AUXIN CONTROL OF LEAF ABSCISSION. I. EXPERIMENTS WITH ERVATAMIA DIVARICATA BURKILL.,
VAR. FLORE-PLENO
M.
AcHARYYA CHOUDHURI
Department
and
S.
K.
CHATTERJEE
if Botany, Burdwan University, Burdwan, West Bengal
SUMMARY
The present study aims to analyse the effects of auxins on
abscission of leaves of Ervatamia divaricata Burkill., var. flare-plena
(Apocynaceae), with special reference to the structure of different
auxins and their relation to abscission activity.
Auxins promote and inhibit abscission of leaves, the effect
is less pronounced in older leaves than in younger ones. The inhibitory effect of A-stage of leaf cannot be traced in B or C stages of
leaves of 3-node twigs. This indicates a lesser degree of auxin
control of abscission in older leaves. This is also confirmed in
experiments with 2-node twigs which are of different physiological
maturation.
The occurrence of two distinct physiological steps in the process
of abscission of A-stage of leaves has been established. Auxins inhibit
the first step and promote the second step of this stage of leaf.
Absence of promotion in older leaves (B and C stages) indicates that
the ageing of leaves has decreased the sensitivity of the second step
towards auxins.
The increasing requirement of induction period to cause 50
per cent abscission of the debladed petioles in spring and summer
months will suggest a possible correlation of the natural occurrence
of two steps of abscission with the metabolic activities of the leaves.
In winter months, weaker metabolic activities may lead to an earlier
completion of the first step, whereas in summer months the first step
of abscission of de bladed petioles is sufficiently prolonged. It appears,
that the second step of abscission gradually loses sensitivity towards
auxin when the leaves enter winter months of the year.
An
analysis of the results of the present study carried out in different
seasons will point out an involvement of auxins in the abscission
process during different seasons.
ABSCISSION OF LEAVES
63
The promotive effects of IPA and NOA may be due to the slow
transport rate; it is likely that these auxins reach the abscission zone
sufficiently later and cause promotion by their effects on the second step.
A distinct gradation in the activity of phenoxy auxins (e.g.,
2, 4-D; d-2, 4-DP; l-2, 4-DP; and 2, 4, 5-T) having different spatial
configurations, has been established by separately following their
inhibition and promotion effects on two steps of abscission. An
analysis of their effects points out that 2, 4-D is the strongest auxm
in the series whereas PAA is the weakest auxin.
INTRODUCTION
'
'
-
Abscission of leaves takes place by the formation of some
specialized cells in the region of abscission zone. The physiological basis of the formation of such cells is of special interest. The
role of endogenous auxins in this phenomenon has been critically
studied by a number of workers (Addicott and Lynch, 1951;
Biggs and Leopold, 1958; Jacobs, 1958).
Influence of amino acids on the abscission process has been
studied by Rubinstein and Leopold ( 1962). Recently Chatterjee
et al. (1965) and Chatterjee (1966) have stressed the significance
of auxin/amino acid balance on the natural ageing of leaves.
Attempts have also been made to isolate some abscission
accelerating substances (Carns et al., 1959; Hall et al., 1961 ;
Addicott, 1965).
The auxin control of Coleus leaf abscission involves the
occurrence of two distinct physiological steps (AcharyyaChaudhuri
and Chatterjee, 1966). The present study aims to analyse the
effect of various auxins on the abscission of Ervatamia divaricata
Burkill., var. jlore-pleno and their abscission activities have been
related to their structures. The abscission behaviour of leaves
in different seasons has also been studied.
MATERIALS AND METHODS
Healthy branches with fully expanded opposite decussate
leaves of Ervatamia divaricata Burkill., var.jlore-pleno (Apocynaceae)
were used in the abscission tests reported here.
The physiological maturation of leaves as reported by the
present authors, was also considered in the present study. 'A'
represents the apical or the youngest pair of leaves, 5-8 days
old; 'B' represents the middle pair of leaves, 15-18 days old;
and 'C' represents the third pair of leaves, 30-35 days old. In
64
M. ACHARYYA CHOUDHURI AND S. K. CHATTERJEE
addition, two-node twigs were also used in some of the experiments. The physiological age of the youngest pair (A,) in such
a twig varied from two to three days and that of the second
pair (B,) varied from 10-12 days.
