijing 100093 , China; 2 School of Biological and
Conservation Sciences, University of KwaZulu-Natal, Durban 4041 , South Africa)
Abstract: Changes in desiccation tolerance of pea ( Pisumsativum) seeds during imbibition were studied in this paper .
Water uptook by seedswas initially rapid, andthen slowly increased until a plateau was reached . Rateof electrolyte leak-
age increased initially until 11 h, and then declined with imbibition . During imbibition, germination percentage of seeds
gradually increased, and water content at which 10 % and 50% of seeds and axeswere killed by subsequent dehydration
significantly increasedwhiledesiccation toleranceof seeds and axeswas progressively lost . Gibberellin and abscisic acid al-
tered seed germination characteristics and improved desiccation toleranceof axes . Theseresults showed that the loss of des-
iccation toleranceof imbibed pea seedswas aquantitativefeature, andthechanges in desiccation tolerance of imbibingor-
thodox seeds could serve as amodel systemfor the study of seed recalcitrance .
Key words: Abscisic acid; Dehydration; Desiccation tolerance; Imbibition; Germination; Orthodox; Pisumsativum; Recalcitrant
Orthodox seeds acquire desiccation tolerance dur-
ing development, and those of most species undergo
substantial drying as the final developmental phase . All
orthodox seeds may be further dehydrated after they
have been shed and will survive in this state for a con-
siderable time . Recalcitrant seeds, however, are shed
at high water contents and are intolerant of dehydration
( Berjak, 2006; Berjak and Pammenter, 2004 ) . There
is considerablevariability in thepost-harvest physiology
of recalcitrant seeds, within as well as among species .
Within a species, variation may occur from harvest to
harvest, or within a single harvest ( Berjak et al. ,
1996; Finch-Savage and Blake, 1994) . Water content
and extent of embryo development at sheddingmay vary
among seasons ( e.g . for Quercus robur; Finch-Sav-
age, 1996; Camellia sinensis; Berjak et al. , 1996 ) .
云 南 植 物 研 究 2009 , 31 (3) : 239～246
Acta Botanica Yunnanica DOI : 10 .3724?SP. J . 1143 .2009.08216
? ?Foundation item: National Research Foundation of South Africa, and National Natural Sciences Foundation of China ( 30870223)
Received date: 2008 - 11 - 30 , Accepted date: 2009 - 04 - 07
作者简介 : 宋松泉 (1957 - ) 男 , 研究员 , 主要从事种子生物学的研究。 E-mail : sqsong@ ibcas. ac. cn; Tel : 010 - 62836484
There are also marked differences in the rate of water
loss from recalcitrant material , which varies both from
species to species, and with the stage of seed develop-
ment (Berjak and Pammenter, 1997) . A further char-
acteristic of recalcitrant seeds ( and one that probably
varies among species) is that they are actively metabol-
ic at shedding, be this a manifestation of ongoing de-
velopment, or of the onset of germination ( Berjak,
2006; Berjak and Pammenter, 2004) . These consider-
able differences among recalcitrant seeds make compar-
ative studies extremely difficult .
It has been suggested that a suit of mechanisms
and processes, under complex genetic control which is
still not fully understood, has been implicated in the
acquisition and maintenanceof desiccation tolerance in
orthodox seeds . These mechanisms and processes in-
clude intracellular physical characteristics, intracellular
de-differentiation and metabolic‘switch off’, reactive
oxygen species and antioxidants and protectants ( late
embryogenic accumulating?abundant proteins, carbo-
hydrates) (Pammenter and Berjak, 1999; Berjak and
Pammenter, 2008) . In fact, themechanismson desic-
cation tolerance of seeds are unknown . Berjak and
Pammenter (2008) recently suggested that whatever the
post-harvest responses of seeds of individual species
may be, they are theoutcome of the properties of pre-
shedding development, and a full understanding of the
subtleties of various degrees of non-orthodox behavior
must await the identificationof, and interaction among,
all the factors conferring extreme orthodoxy .
