Developmental studies on Euphorbia esula. The influence of the nitrogen supply on the correlative inhibition of root bud activity

Regitla Research Station, Canada Departtnent of Agricrrltrrre, Regitla, Saskatche~oat~

Received October 28, 1971

MCINTYRE, G. I. 1972. Developmental studies on Errphorbia esula. The influence of the nitrogen supply on the correlative inhibition of root bud activity. Can. J. Bot. 50: 949-956.

When seedlings of leafy spurge (Errphorbia esrrla L.) were grown at a low nitrogen level (10.5 ppm) the growth of the lateral buds on the shoot was completely arrested by apical dominance while the buds on the roots, although also inhibited, showed considerably greater activity. At a higher nitrogen level (210 ppm) apical dominance was markedly reduced but the resulting outgrowth of the lateral buds in- creased the inhibiting capacity of the shoot, limiting root bud response. When the main shoot was decapitated the degree to which root bud growth was suppressed by a given number of lateral shoots was inversely related to the nitrogen supply. Inhibition of the root buds by the lateral shoots could be sig- nificantly reduced by growing the plants initially at a low nitrogen level so that growth of the lateral buds was arrested. A subsequent increase in the nitrogen supply strongly promoted the growth of the roots buds, some of which were sufficiently released from inhibition to emerge as leafy shoots.

MCINTYRE, G. I. 1972. Developmental studies on Eriphorbia esula. The influence of the nitrogen supply on the correlative inhibition of root bud activity. Can. J. Bot. 50: 949-956.

Lorsque des plantules dlErrphorbia esrrla L. ont CtC cultiv&s en prCsence de faibles quantites d'azote (10.5 ppm), la croissance des bourgeons IatCraux sur la tige a CtC complttement supprimke par la domi- nance apicale alors que les bourgeons sur les racines, bien qu'aussi inhibb, ont montrC une activite beaucoup plus grande. A un niveau d'azote plus Cleve (210 ppm), la dominance apicale a ete fortement reduite mais les ramifications produites a partir des bourgeons lateraux ont augment6 le pouvoir inhibi- teur de la tige diminuant ainsi la reaction des bourgeons racinaires. Quand la tige principale a CtC de- capit&, 1'intensitC avec laquelle la croissance des bourgeons racinaires a ete supprimee par un nombre donne de tiges laterales a CtC inversement proportionnel a I'apport en azote. L'inhibition des bourgeons racinaires par les tiges laterales a pu ttre diminuee de faqon significative en faisant croitre la plante dans un milieu d'abord pauvre en azote de sorte que la croissance des bourgeons latkraux etait arrttee. Une augmentation subsequente dans I'approvisionnement en azote a fortement stim~lle la croissance des bourgeons racinaires dont certains etaient suffisamnient affranchis de l'inhibition pour se developper en tiges feuillkes.

Introduction Leafy spurge (Euphorbia esula L.) is of wide-

spread occurrence in western Canada and throughout northern United States, where it is generally recognized as a serious agricultural weed of considerable economic importance (3, 7, 16). Its ability to resist eradication is attributed largely to its deep and extensive root system and, in particular, to the numerous buds which are produced adventitiously on the roots and which serve as a means of vegetative propagation and regenerative growth (2, 16). When the plant is growing undisturbed most of these buds are relatively inactive. Removal or injury of the above-ground shoots stimulates root bud activity

1Throughout this paper the expression "root bud" is used as a convenient term for buds which are borne on the roots but which, when released from inhibition, develop as shoots. These buds should not be confused with those which are borne at the nodes of the above- ground parent shoot and for which the terms "axillary" or "lateral" bud are used.

causing many of the buds to develop as shoots (7, 16). Cuttings taken from the roots and allowed to regenerate under favorable conditions show a similar stimulation of bud and shoot: development (15). Such observations point to the existence of some form of correlative in- hibition which regulates bud developmeilt in the intact plant. There is, however, no evidence about the nature of the mechanism involved.

