Summary Small Shorea stenoptera Burck. (Dipterocar-paceae) trees of reproductive age growing in an arboretum inwest Java were studied to determine the pattern of vegetativeshoot development, the time and method of floral initiation andthe effect of paclobutrazol on floral enhancement. Vegetativebuds were enclosed by two stipules between which was a leafprimordium, a small axillary vegetative bud and another pairof stipules. This sequence was reiterated five to seven timesbefore the vegetative apex was visible. At the time of floralinitiation, axillary buds developed into floral spikes and com-pound inflorescences formed at the end of drooping branches.A compound inflorescence might bear many floral spikes andeach floral spike bore many flowers. The compound inflores-cence was a modification of the reiterative developmentalpattern observed in vegetative shoots. The time of floral initia-tion began in late June or early July and continued until aboutNovember. Floral enhancement using paclobutrazol as a soildrench was attempted in mid-July, but this was later found tobe after the onset of floral initiation, and the treatment failedto enhance flowering; however, it appeared to enhance the rateof floral and fruit development. The similarity in vegetative buddevelopment among dipterocarp genera suggests that the timeof floral initiation may be easily determined in many speciesbased on simple dissection techniques.

Keywords: dipterocarp, floral enhancement, shoot develop-ment.

Introduction

The family Dipterocarpaceae may account for 10% of all treespecies and 80% of all emergent trees in tropical rain forests(Ashton 1982). Dipterocarps occur in both aseasonal lowlandrain forests where they are evergreen, and in seasonal uplandor more xerophytic forests where they are deciduous (Smiti-nand et al. 1980).

A striking feature of many dipterocarp forests is the phe-nomenon of mass flowering followed by mass fruiting (Ashtonet al. 1988). Mass flowering occurs at intervals of two to 10years, over a period of a few weeks to a few months in eithersmall areas or large regions such as the Malaysian Peninsula.

Large dipterocarps may produce up to four million flowers and120,000 fruits (Ashton 1988). Although the fruits are poorlyprotected and are eaten in vast numbers by wild pigs andheavily parasitized by weevils (Chan 1980, Ashton 1988), theforest floor following a mass fruiting is often blanketed bydipterocarp seedlings. In some regions, reforestation isachieved by lifting and transplanting the wild seedlings.

Managing dipterocarps for seed production is difficult. Firstflowering commonly does not occur until trees are 20 to 30years old (Ng 1966), by which time most are too large for easymanagement or seed harvest. Fruit production is episodic, andbecause seeds of most species are recalcitrant and store poorly(Sasaki 1980), there can be long intervals during which seedfor reforestation is unavailable. Although flowers have threelocules and two ovules per locule, normally only one indehis-cent seed develops per fruit. To overcome these obstacles,methods are needed to induce flowering at regular intervals inrelatively small trees.

Effective control of flowering, requires knowledge of thetime of floral initiation and the development of methods offloral induction or enhancement (Owens and Blake 1985). Thetime of natural floral initiation in dipterocarps has never beendetermined developmentally or experimentally. Dipterocarpsshow episodic shoot growth with distinct periodic vegetativeflushes, followed by periods of shoot elongation. Flushes com-monly precede or coincide with flowering, and inflorescencestend to be borne near the ends of shoots (Burgess 1972).Stimuli that induce flushing and flowering in dipterocarps arenot known. Ashton et al. (1988) proposed that flowering iscontrolled by an endogenous rhythm, perhaps related to epi-sodic shoot flushes and triggered by an external cue. Wycher-ley (1973) suggested that temperature may be responsible forfloral induction in dipterocarps. Ashton et al. (1988) found noclose correlation between flowering intensity of Shorea spe-cies and photoperiod or water availability, and proposed thatexposure to a 2 °C decrease in minimum nighttime tempera-ture for several nights may be sufficient to induce flowering insome dipterocarps.

