Some aspects of daughter bulb growth and development and apical dominance in bulbous Iris

Transcription

1 Plant & Cell Physiol. 20(2): (1979) Some aspects of daughter bulb growth and development and apical dominance in bulbous Iris Robert P. Doss Ornamental Plants Research Laboratory, USDA/SEA, Federal Research, Western Washington Research & Extension Center, Puyallup WA 98371, U. S. A. (Received September 6, 1978) Apical dominance appears to have minimal direct involvement in daughter bulb formation in the bulbous Iris cultivar Ideal. Daughter bulb number and growth relate to the size and reproductive state of the mother bulb and are not markedly influenced by meristem destruction. In contrast, destruction or removal of the apical meristem promotes lateral bud sprouting in intact bulbs, and lateral bud elongation in Iris meristem explants. These results show that, in contrast to certain other bulbous plants, apical dominance does not direcdy limit daughter bulb number in bulbous Iris, but does prevent lateral bud sprouting. Key words: Apical dominance Bulbous Iris Daughter bulbs Shoot growth. Commercial flower bulb propagation sometimes involves artificial removal of apical dominance. For example, with hyacinth, daughter bulb formation is promoted by wounding of the basal plate and bulb meristem (i.e. "scouring" or "cross cutting" 17), and a specific heat treatment, "blindstoken," is said to cause increased daughter bulb yields in tulip by killing the reproductive apex and releasing the lateral buds from correlative inhibition (16, 17, 20, 21). The work reported here is from an investigation of daughter bulb formation in bulbous Iris. Iris daughter bulbs can arise from the lateral buds in the axils of the scale, sheath and outermost true leaf of flowering bulbs. In nonflowering bulbs a daughter bulb arises from the terminal meristem as well as from the lateral buds (2, 3, 17). There is a progressive decrease in daughter bulb weight from the innermost to outermost daughter. Iris bulb mass consists largely of the swollen scale leaves that contain the food reserves. Because planting stock yield is an important criterion for cultivar selection in bulbous crops (10, 13, 17), it is important to have an understanding of the factors involved in Iris bulb morphogenesis and growth. Materials and methods Plant material The bulbous Iris cultivar Ideal was used in all experiments. This widely grown cultivar arose as a mutation of'wedgwood,' itself a cultivar of complex origin (17). Bulbs were obtained from commercial or experimental plantings in Western 387

2 388 R. P. Doss Washington. Bulbs to be grown in growth chambers and bulbs from which meristem explants were to be taken were placed in storage at 30 G and 85% relative humidity 24 to 48 hr after digging. Such "retarded" bulbs show much reduced metabolic and leaf initiation rate and remain in the vegetative state for prolonged periods (5, 6, 11). Bulbs used in field plantings were stored 4 to 6 weeks in a bulb storage shed before replanting into the field. It had previously been found that about half of the bulbs in the 7 to 8 cm circumference rangeflowerwhen grown under field conditions (data not shown). Smaller bulbs flower very infrequently whereas a large percentage of larger bulbs flower (17). Growing conditions Retarded bulbs, 10 cm or more in circumference, were precooled at 10 C for 6 weeks. Precooling was given to insure a goodfloweringpercentage in the forced bulbs (9). Growth chambers used for forcing these bulbs were set for a 16 hr photoperiod (8 hr dark period) with a lights-on temperature of 16 G and a lights-off temperature of 10 C. Mixed fluorescent and incandescent lights provided a photon flux density of 150 J«Em~ 2 sec~ 1 of photosynthetically active radiation at plant height. Relative humidity was maintained at 85%. Plants were grown in pots containing a fine sandy loam field soil. The above conditions had previously been found to be adequate for forcing Iris bulbs in growth chambers. Bulbs were planted in the field on October 7, The field soil was a fine sandy loam. No irrigation or fertilization was applied during the growing season. In vitro culture of merislem explants Meristem explants consisting of the terminal meristem, a few leaf primordia and a portion of the basal plate were excised from surface-sterilized retarded bulbs 9.5 to 11 cm in circumference. These were cultured in the dark at 10 C (or 20 G) on agar solidified Murashige-Skoog medium formulated using the procedure of Gamborg (8). Cultures were grown in 25 X 150 mm plastic-capped culture tubes. These cultural conditions were modified after those used successfully by Rodrigues- Pereira (18). Treatments The field planting was analyzed for fresh weight yield of daughter bulbs, daughter bulb number, flowering percentage and bulb growth. Daughter bulb number included bulbs arising from both the terminal meristem and the lateral buds. All macroscopically visible daughter bulbs were counted. Retarded bulbs were used to determine the influence of meristem destruction on lateral bud activity. In one experiment intact bulbs were pierced through the basal plate with a sterile dissecting needle. Piercing resulted in terminal meristem destruction in one-third of the bulbs so treated. After forcing in the growth chamber the apical meristem and lateral bud behavior were noted. In a set of related experiments, bulb meristem explants were excised from retarded bulbs. The apical dome and young leaves were (a) left intact (b) excised or (c) the young leaves were excised and the apical dome was left intact. After 8 to 12 weeks of culture the explants were analyzed for lateral meristem elongation. Data were statistically analyzed as indicated in tables and figures.