Although the inhibition of root buds1 has been investigated in experiments with other species (5, 17, 18) these investigations have been primarily concerned with the role of auxin in this phenomenon. No attention has been given to the possible involvement of purely nutritional factors. Yet recent work on the related problem of apical dominance has provided considerable evidence of the importance of nutrition (1, 6, 10, 11). Moreover, previous experiments on the influence of nitrogen nutrition on the develop- ment of leafy spurge (12) showed that this factor

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950 C A N A D I A N JOURNAL OF BOTANY. VOL. 50, 1972

had a marked effect not only on apical domi- nance in the shoot but also on the regenerative growth of the buds on the roots. For these various reasons it was decided to obtain further, more precise information on the extent to which the degree of root bud inhibition is determined by the nitrogen supply.

Materials and Methods Seed, collected at Jameson, Saskatchewan (50°25' N,

104"18' W) was germinated as previously described (12). Seedlings, with their primary root about 1 cm long, were planted in 12- to 30-mesh silica sand in plastic pots 12.5 cm in diameter.

The plants were watered with Hoagland's solution (8) in which iron was supplied inchelated form (Sequestrene). The standard solution, containing 210 ppm nitrogen, was used for the "high nitrogen" treatments while the "low nitrogen" level was obtained by equimolar substitution of KzS04 and CaC12 for KNO3 and Ca(N03)~ respectively to give a nitrogen concentration of 10.5 ppm (i.e., 1/20 of its concentration in the standard solution). An excess of nutrient solution (about 200 ml per pot) was supplied at 2-day intervals, evaporation losses being replaced with distilled water on alternate days.

Experiments 1-3 were conducted in the greenhouse where supplementary lighting was provided by a bank of 12 VHO cool-white fluorescent lights which extended the photoperiod to 18 h and gave an additional light in- tensity of ca. 1000 ft-c. The temperature was only partially controlled, the approximate range being from 10 to 15°C at night and 15 to 25'C during the day. In experiment 4 the plants were grown in the greenhouse initially but were transferred to a controlled environment room just before the treatments were applied and were kept there for the duration of the treatment period.

completely arrested. Many of the buds appeared to maintain a slow, continuous growth, some reaching a length of more than 1 cm by the time the seedlings were about 3 months old.

To substantiate these observations and also to determine the effect on bud growth of varying the nitrogen supply, seedlings were grown a t nitrogen levels of 10.5 ppm and 210 ppm and samples, each comprising six plants, were harvested at intervals and the length of the root buds recorded.

The results confirmed the persistence of root bud growth on the intact plant and also showed that the rate of growth was considerably greater at the higher nitrogen level (Table 1). There was, however, a marked variation in bud length in the last two samples from the high nitrogen plants. This was due to the exceptionally rapid elonga- tion of a few buds which appeared to have escaped from inhibition more completely than the others. In the final sample a few such buds had emerged from the surface of the sand and had developed into leafy shoots. These results were well substantiated by subsequent observa- tions on seedlings grown a t the same nitrogen levels in later experiments described below.

As previously reported (12) apical dominance in this species, as measured by the degree of in- hibition of the axillary buds on the shoot, is readily controlled by varying the nitrogen supply. This was also apparent in the present experiment. In the high nitrogen treatment the axillary buds were released from inhibition and developed as Experiments and Results lateral shoots; at the low nitrogen level, on the

A. The Gt'owth of the Root Buds 012 the llztact other hand, the buds were all strongly inhibited Plant and their growth appeared to have been com-

Experiment 1 pletely arrested. Thus, under conditions of Preliminary observations suggested that al- nitrogen deficiency the buds on the shoot were

though the buds on the roots were strongly in- inhibited more completely than those on the hibited by the parent shoot their growth was not roots.