Increased temperature and drought, usually in conjunctionwith gibberellin A4/7 application, are common cultural treat-ments applied to induce cones in Pinaceae (Owens and Blake

Time and method of floral initiation and effect of paclobutrazol onflower and fruit development in Shorea stenoptera (Dipterocarpaceae)

DIDA SYAMSUWIDA1 and JOHN N. OWENS2

1 Forest Tree Improvement Research and Development Institute, Jl. A.M. Sangaji Km. 15, Purwobinangun, Pakem, Sleman, DI Yogyakarta 55582,Indonesia

2 Centre for Forest Biology, University of Victoria, Victoria, B.C. V8W 2Y2, Canada

Received January 11, 1996

Tree Physiology 17, 211--219© 1997 Heron Publishing----Victoria, Canada

1985). The mode of action of these treatments, however, hasyet to be demonstrated. One hypothesis is that these culturaltreatments reduce vegetative growth, and the resulting excessgibberellins promote flowering (Pharis 1976). Gibberellinshave not been found to enhance flowering in hardwood foresttrees. However, flowering in several temperate fruit trees(Lever 1986, Steffens et al. 1993), clove (Martin and Dabek1988) and Eucalyptus (Griffin et al. 1993) is enhanced whenthe growth retardant paclobutrazol is applied as a foliar spray,stem injection or soil drench.

The objectives of this study were to determine the time andmethod of floral initiation and to investigate the use ofpaclobutrazol as a method of enhancing flowering in young,reproductively mature Shorea stenoptera Burck. trees.

Materials and methods

In June 1994, 27 reproductively mature S. stenoptera trees thathad previously flowered were selected from a stand of over 100trees at the Kebun Percorbaan Arboretum in west Java, Indo-nesia (lat. 6° N, long. 106° E). Trees were grouped into threecategories, with nine trees in each group, on the basis ofcircumference at breast height: Group I, 51--55 cm; Group II,46--50 cm; Group III, 41--45 cm. Three trees of each size classwere randomly assigned to each of three paclobutrazol treat-ments: A, high concentration; B, low concentration; and C,control.

Floral initiation

From mid-July through October, branches were sampled fromseveral trees every month. The terminal bud on each branchwas removed and placed in a vial with formalin/acetic acid/al-cohol (FAA) fixative (Johansen 1950). Buds were stored inFAA and taken to the University of Victoria where a centrallongitudinal section 1--2 mm was embedded in ‘‘Tissuemat’’(Fisher Scientific, Ottawa, Canada) using the tertiary-butylalcohol dehydration series of Johansen (1950). Embeddedspecimens were softened in Gifford’s softening reagent (Gif-ford 1950) for 2 to 4 weeks and then serially sectioned longi-tudinally in increments of 6 µm. The median sections weremounted on microscope slides and stained with safranin andfast green and examined to determine whether they were vege-tative or reproductive.

Other buds were dissected by removing successive stipulesuntil the terminal bud or apex and the axillary vegetative budsor inflorescence primordia were visible. The shoot tips, orportions of them, were dehydrated in an ethanol series, criticalpoint dried, mounted on aluminum stubs, sputter-coated withgold and observed with a JOEL JSM-35 scanning electronmicroscope (SEM) (JOEL, Boston, MA) at 10 kV. Other budswere dissected, sketched and photographed with a Wild M 400Photomacroscope (Leica Canada Inc., Willowdale, Canada) toshow bud morphology. In all three types of specimen observa-tion, vegetative buds and buds that had undergone floral initia-tion and various stages of floral development were observedand photographed.

Floral enhancement

In mid-July, sample trees were treated once with one of threepaclobutrazol treatments. A shallow circular trench (about10-cm-deep) was dug at a radius of 1 m from the base of eachtree. Paclobutrazol in the water soluble form ‘‘Cultar’’ wasobtained from ICI (Indonesian Operations, Jakarta). Concen-trations and application methods were based on successfulexperiments with Eucalyptus (Griffin et al. 1993, Moncur et al.1994). Paclobutrazol treatments were either high concentra-tion (1.0 g of paclobutrazol per 15 cm of trunk circumference),low concentration (0.5 g of paclobutrazol per 15 cm of trunkcircumference) or a control (water only). The appropriateamount of paclobutrazol was added to 4 liters of water, whichwas then mixed and poured into the trench around each tree.

Once per month in September, November, December andJanuary 1994, trees were climbed and the numbers of inflores-cences with flowers and with developing fruits counted. Thenumber of flowering compound inflorescences and number offruiting compound inflorescences were counted on each obser-vation date. Numbers of flowers or fruits per compound inflo-rescence or per spike were not counted. Because trees variedin size, numbers of branches and sites in which inflorescencesformed and the number of primary branches per tree werecounted to determine the percentage of branches bearing inflo-rescences.

To observe any treatment carry-over effect, trees were as-sessed for frequency of flowering in November 1995, i.e., theyear following paclobutrazol treatment.

Average rainfall per month was obtained from the weatherstation at the study site.

Figures 1--8. Vegetative and reproductive shoots and buds and fruits ofShorea stenoptera.

