Scientia Horticulturae, 14 (1981) 151--156
151
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
VEGETATIVE PROPAGATION OF SPICE-PLANTS: ROOT FORMATION
IN OREGANO STEM CUTTINGS
A. KUR IS ' , A. ALTMAN 2 and E. PUTIEVSKY'
' Division o f Medicinal and Spice Crops, Agricultural Research Organization, Newe Ya'ar
Regional Experiment Station, and 2 Department o f Horticulture, The Hebrew University
o f Jerusalem, Rehovot (Israel)
(Accepted for publication 3 June 1980)
ABSTRACT
Kuris, A., Altman, A. and Putievsky, E., 1981. Vegetative propagation of spice-plants:
root formation in oregano stem cuttings. Scientia Hortic., 1 4 : 1 5 1 - - 1 5 6 .
Stem cuttings of oregano (Origanum vulgate L.) with pre-formed roots responded to
a range of IBA and IAA concentrations with increased root emergence and without
damage to the cuttings. No similar promotion of rooting was observed in cuttings lacking
pre-formed root primordia. The data are discussed with respect to the differentiation and
elongation phases of adventitious root formation. Root emergence from oregano cuttings
and the continuity of stem vessels into the growing root are described.
INTRODUCTION
It is generally believed that stem cuttings with pre-formed (latent) root
primordia are less responsive to auxin rooting-treatments than are cuttings
which will induce "true" adventitious roots (Hartmann and Kester, 1975).
Moreover, it is claimed that high auxin concentrations may retard rooting
of cuttings with pre-formed primordia. Since the initiation phase of root
primordia is clearly distinguished from the subsequent elongation phase
(Altman, 1972; Haissig, 1974), this implies a difference in the sensitivity of
the 2 phases to auxin.
During a study of rooting and establishment of 3 Labiatae spice plants
(Kuris et al., 1980), it was noticed that this may not always be the case.
The existence of 2 types of stems in Origanum vulgate L., with or without
root primordia protruding from the epidermis, makes it a good experimental
material for assessing the effect of applied auxin on root initiation and
elongation phases in cuttings. In addition, such cuttings, where the sites of
future roots can easily be identified, lend themselves to 3-dimensional observations of the morphology of root development, using a scanning-electron
microscope.
0304--4238/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
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M A T E R I A L S AND METHODS
R o o t i n g e x p e r i m e n t s . - - Apical stem cuttings were removed in December from
1-year-old plants of oregano ( O r i g a n u m vulgare L.) growing in a dark brown
vertisol soil under prevailing o u t d o o r conditions (Newe Ya'ar Regional Experiment station). Two types of cuttings were used: those with visible r o o t
primordia on the surface of the cutting (i.e. " r o o t buds" protruding from
underneath the epidermis), and those without visible primordia. Uniform
cuttings of b o t h types, 12 cm in length, were prepared, the leaves were
removed from the lower half of the cuttings, and they were dipped for 4 s
in solutions of 500--4000 mg 1"1 IBA (~-indole butyric acid). The solution
covered the basal portion of the cuttings to a height of 4 cm. After treatment,
the cuttings were planted in propagation beds in a rooting-medium composed
of horticultural grade peat :perlite (1:1), and maintained in a heated greenhouse.
Each experimental treatment consisted of 5 completely randomized replicates, 8 cuttings in each replicate. At various time-intervals during the
entire rooting-period, 2 cuttings from each replicate (total 10 cuttings for
each treatment), were carefully removed from the medium and their roots
were counted, after which they were inserted back into the propagation beds.
At the end of a 69-day period, all cuttings were removed from the propagating-medium, rinsed in tap water, and the number of roots per cutting was
recorded. Primordia longer than 1 mm were c o u n t e d as roots. Due to the
light texture of the medium, no visible damage to roots occurred when cuttings were lifted.
Scanning-electron m i c r o s c o p y (SEM). - - Specimens were prepared from stem
cuttings with pre-formed roots at various stages of root development. Either
cross-sections, or longitudinal sections, or stems where the cortex was carefully peeled-off, were fixed overnight in a solution of 5% glutaraldehyde in
0.1 M phosphate buffer (pH 7.0), rinsed twice in buffer, and dehydrated in
ethanol series. Specimens were air-dried, affixed to an aluminum stub, coated
with gold (Model 1 2 E 6 / 1 5 7 3 Edwards Vacuum Ltd.) and studied using a
Cambridge Stereoscan (Model 180) at 30 KV.
RESULTS
The pattern of rooting over a period of 69 days was similar in the 2 types
of cuttings, with a considerable increase in the number of roots during the
initial 4-week period (Table I). Nevertheless, while no significant effect of
hormonal treatment on r o o t formation in cuttings devoid of " r o o t buds"
was observed, cuttings with pre-formed roots responded to the relatively
high IBA and IAA concentrations with increased r o o t emergence. No damage
to r o o t primordia, which were present at the time of treatment, was observed
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even at 4000 mg 1"1 IBA (not in the table). The promoting effect of the
hormonal treatments on the number of roots in cuttings with pre-formed
roots was already evident within 7 days of planting.
