PHYSIOL. PLANT. 51: 39-44. Copenhagen 1981
Stem pithiness in tomato plants: The effect of water stress and
the role of abscisic acid
Beny Aloni and Elan Pressman
Aloni, B. and Pressman. E. 1981. Stem pithiness in tomato plants: The effect of
water stress and the role of abscisic acid. _ Physiol. Plant. 51: 39~4.
The pith parerK'h)ma in tomato (LYf{)/lt'TJin/l1l ('SUI/I'Il/II/II) ,telllS "<IS found to be
disrupted in re~pon~ to water stres~ (~tem pithiness), The proces~ of the degr,ldation
~tar(s at the upper p,lrt of the stem and proceed~ do"nwMd, iI, th(' stress b prolonged. The damage to the stem tissue "a, found to be irreversible upon reh)dration
of the strel>sed plal11s. Scanning electron microscopy revealed that the protoplast of
Ihc ,Iffected cells i, dbrupted first. followed b) degradation of Ihe cell ",Ill
Application of ;lb~ci~ic acid (ABA) induced pit hines, in non-,tre~sed planls and
abo enhanced the effeci of a short period of dch)dration. Kinetin. ,L1though e;lu~ing
severe wilting, did not induce pithiness. However, when ~iven after a short period of
"ater stre~~ or wilhin the period of ~tres~, kinetin enhanced pithine~~ development.
In the course of the stres~. ABA levels in the upfH.'r part of thc ~tern and in the
)oung leaves "ere higher than the le\'el' found m the lower part~ of Ihe 1'1,1111 ,hoot.
The increase in ABA leveb "a, detected bdore an) ~ign of pithines~.
II is suggested that ABA might be the triggering agent of Ihe cellular degr,ldiltion
process iniliated b) ",lIer stre~s,
Ke)·word~. "!om<lto stem pithines~ - Pith p,lrenchym,l- Ab,cl~ic ,Kid - Kinetin.
11. Atolli lIlId £/11/1 Prl'sm/{m. A!::riwllllrat l?l'search Or);lIlIi;:III;UIl. The Voklmi
Celller. 1/lI1II/1I1' of Field lIlId Gurdt'll CropJ. Oil"iJiOI! of VI'!::I'/ahk Crop~. lle/ /Jugal/.
l.frat'!.
Introduction
Pithiness in stems of tomato plants grown in the open
field. is a common disorder associated with extreme environmental conditions rather than with pathological
factors (Rylski etal. 1977). A similar disorder is known
to occur in celery petioles (Sayer 1929). In both celery
petioles and tomato stems, the pith parenchyma undergoes rapid degradation which is accompanied by the
accumulation of large air space which then combine to
form large holes (Easu 1963). Petiole pithiness in celery leaves developed rapidly after the plants were subjected to a short period of water stress (Aloni and
Pressman 1979). It could also be induced in celery
leaves by treatment with abscisic acid solutions. It was
suggested that ABA triggers the induction of pithiness.
The morphological similarities between stem pithness
in tomato i.llld petiole pithiness in celery suggest that
environmental stresses might also be the cause of the
tomato disorder.
In the pre~ellt stud) we investigated the effect of water deprivation on the morpholog) of the parenchyma
lis~ue in the ~lcm of tomato pJanl~. We al~o studied the
effect of cxogenou~l) applied ABA and kinetin on stem
ti~suc degradation. Changes in endogenous-free ABA
levels in relation to pithness development were also
determined.
Ahlm'I'WIIOIIS:
ABA.
ab~ci~ic
acid:
PEG,
polyethylene
gl}cerol.
Materials and methods
l'lanl material and growlh conditions
Seeds of tomato (Lycopersicwn esculelllu/1/ Mill.) cv.
Hosen-Eilon were germinated in a greenhouse with day
and night temperatures of 24°C and 18°C, respectively.
After 3 weeks, the seedlings were planted in 10 liters of
Received 27 May. 1980; revised 5 August, 1980
Phj'.iol,
PI~nl.
51 19H1
0031_9317/81/010039-06 $03.00/0 © 1981 Physiologia Plant arum
39
inert volcanic a~h grov.lh medium in pot!>. The plant!>
v.ere !>upplied dail) N:P:K (20:20:20) fertilizer dissol\ed in WI' "':.tter. Dunng the plants' grov.th. side shoots
v.ere remmed :.Ind the main stem ;llIov.ed to elongate
\ertie:.tll~ untIl il v.as 150 em long. at v.hieh time the
experiment v.a~ begun. Fruit!> v.ere 310;0 remO\ed after
fruit !>CI.