Figure 1. Vegetative branch of mature tree showing red stipules (S).

Figure 2. Close-up of vegetative branch showing stipules and emerg-ing leaf (L).

Figure 3. Partially dissected vegetative bud with outer stipule removedto reveal the leaf primordium, axillary buds (A) and small green stipuleinside.

Figure 4a. Vegetative bud with one stipule removed to show leafprimordium and stipule within and axillary bud at the onset of floralinitiation. Figure 4b. Compound inflorescence with outer stipule re-moved to show enlarged axillary inflorescence (AI).

Figure 5. Later stage of a compound inflorescence with two stipulesremoved to show two stages of axillary inflorescence development.

Figure 6. Compound inflorescence during flowering showing redstipules at the base of each spike (SP), and developing fruits ( ).

Figure 7. Distal portion of a compound inflorescence showing severalspikes. Most flowers (F) have been shed and one is aborting ( )leaving the subtending bracts (B). Most stipules have been shed.

Figure 8. Three nearly mature globose fruits showing leaf-like wings.All other flowers abscised.

212 SYAMSUWIDA AND OWENS

FLOWER AND FRUIT DEVELOPMENT IN SHOREA STENOPTERA 213

Statistical analysis

Numbers of inflorescences bearing flowers and fruits wereanalyzed separately, or summed for each date and analyzed bya balanced two-factor analysis of variance (ANOVA) designwith size class and paclobutrazol dosage as fixed effects.Because size class did not influence the number of branchesbearing flower or fruit inflorescences (α = 0.05), all sizeclasses were pooled and subjected to a single-factor ANOVAfor each date. Significant effects were further analyzed withthe Duncan’s multiple range test (Zar 1984).

Observations and results

Shorea stenoptera formed pendant compound infloresences atthe ends of long, often drooping branches in younger trees(Figure 1), and on shorter secondary and tertiary branches inolder trees. The inflorescences, which were initiated and un-derwent early development within the terminal, partiallyfused, protective, stipules on the branch (Figures 2 and 3),commonly had seven to 15 floral spikes (Figure 6). Spikeswere spirally arranged but appeared alternate, and were sepa-rated by long internodes (Figure 7). Spikes matured acropet-ally on the branch and flowers matured acropetally on eachspike (Figure 6). Each spike was enclosed by two stipulesduring early floral development (Figures 4 and 5), and bore oneto usually many spirally arranged flowers (Figure 7), each ona short pedicel. Each flower had two opposite subtendingbracts. Most flowers abscised at the base of the pedicel, leavingthe two bracts that usually abscised a few days later (Figure 7).Rarely did more than one fruit develop per spike. Fruits rapidlyenlarged, and the five sepals elongated, enclosing the globosefruit at their base, and forming three large and two small, broadflat wings. Spikes were pendant and the wings hung downward(Figure 8).

Vegetative bud development

Development within the vegetative bud provided a remarkableexample of reiteration. Each bud was enclosed by two large

spoon-shaped red stipules, each of which attached around halfof the stem and the base of a leaf. The two stipules adheredalong their margins. Between the pair of stipules was a leaf, anaxillary bud and another pair of smaller green stipules (Figures2, 3 and 4a). Within these stipules was, again, a leaf, an axillarybud and a yet smaller pair of stipules. These structures wereoften reiterated five to seven times (Figure 9) as revealed bydissection (Figures 3, 4 and 10) and SEM, before the shootapical meristem was visible (Figure 11). The inner and, to alesser extent, the outer surfaces of the young stipules werecovered by long epidermal hairs (Figure 11) that stuck to theleaf primordia and other stipules.

The apical meristem was a low dome with an oval crosssection, from which successive leaf primordia arose at oppo-site ends (Figure 12). Soon after a leaf primordium was initi-ated, two ridges of meristematic tissue arose at the primordiumbase along the sides of the oval (Figure 12). These ridgesextended around the apical meristem and base of the leafprimordium, and elongated upward to form two oppositestipule primordia (Figure 13). As the stipule primordia elon-gated, they became spoon-shaped and contacted one anotheralong their margins. Epidermal hairs formed on the stipules,causing them to stick together along most of their margins,completely enclosing the leaf primordium and apical meristemwithin (Figure 11). As the leaf primordium elongated and a leafblade began to form, a lateral bud primordium arose in the axilof the leaf primordium (Figure 12). By this time, the next leafprimordium and its stipules had been initiated (Figures 12--14).

