Scientia Horticulturae, 56 (1994) 235-243 235 Elsevier Science Publishers B.V., Amsterdam

Morphology of flower initiation ofbrodiaea

S.S. Han *'a, A.H. Halevyb, 1, R.M. Sachs b, M.S. Reid b aDepartment of Plant and Soil Sciences, French Hall, University of Massachusetts, Amherst, MA

01003, USA bDepartment of Environmental Horticulture, University of California, Davis, CA 95616, USA

(Accepted 20 July 1993)

Abstract

The main axis of dormant brodiaea corms (Triteleia laxa) harvested from the field in the summer comprises a leaf sheath, two leaf primordia, and a vegetative meristem. Floral induction, as indicated by initiation of bracts on the flanks of the apical meristem, occurs after corm dormancy is broken. The bracts enlarge and enclose the reproductive dome, which then divides to form floral primordia of different ages. The daughter corm, which forms from the basal portion of the stem, starts to enlarge early in the growing season. Unlike many other geophytes, there is little competition, in brodiaea, for assimilates between the vegetative propagules (daughter corm and cormels) and the developing inflorescence.

Keywords: Triteleialaxa

Introduction

Flowers of different species of the brodiaea complex are of interest as com- mercial cut flowers, and one, Tri te le ia laxa cultivar 'Queen Fabiola' , is al- ready in widespread commercial product ion (Fig. 1 ). Optimizing cultural techniques for commercial product ion of geophytes, such as the brodiaeas, requires study of growth and development o f the vegetative plant and of the control of flower bud init iat ion and development. Har tsema ( 1961 ), using Blaauw's system, classified geophytes according to the t iming of flower initi- at ion relative to vegetative development and anthesis. Some geophytes initi- ate flowers long before anthesis; in others, flower init iat ion occurs almost si- multaneously with leaf emergence followed almost immediately by flower development. Niehaus (1971 ) showed that members of the brodiaea com- plex were summer-dormant perennials in their native habitats. H e found that when corms break dormancy in September, leaves begin to elongate and root

*Corresponding author. 1Permanent address: Department of Horticulture, The Hebrew University of Jerusalem, P.O.B. 12, Rehovot 76-100, Israel.

© 1994 Elsevier Science Publishers B.V. All rights reserved 0304-4238/94/$07.00

236 s .s . Han et al. / Scientia Horticulturae 56 (1994) 235-243

Fig. 1. Morphological organs of a Triteleia laxa plant: f, individual flower; b, bract; s, scape; 1, leaf; mc, mother corm; dc, daughter corm.

tips protrude. Rapid growth commences after the first heavy rainfall and the leaves grow slowly, reaching their final size during February and March. The scape grows from the apex of the corm and first appears at the soil surface in January. Starch stored in the mother corm is rapidly metabolized during leaf and scape growth, and the corm decreases in volume. A new corm forms at the base of the new leaves and matures before anthesis, and the leaves are dead or nearly so at anthesis. These observations by Niehaus ( 1971 ) have not been complemented by a detailed study of the t iming of morphological events during flower bud initiation and development in brodiaeas. Early growth and development of leaves, roots, reproductive organs and the for- mat ion of the daughter propagules in geophytes are at the expense of carbo- hydrate stored in the propagules. In gladiolus, there is strong competi t ion for storage carbohydrate between the daughter propagules and the developing in-

S.S. Han et aL / Scientia Horticulturae 56 (1994) 235-243 237

florescence (Halevy and Monselise, 1961 ). The daughter corm and cormels of gladiolus enlarge very slowly during inflorescence development, reaching only 10% of their final weight by anthesis and grow rapidly after anthesis (Shillo and Halevy, 1976 ). In commercial production of gladioli corms, de- veloping inflorescences are removed, a practice which significantly increases the production of daughter corms and cormels (Halevy and Monselise, 1961 ). The aims of the study reported here were to examine dry matter partitioning, and gross and microscopic morphology of brodiaea plants during their entire growth cycle.

Materials and methods

Morphological development. - In late fall, the previous season's corms of Tri- teleia laxa (average weight 1.32 g) were planted outdoors at the University of California, Davis. The corms were planted individually 5 cm from the soil surface in PVC containers containing loamy sand. Unless otherwise stated, the flowers were harvested at anthesis. At 2-4 week intervals throughout the entire growth cycle of the plants (until the leaves had senesced), three repli- cate plants were harvested and dissected and the developmental stages of the stems and inflorescences were noted.