The leaves were debladed and 1 em length of the petioles
was left on the main stem. The abscission times (time required
for 50 per cent of the de bladed petioles to abscise) for the de bladed
petioles of different nodes were separately recorded in each case.
The following auxins were used
Indole-acetic acid (IAA)
Indole-propionic acid (IPA)
a-Naphthalene acetic acid (NAA)
a-Naphthoxy acetic acid (NOA)
Phenoxy acetic acid (P AA)
2, 4-Dichlorophenoxy acetic acid (2, 4-D)
2, 4, 5-Trichlorophenoxy acetic acid (2, 4, 5-T)
d-2, 4-Dichlorophenoxy-2-propionic acid (d-2, 4-DP)
l-2, 4-Dichlorophenoxy-2-propionic acid (l-2, 4-DP)
Different concentrations of IAA, IPA, NAA and NOA
were applied in lanolin paste to the freshly cut ends of the
de bladed petioles of both three-node and two-node twigs and the
abscission times were recorded. In the experiments with phenoxy
auxins, only A-stage of leaf of three-node twigs was considered.
In the studies relating to the occurrence of two physiological steps in the process of abscission, the method described
by Rubinstein and Leopold (1963) and modified by the present
authors ( 1966) had been followed. In seasonal studies, an initially
inhibitory concentration of NAA (0·25 per cent) was applied
distally to the petioles after 12, 14, 16 ...... 36, 38 and 40 hours
of deblading. These experiments were conducted at different
times of the year.
In studies with phenoxy auxins, two sets of experiments
were performed. In the first set, 0 · 25 per cent of each of the
auxins was applied to the freshly cut ends ofthe petioles of A-stage
of leaf and later the source of auxins was removed by trimming
off 2 mm portion of the petioles after stipulated periods. In
the second set of experiments, the debladed petioles were subjected to a sufficiently prolonged induction period (36 hours) and
later 0 · 25 per cent of each ofthe auxins was applied distally for
2, 4, 8 and 12 hours. The abscission times were noted as usual.
The plants were maintained on a 12-hour photoperiod
at a temperature of 25°-27°C and the relative humidity of
l
65
ABSCISSION OF LEAVES
70-80 per cent. Abscission readings, i.e. the detachment of
the petioles from the main stem under slight pressure, were
recorded at 12-hour intervals and the time taken for 50 per cent
abscission was calculated. The experiments were replicated at
least thrice with comparable results.
RESULTS
The effects of IAA, IPA, NAA and NOA on abscission of
debladed petioles of three-node and two-node twigs are presented in Figs. la and 1b. Both IAA and NAA inhibited
abscission of debladed petioles of A-stage of leaf markedly.
Complete inhibition of abscission was obtained with IAA in
concentrations of 0 · 25 per cent and 1 · 0 per cent and with
1·0 per cent NAA only. In case of two-node twigs, abscission
of debladed petioles of both A, and B, stages of leaf was inhibited completely by IAA (0 · 25 per cent and 1 · 0 per cent) and by
NAA (1·0 per cent). Partial inhibition was obtained with
0·25 per cent NAA, 0·062 per cent of both IAA and NAA and
•
CEI
-80
• =
113111
..
'
1
. ,6Q
.. ....
+40
ABC A,B,
0
.. ..
..
ABC A,B, ABC A,B,
.\BCA,B,
-20
~~~
""·'· •oc :
~ '' 'oc
'"
IAA
-·40
I
-60
-
I ·O
-~._._____
0"25
-
O·OI5
o·062
I ·O
0•25
0·062
o·OI5
Concentration (per cent)
Fra. Ia. Effects of different concentrations ofiAA and NAA on abscission times
for 50 per cent of the debladed petioles of the two types of twigs. (Solid bars indicate
the abscission times for the petioles of three-node twigs and dotted bars indicate the
abscission times for two-node twigs.)
