Scientia Horticulturae, 32 (1987) 183-193 183 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Ageing-Dependent Responses of Phloem Flavonoids of P r u n u s av ium Graftings: Flavanone-, Flavone- and Isoflavone-Glucosides

D. TREUTTER, W. FEUCHT and P.P.S. SCHMID

Lehrstuhl fi~r Obstbau der Technischen Universiti~t Mi~nchen- Weihenstephan (F.R.G. )

(Accepted for publication 23 December 1986)

ABSTRACT

Treutter, D., Feucht, W. and Schmid, P.P.S., 1987. Ageing-dependent responses of phloem fla- vonoids of Prunus avium graftings: flavanone-, flavone- and isoflavone-glucosides. Scientia Hortic., 32: 183-193.

The qualitative and quantitative patterns of several flavonoid glucosides in the phloem of shoots and stems of Prunus avium during ageing were determined. The flavonoids investigated were the flavanones dihydrowogonin 7-glucoside, prunin and sakuranin, the isoflavone genistin and the flavone chrysin 7-glucoside. All flavonoids increased in senescing interspecific graftings as com- pared to healthy homospecific ones. Increasing levels of prunin and sakuranin were related to advancing age of the branch sections within the crown.

Comparing the 1-year-old phloem from 2- and 8-year-old trees, chrysin 7-glucoside increased about three times in the older crowns. During seasonal shoot ontogeny from February to October, chrysin 7-glucoside levels increased and the values in October for genistin were about 9 times those in April, and they then declined during the dormant season. The seasonal increase ofprunin and sakuranin was comparatively slight. Conversely, dihydrowogonin 7-glucoside declined from April to October. Some other flavonoids did not change in relation to shoot or tree ontogeny.

Keywords: ageing; heterospecific grafts; HPLC; phloem flavonoids; Prunus avium; Prunus cerasus.

INTRODUCTION

Polyphenols should not generally be considered as "waste products" ( Swain, 1977). According to Luckner (1980), secondary metabolites appear to be inte- grated into programs of differentiation and development, and even in organo- genesis they function as markers (Druart et al., 1982; Jalal et al., 1982).

Relationships between polyphenol metabolism and seasonal development of plant tissues have been shown repeatedly (Wiermann, 1970; Feucht and Nachit, 1977; Poessel et al., 1980; Sano et al., 1982; Dangelmayr et al., 1983). In those

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184

cases also the polyphenols could be found to vary even in different cell layers (Swain, 1960; Wellmann, 1974; Strack et al., 1982).

In earlier investigations, changes in the levels ofprunin at the union of grafted stems were found as a response to several treatments (Treutter et al., 1985). Further, prunin has a possible marker function for incompatible heterografts ofPrunus (Treutter et al., 1986). In line with the results cited above, we decided to investigate whether prunin and some other polyphenols also change in the shoot phloem within the tree canopy according to season and shoot- and tree- age. Results are reported for Prunus grafts with different compatibility and therefore also with different longevity.

MATERIAL AND METHODS

Plant material. ~ The field-grown Prunus trees consisted of homospecific ( 'Sam', P. avium L. grafted on the rootstock F 12/1, P. avium L.) and heter- ospecific ( 'Sam' worked on the senescence-promoting P. cerasus L.) grafts. The latter group showed varying symptoms of yellowing senesced leaves. Tree age, sampling data and number of replicates are described in the figure legends.

Extraction of unknown flavonoids. - - Tissue preparation and extraction have been described elsewhere (Treutter et al., 1985 ). In order to get higher amounts of unidentified phenolics, an extract of 3 g dry phloem powder was separated on a column of Polyamid SD-6 (M + N ) into 8 fractions, using as solvents a mixture of 0.5% aqueous formic acid and methanol, the latter ascending to 100%. Each fraction, consisting of several phenolic compounds, was subjected to preparative HPLC by a LDC gradient apparatus ( Latek, Heidelberg, F.R.G. ).