Imbibition of water by orthodox seeds initiates a
series of metabolic steps that lead to germination, and
during this sequence of events, desiccation tolerance
within the seed is lost . Duringthe early stagesof germ-
ination and prior to radicle emergence in most species,
theseeds can be dried to its original moisture content
without causing injury . The same degree of drying,
however, imposed at progressively later stages of germi-
nation, dramatically reduces seed vigour and, if im-
posed after radicle elongation has commenced, usually
results in seedlingdeath (McKersie et al. , 1988 ) . Se-
naratna and McKersie ( 1983) studied germinating soy-
bean ( Glycine max L . Merr cv . Maple Arrow) seeds
and found that seedsgerminated for 6 hwere tolerantof
severedrying, while thosegerminated for 36 h werenot .
Koster and Leopold (1988) found that germinating soy-
bean ( G. maxL . Merr . cv .Williams) seeds lost desic-
cation tolerance between 12 and 18 h of imbibition,
whilegerminating pea ( PisumsativumL . cv . Alaska)
seeds lost tolerance between 18 and 24 h and maize
( Zea mays L . cv . Merit) seeds by 48 h imbibition .
Koster and Leopold (1988) correlated the loss of desic-
cation tolerance with changes in embryo sugar composi-
tion . Leprince et al . (1995) used electrolyte leakageto
determine‘critical moisture contents’ for germinating
bean ( Phaseolus vulgaris L . cv . Pole Kentucky Won-
der) and maize ( Z. mays L . cv . Kelvedon Glory)
seeds . Below these water contents, these seeds were
damaged by desiccation, and the injurywas believed to
be caused by oxygen radicals that resulted fromrespira-
tory metabolism . All of the studies described aboveused
wholeseeds, but themoisture content of embryonic axes
is usually higher than that of thewholeseeds during im-
bibition . In some studies germinating seeds were dried
back to a single water content for the determination of
desiccation tolerance . In other studies, only electrolyte
leakage was used to assess desiccation tolerance of
seeds, without survival data . The criterion for deter-
mining survival also differed among the studies .
Pea seeds imbibed for different times can provide
convenient and reliable experimental materials with dif-
fering desiccation tolerance for thestudy of desiccation-
tolerance and -sensitivity . In the present study, we at-
tempted to better define the time course of the loss of
desiccation toleranceinpeaseeds and to determinehow
this quantitative trait changes during imbibition . To
this end, we looked separately at survival of seeds and
axes, at the changes of electrolyte leakage, and at the
growth rateof seedlings produced by surviving seeds or
axes . The effectsof abscisic acid (ABA) andgibberel-
lin (GA) on thedesiccation toleranceof axes were also
investigated . The results will provide a framework for
the studies of loss of desiccation tolerance in germinat-
ing orthodox seeds and for thelack of this characteristic
in recalcitrant seeds .
Materials and Methods
Plant material
Current harvest of pea ( Pisum sativum cv . Greenfeast)
seedswere obtained fromMcDonalds Seed Company ( Pieterma-
ritzburg, South Africa) and were kept at 16℃ until used . The
042 云 南 植 物 研 究 31 卷
seeds were surface-sterilized in a solution of 1% hypochlorite,
and rinsed three time in sterile water, andthen imbibed by plac-
ing the seeds in a shallow layer of distilledwater or treatment so-
lution (GA andABA) suchthat half the seedwas immersed . Ax-
eswereexcisedfromthe seedsthat had been imbibedfor different
times and were then treated as indicated below . All manipula-
tionswere conducted at roomtemperature (22 - 25℃) .
Water content determinations
Water content of 20 individual seeds or axeswas determined
gravimetrically (80℃ for 48 h) . Water contentsareexpressedon
a dry mass basis [g H2 O ( g dry weight) - 1 , g g- 1 ] .
Desiccation treatments
Dehydration of the differentially pre-imbibed seeds was
achieved by burying them in activated silica gel within closed
plastic buckets for different periods of time . Excised axes were
dehydrated by placingthemina small boat on activated silica gel
in a closed jar for different periods of time .
Assessments of germination and survival
Batches of 40 treated seeds or axes were germinated on
moist filter paper in Petri dishes at 25℃ for 5 days . Seeds were
placed in the dark and the axes in alternating light and dark with
16 h photoperiod ( light intensity, 66 .25μmol m- 2 s- 1 ) . Seeds
showing radicle emergence for 2 mmwere scored as germinated,
and axes showing a marked increase in length and volume were
scored as survived . Although dried seeds and axes take up water
during theearly stages of re-imbibition, those seeds and axes that
were injured by dehydration progressively deteriorated during
continued re-imbibition .