TABLE 1

The growth of buds on the intact seedling in relation to the nitrogen supply - ~ --

Root bud length,* mm

Time (days) from initial sample Nitrogen - -- -- level, ppm 0 10 22 30

*Means(*S.E.)nrebased on the length (measured to nearest 0.1 mm) of the I0 largest buds on the main root of each plant. There were six plants per sample.

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MclNTYRE: DEVELOP.UENT OF EUPHORBIA ESULA 95 1

B. Effect of the Nitrogen Supply on the In- hibition of tlie Root Buds by the Apex and Lateral Branches of the Parent Sl7oot

Experiment 2 Since the previous experiment had shown that

growth of the root buds on the intact plant was significantly affected by the nitrogen supply, a further experiment was designed to compare the inhibiting influence of the apex and lateral branches of the shoot at high and low nitrogen levels.

The seedlings were watered with distilled water only until 1 week after the cotyledons had expanded. They were then divided into two equal groups, one of which was watered with standard Hoagland's solution (210 ppm N) and the other with the modified, low nitrogen solu- tion (10.5 ppm N). When the plants in each group had produced an average of 35 leaves they were assigned at random to the treatments described in Table 2. Decapitation of the shoot (treatment B) involved only the removal of the apical bud; leaves which had started to separate from the bud were not excised. In treatments C and D lateral shoots and all axillary buds which had reached a size of 1 mm were removed. Bud excision was continued throughout the treatment period as younger buds developed. Final data on root bud and lateral shoot length were recorded 6 weeks after treatment.

As in the previous experiment, growth of the root buds and lateral shoots of the intact (control) plants was considerably greater at the higher nitrogen level (Table 2). Root bud and lateral shoot growth was also increased at the lower nitrogen level by decapitation of the shoot. At high nitrogen this treatment, while greatly

increasing lateral shoot development, caused the growth of the root buds to be significantly reduced. That this apparent inhibition of root bud activity in the high nitrogen treatment was due to the increased growth of the lateral branches was evident from the promotion of root bud growth induced by the removal of the laterals and the still greater increase produced when decapitation was accompanied by lateral shoot removal. Root bud growth was also in- creased by lateral shoot excision in the low nitrogen treatment but the combination of decapitation and lateral shoot removal at this low nitrogen level did not produce the further increase which was anticipated. Examination of the data showed that this could be attributed to the inclusion of two plants with abnormally small root buds, none of which was sufficiently large to be recorded. Such variation in root bud production, which was also noted in the previous investigation (12), and from observations on roots collected in the field (15), appears to be a characteristic feature of the species.

Experinzent 3 The inhibiting effect of the lateral shoots on

root bud activity was also investigated by com- paring the degree of inhibition produced by different numbers of lateral shoots in relation to the nitrogen supply. All of the seedlings were grown at the high nitrogen level (i.e., 210 ppm) until they had reached the 8- to 10-leaf stage. The main shoot of each plant was then decapitated just above the eighth leaf and the plants were divided into three groups, each comprising 12 plants. One group (A) had all of the lateral buds removed, thus preventing the occurrence of any lateral growth; a second (B) had all buds re-

TABLE 2

The effect of the nitrogen supply on the response of the root buds to removal of the apex and lateral branches of the parent shoot

--- ~ .- ~ ~ ~ - - . ~ .~ ~ ~ ~- .. . ----A . - .. . --

Length (cm) of root b ~ ~ d s and lateral shoots*

Low nitrogen: 10.5 ppm High nitrogen: 210 pprn -- --

Treatments Root b ~ ~ d s Lateral shoors Root buds Lateral shoots - pp

~ ~

A. Intact control 2.88k0.86 1.16+0.82 18.5 + 3.91 87.8k22.0 B. Main shoot apex renloved 5.26k0.75 25.6 k4.97 6.71+ 2.31 189.0+25.8 C. Lateral branches relnoveri 6.80+0.83 - 37.5 + 12.4 - D. Main shoot apex and

lateral branches removed 5.98k 1.43 - 65.5 k10 .9 - pppp.---.---p--- -. .-

*Data are the mean values (kS.E.)pertreatment of total length per plant of all root buds 5 5 mm and of ;ill lateral shoots 5 2 mm. Means are based on 10 plants per treatment.