\\ :.ttcr !>tre!>~ v. a" applied to group!> of fi\>e plants each.
The planh v.l're depri\cd of v.;tlcr for \arious period~.
At the cnd of each period. thc extcnt 10 "hieh the plant
had been "trC~'oCd "as dctcrmined b) me;l!>urement of
"ater potential of the fir,t expanded leaf "ith a
chamber homh (Seholander 1'/ Ill. IY6~).
for experiment" "ith nutrient .!>Olutions, tomato
seedling... "'cre tran...ferred to I-liter blad, containers
(one \Ccdhng per container) filled "j,h Hoagland's solUlion. 1 he "ater stre!>~ cxperirnenl" ",ere inilialed whcn
the plalll" "'cre flO ern high. For application of water
!'>ITe...... \\e u"cd pol)eth) lcneglycol (PEG 60()(). which
W;I" added 10 lhe nutrient ~olution ...0 thaI an osmOlic
potential of -2.0 bah was maillltltiled. Growlh subsl;lIlce" "'ere applied direClI) to the nUlrienl solutions.
In all Ille trC;ltment". lhc nutrient wlutions "ere \\C1I
iler;lted. WOller stress "as also applied b~ "ithholding
the nutrienl ...,Iution from the root ,,~stem for 20 h.
Pithine"" determination
The degree of plthine..." \\,1" al,.'oC....~ed III a longiludinal
"ection of ll\lernOOC" of the m,lin M .... m and rated from tl
Ihrough 10. Jew lIldieating InlaCt Il::>sue and len indicating "'lrongl~ affected tl~UC. For a more precise deternUTlatl0n of pit hines::> iniliation. 2·mm slice::> "ere
tnlTl ferred 10 I (HI ml of distilled "'ater. Flot:ltion of the
"lie mtlicat . . d that accumulation of air ...paces v.as inilmll'd. v.herca......in\..ing of lhl' "hce" rndicillcd no on"et
of t""lle <kgradalion.
Sunning eteelron
ABA delt'rnlinalion
Endogcllou", free ABA "as extracted b) the method
described by Blumenfeld and Gazit (1970) with an addilional TLC separation. The quantil) of ABA was determined b) gas chromatography (Mizrahi e/ at. 1972).
40
The effeel of "aler deprh'alion
The onset of degnldation in the parench}matous tissue
in tomato stems "as obsened. in most experiments,
afler about 4 da~s of water depri\<uion. "hen lhe "<ller
polenlial of the first full} expanded leaf declined from
ahout -Ul bars in the non-stress conditions 10 belween
-8.0 and -J().O bars. At Ihis lime onl) Ihe young leaves
responded to the stress b} "ihing. "hile the older leaves
maintained full turgor. Plants which "'ere deprived of
'" ater for periods of 2. 4. 6. 8. I() and 12 days could
rapidly regain turgor in their leaves. However. the se·
verity of the induced pithiness in the stem was increased
a... the deh}dnlliol1 period was prolonged (Fig. I). The
damage 10 the Slem which had been caused by various
period . . of "ater stress could not be reversed if lhe
plants were rew,llered ;lIld allowed to grow for an additional 30 days. A ... a resuh of WOller Slress, pithiness was
iniliated h) whitening of the pilh region in the upper
pan of the stem. close to Ihe plant's apex. A:> the stress
"a~ prolonged. Ihe damage proceeded down the stem
and concomitantl~ the "'hite areas became hollowed.
We also ob"ened that 4H h of watcr deprivation
cau~d il decreilsc of 2.0 bars in "ater potcllIiaL but no
sign of pithines~ could be detccted al the end of this
sires.. period. Vi~ible pithiness dcveloped. however. af·
ter the planl" "ere reh~drated and gro"lh continued.
Scanning electron
microscop~
Scanning electron micrographs of transection of pilh)
tomato stems re\ealed thai the cells adjacent to the
hollo"ed core had reached \arious degrees of degradatinn (Fig. 2).
Higher magnification of Ihe deteriorating region and
of a comparable region in an inlact slem are shown in
Fig. 3. The micrograph" show lhal while the paren-
microseop~
One·cm·\ong ~tem ...lice....... ere embedded in 2.5%
glularaldeh}<k for ~S h followed h) 2 h of poslfixalion
In I"" o... nllum tClraoxide according 10 Yasuzumi and
SugIhara (I Y(4). The slice" "ere rin-.cd lhoroughl~.
tran..fl'rred to lIlcrc;l\lIlg conc.... ntralion.. of acetone and
dch~dr;llcd h~ ('firic.. 1 painl dr}lIlg The deh}drated
..!tee.. " .... re Cflh,,-'>Cctloned "ilh a SliMp razor blade and
"ere further processed for scanning electron microscopic observation. A stereoscan 180 scanning electron
micro~pc "a<i used.
