Plant and Soil 98, 53-66 (1987). 9 1987 Martinus Nijhoff Publishers, Dordrecht. Printed in the Netherlands.
Ms. 6883
The allelopathic potential of Coridothymus capitatus L. (Labiatae). Preliminary studies on the roles of the shrub in the inhibition of annuals germination and/or to promote allelopathically active actinomycetes DANIELLA A. KATZ, BARUCH SNEH and JACOB FRIEDMAN Department of Botany and (B. Sneh) Institute for Nature Conservation Research, The George S. Wise Faculty for L f e Sciences, Tel Aviv University, Ramat Aviv 69978, Israel Received 29 May 1986. Revised July 1986
Key words Actinomycetes Essential oils
Allelopathy
Aromatic shrubs
Coridothymus capitatus
Summary Suppression of annuals at various intensities was observed around some shrubs of Coridothymus capitatus growing on kurkar formation in the coastal hills of Israel. The phenomenon was clearly observed as annuals-free belts of 15 20 cm around 'aggressive' shrubs. Quantitatively, density of annuals decreased by 16 fold in the annual-free belts as compared to a distance of 60-80 cm from the canopies of the shrubs. Their dry matter was decreased by 5.4 fold around the shrubs. Suppression rate of emergence of planted seeds of annuals (Plantago psyllium and Erucaria hispanica) early in the season was 45% higher around 'aggressive' C. capitatus than that around 'non-aggressive' ones. In the laboratory, seed germination of the annuals was strongly suppressed by diffusates and volatiles from shoots, as well as from their water extracts and their essential oils. Incubation of fresh shoots of C. capitatus in soil collected from around 'non-aggressive' shrubs, for 7 days, increased population levels of actinomycetes by 9.6 fold and by 36.7 fold when soil was collected from around 'aggressive' shrubs. Isolates of some soil-borne actinomycetes inhibited germination of the test plants Lactuca sativa and Anastatica hierochuntica on agar plates (4-98%). The preliminary results indicate a possible synergistic inhibitory effect induced by essential oils of the aromatic shrub and the phytotoxic activity of actinomycetes.
Introduction
C. capitatus is a semi-dwarf aromatic shrub, common in the Meditteranean phytogeographical territory of Israel. A blend of dried leaves of this shrub, of Majorana syriaca and other additives constitute the traditional Mediterranean spice known as Za'atar (in Arabic). The major essential oils are produced by the leaves (1-2% of leaf fresh weight), and contain: thymol (39.3%), 7-terpinene (19.4%), carvacrol (12.7%) and smaller proportions of p-cymene (6%), caryophyllene (5%), as well as others (< 3%) 27. Allelopathic effects of volatile essential oils of aromatic plants have already been documented 11,12,21,22,23,24, Observations on the annual vegetation on shallow, brown-red, sandy soil on the 'kurkar rock' formation (sand dunes conglomerated with 53
54
KATZ, SNEH A N D F R I E D M A N
CaCO3) 26, revealed that in various regions along the coastal belt of Israel, adult shrubs of C. capitatus demonstrate extremely variable effects of suppression of adjacent annuals. Around so-called 'aggressive' shrubs, annual-free belts are clearly evident in close proximity (10-30cm) to their canopies, whereas, around others, 'non-aggressive shrubs', dense stands of annuals may exist near the canopies. Direct allelopathic effects induced by various aromatic shrubs have already been suggested to occur in the semi-arid regions of California 22 as well as in the Negev desert of Israel 1~ . Volatile essential oils released from shoots of Salvia leucophylla L. (Labiatae), of Artemisia californica (Compositae), or of A. herba-alba, were found to inhibit germination of
various annuals which were rarely present in the close vicinity of those shrubs. In addition to the direct allelopathic effects induced by shrubs of Adenostoma fasciculatum L. 2~ Kaminski 17 has demonstrated that the shrubs stimulated the development of an unidentified micro-organism which increased the aUelopathic effect of suppression of annuals. DeFrank et al. 4 and Heisey et al. 25 indicated the involvement of actinomycetes in allelopathic effects, by the production of herbicidal compounds. The present work summarizes some field observations and preliminary laboratory experiments, attempting to evaluate the potential of C. capitatus shrubs to suppress annuals as well as the possible interaction with allelopathically active soil-borne actinomycetes. Material and methods