Axillary buds on most vegetative shoots remained small.The axillary apex initiated two scales (Figure 12) that elon-gated and enclosed the apex (Figure 11). Development may bearrested at this stage (Figure 15) or the apical meristem mayinitiate leaf primordia and stipules. In the smaller trees, theprimary branches generally lacked secondary branches so ax-illary buds appeared to become latent (Figure 15). In largertrees, secondary and tertiary branches formed from the axillarybuds.

Figure 9. Diagram of a Shoreastenoptera vegetative bud withsuccessive stipules (S) removedto show the enclosed leaf primor-dia (L) and axillary buds (A). Thesequence may be reiterated fiveto seven times as shown in a--e.

214 SYAMSUWIDA AND OWENS

Floral initiation and development

Inflorescence initiation was observed in some buds collectedin July and in all subsequent collections through November. Atinflorescence initiation, the vegetative apex underwent transi-tion to an inflorescence apex. The terminal inflorescence apexwas dome-shaped and initiated a spiral sequence of stem unitsconsisting of a leaf primordium, two stipules and an axillaryapex (Figure 16). Development of the components of the stemunits differed depending on the time of inflorescence initia-tion, with leaf primordia being smaller or absent in stem unitsinitiated later, whereas stipules and internodes were longer and

axillary buds enlarged more rapidly (Figures 16--18). All axil-lary buds initiated in the inflorescence developed into floralspikes (Figure 18).

An axillary bud that developed into a floral spike was en-closed by two opposite stipules (Figures 4, 5 and 19). A leafprimordium was present in the first few stem units, which werereproductive (Figures 4 and 5), but was absent in subsequentmore distal stem units (Figures 6 and 7). Spike primordiaelongated and initiated a series of lateral apices. Each lateralapex initiated two large bract primordia (Figures 19--21), thenfive sepals that remain fused at the base (Figures 20--22), five

Figures 10--15. Vegetative Shoreastenoptera buds using light (LM)and scanning electron microscopy(SEM).

Figure 10. Fixed and partially dis-sected bud viewed with a dissect-ing LM. Outer stipules wereremoved to show an inner stipule(S3), its leaf primordium (L3) anda bud (A4) in the axil of each leaf.Different subscripts indicatestages of development.

Figure 11. Enlarged SEM of a budas in Figure 10. The stipules,leaves and axillary buds havingthe same subscript as in Figures10 and 11 are at the same stage.The stipule (S2) has been removedto reveal its leaf (L2), the apicalmeristem (arrow) and the young-est leaf primordium (L1).

Figure 12. Enlarged SEM of theapical meristem ( ) and leaf pri-mordia (L1, L2, L3) and the axil-lary bud (A3) as shown in Figure11. Two stipules (S1) are thosethat belong to L1. Outer stipules(S2) have been removed. An axil-lary bud (A2) has just begun to beinitiated in the axil of L2. Axillarybud (A3) has two lateral scales (ar-rowhead).

Figure 13. A later stage of stipuledevelopment to that shown in Fig-ure 12. Stipules (S1) for L1 haveenlarged covering the apex. Thenext leaf primordium (L0) is beinginitiated.

Figure 14. Median longitudinalsection of a vegetative bud as inFigure 13 showing the apical mer-istem ( ) leaf primordia (L1, L2)and the axillary bud (A3) for leafthree.

Figure 15. Median longitudinalsection of an axillary vegetativebud that has become latent soonafter being initiated.

FLOWER AND FRUIT DEVELOPMENT IN SHOREA STENOPTERA 215

petals fused along most of their length forming a coronateflower bud (Figures 20--22), fifteen stamens (Figure 23) fusedto the base of the petals, and finally, a central pistil with threelocules and a single style. Proximal spikes were short with onlyone or few flowers, central spikes bore many flowers and distalspikes decreased in length and number of flowers acropetally

until the most distal spikes usually bore only one flower (Fig-ure 7). The inflorescence usually terminated in a single or pairof flowers (Figure 7). Occasionally, the inflorescence apexreverted back to a vegetative apex and resumed leaf initiation.This resulted in one or more floral spikes along an otherwisevegetative branch. Floral spikes had no arrested period of

Figure 21. SEM of newly initiated floral spike showing distal spike apex (arrowhead), paired bract primordia (one removed) and flower primordiumshowing five sepals and onset of petal initiation.

Figure 22. SEM of flower primordium at slightly later stage than in Figure 21, showing five sepal primordia ( ) and five petal primordia onstar-shaped flower primordium.