In subsequent experiments, corms were planted in PVC containers contain- ing commercial soilless mix and grown in a greenhouse ( 18 ° C night temper- ature). At various times after planting, plants were harvested and dissected. Shoot axes and reproductive domes were fixed in 2% glutaraldehyde (in 0.1 M phosphate buffer, pH 7), dehydrated in an ethanol series, critical-point dried in CO2, and photographed using a scanning electron microscope (ISI DS 130, International Scientific Instruments, Pleasanton, CA).

Dry weight changes o f various organs. - Plants were harvested throughout de- velopment and dry weights of separated leaves, mother corms, and daughter corms plus cormels of three replicate plants were measured at each sampling. The dry weight of the inflorescence was measured until anthesis of the first flower.

Results and discussion

Growth and development. - Brodiaea corms lifted from the field in the sum- mer remain dormant for a few months, then start to grow in early fall, even if held dry at room temperature (Han, 1988 ). Two weeks after planting corms that had broken dormancy, shoots began to emerge from the soil and root growth commenced (Fig. 2). Each brodiaea shoot comprises a leaf sheath and two narrow leaves. The leaves grow rapidly after emergence and continue to elongate until the end of the growing cycle (Fig. 3 ). At 9 weeks after plant-

238 s.s. Han et al. I Scientia Horticulturae 56 (1994) 235-243

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Fig. 2. Dry weight partitioning of (a) mother corm, (b) leaves, (c) daughter corm and cormels, and (d) inflorescence ofbrodiaea during the growing period.

S.S. Han et aL /Scientia Horticulturae 56 (1994) 235-243 239

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ing, a swelling just above the mother corm indicated the future location of the replacement daughter corm. Concurrently, the axillary buds in each plant were growing rapidly and the flowers had initiated. The mother corm had almost disintegrated 15 weeks after planting, tips of the leaves had begun to senesce, and a cormel had formed at the base of each of the axillary shoots (Fig. 2). The number ofcormels formed depends on the size of the mother corm (Hail et al., 1991 ). The scape started to elongate rapidly 21 weeks after planting and continued to grow until anthesis (26 weeks after planting). Complete senescence of the leaves occurred 4-5 weeks after anthesis.

Storage reserves in the mother corm, indicated by corm dry weight, were continuously depleted throughout the growing cycle (Fig. 2 (a) ) . Depletion was most rapid between 4 and 9 weeks after planting, when the roots and the axillary buds in each plant were growing rapidly and the flowers and the daughter corm had initiated. Depletion of the storage reserves between 9 and 15 weeks was accompanied by rapid growth of the leaves. By 15 weeks after planting, storage reserves in the mother corm were almost gone and the leaves had reached half of their final length (Fig. 3 ). The leaves began to transport carbohydrates basipetally for the enlargement of the daughter corm (Fig. 2 (c) ), as evidenced by rapid accumulation of dry weight of the daughter corm and cormels. In cultivation, the combined fresh weight of the daughter corms and cormels examined was always greater than that of the mother corm (Han et al., 1991 ). Major dry weight accumulation in the inflorescence commenced

240 S.S. Han et al. / Scientia Horticulturae 56 (1994) 235-243

when the scape started to elongate rapidly 21 weeks after planting and contin- ued until anthesis, when the inflorescence was removed (Fig. 2 (d ) ) . The competition for assimilates between the inflorescence and the daughter corm and cormels during this period slightly reduced the rate of growth of the veg- etative propagules (Fig. 2 (c) ) .

Microscopic observation of flower bud initiation and development. -The apical meristem of freshly harvested brodiaea corms is vegetative (Fig. 4 (A)) . The shoot axis, comprising two leaf primordia and a vegetative meristem, sur- rounded by a leaf sheath (Fig. 4(B) ) is embedded in the mother corm, ap- proximately 5-10 mm from the surface. Floral induction, indicated by initi- ation of bracts on the flanks of the apical meristem, occurred 1 month after planting. Concurrently, the apical meristem of the daughter corm appeared near the apex of the mother meristem (Figs. 4 (C) and 4 (D) ) . Six weeks after planting, the two outside bracts enclosed the developing inflorescence (Fig. 5 (A)) and another three to four bracts were initiated. Florets were not yet visible. By 10 weeks after planting, however, three to four floret primordia

Fig. 4. Scanning electron micrograph of Triteleia laxa: (A) vegetative apical meristem; (B) a shoot axis showing the leaf sheath (Is) and the tip of a leaf (1); (C) initiation of two bracts (b) on the flanks of the meristem; (D) relative position of the inflorescence (i), daughter corm meristem (dm), and the daughter corm (dc), all embedded in the mother corm.