Field observations In order to evaluate the suppression effect of C. capitatus shrubs on annuals, observations of the phenomenon were carried out on a 'kurkar' formation 26hill, in the coastal region, north of Tel Aviv University campus, and with some complementary observations in two other locations, one 40 km to the north and the other 20 km to the south. In addition, density of annuals was recorded as well as their dry matter production, taken from 0-20 cm and further away, from 60-80 cm, from the shrubs' canopies. The annuals were collected from 15 x 5cm during the winter of 1986 (February 25). Ten samples were harvested close to the canopies (5-15cm) and 10 samples from a further distance (60-80cm) around each of 3 'aggressive' as well as 3 'non-aggressive' shrubs. Seed samples of the annuals which were rarely present in the annual-free belts around the adult shrubs, but common outside the annual-free belts, e.g. Plantago psyllium and Erucaria hispanica, were collected during July 1985. In order to evaluate the effect of the shrubs on seedling emergence, those seeds were planted at distances o f 5-15 and 60-80 cm from the shrubs' canopies on Nov. 1985, in groups of 10, in 20 sites for each distance, around each of 3 'aggressive' and 3 'non-aggressive' shrubs. Germination rates were recorded 13 and 30 days after planting.
Inhibition tests of annuals seed germination by shoots of C. capitatus Fresh green shoots of C. capitatus collected from the observed field, were cut into 0.5 cm segments and blended for 30 see. in a Waring blender (8 g in 100 ml H2 O) and desired dilutions were made in water. Fifty seeds of either P. psyllium or E. Hispanica were placed on 2 layers of Whatman No. 1 filter paper per 9cm diam. plate. Aliquots of 4ml distilled water or shoot suspension were
ALLELOPATHIC POTENTIAL OF CORIDOTHYMUS CAP1TATUS L. (LABIATAE).
55
added to each plate for tests of inhibition of germination. Plates were sealed with parafilm and incubated 4 days at 25~ For the effect of shoot diffusates, distilled water was replaced by various concentrations per plate of the shoots' suspension, whereas, for volatile inhibitors suspensions were placed in 5 cm diam. open plates set in the centre of the 9 cm diam. plates with moistened filter paper. Thus, the seeds placed in the large plates had no direct contact with the shoots or their extract but with their volatiles. Water extracts of 10 g shoots in 100 ml distilled water were carried out in a reflux for 30min. The extracted plant material was removed by centrifugation for 10rain at 15000rpm. Essential oils were extracted from 250 g green shoots with 11 sterile distilled water by a Clevenger apparatus 2 and tested as described for shoots or suspensions. The experiments were carried out in triplicate.
Isolation of allelopathically active soil-borne actinomycetes Actinomycetes were isolated from soil samples collected from various distances from the canopies of C. capitatus shrubs, with or without annuals-suppression belts, on the selective medium developed by Huang and Ko tr. The medium contained 2% agar in tap water (TWA), adjusted with I NNaOH to pH 10.5. The isolates were grown and maintained on GMP agar4 which contained: glucose - - I 0 g, Black strap molasses - - 20 g, peptone - - 5 g and agar 15 g, in 1000 ml H 20. pH was adjusted to 7.0 with 1 NNaOH. In order to evaluate the effect of actinomycetes on seed germination, each isolate was streaked on 4 GMP plates. After 2 days incubation at 28~ 225 seeds of the test plants, e.g. Lactuca sativa L. (Compositae) or Anastica hierochuntica L. (Cruciferae) were placed in each plate. Non-inoculated plates served as controls. Seed germination and rootlet length were determined after additional incubation for 24 h at 25~ under continuous fluorscent and incandescent light. The effect of C. capitatus leaves on population levels of actinmomycetes in soil was determined by mixing 50 g soil with 0.25 g green leaves of the shrubs, previously blended in 5 ml of deionized water in 150 ml glass beakers covered with polyethylene. Similarly, moistened soil without leaves served as control. The beakers were covered with polyethylene sheets to minimize evaporation and incubated for 5 days at room temperature (18 24~ Population levels of actinomycetes were counted by plating soil dilutions on the selective TWA.