Conductivity tests
Electrolyte leakage from 7 - 20 replicate seed or axis was
measured individually for 12 h in 2 .5 ml distilled water using a
CM100 multi-cell conductivity meter ( Reid and Associates,
Durban, South Africa) . The conductivity of the leachates was
measured immediately after the dryingtreatmentsand leakage rate
was expressed asμS cm- 1 ( g dry weight) - 1 h- 1 .
Statistical analysis
All data wereanalysed usinga one-way ANOVA model from
the SPSS 11 .0 package for Windows (SPSS Inc . , 2006) .
Results
Changes occurring during imbibition of seeds
Germination commences with the uptake of water
by the dry seed and is completed when a part of the
embryo, usually the radicle, extends to penetrate the
structures that surround it . Water content of pea seeds
increased from0 .13 g?g in the dry seeds to 1.82 g?g at
12 h of imbibition, and then slowly increased to 2 .62
g?g after 96 hof imbibition (Fig. 1a) . Water contentof
axes, however, exhibited a typical triphasic pattern of
water uptake with a marked increase during the initial
phase, and then a slow increase until 64 h of imbibi-
tion, followed by a second substantial increase
(Fig. 1a) . The water uptake by axis was greater than
that by the whole seed; for example, when seeds had
imbibed for 12 h, the water contents of seeds and axes
were 1 .82 and 2 .24 g?g, respectively, and for 96 h,
2 . 62 and 5 .55 g?g, respectively .
The germination percentage of seeds increased
with imbibition time ( P value≤ 0 .001 ) ; the first
seeds germinated after about 40 h, and full germination
was achieved by 96 h . The time taken for 50% seeds
to germinate ( T50 ) was about 74 h ( Fig. 1b) .
The rateof electrolyte leakage fromseeds dramati-
cally increased until 11 h of imbibition, fromabout 5μS
cm- 1 (g DW) - 1 h- 1 at 1 h to 94μS cm- 1 (g DW) - 1
h- 1 at 11 h, and thenobviously declined to 29μS cm- 1
(g DW) - 1 h- 1 at 49 h ( P value≤0.001, Fig. 1c) .The
leakage of seed, however, increased with imbibition up
to 60 h ( P value≤0 .001 , Fig.1d) .
Effect of pre-imbibition on desiccation tolerance of
seeds and axes
Dehydration of partially imbibed seeds was achie-
vedby burying theminsilicagel for 24 h toawater con-
tent of approximately0 .1 g?g; axesweredehydratedover
silicagel for 12 h to approximately 0 .05 g?g . Seeds and
axes of pea were tolerant on dehydration during the first
24 h of pre-imbibition (data not shown) . But the toler-
ance was progressively lost with pre-imbibition times
longer than 28 h ( P value≤0.001, Fig.2a) . The pre-
imbibition times at which 10% and 50% of seeds were
killedby subsequent dehydrationwere about 32 h and 55
h, respectively; and 10% and 50% of axes were killed
about 36 h and 42 h, respectively (Fig. 2a) . Pea seeds
and axes lost completely desiccation tolerance after imb-
ibition for 96 h and 60 h, respectively . The desiccation
tolerance of the epicotyl was greater than that of the
radicle ( data not shown) .
The fresh weight of seedlings integrates the desic-
cation toleranceof the seed or axis producing the seed-
ling, and thesubsequent metabolismassociatedwithger-
mination and growth . The fresh weight of seedlings pro-
duced by survivingseeds or axes that hadbeen dried after
pre-imbibition declinedwhen thepre-imbibitionperiodsof
1423 期 SONG Song-Quan et al. : Temporal Pattern of Changes in Desiccation Tolerance during Imbibition of . . .
Fig . 1 Changes during imbibition of pea seeds . (a) Time courses of water content in seeds and axes . Values are means±SD of three replicates of 20
seeds or 20 axes each and expressed on adry mass basis . ( b) Changes in germination . Seeds showing radicle emergence for 2 mmwere counted as germi-
nated . All values are means±SD of three replicates of 40 seeds each . (c) Changes in rates of electrolyte leakage . ( d) Changes in leakage . Individual
seed was placed in 2 .5 ml distilledwater and conductivity of the leachatewas measured . All valuesaremeans±SD of 20 replicates of individual seed each .
time increased ( P value≤0.001 , Fig.2b) . Following 36
h pre-imbibition and dehydration, fresh weight of seed-
lings produced by surviving seeds and axes decreased by
32 and 53% , respectively, relative to controls .
