A ROLE FOR CARBOHYDRATE LEVELS IN THE CONTROL OF FLOWERING IN CITRUS

Transcription

1 Scientia Horticulturae, 26 (1985) Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands A ROLE FOR CARBOHYDRATE LEVELS IN THE CONTROL OF FLOWERING IN CITRUS E.E. GOLDSCHMIDT, N. ASCHKENAZI, Y. HERZANO, A.A. SCHAFFER and S.P. MONSELISE Hebrew University of Jerusalem, Department of Horticulture, Rehovot (Israel) (Accepted for publication 19 February 1985) ABSTRACT Goldschmidt, E.E., Aschkenazi, N., Herzano, Y., Schaffer, A.A. and Monselise, S.P., A role for carbohydrate levels in the control of flowering in citrus. Scientia Hortic., 26: Girdling in October of small or large fruitless branches increased 2--3-fold both starch content of leaves and flower numbers as compared with ungirdled 'Murcott' mandarin trees. Autumn girdling and GA 3 treatments were both effective and additive in increasing starch contents of leaves and twigs of 'Shamouti' orange trees. GA3, however, had the expected effect of depressing the reproductive inflorescences in both girdled and ungirdled branches, while girdling had the opposite effect. Girdling and fruit removal in October also additively and dramatically increased flower production in 'Murcott'. Lowtemperature regimes in a phytotron caused young 'Minneola' budlings to flower earlier in the season and more profusely, while having no effect on starch content of leaves and twigs. The interactions of increased carbohydrate content and gibberellin in the control of flower formation in citrus are discussed. Keywords: citrus; cold temperatures; de-fruiting; gibberellin; girdling. INTRODUCTION Carbohydrate levels have been suggested as a limiting factor for flower formation in citrus (Ogaki et al., 1963; Goldschmidt and Golomb, 1982) as well as in other fruit trees Harley et al., 1942; Worley, 1979; Monselise and Goldschmidt, 1982). Promotion of flower formation following girdling treatments has been taken as major evidence supporting this notion (Cohen, 1981), since girdling is known to cause accumulation of carbohydrates within the girdled branch (Furr and Armstrong, 1956; Fishler et al., 1983). Carbohydrates have also been assumed to play a dominant regulatory role in the nutrient diversion hypothesis of flowering, advanced recently by Sachs and co-workers (Sachs, 1977; Sachs and Hackett, 1983). On the other band, reduced levels of gibberellins appear to be the prerequisite for flower formation in citrus (Goldschmidt and Monselise, 1970; Guardiola /85/$ Elsevier Science Publishers B.V.

2 160 et al., 1982) and numerous other woody perennials (Bradley and Crane, 1960; Griggs and Iwakiri, 1961; Guttridge, 1962; Goldschmidt and Monselise, 1970; Landsberg and Thorpe, 1975). The presence of fruit has also been shown to reduce or even prevent flower formation in citrus trees (Moss, 1971; Jones et al., 1974). Cool winter temperatures are another factor which seem to enhance flower formation in citrus (Lenz, 1969; Moss, 1976). In the present report, we shall examine several lines of evidence bearing upon the role of carbohydrates and their possible interaction with other factors in the control of flowering in citrus. MATERIALS AND METHODS Twelve mature, shy-bearing 'Shamouti' orange trees (Citrus sinensis (L.) Osbeck) on sour orange stock (C. aurantium L.) were girdled in late October and 12 were left as controls; 6 girdled and 6 control trees were sprayed twice with 72 pm (25 mg 1-1) GA3 in November and December. Starch content of leaves and twigs was determined on 1 December. On the same trees, distribution of different spring shoot types was recorded in April. Six mature 'Murcott' mandarin trees (a natural C. reticulata Blanco hybrid) on sour orange stock were selected and on each tree branches were girdled and some of these were also defruited on 26 October; sampling for starch determinations was carried out 1 month later, and the flowers produced were counted in April. Three-year-old potted 'Minneola' tangelo budlings (a C. reticulata C. paradisi Macf. hybrid), on sour orange stock (which had been sparsely flowering the previous season), were held in an unheated greenhouse or at various day/night temperatures in a phytotron. After 48 days, starch determinations of different organs were carried out on 3 out of 6 budlings per temperature regime, and the rest were left under the same conditions for 39 additional days and then transferred to the greenhouse on 7 March. Total flower numbers until 31 May were recorded. Starch determinations were made as follows: after fresh weight determinations, tissues were dried in a draught oven at 65 C and dry weight measurements were taken. Soluble sugars were removed by extraction in 80% ethanol from 100-mg samples of ground, dried material. The remaining insoluble material was autoclaved in 6 ml dd water at 120 C for 1 h, and each sample was incubated with 50 mg amyloglucosidase (Sigma Chem.) at 60 C for 2 h (Thievend et al., 1972). Released glucose was measured with the anthrone reagent (Yemm and Willis, 1954). Enzyme blanks were used to correct for enzyme contamination. RESULTS The correlation between elevated carbohydrate (=starch) levels and flowering is again demonstrated by the data in Table I. The separation of