Plate 1. A 'non-aggressive' shrub of Coridothymus capitatus surrounded by annuals mainly Stipa capensis. Photographed by J. Friedman, April 10th, 1986.
56
KATZ, SNEH AND FRIEDMAN
Plate 2. An annual-free belt of a horseshoe pattern around an 'aggressive' shrub of Coridothymus capitatus, most dearly evident on the western side and fading out on the eastern (lee) side. The annuals stand around the belt was dominated by Stipa eapensis. Photographed by J. Friedman, April 10th, 1986.
Results
Qualitative field observations The association between annuals and C. capitatus shrubs was found to be extremely variable (Plates 1-3). Around some shrubs, annuals, mainly Stipa capensis, occupied all the areas among the scattered shrubs, up to its margins (Plate 1). Around some other shrubs, annual-free belts appeared near their canopies, forming a horseshoe pattern with the strongest effect towards the western side (5-15cm wide) and the least effect on the eastern side (Plate 2). Areas mostly bare of annuals, with scattered shrubs were observed on shoulders of a few western aspects (Plate 3).
Quantitative field observations Density and yield of annuals expressed as plants.cm -2 or as g dry matter, cm -2, near 'aggressive' shrubs (5-15 cm) was reduced by 16 and 5.4 fold respectively at a distance of 60-80 cm from the same shrubs (Table 1). It seems that germination and seedling emergence are the most susceptible phase of the annuals in their association with C. capitatus shrubs.
ALLELOPATHIC POTENTIAL OF
CORIDOTHYMUS CAPITATUS L. (LABIATAE).
57
Plate 3. Annuals-free areas with scattered shrubs of Coridothymus capitatus on a western aspect. Photographed by J. Friedman, April 10th, 1986.
Table 1. Density (number of plants.cm -2) and dry matter yield (mg.cm -2) of annuals in the vicinity (5-15 cm) and at a distance (60-80 cm) from canopies of Coridothymus capitatus 'aggressive shrubs' near Tel Aviv on February 25th 1986 Site
Distance from canopy (cm)
Density a (plants era- 2)
Dry matter a (mg. cm-2)
a b
5 15 60-80 -
0.211 _+ 0.058 3.32 _+ 0.330 16.69 + 5.699
14 + 9.073 76 _+ 15.275 5.4 _+ 1.683
b/a
a Mean + standard deviation of 10 samples per shrub of 15 x 5 c m collected around 3 shrubs.
Seedling emergence around canopies of 'aggressive' and 'non-aggressive' shrubs of C. capitatus The data on the effect of C. capitatus on emergence of deliberately planted annuals (Eo hispanica and P. psyllium) are summarized in Fig. 1, and Tables 2 and 3. Seedling emergence of both annuals planted close (5 cm) to 'aggressive' shrubs was considerably reduced on the 13th day after planting (Tables 2 and 3), both when compared with those planted as controls (10m away from any shrub) and those planted close to 'non-aggressive' shrubs. Thus, extra reduction of 45% was induced by the 'aggressive' shrubs. After a longer period (a further 17 days) suppression rate was only slightly increased around 'aggressive' shrubs,
58
KATZ, SNEH A N D F R I E D M A N
30
uJ O Z uJ
o n,
uJ
--
CONTROL
20
15
3O
uJ
o z
lO
r~ uJ uJ o0
5
s
0 12.11.85
25.11.85
0
10
I 3O ._
I 12.12.85
Date
30
Time (daysl
E. h i s p a n i c a CONTROL
25
| 30
z 15 LU
3O
10
5 t/)
5
0 12.11.85 0
25.11.85 13
12.12.85 Date 30 Time (days}
Fig. 1. Inhibition of emergence of Plantagopsyllium (a) and Erucaria hispanica (b), planted at 5 and 30 cm distances from canopies of 'aggressive' Coridothymu~ capitatus shrubs, as compared to the control (planted 10m away from shrubs).