Fre~
RcslJll!>
o
2
4
6
8
10
12
4.7
6.3
8.3
9.1
12.5
13.3
15.1
Fig. I. The effccl of dU!"""dlion of ",alcr stress on !eaf waler
polcnllal and stem pithiness in lomato planls. The numerals
aoovc the stem sections indicate lhe number of days during
.... hleh the plant had been slressed. The numerals below jndie,lIe lhe .... ater potenlial (-baa). measured in the leaves at the
indica led limes.
Physior.
Plant. SI.
1981
Fig. 2. Cross-l>Cclion of tomato stem vle.... ed In a scanning
electron microscope (x22). The hollo", pith and the adJacelll
di~rupled cells ,lfe seen on the lo.... er righl ~ide.
eh) malOu~ cells in lhe inlaCI stem contain large numbers
of organelles. mostly chloropJast~. the "'llter-slressaffccled cells arc essentially dc'oid of the~ struclures.
Allhough the cell walls arc slill defined. the~ appear to
be less rigid lhan the walls of celts from an intact slem.
medium for !>even more days. Mild pithiness developed
ani) in the upper internodes of the strc!>Scd plants. while
lhe control plants wcre not Ilffeeled at all. Howevcr.
"hen 2 x lo-5M ABA was included in lhe stress sof~
ution. pithiness development along the enlire Slem "'as
greatl) enhanced. ABA gi'en at the So1me concentration
for ~ h to non*stressed plants increased pilhiness bUI
nOllO the same eXlent as did ABA in combination wilh
deh)dralion (Tab. I). Plants stressed b) PEG 6UOO
(-1.0 bars) which comained :2 x !O-5M kinetin. exhibited sc\ ere pithiness along thc entirc slcm. Pithiness
development under these condition.. "113 accompanied
b) rapid dcsiccillion of the shoot .... hiC"h ",as not relievcd
after lhe plams had been Iransferred 10 a PEG-pluskinelin·frce medium. In order to investiga1e further thc
intcrllclion between waler stress. ABA. kinelin and
Till>. I. '1 he effcet of combination of .... atcr Mrc~s '" itll ABA or
"itll linctin on pithiness dC\'c!opmellt along ,tcm~ of IOmatn
plant~. PJ(lIlb grown In HOllgJand's solUlion .... ere stressed b)
PEG 6000 l-1.tl bars) in the prcsence of 2 x I n-sM ABA or
I.lOctlO and Ihen ",ere transferred 10 non-,lresslnll Hoagl:md's
solullon After.5 da}s the !ilem~ ",ere exanuned for pilhmess.
Stem
IOlernodc no,
Pilhincss ckgrec (rallng 0-10)
PEG
Effeet or ABA llnd kinetin
Water stress in planls is associated "'ilh profound
changes in the endogenous le,e1!> of ABA and c)toJ..inins (Uvnt and Vaadia 1972). Therefore. ",e imeSligated a possible interaction bct"'een ABA and kinetin
and the process of parenchyma degradation in stressed
plan IS.
Young tomato plants (60 em high) werc dehydrated
for 48 h b) PEG 6000 in Hoagaland's solution so thai
an osmotic potential of -2.0 ban. was ohlained. Then
the) were allowed to conlinue growlh in a PEG-free
lop
,
I
3
,
•
4
7
8
Ba'iC
9
1(\
Conlrol ABA KIOCllO
PEG
,,
0
0
0
0
0
0
0
0
0
0
0
0
0
,,
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
••
0
0
0
0
0
U
PEG
+
+
ABA kinclin
,
J()
10
10
10
J()
3
0
0
0
,
III
10
III
J()
•
8
8
8
8
_
Fig 3 Scannmg electron micrographs of cross*scclions of 10malO siems (x235). lotaci slem is seen on the rig~t side: T~e
parenchyma cells cootalO a large number or organelles. mosll) chloroplasls. and some crystals Cell~ r~om the ....' hllcmng region In
pIthy Slem (len photograph) are de"old or organelles The cell walls of Ihesc cells appeared Jess rigid than those of the control
cells.