3 Daughter bulb growth and apical dominance in Iris 389 Table 1 Influence of Iris 'Ideal' bulb size on daughter bulb yield Mother bulb circumference (cm) Planting weight (g)' 4427 ± ± ± Daughter bulb yield, fresh weight (g)* A 8337 B 4258 C 2009 C Daughter bulb number per mother bulb * A 2.61B 1.81 C D Bulbs were harvested in late summer and stored in a bulb storage shed under ambient conditions until replanted on October 7, Data were taken at harvest in early August of " Values are means and standard errors for 3 blocks each consisting of 483 bulbs. * Data analyzed as randomized complete blocks. Means within columns bearing different letters differ significantly (P=0.05) as determined by Duncan's multiple range test. Results Daughter bulb number and growth were influenced by the size and the reproductive state of the mother bulb. Larger mother bulbs gave rise to a greater number of daughter bulbs of greater total fresh weight and larger mother bulbs produced larger daughter bulbs (Table 1). Daughter bulb number and average fresh weight were influenced by the reproductive state of the mother bulb. Flowering bulbs produced more daughters 0) CD to 0) 111 Nonflowering Months after Planting Fig. 1. Influence offloweringand harvest date on fresh weight of largest (innermost) daughter bulb in Iris 'Ideal.' Planting stock consisted of a single lot of bulbs 7 to 8 cm in circumference. Sample sizes varied from 9 to 21. Open symbols indicate significant different (P=0.05) in yield at the indicated harvest time as determined by Student's t-test. See Table 1 for conditions.

4 390 R. P. Doss than nonflowering bulbs of the same size range (Table 2). Although the increase in the largest daughter bulb (innermost lateral daughter bulb) fresh weight in flowering plants lagged that (innermost daughter bulbj in nonflowering plants, the bulb weights a t harvest, 10 months after the Oct. 7 field planting, from flowering and nonflowering mother bulbs were not significantly different (Fig. 1). The daughter bulbs that arose at the innermost bulb-forming lateral meristems were much larger in flowering plants than in nonflowering plants (Table 2). Note that the terminal meristem in nonflowering plants gives rise to the largest daughter bulb whereas a flower is produced by the terminal meristem in flowering plants (2). Apical meristem removal or destruction caused lateral shoot growth in both whole bulbs and in bulb meristem explants (Fig. 2 ). I n meristem explants apical meristem removal resulted in lateral shoot elongation (Fig. 2c). Removal of leaf

5 Daughter bulb growth and apical dominance in Irir Fig. 2. InJwue of apex &sf~tion on laha1 shoot growth in whole bulbs (a and 6) and mnisfem explants (c and d). The result of meristem destruction or removal is illustrated in a and c. Whole bulbs were removed from retarding temperature (30 C) and subjected to 6 weeks of precooling (10 C) prior to planting. Apex destruction was accomplished immediately after precooling. Plants were grown in a growth chamber using a lights-on temperature of 16'C and a lights-off temperature of 10 C with 16 hr photoperiods and 8 hr dark periods. See Table 3 and tact for discussion of conditions used with bulb acplants. primordia alone stimulated some lateral bud growth (Table 3). Excision of both the apical dome and leaf primordia resulted in additional lateral bud activation and elongation. Daughter bulb number in whole bulbs (9.0 to 9.5 cm circumference) was not influenced by meristem destruction with an average of 3.5 daughters per bulb observed in both the pierced and unpierced (control) treatments (data not shown).