A L L E L O P A T H I C P O T E N T I A L O F C O R I D O T H Y M U S CAPITATUS L. (LABIATAE).
59
Table 2. Emergence of Erucaria hispanica as affected by 'aggressive' and 'non-aggressive' Corido-
thymus capitatus ''a Distance from
Nature of shrub
canopy (cm)
I000 30
Control Nonaggressive Aggressive Net c suppression Nonaggressive Aggressive Net c suppression
13 days
30 days
Emergence %
% of control
12.5 12.5
100.0 103.1
6.3 -
50.0 -
Suppression rate b
Emergence
Suppression rate b
%
% of control
0 -3.1
25.7 21.7
100.0 80.4
0 19.6
50.0 53.1
9.6
38.3
-
61.7 42.1
11.3
87.5
12.5
12.5
49.8
50.2
3.6
29.6
70.4 57.9
8.1
30.6
69.4 19.2
-
Ten seeds were planted in 2-4 cm 2 sites. 20 sites for each distance per shrub, around 3 'aggresssive' and 3 'non-aggressive' shrubs. b Suppression rate = 100 - % of control. r Net suppression = suppression rate of 'aggressive' minus that of 'non-aggressive' shrubs. a
Table 3. Emergence of Plantago psyllium as affected by "aggressive" and "non-aggressive" Cor-
idothymus capitatus~ Distance from
Nature of shrub
canopy (cm) 1000 30
Control Nonaggressive Aggressive Neff suppression Nonaggressive Aggressive Neff suppression
13 days
30 days
Emergence
Suppression rate b
Emergence %
% of control
Suppression rate b
%
% of control
17.9 10.0
100.0 55.8
0 44.2
29.7 10.7
100.0 36.0
0 64.0
3.8
21.2
78.8 34.6
12.5 -
42.0
58.0 - 6.0
14.0
78.2
21.8
10.0
33.6
66.4
1.9
10.6
89.4 67.6
10.3
34.6
65.4 - 1.0
a Ten seeds were planted in 2-4 cm 2 sites, 20 sites for each distance around each of 3 'aggressive' and 'non-aggressive' shrubs. b Suppressive rate = 100 -- % of control. c Net suppression = suppressive rate o f 'aggressive' minus that o f 'non-aggressive' shrubs.
while it was markedly increased around 'non-aggressive' ones. Thus, the additional suppressive effect of the aggressive shrubs on emergence o f E. hispanica relative to that induced by the 'non-aggressive' shrubs was reduced while that o f P. psyllium was not manifested.
60
--
KATZ, SNEH AND FRIEDMAN
Plantago
100
psyllium
|
Z
O F-
Z "'
50
a uJ LU
u~ 0L 0
20
40
60
80
FRESH PLANT MATERIAL (mg.m1-1) .-.
1ooI
Erucaria hispa n i c ~
o~
_z
0
0
20
40
60
80
FRESH PLANT MATERIAL(mg.m1-1) Fig. 2. Inhibition of seed germination (4 days after wetting) of Plantago psyllium (a) and Erucaria hispanica (b) by shoots of Coridothymus capitatus applied as diffusates (e) and volatiles (O) or shoot extracts applied as diffusates (A) and volatiles (A).
A L L E L O P A T H I C P O T E N T I A L O F CORIDOTHYMUS CAPITATUS L. (LABIATAE).