The separation parameters were as follows: column, Hyperchrome 150 X 20.5 mm I.D. Merck LiChrosorb RP 18, 5 zm; solvents, (A) acetic acid (1%), (B) methanol/butanol (5/1 v/v) , p.a. quality, gradient 8-80% B in A, concave ( m = 2 ), 100 min, flow rate 9 ml rain- 1, 2500 PSI, detection 290 nm.

The peaks were collected with the LKB "Superrac". This procedure yields prunin and dihydrowogonin-glucoside from the aqueous polyamid-fraction. Sakuranin was eluted with 10% methanol, genistin with 50% methanol, and chrysin with the last fraction.

HPLC-analysis of the flavonoids

The phloem flavonoids. - - The phloem flavonoids were separated by means of a gradient apparatus (Kontron, Eching, F.R.G. ).

The separation parameters were as follows: column, Shandon Hypersil ODS, 3 #m, 250 X 4.6 mm I.D.; solvents, as above, but LiChrosolv quality (Merck, Darmstadt, F.R.G. ), gradient 10-80% B in A, concave (m = 2), 85 min, flow rate 0.5 ml min- 3, 1000 PSI, detection 280 nm, full scale 0.2 absorbance units.

185

An internal standard method was employed for quantification. The glyco- sides prunin (available from Roth, Karlsruhe, F.R.G.) and dihydrowogonin- 7-glucoside could be quantified directly; the other flavonoids were calculated as the corresponding aglycones ( available from Roth, Karlsruhe ).

Automatic injection was carried out by an autoinjector (Abimed, Dilssel- dorf, F.R.G. ).

Detection was achieved with a filter detector (Kontron, Eching, F.R.G.) operating at 280 nm. A reporting integrator ( Milton Roy, Hasselroth, F.R.G.) was employed for printing.

Iden t i f i ca t ion procedure . - - The samples were hydrolyzed by refluxing in a waterbath with 2N HC1 for 20 min and the aglycones were extracted with ethyl acetate. The UV-absorbance was measured on a Beckman spectral photo- meter using shift reagents according to Mabry et al. (1970). The sugar iden- tification was made by thin-layer chromatography with authentic sugars, as described by Harborne (1973, pp. 212-232), and by an enzymatic assay with hexokinase ( Boehringer, Mannheim, F.R.G. ). With the exception of dihydro- wogonin, the aglycones were available from Roth (Karlsruhe, F.R.G. ) and could be employed as standards.

RESULTS

Iden t i f i ca t i on of the f lavonoid-glycos ides . - - With the HPLC-method described, the phloem polyphenols could be separated, giving two main peak groups (Fig. la, b ). The first group consisted of phenolic acids and catechins (D. Treutter and O. Bayer, 1985, unpublished results). The second group, with retention times (Rt) > 20 min, included flavonoids. The identification of the flavanones dihydrowogonin 7-glucoside and prunin (naringenin 7-glucoside) has been published in a previous paper (Treutter et al., 1985 ). By means of UV-spec- troscopy of both the glycosides and the corresponding aglycones after hydrol- ysis in comparison with authentic samples, the glycosides which were isolated further could be identified as glucosides of the flavone chrysin, the isoflavone genistein and the flavanone sakuranetin. These data are given in Table I.

The bathochromic shift of Band II in the presence of A1C1JHC1, shown by four of the identified flavonoids (Table I), leads to the detection of a free 5- hydroxyl group (Mabry et al., 1970) and thus a glucosylated position 7.

S e n e s c e n c e - p r o m o t i n g graf t ings . - - The phloem flavonoids analyzed just above the union of grafted stems differed markedly. In general, the senescence-pro- moting graft ( 'Sam'/P. cerasus W) showed higher amounts of flavonoids com- pared to the control ( 'Sam'/F 12/1), especially during the growing season in July (Fig. 2a). However, the particular polyphenolics did not change in a sim- ilar manner. Dihydrowogonin 7-glucoside rose only by 1.4 times, whereas gen-