>14
Control times for 50 per cent abscission of th(debladed petioles of
A
B
C
A1
B1
-stage of leaf - ........... - ........... -
••••••••••• -
-
.• , •••••••• -
96 hours
88 hours
78 hours
98 hours
90 hours
66
7\L ACHARYYA CHOUDHURI AND S. K. CHATTERJEE
NOA
IPA
A,H ABC .~.ll,
0·25
1 ·U
·\BC A,ll, ABC
0·062
ABC .\,B, ABC .\,8,
.\,B.
I 0
l)
l'•
0·062
0 015
Concentration (per cent)
FIG. I b. Effects of different concentrations of IPA and NOA on abscission
for 50 per cent ofthc debladed petioles of the two types of twigs. (Solid bars for
three-node twigs and dotted bars for two-node twigs.)
with 0·015 per cent IAA and NAA in A., A, and B, stages of
leaves. The petioles of B and C stages of leaves of three-node
twigs showed little or no change in their abscission behaviour
in different treatments of these t\VO auxins.
Both IPA and NOA were effective in promoting abscission.
The promotion of abscission was obtained with all the concentrations used and the extent of promotion was maximum with
1·0 per cent IPA and NOA and minimum with 0·015 per cent
of these auxins in A, B and C stages of leaves of three-node twigs
and A, and B, stages of leaves of two-node twigs.
Table I shows the effects of phenoxy auxins having different
spatial configurations on the abscission of debladed petioles of
Table 1. Effects of dijferent concentrations ofphenoxy auxins of varying
configurations on abscission times for 50 per cent of the deb laded petioles
of.first node (A-stage of leaf)
Phenoxy auxins
___________
···-·--
l·O·;~
Chang<' in abscission time in hours over controls
at concentration,;
--··-·--------------·------------...
···•-'""'
·-----·----------..
- ..------···----- ' " · - · · - - - 40
38
--18
PAA
28
2, 4-D
1-0
2, +.:i-T
30
2!i
')6
-70
d-2, 4-Dl'
..,-32
-:-60
H7
4-DP ___ --~T:. 7'2 __ .
Control time for 50 per cent abscission of the de bladed petioles: 96 hr.
>+=Complete inhibition
L =Inhibition in hours over control
.....-Promotion in hours over control
..
1-_;
,
...
I'
~~
,,
tfll
l
I.
t
it
j
I
ABSCISSION OF LEAVES
~r
J
I
r
A-stage of leaves. 2, 4-D caused total inhibition of abscission
with all the concentrations tried, whereas d-2, 4-DP produced
total inhibition in 1 · 0 per cent and 0 · 25 per cent concentrations
only. Abscission was partially inhibited by l-2, 4-DP and 2,
4, 5-T at all the concentrations and was promoted by PAA.
The requirements of varying induction periods for 50
per cent promotion of abscission in A-stage ofleaves of three-node
twigs in different seasons are presented in Fig. 2a. The induction period was maximum in late summer (36 hours) and was
minimum in winter (18 hours) and was medium during spring
45
'/"
"'
6
1-;
I
,.
t
-
"0
.8
1-;
v
0..
::
·t 30
g
"0
.a
'0
"
~
;:l
15
::
~
~
Season
Fra. 2a. Seasonal variations in the requirement of induction period to promote
50 per cent abscission of the debladed petioles (A-stage) when an initially inhibitory
cone. of NAA (0· 25 per cent) was applied distally.
S, = Mean value of induction period during winter (Dec.-Feb.)
S2 =
spring (Mar.-May)
Ss =
late summer (June-Aug.).
(26 hours). When a constant induction period of 40 hours was
applied to the explants in different seasons, the extent of
promotion was maximum during late summer (66 hours) and
minimum during winter months-42 hours (Fig. 2b).
The effects of 0 · 25 per cent of different phenoxy auxins
when applied for different durations just after deblading,
J
'
67
68
!\1. ACHARYYA CHOt'DHURl AND S, K.
CHATTERJEE
0
U'l
'c 75
60
.g
~
45
\..,
::...
30
15
~
·•
Season
FIG. 2b. Seasonal variations in the extent of promotion of 50 per cent abscission
of the debladed petioles (A-stage) when an initially inhibitory cone. of NA.\
(0· 25 per cent) was applied distally after a constant induction period of 40 hours.