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952 CANADIAN J O U R N A L OF BOTANY. VOL, 50. 1972

moved except for the two buds at the cotyledon- ary node; while in the third group (C) the two buds at the cotyledonary node together with the buds in the axils of the four oldest leaves were left to grow, the remaining buds being excised. Ten days later all the pots were leached with distilled water and half of the plants in each group were supplied with the same high nitrogen solution as before while the other half were watered with the low nitrogen solution (10.5 ppm N). The plants continued to receive these solutions for the following 30 days. They were then removed fro111 their pots and the length of the root buds recorded.

The degree of root bud inhibition increased with the number of lateral shoots and was in- versely related to the nitrogen supply (Table 3). In the low nitrogen treatment the inhibition of root bud growth by only two lateral shoots was Inore than three times greater than that pro- duced by six shoots at the higher nitrogen level.

TABLE 3

The effect of the nitrogen s ~ ~ p p l y on the degree of root bud inhibition produced by different numbers

of lateral shoots ~p

Root bud length,* cm No. of lateral Low nitrogen, High nitrogen,

Treatment shoots 10.5 pprn 210 ppm

Ratio A/B 18.4 4 .9 Ratio A / C 38.9 6.0

*Mean values (?S.E.), based on 12-plants per treatment. are lor total length per plant of all root buds > 5 nim. Data sheets for treat- ment C were accidentally discarded before the standard error had bcen computed.

C. E8ect of Increasing the Nitrogen SuppIy to Plants Grown InitiaIIy at a Low Nitrogen Level

Experiment 4 It was apparent from the previous experiments

that when the seedlings were grown at a high nitrogen level the growth of the root buds, although considerably greater than in the low nitrogen plants, was much reduced by the out- growth of the lateral shoots. An experiment was therefore designed to determine whether this inhibition by the lateral shoots could be avoided by growing the seedlings initially at a low nitro- gen level to arrest the growth of the lateral buds and then subsequently increasing the nitrogen supply.

All the seedlings were grown at a low nitrogen level (10.5 ppm) for 6 weeks. Plants for experi- mental treatment were then selected for uni- formity of shoot and root-bud development. Each of the selected plants had 40-50 fully expanded leaves and ranged from 20 to 30 cm in height. There were no lateral shoots, the growth of all the axillary buds having apparently been arrested when only 1-2 mm long. The root buds, which were examined by washing away the sand from the upper part of the root system, ranged from about 1 to 3 mm in length.

To obtain greater environmental control the selected plants were transferred to a growth chamber immediately before treatment and were kept there for the duration of the treatment period. The temperature was held constant a t 15 + 1°C; the light, provided by a combination of cool-white fluorescent and incandescent lamps, had an intensity of ca. 3200 ft-c; and the daylength was 16 h. The plants were divided into three sets of 12. Two sets continued to

TABLE 4

The effect on root bud growth of increasing the nitrogen supply to plants grown initially at a low nitrogen level

Root buds* Main shoot No. of

Treatments growth, crn Length, cm Dry wt., mg emergent shoots

A. Control 6.78k0.60 3.1k0.82 24.35 8.53 0.08k0.08 B. Shoot removed - 5.9k1.89 46.8k25.7 0.93k0.31 C. Nitrogen supply increased

from 10.5 to 210 ppin 17.7 k1.59 26.0k4.73 245 k54.3 3.08k0.98 --

. . . .

*Mean values (cS.E.) based on 12 plants per treatment are for total length and dry weight per plant of all root buds 5 5 mm. The main shoot and root-bud data'for treatment C are all significanily different (P < 0.01) from the other two treatments.