6 392 R. P. Doss Table 2 Influence of Iris 'Ideal'Jlowering on daughter bulb development Reproductive state" Flowering Nonflowering See Table 1 for conditions. 0 Data obtained from a planting'of a single lot of mother bulbs 7 to 8 cm in circumference. * Means within columns differ very significantly (P=0.01) as determined by Student's t-test. ' Determined on a sample consisting of 47floweringand 59 nonflowering bulbs. d Values determined at harvest using a sample consisting of 11 flowering and 13 nonflowering plants. Table 3 Influence of apex destruction of Iris 'Ideal' meristem explanls on lateral shoot growth Number of Terminal Number of Length of longest Treatment explants shoot length lateral lateral shoot" in sample (mm)' shoots/explant" (mm) A Apical dome and true leaf primordia intact B Apical dome intact, true leaf primordia excised C Apical dome and leaf primordia excised ± ± ± ± ± Explants were obtained from 9.5 to 11 cm circumference retarded bulbs. Cultures were maintained in the dark at 10 C (or 20 C) for 8 to 12 weeks. " Values are means and standard errors. Discussion Daughter bulb number or size in several bulbous plants is clearly influenced by apical dominance (e.g. in hyacinth, 17; and tulip, 16, 17, 20, 21). In contrast the number of daughter bulbs that develop in bulbous Iris relates principally to mother bulb size and flowering behavior. Larger mothers produce more daughter bulbs (Table 1) and flowering increases daughter bulb number while reducing daughter bulb growth rate (Table 2, Fig. 1). It could be argued that increased daughter bulb number in flowering Iris plants results from early removal of the apical dominance exerted by the terminal meristem since in a number of plants floral evocation is followed by sprouting of lateral buds (7). This hypothesis is not supported by the finding that meristem destruction in intact Iris bulbs and in explants leads to lateral shoot elongation without influencing the total number of daughter bulbs formed (Table 3, Fig. 2). A similar phenomena has been described for tulip bulbs subjected to excessively high temperatures during heat treatment (10, 17). In Iris (Table 1) and tulip (15) daughter bulb size is strongly influenced by the size of the mother bulb. As with tulip (13) and Narcissus (14) correlative factors are probably involved in the gradation in the size of daughters. The innermost daughter bulb is the largest regardless of whether it is formed by a larteal or the terminal meristem (16). In a plant where the terminal meristem gives rise to a flower the