S
s21
s3
~ ;vit-an Yalue of the extent of promotion during winter
~'
. . spring
=
.. late summer.
on abscission of A-stage of leaves are shown in Table II.
2, 4-D showed a gradual increase in the extent of inhibition of
abscission which was complete in 12 and 16 hours of treatments.
d-2, 4-DP; l-2, 4-DP; and 2, 4, 5-T induced partial inhibition of
abscission in 8,12, and 16 hours of treatments and the extent of
inhibition successively decreased. P AA clearly promoted
abscission which increased with the duration of treatment.
Table III shows the effects of phenoxy auxins when applied
after 36 hours of induction period for different durations.
Abscission of A-stage ofleaves was promoted in all the treatments
and the effect was maximum with 2, 4-D and minimum with
PAA, the response to other auxins falling in between. d-2, 4-DP;
l-2, 4-DP; and 2,4, 5-T also produced promotion of abscission in
different treatments but the extent of promotion was successively
reduced.
G9
ABSCISSION O'F LEAVES
Table II. Effects of different phenoxy auxins on inhibition ( +) or
promotion ( - ) of 50 per cent abscission of the debladed petioles
(A-stage) over control during varying induction periods
Phenoxy auxins
Extent of inhibition ( +) or promotion ( --) after different
durations (hr) of auxin application (hr)
2
PAA
2, 4-D
2, 4, 5-T
d-2, 4-DP
l-2, 4-DP
4
-r8
-2
0
0
·--·~-~-
4
8
--20
+30
0
0
0
-28
+44
+24
+34
-t-26
---
12
30
-T -)-
+26
+54
+40
16
35
'-+
+30
+60
+48
- - - - - - -·--·------ - - - - -
Control times for 50% abscision of the debladed petioles of A-stage of leaf: 98 hr.
( + + indicates complete inhibition of abscission.)
Table III. Effects of different phenoxy auxins on promotion ( - ) of
50 per cent abscission of the debladed petioles (A-stage) over control
when applied for different duration after 36 hours of induction period
Phenoxy auxins
PAA
2, 4-D
2, 4, 5-T
d-2, 4-DP
1-2, 4-DP
Extent of promotion H when applied for different
durations (hr)
2
4
8
12
0
4
--36
10
-25
-17
12
56
-20
31
-20
-30
-66
-48
- 36
-33
10
0
-6
-4
Control times as indicated in Table II.
l
DISCUSSION
Both promotion and inhibition of abscission of leaves by
auxins were reported in bean by Gaur and Leopold ( 1955) and
Biggs and Leopold ( 1958), and in cotton by Addicott et al. ( 1955).
Chatterjee et al. ( 1955) analysed the effects of auxins on abscission of bean leaves of varying ages and established that auxin
effects were less pronounced in older leaves than in younger ones.
In the present study with three-node twigs, the inhibition of
abscission of A-stage of leaf could not be traced in B or C stages
of the same. This indicated the possibility of a lesser degree
of auxin control of abscission in older leaves. Experiments with
two-node twigs confirm this observation.
70
M. ACHARYYA CHOUDHURI AND S. K. CHATTERJEE
The presence of two distinct physiological steps in the
process of bean leaf abscission was shown by Rubinstein and
Leopold (1963). Similar studies extended by the present authors
in Coleus leaf abscission (1966) also indicated that the abscission
process involved two distinct physiological steps of which the
first step could be inhibited and the second step could be promoted
by auxins. In the present study with Ervatamia the occurrence
of similar steps in the process of abscission of debladed petioles
has been followed with NAA and other phenoxy auxins.
There was no inhibition of abscission of older leaves (B
or C stages) when auxins were applied on the cut ends of the
petioles. This will point out that the leaves are no longer in
the first step. On the contrary, the absence of distinct promotion
in these leaves will indicate that the leaves which were supposed
to be in the second step, have become less sensitive towards
auxins. It is likely that the natural process of ageing in these
leaves has decreased the sensitivity of the second step of abscission
towards auxins. This is in complete agreement with the observations of Chatterjee and Leopold ( 1955) with bean leaves where a
decrease in overall sensitivity of abscission process towards
auxins with increase in leaf age has been shown.