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MclNTYRE: DEVELOPMENT O F EUPHORBIA ESULA 953

F I G . 1. Seedlings of leafy spurge illustrating the effect on root bud growth of an increase in the nitrogen supply. (A) A control plant grown at a low nitrogen level (10.5 ppm) throughout the experiment. (B) A plant grown initially at the same low nitrogen level but which received a n increased nitrogen supply of 210 ppm (i.e., standard Hoagland's solution) for a period of 35 days before harvest. Note the increased growth of the root buds, several of which have emerged as leafy shoots. Magnification, X 113.

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954 CANADIAN JOURNAL OF BOTANY. VOL. 50, 1972

receive the same low nitrogen solution (i.e. 10.5 ppm). One of these (A) served as controls while the other (B) had the main shoot cut off at its junction with the root. The third set of plants (C) had the low nitrogen solution replaced by standard Hoagland's solution, increasing the nitrogen level from 10.5 to 210ppm for the duration of the treatment period. The experi- ment was terminated after a further 35 days and the growth of the shoots and root buds recorded.

Removal of the shoot from plants which had continued to receive the low nitrogen solution resulted in some stimulation of root bud growth and in the subsequent emergence of an average of about one shoot per pot (Table 4). This rather weak response agrees with results pre- viously reported (12) which showed that the amount of regenerative growth induced by shoot removal was greatly affected by the nitrogen supply, being severely depressed at low nitrogen levels. The treatment in which the nitrogen supply was increased, however, had a much more striking effect. Not only did the additional nitrogen produce an 8- to 10- fold increase in root bud growth (as measured by both length and dry weight increments) but also resulted in the emergence of an average of three shoots per plant (Fig. 1). In one pot as inany as 12 shoots had emerged and it was evident from the number of elongating buds close to the surface of the sand that shoot emergence in the other pots would have been considerably greater if the experiment had been terminated at a later date.

Growth of the parent shoot also responded to the increased nitrogen supply. Apical growth was significantly increased and renewed activity of the axillary buds was apparent about 7 days after treatment. No measurements of lateral shoots were recorded but there was evidence of considerable variation, some plants producing numerous lateral branches ranging from 0.5 to 1.0 cm in length, others (see, for example, Fig. 1) showing relatively little response.

This experiment was later repeated with essentially the same result.

Discussion The results of this investigation suggest: that,

under the experimental conditions, the degree of root bud inhibition was largely determined by

the ability of the buds to compete with the dominant shoots for a limited nitrogen supply. In experiment 1 the root buds of the high nitro- gen plants not only grew much more rapidly than at the lower nitrogen level but, in a few instances, they escaped sufficiently from in- hibition to develop into shoots. It was also shown, however (experiment 2), that when the plants were grown at a high nitrogen level the outgrowth of the axillary buds resulting from a loss of apical dominance caused a significant reduction of root bud development. This in- hibiting effect of the lateral shoots may well have been due to increased competition for nitrogen but the evidence on this point is by no means conclusive. Although such an explanation is consistent with the results of experiment 3 the conditions created in that experiment were somewhat artificial. Because of the high nitrogen supply initially provided, decapitation of the shoot resulted in extremely vigorous lateral shoot growth. In these circumstances the subsequent reduction in the nitrogen supply may have in- duced a greater degree of nitrogen competition than would be likely to occur under normal conditions.

The possibility that the inhibiting effect of the lateral shoots may have been caused, a t least partly, by increased competition for carbohvdrate should also be considered. It is generally accepted that the high shoot/root ratio typically shown by plants receiving an abundant nitrogen supply, and which was a notable feature in previous experiments with leafy spurge (12), is partly attributable to in- creased use of carbohydrate by the shoot and a consequent reduction in the amount available for root development. Thus, in the present in- vestigation the vigorous growth of the lateral shoots of the high nitrogen plant may have diverted carbohydrates from the roots in amounts sufficient to limit root bud response. Further experiments designed to test this hy- pothesis and to assess the importance of the carbohydrate supply in the mechanism of root bud inhibition are at present in progress.