Rate of electrolyteleakage frompea seeds and ax-
es after dehydration markedly increased with increasing
pre-imbibition time ( P value≤0 .001) , but the leak-
age rate of axes was much higher than that of seeds
(Fig. 2c) . Leakage rateof axes increased from an ini-
tial value of 141μS cm- 1 ( g DW) - 1 h- 1 to 316μS
cm
- 1 (g DW) - 1 h- 1 at 48 h, and to 785μS cm- 1 (g
DW) - 1 h- 1 at 96 hof imbition (Fig. 2c) .
Relationship among pre-imbibition time , water
content and desiccation tolerance of axes
To assess and quantify the loss of desiccation tol-
erance during imbibition, pea seeds were pre-imbibed
for various times, the axeswere excised and then dried
for increasing periods of time . Water contents of axes
fromseeds imbibed for 40 , 56 and 64 h were 3 .04 ,
3 .24 and 3 .77 g?g, respectively, and declined rapidly
with dehydration over silica gel , all have been de-
creased inwater content to0 .05 g?gafter 12 hdehydra-
tion ( P value≤0 .001 , Fig. 3a) .
For axes fromseeds imbibed for 40 , 56 and 64 h,
respectively, their survival and seedling fresh weight
produced decreased obviously, and their leakage rate
increased notably, with dehydration ( P value≤0 .001 ,
Figs. 3b-d) . With increasing pre-imbibition time, de-
hydration damage of axes, as measured by survival ,
seedling fresh weight produced and leakage rate, did
become apparent; and the water contents at which
10% of axes (W10 ) or 50% of axes ( W50 ) werekilled
by dehydration increased (Fig. 3e) .
Effects of GA and ABA on germination of seeds
and desiccation tolerance of axes
Germinating peaseeds in 1 and 10μmol?L GA mar-
ginally enhancedgermination froman initially highvalue,
but a GA concentrationof 100μmol?L partially inhibited,
and a concentration of 1000μM completely inhibitedger-
mination (Fig.4) .Treatment of seeds with 1 - 100μmol?L
ABA increasingly inhibited, and a concentration of 1000
μmol?L ABA totally inhibited germination (Fig. 4) .
242 云 南 植 物 研 究 31 卷
Fig . 2 Changes in desiccation tolerance of seeds and axes during imbibi-
tion . After imbibition for various periods of time, seeds and excised axes
were dehydrated (seedswereburied insilicagel for 24 h, water content was
approximately 0 .1 g?g; axes were placed above silica gel for 12 h, water
content wasapproximately 0 .05 g?g) and then re- imbibed onmoist filter pa-
per for 5 days . (a) Survival ; seeds showing radicle emergence for 2 mm
were counted as germinated, and axes showing increase in length and vol-
ume were counted as survived . All values are means± SD of three repli-
cates of 40 seeds or 40 axes each . (b) Fresh weight of seedlings produced
by surviving seeds or axes . Seedling fresh weight does not included cotyle-
dons . (c) Rates of electrolyte leakage . Individual seed or axis was placed
in 2 .5 ml distilled water and conductivity of leachate was measured . All
values are means±SD of seven replicates of individual seed or axis each .
Axes fromseeds treated in 10 and 100μmol?L GA
for 44 h showed improved desiccation tolerance, in that
survival and seedling fresh weight increased, relativeto
water-treated controls, but the rate of electrolyte leak-
age also increased (Table1 ) . ABA at 1 and 10μmol?L
also increased desiccation tolerance of axes, the axes
showing a decreased rate of electrolyte leakage as well
as increasedsurvival andseedling freshweight . For ex-
ample, survival and seedling fresh weight of axes from
seeds treated in 10μmol?L ABA increased by 36% and
12% , respectively, and leakage rate decreased by
21% , relative to controls (Table 1) .
Table 1 The effect of GA andABA on desiccation toleranceof pea axes .