61
Inhibition of seed germination of annuals by shoots of C. capitatus Seed germination of P. psyllium and E. hispanica was strongly inhibited by diffusates or by volatiles derived from blended shoots of C. capitatus (Fig. 2). P. psyllium was slightly more susceptible to the inhibitors the E. hispanica. Blended leaves applied either as diffusates or as volatiles were more effective in suppressing germination of both annuals tested than the volatiles of their extracts. A complete inhibition of seed germination of both annuals by diffusates of blended leaves was obtained by a concentration of 20-40 mg shoots per ml.
Inhibition of seed germination of annuals by essenthTl oils recovered from shoots of C. capitatus When essential oils recovered from shoots of C. capitatus were added to plates with seeds ofE. hispanica and P. psyllium, their germination was significantly suppressed by diffusates as well as by volatiles (Table 4). Amounts of 2/d oil per plate markedly reduced germination (4 days after wetting). E. hispanica seems to be somewhat more sensitive to the essential oils applied than P. psyllium.
Inhibition of seed germ&ation of test plants by soil-borne act&omycetes Various isolates of actinomycetes inhibited (on plates) germination of L. sativa and A. hierochuntica seeds at various rates from 0 to 96% (Table 5). Isolates demonstrating strong inhibition were isolated from soil samples of both annual-suppressive belts around C. capitatus and non-suppressive areas. A strong inhibitory effect ( < 70%) was demonstrated by 8 out of 17 isolates tested. Those seeds that had germinated on the agar with the inhibitors were also affected. Their rootlets, root hair, hypocotyls and first leaves were deformed. Table 4, Inhibition of seed germination o f Plantago psyllium and Erucaria hispanica by essential oils extracted from Coridothymus capitatus shoots Essential oil Cul. plate- 1 )a
Seed germination (%)
1 day
0.0 2.0 4.0 9.2
P. psyllium
E. hispanica 4 days
1 day
4 days
d
v
d
v
d
v
d
v
25 0 0 0
5 0 0 0
75 20 0 0
70 30 0 0
55 5 0 0
60 I0 20 0
95 35 0 0
80 70 65 0
The a m o u n t o f essential oils in the shoots was 2% of the shoots' fresh weight. d = diffusate. v = volatiles.
62
KATZ, SNEH AND FRIEDMAN
Table 5. Inhibition o f seed germination in vitro of lettuce and Anastatica hierochuntica by actinomycetes isolated from soil samples collected around Coridothymus capitatus shrubs Isolate N o a
AI A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 C1 C2 DI D2 D3 E1 Control
Inhibition (%)b
Lactuca sativa
Anastatica hierochuntica
80 48 20 68 76 56 48 52 32 52 16 84 84 76 68 64 48 0
72 64 32 84 84 92 24 44 36 0 4 76 92 88 64 72 28 0
a Isolates designated A, B, C, and D were collected from annuals suppressive soil belts around different C. capitatus shrubs while E was from a non-suppressive area. b Germination was recorded after 48 h incubation. Table 6. Effect of green shoots of Coridothymus capitatus on population levels of actinomycetes in soiP Soil treatment
N o t amended 0.5% shoots Fold increase
Average numbers of actinomycetes (cfu.g-t)b Soil A c
Soil W
1.9 • 106 1.5 • 107 9.5
1.2 • 106 4.4 x 107 36.7
a Population level counts of actinomycetes were carried out 7 days after shoot incorporation in soil. b cfu = colony forming units. c Soil A - - was collected from the vicinity of 'non-aggressive' shrubs; Soil B - - was collected from the vicinity of 'aggressive' ones.
Effect of shoots of C. capitatus on populations of soil actinomycetes Incubation of green shoots of C. capitatus (0.5%) in soil for 7 days induced a considerable increase in population levels of actinomycetes - - by 9.5 fold in soils sampled from 'non-aggressive' shrubs and by 36.2 fold in samples from 'aggressive' ones (Table 6). Discussion Chopped shoots or essential oils extracted from shoots of C. capitatus collected from the field, inhibited germination of annuals (Erucaria
ALLELOPATHIC POTENTIAL OF CORIDOTHYMUS CAPITATUS L. (LABIATAE).