TABLE I
The effect of applied I A A or I B A on r o o t f o r m a t i o n ( n u m b e r o f roots per cutting) in
oregano stem cuttings with or w i t h o u t pre-formed roots. The standard error of the means
is indicated
Treatment
Days after planting
7
27
69
3.5±1.4
11.4±2.6
9.3+_2.1
11.1+_2.0
22.5±2.7
30.0±3.1
24.5-+2.6
31.5±2.9
32.5+3.0
41.0+3.2
40.3+_2.8
42.0±3.1
4.2±1.5
3.5±1.3
5.1-+1.6
1.7±0.8
18.5+_2.3
21.0+_2.0
18.1+_1.9
21.5±2.5
33.8+_3.1
33.8±3.6
34.3±3.8
36.0±3.0
Cuttings with pre-formed roots
Control
IBA, 1000 mg l "
IAA, 1000 m g l "
IAA, 2000 mg 1"1
Cuttings devoid of pre-formed roots
Control
IBA, 1000 mg 1"~
IAA, 500 mg l "1
IAA, 2000 mg l "
The detailed anatomy of root formation in cuttings of many species being
well documented, the present SEM study underlines some morphological
features which accompany the growth, emergence and functions of adventitious roots. R o o t buds (otherwise known as pre-formed root primordia) are
clearly seen on the surface of oregano stems, some being present before
taking the cuttings (Fig. 1A). The initial growth of primordia is characterized
by a cap-like structure on the stem surface, covered b y a sheath of several
layers of epidermis and adjacent cortex cells which was ruptured (Fig. 1B).
Further elongation of the r o o t is accompanied by a complete rupture of the
cortical sheath, resulting in r o o t emergence (Fig. 1C). The origin of roots
from underneath the outermost region of the newly-formed xylem, creating
a crater-like structure (which can be seen only after peeling off the bark) is
clearly evident, as are the vessel continuity and connections between the
stem and the elongating roots (Fig. 1D, E). The fully developed root, upon
emerging from the cutting, is characterized b y a well-defined root cap. A
transverse section of the stem at the region of r o o t development reveals some
details on the c o m p o n e n t tissues (Fig. 1F).
DISCUSSION
Pre-formed adventitious roots ( " r o o t buds") are distinguished from " t r u e "
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adventitious roots (otherwise referred to as "induced") by their being developed
prior to cutting removal, i.e. during stem formation (Haissig, 1974; Hartmann
and Kester, 1975). Since auxin concentrations which induce root primordia
formation on cuttings are usually supra-optimal for root elongation (Audus,
1959; Hess, 1969; Went and Thimann, 1937), standard hormonal treatment
of cuttings with pre-formed roots often results in no response or in damage.
The present data indicate that high concentrations of IBA or IAA may, in
fact, promote rooting of cuttings with pre-formed roots with no similar
promotion in cuttings of the same plant which lack root buds (Table I). This
may be a special response of oregano (or of Labiatae in general). Alternatively,
it is possible that hormone treatment induces a number of otherwise too
immature primordia to develop without damage to existing ones. The latter
seems more plausible since no clear-cut distinction between the phase of
primordia differentiation and the elongation phase, and hence their response
to auxin concentrations, can be made. This is further supported by the
findings that the promoting effect of IBA and IAA was most evident at the
initial rooting-period, when the developing primordia are still immature
(Table I), and by the gradual development and emergence of the roots (Fig. 1).
Similar findings, in willow cuttings, indicate that auxins do not induce primordium initiation, but rather enhance primordium development (Haissig,
1972).
Several investigators have claimed that rooting of stem cuttings of certain
plant species can be hindered by a highly sclerified bark which presents a
mechanical barrier to root emergence (Beakbane, 1961). Thus, the ease with
which oregano cuttings root, may be related to the (presumably) easy penetration of the bark, as judged from the relatively thin sheath of cortex tissues
above the primordium, and from the mode of penetration, which resembles
a mechanical rupture rather than enzymic degradation (Fig. 1).
Fig. 1. R o o t f o r m a t i o n and emergence in oregano stem cuttings - - a s c a n n i n g - e l e c t r o n
m i c r o s c o p e study. Specimens were photographed at the linear scale given for each
picture ( 3 0 - - 1 0 0 0 # m ) . A. Surface view of an o r e g a n o s t e m cutting with 2 e m e r g i n g r o o t s
( R ) ( a r r o w s ) and numerous e p i d e r m a l hairs. B. L o n g i t u d i n a l s e c t i o n o f s t e m (S; b o t t o m )
w i t h a n i n t a c t r o o t p r i m o r d i u m ( R ) c o v e r e d b y a r u p t u r e d e p i d e r m a l s h e a t h . H = epidermal hair. C. A n e m e r g i n g r o o t ( R ) w i t h t h e r o o t c a p visible just above the surface o f t h e
s t e m ; a flap o f b a r k (B), ruptured and pushed aside by the f o r m i n g r o o t , is seen t o its
right. D. S u r f a c e view o f the inner part o f a stem cutting, a f t e r r e m o v a l of t h e b a r k ; a
depression in the external surface of the w o o d (W) w i t h an emerging r o o t , b r o k e n a t its
tip. E. S a m e as D, b u t a fully developed intact r o o t w i t h a r o o t cap in place. F. Transverse
s e c t i o n o f a s t e m . P = p i t h ; W = w o o d x y l e m ; C = c o l l e n c h y m a ; B = bark.
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