7 Daughter bulb growth and apical dominance in Iris 393 innermost lateral bulb is much larger than the innermost lateral bulb in a nonflowering plant (Table 2). In the nonflowering plant the terminal meristem gives rise to the largest (innermost) daughter bulb (Table 2). Apical dominance in Iris, as manifested by inhibition of lateral shoot elongation in apex explants, depends in part on the presence of young leaves and thus resembles the phenomenon as described for most other plants (Table 3) (12). A rapid onset of leaf formation probably results in an intermediate degree of lateral meristem inhibition in explants having only an intact apical dome. The inhibition of sprouting of lateral buds of Iris is a correlative phenomena (Table 3, Fig. 2) that should not be confused with the inhibition of sprouting of mature Iris bulbs (1, 4, 22). Mature bulbs of Iris and several other bulbous plants exhibit dormancy that is controlled by characteristics of the bulbs themselves, presumably involving inhibitory compounds (19, 23). The inhibition of sprouting of lateral buds while attached to the mother bulb in Iris more nearly relates to the premature sprouting of immature bulbs of lily (23), Laportea (19) and other bulbous plants upon removal from the mother plant. It would be interesting to determine if the lack of dormancy in immature bulbs from these plants is due to release of the lateral meristem from apical dominance and similarly to determine whether precocious sprouting would occur if immature Iris bulbs were removed from the mother bulb. It should be emphasized that daughter bulbs arise at the lateral buds in whole Iris bulbs with or without destruction of the terminal meristem (Fig. 2a and 2b). Thus, it is lateral bud sprouting that is clearly controlled by apical dominance; whereas daughter bulb formation is influenced by other factors (2, 3). The technical assistance of J. K. Christian and review of the manuscript by Ms. JoAnn Robbins, and Drs. Ralph Byther, Robert Linderman, and John Potter is appreciated. References ( / ) Alpi, A., N. Ceccarelli, F. Tognoni and G. Gregorini: Gibberellin and inhibitor content during Iris bulb development. Physiol. Plant. 36: (1976). ( 2) Aoba, T.: Studies on bulb formation of bulbous-iris plant (I) On process of bulb formation and structural state of bulb. Bull. Yamagata Univ. (Nat. Sci.) 5: (1967) (in Japanese). ( 3) Aoba, T.: Effects of temperature on bulb- and tuber-formation in bulbous and tuberous crops. VI. On the bulb formation in bulbous iris. J. Jap. Hort. Sci. 43: (1974) (in Japanese). ( 4 ) Ando, T. and Y. Tsukamoto: Capric acid: A growth inhibiting substance from dormant Iris hollandica bulbs. Phytochemistty 13: (1974). ( 5) Beijer, J. J.: Experiments on the retardation of Dutch irises. Ada Bot. Neerl. 1: (1952). ( 6) Blaauw, A. H.: On the relation between flower formation and temperature. I. and II. (Bulbous Irises). Nederl. Mad. Wetenschappen, Proc. 44: , (1941). ( 7) Evans, L. T.: The nature offlowerinduction. In The Induction of flowering. Edited by L. T. Evans, p Cornell Univ. Press., Ithaca, New York, ( 8) Gamborg, O. L.: Callus and cell culture. In Plant Tissue Culture Methods. Edited by O. L. Gamborg and L. R. Wetter, p National Res. Council Canada, Saskatoon, Saskatchewan, ( 9 ) Gould, C. J., N. W. Stuart and T. Sabelis: Suggestions and comments on forcing Washingtongrown iris in greenhouses in the U. S. Florists' Rev. 154 (3996): 33-34, 81 (1974).

8 394 R. P. Doss (10) Hekstra, G.: Selectieve teelt van tulpen gebasseerd op produktie-analyse. Versl. landbouwk Onderz. 702 (1968) (Cited in 13). (11) Kamerbeek, G. A.: Respiration of the iris bulb in relation to the temperature and growth of the primordia. Ada Bot. Need. 11: (1962). (12) Phillips, I. D.J.: Apical dominance. Ann. Rev. Plant Physiol. 26: (1975). (13) Rees, A. R.: The initiation and growth of tulip bulbs. Ann. Bot. 32: (1969). (14) Rees, A. R.: The initiation and growth of Narcissus bulbs, ibid. 33: (1969). (15) Rees, A. R.: The effect of bulb size on the growth of tulips, ibid. 33: (1969). (16) Rees, A. R.: The effect of high-temperature treatment of tulip bulbs ("blindstoken") on flowering and bulb yield. Rep. Glasshouse Crops Res. Inst. 1966, p (1967). (17) Rees, A. R.: The Growth of Bulbs. Academic Press, London, (18) Rodrigues-Pereira, A. S.: Physiological experiments in connection with flower formation in Wedgwood iris (Iris cv. 'Wedgwood'). Ada Bot. Need. 11: (1962). (19) Tanno, N.: Dormancy in Laportea bulbils: experiments with bulbs cultured in vitro. Plant & Cell Physiol. 18: (1977). (20) Toyoda, T. and K. Nishii: Studies on high temperature treatment of tulip bulbs to prevent flowering. I. Effects of date and length of treatment onflowering,growth and bulb^roduction J. Hort. Assoc. Japan 16: (1957). (21) Toyoda, T. and K. Nishii: Studies on high temperature treatment of tulip bulbs to prevent flowering. II. Relation between degree of injury in theflowerbud and the yield of new bulbs, ibid. 27: (1958). (22) Tsukamoto, Y. and T. Ando: The change of amount of inhibition inducing dormancy in the Dutch iris bulb. Proc. Japan Acad. 49: (1973). (23) Wang, S.-Y.: Physiology of dormancy in Lilium longiflorum Thunb. Diss. Abstr. B30, (1969).