63
hispanica and Plantago psyllium) when applied as diffusates or as volatiles. These annuals were rarely found in the vicinity of C. capitatus shrubs. Inhibition of seed germination in plates by substances of a natural plant origin by itself, is not a sufficient evidence for their actual ecological significance. However, it supports the assumption that adult shrubs possess an allelopathic potential. Volatiles from living leaves were more effective in inhibiting seed germination when compared to essential oils extracted from equivalent amounts of plant material (Fig. 2). The results suggest the presence of an active volatile substance(s) in the leaves which was not as effectively obtained by extraction with the Clevenger apparatus. Yield of annuals (as dry matter, cm 2) and particularly annuals density, were considerably lower in the annuals-free belts around aggressive shrubs, suggesting that seedlings emergence, subsequent growth or survival were considerably suppressed. In deliberately planted annuals seeds (of species usually absent in the vicinity of C. capitatus) in the field, there was a considerable inhibition of emergence, especially in the vicinity of the aggressive shrubs (13 days after planting). However, 30 days after planting, the specific inhibitory effect was considerably reduced to a partial recovery in annuals germination in the vicinity of the aggressive shrubs. In order to get a better perspective of the processes involved in the suppression of annuals around canopies of C. capitatus, rate of seedling mortality should also be examined. Incubation of C. capitatus leaves in soil collected from the vicinity of the shrubs, considerably increased population levels of soil actinomycetes. Among those, some isolates efficiently inhibited seed germination of test plants. In nature, leachable compounds from either living or from fallen leaves, may stimulate the activity of actinomycetes in soil, possibly by supplementing the upper soil layers with available organic matter 13'18'19'25.DeFrak et al. 4 and Heisey et all 5 have indicated the potential of soil-borne actinomycetes to enhance allelopathic effects in soil. In the present study, some isolates of actinomycetes obtained from soil, inhibited germination of seeds of test plants and subsequent seedling development in vitro. Such isolates were found close to aggressive, but also around non-aggressive shrubs. It is quite likely that annuals suppression around the canopies of C. capitatus, is the consequence of a direct allelopathic effect of diffusates and/or volatiles emanated from the shoots, associated with a synergistic interaction of an indirect inhibitory effect of secondary metabolites produced by soil-borne actinomycetes (or by other soil micro organisms), enhanced by the shrubs' emanated metabolites. Addition of organic amendments to soil is known to enrich popula-
64
KATZ, SNEH AND FRIEDMAN
tion levels and increase the activity of various groups of soil microorganisms9:3'~6:8'25. Thus, increasing the production of inhibitory substances 32. A variety of soil micro organisms were shown to possess deleterious effects on plants (minor pathogens)34. These effects are often incited by phytotoxic metabolites produced by these micro organisms. Fredrickson and Elliott7'8 reported production of a growth inhibitor of wheat by a non-fluorescent Pseudomonas sp. Enrichment of populations of actinomycetes in soil by the addition of chitin was associated with a strong phytotoxic effect on bean seeds during 2 weeks after chitin incorporation (Sneh, unpublished). Phytoxins were also produced by crop residues. Inhibitors produced by soil isolates in vtiro are frequently inactive in soil. A heptaene antibiotic produced in vitro by a considerable proportion of the actinomycetes isolates enriched by chitin in soil, could not be detected in soil, nor its activity as tested in soil on yeasts (highly sensitive to heptaenes)32"33. Antibiotics produced in vitro by soil microorganisms may either not be produced by them in soil due to inappropriate nutritional or other ambient conditions, or in cases where they are produced, they may be quickly inactivated by adsorption to soil colloids, by chemical or by microbial degradation5. On the other hand, polar lipids, and to some extent volatile ammonia, produced in soil by actinomycetes and other microorganisms during chitin microbial decomposiiton, inhibited the saprohytic and pathogenic activities of Rhizoctania solani Kuhn in soil32. In addition to allelopathic activity in soil, competition or reduction in essential growth factors which exist in limited amounts by the interacting plants, could also contribute to the elimination of annuals in the vicinity of the shrubs. Since these shrubs are dwarf, their shading effect is meagre and their major competitive interactions may result through their root systems. Field observations showed that the concentration of roots systems of the shrubs in the upper soil layer gradually decreases towards the circumference, suppression of annuals by roots of the shrubs could manifest itself in a gradual pattern of annuals suppression. However, the pattern of the annuals-free belts and the border with the unaffected annuals was found to be very defined (Plate 2). Therefore, in addition to competition, allelopathy may be considered as an important factor governing the formation of the annual-free belts. The components determining the variability of annuals suppression observed in nature has been a challenge which has not yet been completely understood during the present study. However, few clues were probably found. Field observations suggested that there is a considerable variability in the soil layer on the kurkar parent material. In areas where kurkar parent material dominated most of the soil, annuals were often
ALLELOPATHIC POTENTIAL OF C O R I D O T H Y M U S CAPITATUS L. (LABIATAE).