186

c o

Q

hl

Lq

b

i

s c d

G p ~ D ngl s' c

I i i t t i Rt 2() 40 6'0 8'0 rain

Fig. 1. HPLC-separation of the phloem flavonoids of (a) P. avium 'Sam' and (b) a mixture of the identified glucosides and their corresponding aglycones. Separation was performed on Shan- don Hypersil ODS, 3 ~m (for details see methods). Glucosides: G, genistin; P, prunin; S, saku- ranin; D, dihydrowogonin 7-glucoside; C, chrysin 7-glucoside. Aglycones: n, naringenin; g, genistein; d, dihydrowogonin; s', sakuranetin; c', chrysin; I, internal standard (6-methoxy-flavone).

istin and chrysin 7-glucoside increased 2.5 and 4.0 times, respectively. These differences were somewhat reduced in the older unions (Figs. 2b and 3c ), which was partly due to elevated levels of the control stems ( 'Sam' /F 12/1 ). Overall, the values of the 4- and 6-year-old composite stems show that the senescence- promoting grafts, with one exception, yielded higher values of polyphenols.

In the November phloem of 5-year-old stems, the values were not signifi- cantly different from each other except for prunin (Fig. 3a). However, in the 6-year-old stems, accelerated ageing was clearly related to higher amounts of the polyphenolics investigated (Fig. 3b).

Seasonal development of young shoots. - - In young but woody shoots of P. avium, the contents of the est imated flavonoids varied differently depending on the seasonal phloem development (Fig. 4). The two flavanones, prunin and sak- uranin, remained nearly constant during the period from February to Novem- ber, whereas the concentrat ion of the structurally more distinct flavanone dihydrowogonin-glucoside decreased during the summer.

Starting with a very low level in spring, the isoflavone genistin content con-

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Fig. 2. Phloem flavonoids of the stem above the union (P. avium 'Sam') sampled in July. Age of the trees: (a) 3 years; (b) 4 years; (c) 6 years. Open bars: homospecific grafts, with (a) 1, (b) 3 and (c) 3 replicates. Hatched bars: heterospecific grafts, with (a) 13, (b) 10 and (c) 7 replicates. The vertical lines represent the standard deviation of the mean. D, dihydrowogonin 7-glucoside; P, prunin; G, genistin; S, sakuranin; C, chrysin 7-glucoside.

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Fig. 3. Phloem flavonoids of the stem above the union (P. avium 'Sam') sampled in November. Age of the trees: (a) 5 years; (b) 6 years. Open bars: homospecific grafts, each with 3 replicates. Hatched bars: heterospecific grafts, with (a) 9 and (b) 11 replicates. The vertical lines represent the standard deviation of the mean. D, dihydrowogonin 7-glucoside; P, prunin; G, genistin; S, sakuranin; C, chrysin 7-glucoside.

tinued to increase until autumn, with a maximum value in October, and declined thereafter. The flavone chrysin-glucoside remained constant during the spring and started to rise in summer, reaching a value about 50% higher than in spring.

189

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Fig. 4. Influence of the seasonal development of the phloem from P. a v i u m 'Hedelfinger' on the pattern of flavonoids in 1-year-old shoots. Age of the tree: 8 years. Each value is the mean of three replicates. The vertical bars represent the standard deviation.

S h o o t a g e . - - The phloem flavonoids estimated in the 1-, 2- and 4-year-old wood of a branch during November varied considerably (Fig. 5). The 1-year- old section was markedly different from the older ones. Prunin, sakuranin and genistin showed their lowest values and the glucosides of dihydrowogonin and chrysin their highest. Sakuranin and prunin increased continuously towards the oldest shoot section, the 3- to 4-year-old section exhibiting a 2-fold increase in prunin. With shoot ageing, the flavone chrysin-glucoside stayed constant. Genistin and dihydrowogonin-glucoside (Fig. 5) showed convex and concave curves, respectively.

T r e e a g e . - - When comparing 2- and 8-year-old trees, it was found that the flavonoids changed in only two cases in the phloem of 1-year-old shoots (Fig. 6). There was a slight reduction of dihydrowogonin 7-glucoside in the older

190

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Fig. 5. Effect of shoot age (branch section) on the content of flavonoids from P. avium 'Sam'. Three replicates for 1-, 3- and 4-year-old branch sections; 10 replicates for 2-year-old branch sections. Age of the tree: 5 years, The vertical bars represent the standard deviation.