When the plants were grown at a low nitrogen level the growth of the axillary buds on the shoot was completely arrested while the buds on the roots, although also inhibited, maintained a slow but continuous growth. This difference might also be explained by reference to the

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McLNTYRE: DEVELOPMENT OF EUPHORBIA ESULA 955

known effects of nitrogen nutrition on the shoot / root ratio. The low values for this ratio typically produced by nitrogen deficiency are generally considered to reflect the superior ability of the root to compete for the limited nitrogen supply. Thus, in the present experiment, the root may have been capable of retaining sufficient nitro- gen to maintain all of the root buds in an active condition while the amount exported to the shoot was sufficient only for the growth of the main shoot apex and for the early development of the lateral buds.

It is also possible that anatomical differences are involved. In contrast to the buds on the shoot, which are presumably exogenous in origin, the root buds are reported to arise endogenously in the outermost layer of the stele in a similar manner to lateral roots (13). Thus, the root buds, by virtue of their initial proximity to the conducting tissues of the parent axis, may well have an advantage over the buds on the shoot in competing for any nutrients which are in short supply. There seems little doubt, how- ever, that (in expt. 4) the persistence of root bud growth, whatever its explanation, enabled the buds on the roots to compete effectively with the relatively inactive buds on the shoot for the increased nitrogen supply and was thus a major factor in their release from inhibition.

The mechanism of root bud inhibition has also been investigated in experiments with aspen (Populus spp.) (5), black locust (Robirzia pseudo- acacia L.) (18), and poison ivy (Rhus radicans L.) (17). The main object in all of these in- vestigations was to test the hypothesis that the inhibition of root bud growth was caused by auxin produced in the dominant shoot. It was shown that when the inhibiting shoot was re- moved the application of indoleacetic acid in lanolin to the cut surface of the root or stump of the stem caused a significant inhibition of root bud activity. This evidence, however, can- not be regarded as conclusive. The application of auxin preparations to decapitated shoots is a technique which has been widely used in the study of apical dominance but the significance of growth inhibitions produced in this way is open to question (4, 9) and has been the subject of considerable discussion (reviewed by Phillips in ref. 14). Farmer ( 9 , who used the same tech- nique of exogenous auxin application in experi- ments on root bud inhibition in Populus spp.,

noted, that while the isolation of roots from the parent shoot may cut off the supply of bud growth inhibitors, it may also be expected to have other far-reaching effects on the physiology of the plant. In particular, he suggested that water and other nutrients normally translocated under the influence of the parent shoot will tend to accumulate in isolated roots and that their increased availability could equally well account for the observed stimulation of root bud activity. This suggestion would seem to be well supported by the results of the present investigation.

Acknowledgments I thank Mr. William Fleming for technical

assistance and Dr. J. R. Hay for helpf~~l dis- cussion.

I . ASPINALL, D. 1961. The control of tillering in the barley plant. I. The pattern of tillering and its relation to nutrient supply. Aust. J. Biol. Sci. 14: dc)?-sns . < - - - - .

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3. COUPLAND, R . T., and J. F . ALEX. 1954. Distribution of the underground parts of leafy spurge (E~rpl~orDin eslrla L.). Can. J . Agric. Sci. 34: 161-176.

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15. RAJU, M. V. S., T. A. STEEVES, and R. T. CCJUPLAND. 17. STERRETT, J. P., C. L. FOY, and S. W. BINGHAM. 1964. On the regeneration of root fragments of leafy 1970. The effect of auxin on sprouting in poison ivy. spurge (Euphorbia esula L.). Weed Res. 4: 2-11. Weed Sci. 18: 199-201.

16. SELLECK, G. W., R. T. COUPLAND, and C. FRANKTON. 18. STERRETT, J. P., and W. E. CHAPPELL. 1967. The 1962. Leafy spurge in Saskatchewan. Ecol. Monogr. effect of auxin on suckering in black locust. Weeds, 32: 1-29. 15: 323-326.

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