Figure 23. SEM of a flower bud at latest stage shown in Figure 20 but with bracts, sepals and petals removed to show the 15 stamen primordia.

Figures 16--23. Inflorescence de-velopment in Shorea stenopterausing light (LM) and scanningelectron microscopy (SEM).

Figure 16. Fixed and partially dis-sected compound inflorescencebud viewed with a dissecting LMafter outer stipules were removedto reveal inner stipule (S), leaf pri-mordia (L1--L4), large axillary flo-ral spike buds (SP) and terminalinflorescence bud (arrowhead).

Figure 17. LM of median longitu-dinal section of a young floralspike in the axil of a leaf (L)showing bract (B) primordia. Ax-illary flower primordia are out ofthe plane of view or not yet initi-ated.

Figure 18. LM of median longitu-dinal section of an inflorescencebud showing floral spike primor-dia at various stages of develop-ment.

Figure 19. SEM of a floral spikeprimordium elongating beyondbasal stipules showing bract pri-mordia with axillary flower api-ces (arrows) and the distal spikeapex (arrowhead) starting to initi-ate more bracts.

Figure 20. SEM of a developingspike after most bracts were re-moved. Bracts on lower side ofphotograph cover spike apex andyoungest flower primordia.Shown clockwise, three stages offlower development in aboveflower primordia. Five sepals ( )have been initiated, three largeand two small, and inside theseare five petal primordia.

216 SYAMSUWIDA AND OWENS

development. Within an inflorescence, they developed, elon-gated and burst from the stipules in an acropetal manner overseveral weeks (Figure 6).

Floral enhancement

Dissections of buds collected in July showed that floral initia-tion had begun in some branches of some trees before thepaclobutrazol treatments were applied in mid-July. In Septem-ber, nonfruiting inflorescences were visible, but there were nosignificant differences among treatments. No observationswere made in October. In November, there were abundantflowering and fruiting inflorescences per tree. Statisticalanalyses showed that paclobutrazol concentration had a sig-nificant effect (P = 0.0463) on the number of branches bearingflowering inflorescences in November only. The Duncan’smultiple range test (α = 0.05) verified that paclobutrazol ap-peared to depress the number of branches bearing floweringinflorescences relative to the control treatment. However, thenumbers of fruiting and flowering inflorescences combinedwere not influenced by paclobutrazol on any observation date.The treatment differences in November suggest that, althoughpaclobutrazol did not influence the total number of floweringand fruiting inflorescences, it did hasten fruit development sothat control trees had significantly more flowering inflores-cences but fewer fruiting inflorescences than paclobutrazol-treated trees. In January, there was a marked decrease inflowering and fruiting inflorescences because flowering hadceased and many inflorescences had shed their fruits (Fig-ure 24).

There was no carry-over effect of paclobutrazol in the 1995flowering--fruiting season. Flowering was sparse and randomamong the control and treated trees.

Discussion

The development of vegetative buds and the initiation or devel-opment of inflorescences have not been described for anydipterocarp. Based on our observations of species of Diptero-carpus, Hopea, and Shorea, at least these major genera havevery similar vegetative bud and shoot development. Shoreastenoptera vegetative buds were enclosed by two large, red,opposite, obtuse stipules, the bases of which encircled the stemjust below the leaf axil. These stipules adhered along theirmargins, and opened as the leaf, shoot axis and reiteratedterminal bud inside elongated. Five to seven reiterative budswere enclosed within the outermost stipules (Figure 9). Reit-eration is a well documented developmental pattern in manywoody perennials, and is well represented in the above generaand easily observed in S. stenoptera. The predictability of thispattern of development means that the position of potentialinflorescence primordia as axillary buds is easily seen and thetime of floral initiation can be easily determined by dissectingterminal buds.

We found that shoot elongation and flowering in S. stenop-tera began during the dry season, June through September, butcontinued through the rainy season, December and January(Figure 25). This agrees with the results of van Schaik et al.(1993) who found that peaks of flushing and flowering intropical forests occur preceding and during the early rainyperiod. Water availability is thought to be important in control-ling the phenologies of many tropical forest trees (Reich andBorchert 1984). Vegetative phenology, in turn, may affectfloral initiation as it does in temperate trees (Owens and Blake1985).