65
more eliminated than in areas where the layer of brown-red sandy soil on the kurkar formation was deeper. Annuals elimination on the western side of the aggressive shrubs was more significant than that on the eastern side (Plate 2). In each situation it is possible that water stress differs and thus it probably results in changes in amounts or in composition of essential oils in inducing allelopathy. Effects of water stress on essential oil production have already been shown to occur in some members of Labiatae 1'6'~4'2~176 . Water stress may also increase susceptibility of annuals. Genetic variability determining the chemistry of essential oil composition may also account for differences among individual plants and has been demonstrated in various members of Labiatae. Individual plants of M. syriaca and C. capitatus were found in Israel to contain high amounts of thymol with small levels of carvacrol or vice v e r s a 27"28. Similar differences were found along the Pacific coast of the United States of America for Satureja douglasii 14. The variability in annuals suppresion rate by the shrubs as a result of their chemical variability, as well as the interaction (additive/synergistic) between volatile essential oils and prevalence of phytoxic activities of soil actinomycetes, are currently under investigation. Acknowledgements We would like to express our cordial thanks to Professor Alan Putnam, who provided us with the challenge to look for allelopathically active soil-borne actinomycetes.
References 1 2 3 4 5
6 7 8 9 10
Clark R J and Menary R C 1980 The effect of irrigation and nitrogen on the yield and composition of peppermint oil. Aust. J. Agric. Res. 31,489-498. Clevenger R 1928 Apparatus for the determination of volatile oil content of plant materials. Am. Perfumer. 23, 467. Cochran V L, Elliott L F and Papendick R I 1977 The production of phytotoxins from surface crop residues. Soil Sci. Soc. Am. J. 41,903-908. DeFrank J and Putnam A R 1985 Screening procedures to identify soil-borne actinomycetes that can produce herbicidal compounds. Weed Science 33, 271-274. Elad Y and Misaghi I J 1985 Chemically mediated interactions between plants and other organisms. In Recent Advances in Phytochemistry. G A Cooper-Driver, T Swain and E E Conn, Eds. Vol 19, pp. 21-45. Plenum Press, New York and London. Fluck H 1963 Intrinsic extrinsic factors affecting the production of secondary plant products. In Chemical Plant Taxonomy. T Swain, Ed. p. 167-186. Fredrickson J K and Elliott L F 1985a Colonization of winter wheat roots by inhibitory rhizobacteria. Soil Sci. Soc. Am. Proc. 49, 1171-1177. Fredrickson J K and Elliott L F 1985b Effects on winter wheat seedling growth by toxinproducing rhizobacteria. Plant and Soil 83,399-409. French R C 1961 Stimulation of uredospore germination in wheat stem rust by terpens and related compounds. Bot. Gaz. (Chicago) 122, 194-198. Friedman J 1986 Allelopathy in desert ecosystems. In Allelochemicals in Agriculture, Forestry and Ecology. G R Waller, Ed. American Chemical Society, Boston, pp. (in press).
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ALLELOPATHIC POTENTIAL OF CORIDOTHYMUS CAPITATUS L. (LABIATAE).
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