Seeds were imbibed in GA or ABA solution for 44 h, the axes were ex-
cised and then desiccated to 0 .06±0 .003 gH2 O (gDW) - 1 . For surviv-
al and seedling fresh weight, values are themeans±SD of threereplicates
of 40 axes each . For electrolyte leakage, values are the means± SD of
seven replicates in a single axis
Treatment
Survival
( % )
Leakage rate
(μS cm- 1 ?
( gDW) - 1 h - 1 )
Seedling fresh
weight
( mg seedling - 1 ?)
Water 19 ?±1 399 @±28 41 .6±2 G. 2
10 ?μM GA 25 ?±2 464 @±30 49 .6±2 G. 9
100 ?μM GA 36 ?±2 421 @±28 48 .2±2 G. 9
10 ?μM ABA 55 ?±3 355 @±26 46 .6±1 G. 8
100 ?μM ABA 77 ?±3 304 @±12 50 .6±2 G. 3
Discussion
Seed germination incorporates those events that
commence with the uptake of water by the quiescent
seed and terminate with the elongationof the embryonic
axis (Bewley, 1997) . Water uptake by pea seeds oc-
curred rapidly during the initial phase of imbibition,
and then more slowly with a plateau phase ( Fig. 1a) .
The initial rapid hydration phase (phase I ) was related
to the matric potential of dry seeds; the second slow
hydration phase ( phase II ) corresponded to the period
of germination . The length of phase II is affected by
imbibition temperature and the water potential of the
medium in which the seeds are imbibed ( Bewley,
1997; Bradford, 1995) . A pea axis is locatednear the
surfaceof the seed and can imbibe water more rapidly
than the whole seed . This, together with a different
chemical composition, leads to a higher water content
of the axis than of whole seed ( Fig. 1a) . By 64 h of
imbibition over 20% of the seeds had germinated
(Fig. 1b) ; that is, the cellsof some axeshad extended,
taking up more water, and so water uptake by axes ex-
hibited atypical triphasic patternover 96 hof imbibition
3423 期 SONG Song-Quan et al. : Temporal Pattern of Changes in Desiccation Tolerance during Imbibition of . . .
(Fig. 1a) , as reviewed by Bewley (1997) , Bewleyand
Black (1994) , and Bradford ( 1995 ) . The axes com-
prised onlyabout 3% of the drymass of theseeds, and
so the pattern of water uptake by axes would not influ-
ence the pattern observed in whole seeds .
Rate of electrolyte leakage from pea seeds in-
creased duringthe first11 hof imbibition, and then sl-
owly declined ( Fig. 1c) . The influx of water into the
cells of dry seeds during phase I results in temporary
structural perturbations, particularly to membranes,
which lead to an immediate and rapid leakageof solute
ands low molecular weight metabolites into the sur-
rounding imbibition solution . This is a consequence of
thetransitionof themembranephospholipid components
fromthe gel phase formed during maturation drying to
the normal , hydrated liquid-crystalline state . Within a
short timeof rehydration, themembranes return to their
more stable configuration, at which time solute leakage
is curtailed ( reviewed by Bewley, 1997) .
Increasing periods of pre-imbibition of seeds prior
to thedehydrationof wholeseeds and axes, lead to de-
clining survival and fresh weight of seedlings ( Fig. 2a,
b) and progressively increasing rates of electrolyte
leakage (Fig. 2c) , showing that desiccation tolerance
442 云 南 植 物 研 究 31 卷
Fig . 4 Effects of different concentrations of GA andABA on ger-
mination of pea seeds . Seeds were germinated at 25℃ for 5 days .
All values are means±SD of three replicates of 40 seeds each
in imbibing pea seeds was gradually lost . The data in
Fig. 2 suggest that excised axes are more sensitive to
dehydration than whole seeds . However, the drying
treatments reduced thewater content of the axes to lev-
els lower than that of wholeseeds (0 . 05 g?g and 0 .1 g?
g, respectively) . The progressive loss of desiccation
tolerance during imbibition of pea seeds is similar to
findings for bean andmaize by Leprince et al . (1995)
and for mung bean ( Vigna radiata) by Sun (1999 ) .
Senaratna and McKersie ( 1983 ) showed that soybean
seeds imbibed for 36 h were able to tolerate drying to
20% but not to 10% moisture content ( fresh weight
basis) , andVertucci and Farrant ( 1995 ) have suggest-
ed that desiccation tolerance is a quantitative trait .