Floral initiation began in June or July and continued at leastinto December. Inflorescences were at all stages of develop-ment on a tree, and many stages of development were apparentwithin an inflorescence. Therefore, a single time of floralinitiation, as found in most temperate trees (Owens and Blake1985), does not exist in S. stenoptera. As a result, it is difficultto determine particular climatic factors causing flowering.Several factors may be involved. Even in temperate trees, inwhich climate is seasonal and time of initiation precise, manyclimatic factors, including rainfall, temperature and light in-tensity, affect and interact to stimulate floral initiation. Thiscommonly occurs during or after lateral branch elongation, thebranches on which flowers are initiated (Owens and Blake1995). Because floral initiation in S. stenoptera begins andpredominantly occurs during the early rainy season, the pre-ceding drier period could be a factor. Vegetative growth, asshoot elongation, may be reduced during the dry season. Dur-ing the rainy season, floral initiation stops but flowering con-tinues, though less abundantly. van Schaik et al. (1993)demonstrated major roles for irradiance and water stress in thecontrol of phenology of tropical woody plants. Chaikiattiyoset al. (1994) describe how floral induction in tropical treesfollows a check in vegetative growth, which normally occursduring the dry season. Ashton et al. (1988) considered droughtto be an unlikely factor in floral initiation in dipterocarps; but,until shoot water potentials are measured, as has been done insome tropical fruit trees (Chaikiattiyos et al. 1994), drought

Figure 24. Histogram showing percent of branches bearing floweringand fruiting inflorescences for control (C) and low (L) and high (H)concentrations of paclobutrazol over four months.

FLOWER AND FRUIT DEVELOPMENT IN SHOREA STENOPTERA 217

cannot be ruled out as a factor in floral initiation in diptero-carps. The dry season is also probably accompanied by in-creased periods of sunshine because of less cloudy conditions.Ng (1977) suggested that an increase of 2 h per day of sunshinecould induce flowering in dipterocarps. Less cloud may alsoresult in slight increases and decreases in day and night tem-perature, respectively.

Although we were unable to identify the mechanism bywhich environmental cues affect floral initiation in diptero-carps, we demonstrated that the time of floral initiation can bedetermined by bud dissection. Floral spikes arose from theaxillary buds and the latter were always present within thereiterated buds. Also, the early axillary inflorescence budswere easily distinguished from axillary vegetative buds. Al-though bud dissection will reveal only after the fact that floralinitiation has begun, this information is useful because floralinitiation likely occurs at about the same time each year andover several months. Therefore, floral enhancement may bepossible before or during the time when inflorescence budsbecome visible.

Paclobutrazol affected flowering and fruiting, but not in themanner we expected. In September, there was a trend towardincreased flowering with increasing concentration of paclobu-trazol (Figure 24); however, this trend was not statisticallysignificant. In November, control trees had more flowers thantrees in either paclobutrazol treatment, but no significant treat-ment effects were found when numbers of flowering andfruiting inflorescences were combined. This, together with ourdevelopmental data, leads us to conclude that the applicationof paclobutrazol was either too late or at too low a concentra-tion to affect floral initiation, but that it hastened floral and fruitdevelopment. This is consistent with effects of paclobutrazolon fruit trees where it acts as a growth retardant by suppressinggibberellin production, thereby reducing cell division and cellexpansion (Dalziel and Lawrence 1984). Reduction in vegeta-tive growth allows greater partitioning of assimilates to repro-ductive growth, flower and fruit formation and fruit growth(Lever 1986). In apple (Quinlan and Richardson 1986), peach(Erez 1986), clove (Martin and Dabek 1988) and Eucalyptus(Griffin et al. 1993, Moncur et al. 1994), paclobutrazol reduced

vegetative growth and enhanced fruit production, but in allcases, the time of treatment was important. Our results togetherwith those listed above indicate that paclobutrazol may beuseful for enhancing fruit production in some dipterocarps. Itmay be especially beneficial if applied before floral initiationto young dipterocarps thereby reducing vegetative growth overseveral years and thus making the trees a more manageablesize for seed collection (Erez 1986).

Acknowledgments

This research was supported by the ASEAN Forest Tree Seed CentreProject (Muak Lek, Saraburi, Thailand), the Forest Tree ImprovementResearch and Development Institute (Yogyakarta, Indonesia) andthrough a Canadian NSERC Operating Grant (No. 1982) to J.N.O.Appreciation is extended to Parlin Tambunan for collection of speci-mens and flowering data, to the staff at The Kebun Percorbaan Arbo-retum, west Java, Indonesia for use of the trees and assistance incollection of specimens, Glenda Catalano for technical assistance, Dr.Derek Harrison for statistical assistance and Diane Gray for typing themanuscript.

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