3 161 TABLE I Effects of girdling on starch and flowering of fruitless branches of different sizes of 'Murcott' mandarin (girdling on 26 October, sampling after 1 month) Branch Girdled Leaves starch Flowers (10 April) size 1 mg g 1 Ratio 2 Number Ratio -~ Small a ~ 70 c Small 42.7 b 25 d Large a 810 a Large b 280 b "Small" branches, approximately 0.7 cm diameter; "large" branches, approximately 3.5 cm diameter. Girdled/control. 3 Mean differentiation within columns by Duncan's multiple range test at 5% level; each number Ls an average of 6 replications. a branch from the rest of the tree by a girdle in autumn resulted in a high rate of starch accumulation and a corresponding nearly 3-fold increase in flower number per branch. The effects were of similar magnitude whether small branches or large, scaffold branches were girdled, indicating that girdling effects were not associated with root starvation which may be occurring when large portions of the canopy are enclosed by a girdle. The possibility that gibberellins interfere with flowering by lowering carbohydrate levels has also been examined (Table II). GA caused a small, but nevertheless significant, increase of starch levels in leaves of girdled branches as well as of ungirdled control trees. Nevertheless, these same TABLE li Starch contents of 'Shamouti' orange leaves and twigs as affected by GA and girdling. Girdling on 20 October, samples from starch determinations collected on 1 December, means of 6 trees per treatment, mg g- ~ dry matter Plant organ Girdled Ungirdled Significance of main effects' +GA GA +GA -GA GA Girdling Leaves 71.70m/t n/r 58.92a/u b/r + + Twigs m/t 92.33m/r a/u 62.54a/r - + 1Mean differentiation within plant organ by Duncan's muntiple range test at 0.05 level: a,b, for comparing + GA, within ungirdled; m,n, for comparing + GA, within girdled; r,2, for comparing ± girdled, within -GA; t,u, for comparing ± girdled, within +GA.

4 162 TABLE III Percentage of vegetative shoots, mixed and reproductive inflorescences in the spring flush of 'Shamouti' orange trees as influenced by GA 3 and girdling. Averages of 6 branches per treatment. Results presented in percentages, but statistics calculated after arc/sin transformation ~ Shoottype Control trees, girdled Ungirdled trees +GA GA +GA GA Vegetative bc 2.95 c a b Mixed a b ab ab Reproductive b a 2.07 c b ~Mean differentiation by Duncan's multiple range test at 5% level, within each shoot type. TABLE IV Effect of girdling and fruit removal from heavy bearing 'Murcott' mandarin on flower production. Treatments carried out 26 October, 1983, measurements made 10 April 1984 Treatment No flowers/ branch ~ Control + fruit 90 c Girdled + fruit 130 c Control - fruit 280 b Girdled fruit 810 a 'Mean differentiation by Duncan's multiple range test at 5% level; each number is the average of 6 replicates. trees responded to GA, as anticipated, by markedly reduced flowering and a shift towards the production of vegetative shoots (Table III). Girdling caused a strong, reverse shift toward the production of flowering. In the combined treatment, GA counteracted the girdling effect. The combined effects of fruit presence and girdling on flowering have been tested in heavily bearing 'Murcott' mandarins (Table IV). Girdling had little effect in the presence of fruit. Removal of fruit alone caused a 3-fold increase in the flowering of non-girdled controls and a 7-fold increase in the flowering of girdled branches. Thus the presence of fruit almost nullified the effect of girdling. Profuse flowering and suppression of leaf development following early summer girdling and fruitlet removal are shown in Fig. 1. Quantitative effects of cool temperatures on the promotion of flowering have been obtained in the present study with young 'Minneola' trees held in a phytotron under various temperature regimes (Table V). Starch levels

5 163 Fig. 1. 'Murcott' mandarin branches. Left, control; right, a branch girdled and de-fruited in May ~:983. Picture taken in March Girdled, sink-less branches naturally shed their leaves. TABLE V Starch content (mg g-~ dry weight) of organs of budlings after 48 days at different temperature regimes (day/night~ C ). Flower differentiation in the phytotron (Ph) or after transfer to the greenhouse (Gr) on 7 March, and total flower numbers until 31 May Parameters Control ~ 17 / 12 22/17 27/12 27/22 Degree hours (48 days) Starch Mature [eaves c b a b b Mature Lwigs b a bc bc 97.8 c Thick roots a a a a a Date of flower bud appearance 7 Mar. (Gr) 26 Jan. (Ph) 7 Mar. (Ph) 20 May (Gr). July (Gr) 3 Total flower No. until 31 May ' Kept in a~] unheated greenhouse during the winter months. 2 Mean differentiation within plant organs by Duncan's multiple range test at 5% level. 3 Flower b~ds appeared in July in this treatment.