The inhibition of abscission of leaves was erased and there
was promotion when NAA was applied after a suitable induction period. The increasing requirement of this induction
period to cause 50 per cent abscission of the debladed petioles in
spring and summer months will suggest a possible correlation of
the natural occurrence of two steps of abscission with the metabolic activities of the leaves. In winter months, weaker metabolic
activities may lead to an earlier completion of the first step
whereas in summer months the first step of abscission of de bladed
petioles is sufficiently prolonged. By examining the extent of
promotion caused by the application of NAA after 40 hours of
induction period in different seasons, it appears that the second
step of abscission gradually loses sensitivity towards auxin when
the leaves enter in winter months of the year. Different authors
(Myers, 1940 and Kaushik, 1965) found that IAA was more
effective in inhibiting abscission in summer than in winter.
Their results might be interpreted in terms of weaker sensitivity
of the abscission process towards auxin in winter months. An
analysis of the results of the present study carried out in different
seasons will point out a similar involvement of auxins in the
abscission process during different seasons.
.\.BSCISSION OF LEA\'ES
iI
Th(' promotive effects of auxins, like IPA and NOA on
abscission of debladcd petioles and the complete absence of
inhibition in these cases lead to some interesting speculation
regarding the two step hypothesis of abscission. It can be
argued that due to the slow transport rate, these auxins reach the
abscission zone sufficiently later and cause promotion by their
dfects on the second step.
Among the five phenoxy compounds used in the present
investigation the effects caused by 2, 4-D; d-2, 4-DP; l-2, 4-DP;
and 2, 4, 5-T will suggest that all these substances possess considerable auxin activity and 2, 4-D is the strongest in the series.
A somewhat similar indication has been put forward by
Chatterjee et al. (1963) in their studies with bean leaf explants.
Abscission effects caused by P AA are, however, contradictory to
their observation in the sense that this weak auxin fails to
cause even slightest inhibition when applied during the first
step. On the other hand, PAA induces promotion and it can
be argued that a variety of other chemical agents (Gaur and
Leopold, 1955 and Biggs, 1957) which are not at all auxins cause
promotion of abscission. Weak auxin activity of PAA can,
however, be tested if it is applied in the second step of abscission.
When different phenoxy auxins are applied after 36 hours of
deblading 2, 4-D appears to have maximum auxin activity and
PAA the minimum. A distinct gradation in the activity of
phenoxy auxins having diflerent structural configurations ha:;
been, thus, established by separately following their inhibition
and promotion effects on two steps of abscission. An analysis
of their effects will point out that auxins are playin~ a very
significant role in the control of abscission of leaves.
ACKNOWLEDGEMENT
The authors wish to express their deep sense of gratitude
to Professor A. C. Leopold for going through the manuscript
and for his valuable suggestions. Sincere thanks are also due to
Professor P. N. Bhaduri, Head of thr. Department of Botany,
University of Burdwan . for providing laboratory and other
facilities and to Dr. S. C. Dutta, Lecturer in Botany, University
of Calcutta, for kindly supplying a small quantity nf 2,4,5-T
used in the present investigation.
REFERENCES
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53rd Ind. Sc. Gong. Asscn. I Abstract\. Part III, 252.
l'ror.
72
M. ACHARYYA CHOVDHVRI AND S. K. CHATTERJEE
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Addicott, F.T. (1965). Physiology of abscission. Handbuch der Pflanzenphysiologie, XV, Part II, 1094-1122.
- - - - a n d Lynch, R.S. (1951). Acceleration and retardation of abscission by
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Biggs, R.H. (1957). Physiological basis of abscission in plants. Doct. Thesis, Purdue
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- - - - and Leopold, A.C. (1958). Two-phase action of auxin on abscission.
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Carns, H.R., McMeans, J.C. and Addicott, F.T. (1959). An abscission accelerating
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- - - - and - - - - (1963). Auxin structure and abscission activity. Plant
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Rubinstein, B. and Leopold, A.C. (1962). Effects of amino-acids on bean leaf abscission. Plant Physiol., 37: 398-401.
- - - - and
(1963). An analysis of auxin control of abscission. Plant
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