Figs. 3b-d indicate that desiccation sensitivity of pea
axes (on thebasis of the influence of water content on
survival, seedling fresh weight and rate of electrolyte
leakage) gradually increased with increasing imbibition
time of seeds . The water contents corresponding to
10% and 50% loss of viability more precisely express
the changes in desiccation sensitivity of imbibed seeds
(Fig. 3e) . This indicates that the change in the desic-
cation toleranceof imbibed pea seeds is not an“all-or-
nothing”response . These data are in agreement with
the concept of Vertucci and Farrant (1995) concerning
the quantitative natureof desiccation tolerance .
One of the earliest symptoms of injury following
dehydration is the loss of function or structure of either
the plasmalemmaor organellemembranes (McKersie et
al. , 1988) . Consequently, sensitivity to desiccationof
both orthodox and recalcitrant seed tissues has been
quantitatively expressed in terms of a‘critical water
content’determinedby a leakage assay ( Berjak et al. ,
1993; Vertucci et al. , 1993; Leprince et al. , 1995) .
However, each of these laboratories quantified electro-
lyte leakage differently, makingdirect comparisons dif-
ficult . The growth rate of seedlings, reflecting both
desiccation tolerance and subsequent activities of germ-
ination and?or growth, could also be used a parameter
to assess desiccation tolerance . Additionally, Pam-
menter et al . ( 1998 ) have pointed out that the re-
sponse to desiccation can depend on the rateof drying,
and consequently have questioned the concept of a
‘critical water content’. Therefore, it seems important
to include survival , leakage rate and growth rate of
seedling when discussing desiccation tolerance .
The marginal enhancement of germination of pea
seedsby 1 - 10μmol?L of GA (Fig.4) might be because
GA promoted the de novo synthesis ofα-amylase mRNA
andα-amylase (Bewley and Black, 1994) . The mecha-
nismby which GA improved desiccation tolerance of im-
bibing peaaxes (Table 1) may beindirectly viaα-amylase
production, which would result in sucrose being translo-
cated fromthe cotyledons to the axis . However, this as-
pect requires further study . Exogenous ABA treatment
markedly inhibitedgerminationof peaseeds (Fig. 4) , and
increased desiccation tolerance of axes (Table 1) . Pre-
vention of embryo radicle extension can be achieved by
incubatingmature seeds in a solutionof ABA .This inhi-
bition canoccur even when ABA is introduced lateduring
germination, anhour or so before radicleextension . ABA
prevention of radicle extension is caused by inhibition
cell wall loosening ( reviewed by Bewley, 1997) . The
mechanism by which ABA increased desiccation toler-
ance of imbibing peaaxes is not clear . It is known that
ABA can change thedesiccation toleranceof developing
immature seeds through late embryogenic abundant
(LEA) protein synthesis and prevention of precocious
germination (Kermode and Finch-Savage, 2002) .
Pea seeds progressively lost desiccation tolerance
with imbibition; this desiccation tolerance is a quanti-
tative feature which is mainly dependent on the germi-
nation activity of the seed or axis . Seeds germinated at
5423 期 SONG Song-Quan et al. : Temporal Pattern of Changes in Desiccation Tolerance during Imbibition of . . .
28 - 30℃ are more desiccation sensitive than at 22 -
23℃ (Sun, 1999) . The expression of desiccation-in-
duced damage is dependent on metabolism, which is a
function of oxygen concentration and temperature (Lep-
rince et al. , 1995) . Many characteristics of germinat-
ing orthodox seeds are similar to those of recalcitrant
seeds . These include desiccation sensitivity, high de-
gree of subcellular development and metabolic activity,
thedegreeof sensitivity being affected by developmen-
tal stage and drying environment, and desiccation inju-
ry possibly being associated with free radical-mediated
oxidativedamage (Song et al. , 2004; Berjak, 2006;
Berjak and Pammenter, 2008 ) . Germinating seeds of
different desiccation sensitivity and developmental stage
can be prepared using the same seed lot, andgermina-
tion activity and desiccation sensitivity of seeds can be
controlled by altering temperature, oxygen and water
content during imbibition . Consequently, desiccation
sensitive imbibing orthodox seeds can serve as amodel
system for the studies of seed recalcitrance .
Acknowledgements: We are grateful to the National Research
Foundation of South Africafor providingpostdoctoral fellowship to
Songquan Song .
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