6 164 did not correlate well with flowering in this case, however. Starch levels were rather high in general, and the differences between temperature regimes were minimal. Nevertheless, the intensity of flowering was in strict accordance with the exposure to cold temperatures. DISCUSSION The inhibitory role of gibberellins in citrus flower formation has been convincingly demonstrated through exogenous applications of gibberellins (Monselise and Halevy, 1964; Goldschmidt and Monselise, 1970; Guardiola et al., 1982) and growth retardants (Monselise et al., 1966). Endogenous gibberellin levels have, therefore, been assumed to play a regulatory role in the modulation of flowering of citrus, although estimates of gibberellin levels during flower differentiation have not been reported. The involvement of carbohydrates in the flowering of citrus, on the other hand, has so far only been indirectly implied, and it is difficult to visualize how direct, unequivocal evidence can be produced. Although girdling experiments provide seemingly solid correlations between starch levels and flowering, the possibility that other, more specific, factors accumulating above the girdle may control flower formation must not be neglected. We may nevertheless continue to use the term "carbohydrates" until some further characterization of the factors involved emerges. Data from various sources (Bodson, 1977; Goldschmidt and Golomb, 1982), including our own (Tables I--III), suggest that the carbohydrate factor invokes a quantitative flowering response over a range of starch concentrations, and not merely an all-or-none effect. The increased flowering following fruit removal (Table IV) is inconclusive, since it involves the removal of an important carbohydrate sink but, at the same time, eliminates a major natural source of gibberellins (Moss, 1971; Monselise, 1973). The temperature experiment (Table V) clearly indicates, however, that carbohydrate levels are not always the limiting factor for flowering of citrus. The cool temperature effect may, however, be explicable in terms of the gibberellin factor. Conditions which restrict root growth have previously been shown to be favourable for the flowering of citrus (Monselise and Halevy, 1964). The restriction of root growth imposed by cool temperatures may limit the amount of endogenous gibberellins within the canopy, thereby permitting more intense flower formation. Thus, the available evidence suggests that both gibberellins and carbohydrates play some role in the control of flowering in citrus, and the question should therefore be asked: are gibberellins and carbohydrates two regulatory factors acting independently, in two opposite directions, or do they act via each other? Our data (Tables II and III) show that GA did not exert its inhibitory role by lowering carbohydrate levels; in fact, GA treatments increased starch levels to some extent in both girdled and non-

7 girdled branches. The reverse hypothesis, that girdling acted by affecting endogenous levels of gibberellins during the time of flower bud initiation, has not been examined in the present study. However, we see no reason why autumn girdling should result in the lowering of GA levels above the girdle. On the contrary, if seedy fruit do indeed act as a source of gibberellins (Moss, 1971), girdling of branches bearing fruit would be expected to further reduce flowering, which was not the case (Table IV). We tend, therefore, to conclude that gibberellins and "carbohydrates" operate as independent regulatory factors in the flowering of citrus. A somewhat similar conclusion has recently been reached by Ross et al. (1984), who found, in their Pinus radiata system, that the specific flower-promoting effect of gibberellin A4 7 could not be explained in terms of carbohydrate sink acl~ivation. 165 ACKNOWLEDGEMENTS This work was supported by the United States--Israel Binational Agriculture and Development Fund (BARD). The skilled technical help of Messrs. J. Costo and D. Galili is gratefully acknowledged. REFERENCES Bodson, M., Changes in the carbohydrate content of the leaf and the apical bud of Sinapis during transition to flowering. Planta, 135: Bradley, M.V. and Crane, J.C., Gibberellin-induced inhibition of bud development in some species of plums. Science, 131: Cohen,,%., Recent developments in girdling of citrus trees. Proc. Int. Soc. Citriculture, 1981: Fishler, M., Goldschmidt, E.E. and Monselise, S.P., Leaf area and fruit size in girdled grapefruit branches. J. Am. Soc. Hortic. Sci., 108: Furr, J.]?~. and Armstrong, W.W., Flower induction in grapefruit in the Coachella Valley, California. Proc. Am. Soc. Hortic. Sci., 67: Goldschmidt, E.E. and Golomb, A., The carbohydrate balance of alternate-bearing citrus trees and the significance of reserves for flowering and fruiting. J. Am. Soc. Hortic. Sci., 107: Goldschmidt, E.E. and Monselise, S.P., Hormonal control of flowering in citrus and some other woody perennials. In: J.D. Carr (Editor), Plant Growth Substances. Springer, Berlin, pp Griggs, W.H. and Iwakiri, B.T., Effects of gibberellin and 2,4,5-trichlorophenoxypropionic acid sprays on Bartlett pear trees. Proc. Am. Soc. Hortic. Sci., 77: GuardioLa, J.L., Monerri, C. and Agusti, M., The inhibitory effect of gibberellin on flowering in citrus. Physiol. Plant., 55: Guttridge, C.G., Inhibition of fruit bud formation in apple with gibberellic acid. Nature (London), 196: Harley, C.P., Magness, J.R., Masure, M.P., Fletcher, L.A. and Degman, E.S., Investigations in the causes and control of alternate bearing in apple trees. USDA Tech. Bull. 792, pp Jones, W.W., Embleton, T.W., Barnhart, E.L. and Cree, C.B., Effect of time and amount of fruit thinning on leaf carbohydrate and fruit set in 'Valencia' oranges. Hilga:rdia, 42:

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