PhyWol Planl
!il 198t
41
pithiness, plants grown in Hoagland's solution were deprived of the nutrient medium for 20 h, after which a
water potential of -I 7 bars was measured in their upper
leaves. At this lime. no pithiness could be observed in
the slem of the stressed plams. After the stress period.
the plants' rOOIS were placed for five more days in
Hoagland's solution which contained 2 x JO-sM ABA,
or kinetin, or none. Following the stress, during 5 days
of rehydration, 38% of the internodes in the plants'
stems were pithy. However, when rehydration was applied in the presence of ABA, the siems were almost
entirely pithy (Tab. 2). Plants which were treated with
kinetin for 6 days, wilted severely bUI showed no sign of
pithiness. However, the parenchyma degradation was
greatly enhanced in plants which were treated for 5 days
with kinetin after being stressed for 20 h (Tab. 2).
If ABA is part of the inducing mechanism of the
parenchyma degradation, thcn it is expected that following the onset of water stress. levcls of ABA at the
upper part of the plant would be higher than in the
lower parI. Tab. 3 show:. that this appears to be (he
ca~. Furthermorc. the rise in free ABA level significanti) preceded an) sign of pithiness initiation in both
the upper and lower stem internodes. Based on the
same logic. it was anticipated that the pith region of thc
stem \loould produce higher levels of ABA Ihan thc
adjacent peripheral cells. In order 10 cxamine this as·
sumption. sections of the four upper internodes \loere
preparcd from plams \lohich \locrc \loater·stresscd for 2
Tab. 2. Thc effect of water SlTess. and rch)dration in the presence of ABA or kinetin. on pithmess in tomato sIems. Plants
gro~n m t-Ioagland's solution ""ere stressed b) ""ithholding the
nutrient solullon for 20 h and then reh)drated with either 2 x
1U-$,\J ABA or kinetin, given to the rOOt system. Mean ± SE.
Trcalmenl
Hollo"" internodes, %
Control
Water ~tres., ..... HO:lgland's solution
ABA
Kinetin
Waler stress
Waler slres~
Hoagland's + ABA
Hoagland's + kinetin
o
38±4
20± I
o
89±7
82±3
Tab. 3 The effect of ~ater stress on the free ABA levels in
upper (}oung) 1ea\es and lo""cr (old) Iea\es of tomato plants.
Water stress was applied by ~ithholdmg water from the plants
durmg the mdlcated periods. after y,hich the upper fully expanded 1ea\esand the lower lea\es y,ere sampled. Mean ± SE.
Days in ""aler
SIr~SS
o
•6
42
Young lea\'es
Old leaves
97.1 ± 10.3
417.6±30,1
916.0±57.3
71.8± 10.3
160.0±15.5
265.0± 7.8
Tab. 4. Free ABA levels in the pith parenchyma and its
neighbouring tissue in lhc Slem of water-stressed IOmalO
plants. Water stress was applied by withholding water from the
plants during the indicated periods. arler which Ihe middle
internodes wcre sampled.
Da}"s in water stress
ABA. nglg dry w\.
Pith parench)ma Surrounding tissue
,o
•
.95
276
814
879
1468
271
and -4 d:t)'s before pithiness could be observed. The sections were separated into the outer ring and Ihe inner
parenchymatous cylinder and free ABA levels were
determined in both. Tab. 4 shows that on a dry weight
basis, the levcls of ABA were significantly higher in the
ouler tissue than in the pith cylinder, implying that the
cells, which produce more ABA arc not necessarily the
more scnsitive to the hormonc.
Distussion
The parench) rna cells in the tomato stem were found to
be most sensitive 10 aler deprivalion.
Microscopic obser ations indicated thai these cells.
.... hich are localed al the core of the stem, were rapidly
disrupted in response to a small decrease in waler potentiaL .... hereas olher cell t) pes \loere less sensitive. The
disruption starts at Ihe pith of the stem and as Ihe stress
is prolongcd the surrounding cells are also affected.
Funher support for the supposition of hypersensitivity of the stem parenchymatous cells to water stress
is that relatively to other cell types, they lost more
readily their stainability with 2:3:5-triphenyl·tetrazolium chloride (n'C) in the course of dehydmtion
(Aloni, unpublished data). Fellows and Boyer (1978)
have also indicated that in sunOower leaves the parenchymatous cells were disrupt cd in the course of watcr
stres!'>, whereas the adjacent sieve clements and companion cells were more preserved. These authors have
also indicated thai irreversible disruption of the cells
occurs when breakage of tonoplast and plasmalemma
takes place. They suggested that this breakage may be a
ph)sical disruption rather than a biochemical degradation. With the melhodology used here, we could not
distinguish breakage of membranous structures: however. il is clear that cell organelles as .....ell as other cellular components are disrupted prior to the degradation
of the cell wall.