XS. Han et al. / Scientia Horticulturae 56 (1994) 235-243 241

Fig. 5. Scanning electron micrograph of Triteleia laxa reproductive meristem: (A) two bracts enclosing the whole inflorescence 6 weeks after planting; (B) formation of four flower primor- dia (f) 10 weeks after planting; (C) formation of the outer tepals (ot) 12 weeks after planting; (D) initiation of the inner tepals (it) 15 weeks after planting; (E) a developing inflorescence with different stages of flower buds.

had formed (Fig. 5 ( B ) ) . The outer tepals o f the largest pr imordia were ap- parent 12 weeks after planting (Fig. 5 ( C ) ) and the inner tepals formed by 15 weeks (Fig. 5 ( D ) ) . At 18 weeks after planting, many florets at different de- velopmental stages were observed (Fig. 5 (E) ). Rapid scape elongation began

242 S.S. Han et al. / Scientia Horticulturae 56 (1994) 235-243

20 weeks after planting. The first floret reached anthesis approximately 16 weeks after its initiation (Fig. 2 ), or about 26 weeks after planting, at which t ime the scape was at max imum length.

Formation and enlargement of the daughter corm and cormels. -The daughter corm, formed by enlargement of the basal port ion of the stem (Fig. 4 (D) ), started to enlarge early in the growth cycle (Figs. 2 and 4 (D) ). Its absolute growth rate, as measured by dry weight gain, was slow initially, but increased rapidly from 15 weeks after planting. Unlike other geophytes, where the main development of the daughter propagules occurs after anthesis, the daughter corms of brodiaea reached 80% of their final weight by anthesis, and contin- ued growing until leaf senescence (Fig. 2 (c) ) . These data suggest that, in con- trast to other geophytes (Halevy and Monselise, 1961 ), there is much less competi t ion for assimilates in brodiaea between the daughter corm and cor- mels and the developing inflorescence. This hypothesis is consistent with the observation that removing the developing inflorescence did not result in in- creased weight of the propagules compared with removing it at anthesis (data not shown). In contrast to brodiaea, the daughter corm and cormels of gladi- olus enlarge very slowly during inflorescence development, reaching only 10% of their final weight by anthesis (Shillo and Halevy, 1976 ). The difference in behavior of these two geophytes may reflect the difference in sizes of their inflorescences in relation to total leaf area; the brodiaea inflorescence is rela- tively small with little dry weight, whereas the gladiolus inflorescence is quite massive. For commercial production of gladiolus corms, removal of the flower scape at an early stage of development is required for increasing the size of the daughter corms (Halevy and Monselise, 1961 ) but this practice should not be required for brodiaea corm production. When growing brodiaea for commercial corm production, growers could therefore harvest cut flowers for marketing without reducing the corm yield.

References

Halevy, A.H. and Monselise, S.P., 1961. Influence of flower removal on yields of corms and cormels and successive flower quality of gladiolus var. Sans Souci. The Gladiolus Year Book, 36:71-76.

Hart, S.S., 1988. Control of growth, flowering and flower senescence in Triteleia laxa. Ph.D. Dissertation, University of California, Davis, CA, 105 pp.

Han, S.S., Halevy, A.H., Sachs, R.M. and Reid, M.S., 1991. Flowering and corm yield of bro- diaea in response to temperature, photoperiod, corm size and planting depth. J. Am. Soc. Hortic. Sci., 116 ( 1 ): 19-22.

Hartsema, A.M., 1961. Influence of temperature on flower formation and flowering of bulbous and tuberous plants. In: W. Ruhland (Editor), Encyclopedia of Plant Physiology, Vol. 16. Springer, Berlin, pp. 123-161.

S.S. Han et aL / Scientia Horticulturae 56 (1994) 235-243 243

Niehaus, T.F., 1971. A biosystematic study of the genus B r o d i a e a (Amaryllidaceae). Univ. Calif. Publ. Bot., 60:1-67.

Shill0, R. and Halevy, A.H., 1976. Inflorescence development of flowering and blasted gladiolus plants in relation to development of other parts. Sci. Hortic., 4:79-86.