Fig. 6. Effect of tree age on the content of flavonoids of the 1-year-old shoots. (a) age 2 years; (b) age 8 years. Open bars: P. avium 'Sam' 3 replicates. Hatched bars: P. avium 'Hedelfinger' 6 repli- cates. D, dihydrowogonin 7-glucoside; P, prunin; G, genistin; S, sakuranin; C, chrysin 7-glucoside.

trees and a reduction of chrysin-7-glucoside in the younger ones. It has already been shown in an earlier paper (Treut ter and Feucht, 1985 ) that 'Hedelfinger' produces more dihydrowogonin 7-glucoside than 'Sam'.

DISCUSSION

The occurrence of the flavonoids dihydrowogonin-7-glucoside, prunin, sak- uranin, chrysin-7-glucoside and genistin in the genus P r u n u s was shown by Hasegawa (1957) and Asahina et al. (1927). The first author quoted two com- pounds which were recently identified in early summer phloem and in vitro callus of P r u n u s av ium and P. cerasus (Treut ter et al., 1985). In the present extension of this work, all five flavonoids mentioned were further character- ized in the phloem ofP. avium. However, in this paper special emphasis is given to the changing phloem activity, taking into account seasonal variations as well as variations within the tree, both implying age-related processes. Since two such important phloem translocates as sugars and hormones influence the synthesis of polyphenols (Creasy and Swain, 1966; Shah et al., 1976), it is at least possible to expect changing quantities of flavonoids, not forgetting that

191

environmental parameters may also modify further the accumulation of poly- phenols (Dittrich and Kandler, 1971; Piretti et al., 1980; Duteau et al., 1981 ).

The dry matter of Prunus phloem contains up to 13% soluble sugars ( Schmid and Feucht, 1986), and it has been shown that sucrose promotes the accumu- lation of prunin and eriodictyol 7-glucoside (Treutter et al., 1985).

In general, fruit trees accumulate carbohydrates in late summer and autumn when both shoot and fruit growth have ceased ( Sakai, 1966). Possibly for this reason, all flavonoids in the present study, except dihydrowogonin 7-glucoside, showed an increasing trend, especially at the end of the season. It should be noted that the same seasonal pattern was found in the phloem of Prunus domestica for total phenols (Hillis and Swain, 1959). As with all dwarfing rootstocks, Prunus cerasus also has an age-accelerating effect on the whole graft symbiont. Practically all the flavonoids investigated showed a consistent, albeit not always significant, increase in concentration in the senescing trees as compared with the vigorously growing homospecific grafts. The physiology of natural ageing is complex. However, it is known that in Prunus species there is an overall decrease of minerals in the crown of interspecific grafts (Breen and Muraoka, 1975) and, in turn, mineral deficiency is one of several factors which enhance polyphenol accumulation (Margna, 1977).

The course of natural ageing is further demonstrated by the 1-, 2-, 3- and 4- year-old branch sections. Prunin and sakuranin invariably increased with advancing chronological shoot age. Genistin followed the same pattern until Year 3, and then exhibited great variability. Fruit trees exhibit a well-known general tendency to transfer water, nitrogen and phosphorus to the younger shoots ( Gardner et al., 1952 ), and it may be well to point out that the reduction of N in the older tissues may account for carbon shuttling at the PAL level to the phenolic pathway (Margna, 1977).

A decrease of phosphorus enhances the formation of flavonoids (Rossiter and Beck, 1966; Murali and Teramura, 1985).

It should be emphasized, however, that the distributional pattern of the fla- vonoids investigated is not uniformly related to age. For instance, both chrysin 7-glucoside and dihydrowogonin 7-glucoside were highest in the 1-year-old branch section as compared to the older ones. Thus, the destiny of the flavon- oids studied here can be different, and appears to be matched to diverging regulatory mechanisms which operate when the phenolic pathway has already traversed the primary flavanones.

Thus, the present data suggest that only some of the flavonoids can be used as indicators of advancing senescence. Any physiological involvement of the phenolics in premature ageing has yet to be established experimentally.

ACKNOWLEDGEMENT

This work was kindly supported by the Deutsche Forschungsgemeinschaft.

192

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