The stem appeared not to be homogeneous in respect
10 its sensitivily 10 waler stress. The upper internodes
are the first to be affecled, whereas in Ihc lower internodes pithiness developed only afler a significant decrease in the leaves' waler potential.
The nature of the waler-Slress-related degradation.
Ph)'1io1. Plam.
~l.
t981
be it either a physical or biochemical event. is markedly
affected by ABA or kinetin. Application of ABA
through the root system induces stem pithiness without
any effeel on the leaf turgor. suggesting that ABA might
be part of the disruption mechanism.
ABA and dehydration seem to have a synergistic effecI. indicating that ABA is more efficient in inducing
cell degradation when the tissue is dehydrated (Tab. 2).
A similar relationship has been found for ABA and
various plant stressed in the induction of senescence
(Osborne 1968, Smith el al. 1968). It has been proposed that ABA exerts its senescing effect only in
stressed plants or in plants in which senescence has already started (Lindoo and Noodcn 1978). In our case,
cellular degradation was induced by ABA in nonstressed plants. suggesting that stress is not a prerequisile for ABA action, but that under stress conditions
ABA acts more efficiently.
The site of ABA synthesis is the cell plastids, as proposed by Milborrow (1978). The rate of the ABA elevation was found to be proportional to the severity of
the stress. Therefore, it is expected that the upper leaves
as well as the upper stem internodes would produce
higher levels of ABA following water stress, since these
tissues are the first to wilt upon dehydration. Indeed. it
is shown that the ABA level in the upper part of the
plant is higher than that of the lower parts (Tab. 3).
However. the rise in ABA in the pith parenchymatous
cells in the course of dehydration was found to be
slower than that of the surrounding neighbouring cells.
whereas the latter were more sensiti\c to the stress. The
difference in ABA production can be attributed to
either fewer or less aelive plastids in these cells. On the
other hand. the difference in the stress·related cellular
disruption might be more related to the higher sensitivity of the parenchymatous cells to the combined
effect of ABA and desiccation.
The effect of kinetin is marc difficull to comprehend.
Application of kinetin through the roOt system had a
rapid wilting effect. Neverthclcs~. the sevcrely wilted
plant~ were entirely unaffected with pithiness (Tab. I).
On the other hand. if applied after the degflldiltion process has been induced b} "ater stress. kinetin could not
eliminate pithiness. but un expect cd!} enhanced its development (Tab. 2). In recent experiments (unpublished) \\e hil\e observed that in celef) plants. kinetin
(at concentrations of IO- s and 10-c.M. gi\en either to
the roots of intact plants or to del ached lea\es) pre\ented petiolc pithiness dc\elopment induced b} ABA
but "'a!'> inefficient in protecting the plants or the detached lea"cs agalllst petiole pithine!'>S induced b} \\ater
stre~. This suggests that kinetin rna} direell} protect
the parench}ma tissue and does nOl exert ils effeci b}
indireci innuence on the root functions.
In bolh tomalO and celcf)' plants. the effeci of kinetin
may be attributed 10 its inability to protect against the
assumed combined effect of ABA and desiccation.
The nature of ABA and kinetin interactions with the
Ph)-..ol. PlanL
~1.
1981
process of thc parenchymatous tissue degradation is
now under further investigation. It should be stated that
the pith of man} plants develops stem hollowness during their natural grmHh (El.klu 1965). Recently.
Mikesell and Schroeder (1980) described the anatomical featurcs of stem hollo.....ness. classified as chambered
pith in PllYlOlacca americana L. Thc) suggested that the
disruption of the pith cells is achieved by l}sigenous and
schizogenous mechanisms.
Whether the "ater stress induced pithiness in tomato
~tems and in ecler~ petioles and the naturall~ occurring
hol1o\\ne~ arc initiated b~ the l.klmc mechanism and are
similarJ~ affected b~ ABA and ~lIletin is not kno.....n.
Ne\'erthele~s. it is expected that naturally hollo\\ plants.
gro\\ n in their natural habitat arc often cxposed 10 various environmental stresses which may affect the process of their pith pilrench) rna degradation.
AC~J10l\'1t'(lg('lIu'm~ -
Wc wish lO thHnk Mrs. Rachel Sh.tked
for her ~killful llssi~t:lnce. and Mrs. Rivk,ll Lev for her help in
carr)ing out the M.'<lnning electron micro!'oC0py.
This \\ork \\'I~ supported b) a gr:mt from the Umtcd StatesIsrael t\gricultural Re!'oCarch and De\l'lopmcnt Fund (BARD).
Contribution from tht' Agricultural Re"Carch Org'lIlizmion.
The Volcani Center. No. 1IJ2-E. l'JI$O Series.
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