Planta (Berl.) 124, 93--97 (1975) 9 by Springer-Verlag 1975

Short Communication Control of Branching Pattern in Microgramma vacciniifolia, an Epiphytic Fern Ann M. Hirsch* Department of Botany, University of California, Berkeley, California 94720, USA l~eceived 19 August 1974; accepted 26 February 1975

Summary. The rate of expansion of lateral buds of Microgramma vaccinii]olia (Langsd. et Fisch) Copeland (Polypodiaceae) does not differ in intact plants, in plants with the rhizome apex removed, and in plants with the apex removed and replaced with indole-aeetic acid (IAA). t~oot elongation is implicated in bud outgrowth possibly through the uptake of water and nutrients. The phenomenon of apical dominance has been observed in nearly all groups of plants. Generally, it has been assumed t h a t pteridophytes show the same type of correlative inhibition of lateral bud growth as exhibited b y higher plants. However, there have been very few investigations of apical dominance in ferns and fern allies (Lang, 1915; Williams, 1937; Wardlaw, 1946; Allsopp, 1956; Albaum, 1938; Laetsch and Briggs, 1963). The morphology of the fern shoot system seems to support classical assumptions of correlative bud inhibition. Many ferns exhibit an aerial or subterranean creeping rhizome with a leading terminal apex, and although lateral buds in ferns are not often found in an axillary position, their development is similar to t h a t of the axillary buds of dicotyledons : the lateral buds of m a n y fern rhizomes commonly do not elongate until they are displaced some distance from the apex of the rhizome. Microgramma vaccinii/olia is a prime example of a fern with a terminal leading habit. Because bud expansion does not begin until 1-2 cm behind the apex with bud position 6 or 7 (Hirsch and Kaplan, 1974), a correlative relationship between the lateral buds and the main apical bud has been suggested (Troll, 1937). This study explores t h a t suggestion. Plants of Microgramma vaceinii/olia (Langsd. et Fisch) Copeland (Polypodiaceae) were originally obtained from the University of California Botanical Garden at Berkeley. They were subdivided and grown under greenhouse conditions with supplemental lighting to have 16 h light per day. Uniform plants with 10-12 maeroscopically visible leaf-bud pairs were selected for study. The rhizomes were either decapitated or left intact. When a rhizome was decapitated, 0.5 cm of the tip with five primordial leaf-bud pairs (Hirsch and Xaplan, 1974) were removed.

* Present address: Department of Botany, University of Minnesota, St. Paul, Minnesota 55108, USA.

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Indole-3-acetie acid (IAA) was applied to the cut rhizome surfaces in either lanolin or water at concentrations from 10-6 to 5.7 • 10-3 M (1%). Aqueous IAA solutions were applied in 12 • 35 mm glass vials which had been previously spray-painted black. The vials fitted over the cut rhizome stump and all solutions were changed daily. Ten macroseopically visible lateral buds along the lengths of decapitated and intact rhizomes were measured at 7, 20, 30 and 45 days. Decapitation often resulted in a marked decrease in vigor of the plants; such plants were not included in the measurements. N o significant difference in t h e m e a n increase in l e n g t h of 10 buds of i n t a c t p l a n t s as c o m p a r e d t o 10 b u d s of d e c a p i t a t e d p l a n t s was f o u n d d u r i n g t h e course of t h e e x p e r i m e n t . This is consistent for each of t h e 10 b u d positions along t h e l e n g t h of t h e r h i z o m e a n d for each p e r i o d of m e a s u r e m e n t . T a b l e 1 lists t h e m e a n increase in l e n g t h for t h r e e i n d i v i d u a l b u d positions a t 30 days. I n T a b l e 1, i t can be seen t h a t t h e t e r m i n a l b u d of i n t a c t rhizomes grows a t a m u c h higher r a t e t h a n t h e l a t e r a l buds. Table 1. Mean increase in length (in mm) of buds of intact and decapitated rhizomes of Microgramma vaccinii/olia after 30 days Values are mean 4-standard error in mm; in (), sample size. Bud No. 1 6 10 Terminal

Intact rhizomes 0.97 4- 0.26 (15) 1.83 4-0.66 (13) 2.68 4-1.33 (11)

Decapitated rhizomes 1.68 4-0.45 (11) 1.62 :t: 1.06 (10) 2.114-1.28 (9)

27.43 4- 5.34 a (7)

a Statistically different at the 0.05 level from the lateral buds of both intact and decapitated rhizomes.

N o effect of I A A on b u d e x p a n s i o n was noted. A f t e r 6 weeks of t r e a t m e n t , l a t e r a l b u d s e x p a n d e d a t a r a t e c o m p a r a b l e to t h a t of t h e controls. To d e t e r m i n e if diffusible I A A was even p r e s e n t in t h e fern apex, t h e Arena coleoptile s t r a i g h t g r o w t h b i o a s s a y was used following t h e m e t h o d of Eliasson (1969). Seeds of Avena sativa L. ev. Montezuma (Lot No. 70-72, University of California College of Agriculture) were germinated on 3 layers of cheesecloth in the dark except for a 5 h exposure to weak red light (600-750 nm, 6.35 ergs cm -2 sec-1) approximately 20 h before use. Apices of M. vaccinii/olia 0.5 cm long were removed and placed on agar blocks which had been treated with either one drop of 5 mM KCN to prevent auxin inactivation (Steeves et al., 1953) or distilled water. After diffusion, the agar blocks were placed in vials containing 2 ml of buffer solution (5 mM citric acid, 10 mM K2HP04, 1.6% glucose; pH 5.0) and 10 randomly selected 5-ram coleoptfle segments (Eliasson, 1969). The vials were rotated on a clinostat in the dark at 25 ~ for 20 h. The length of the coleoptiles was measured to the nearest 0.1 mm. W h e n t i p s of M. vaccinii]olia were allowed to diffuse on a g a r for 4 h a n d t h e a g a r t h e n m o n i t o r e d for a u x i n a c t i v i t y b y m e a n s of t h e Avena s t r a i g h t g r o w t h bioassay, t h e a g a r blocks containing t h e diffusate d i d n o t cause significantly different g r o w t h from t h e control blocks w i t h o u t diffusate. Cytokinins, along w i t h auxins, m a y p l a y a p a r t in t h e h o r m o n a l r e g u l a t i o n of apical d o m i n a n c e (Sachs a n d T h i m a n n , 1967). Since t h e r o o t s of Microgramma

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95

are located on the ventral surface of the rhizome in close proximity to the buds, and since roots have been implicated as a source of eytokinins (Kende, 1964), outgrowth of lateral buds in M. vaccinii]olia might be enhanced by elongation of the roots and their production of cytokinins. Kinetin at 5.8• -9 mmol in 2.5% polyethylene glycol 1500 (PEG) and 50% ethanol (Sachs and Thimann, 1967) was applied once daily to the first or ninth macroscopically visible bud. 2.5% PEG 1500 and 50% ethanol were applied to control buds. Kinetin and control solutions were also applied to the second axillary bud above the cotyledons of 10-day-old Pi~um sativum L. cv. Alaska to serve as controls for the reagents. Growth and measurement conditions for the peas were the same as for the ferns. When 5.8 X 10-9 mmol kinetin are applied directly to axillary buds of 10-dayold pea plants, there is a two-fold increase in the length of buds after 7 and 14 days of treatment as compared with the controls (data not shown). However, in contrast to the effect of kinetin application on buds of peas, there is no significant increase in the length of either bud 1 or bud 9 of M. vaccinii]olia after 7 (Table 2) or 14 (data not shown) days. When buds from both controls and kinetin treatments were fixed, embedded in paraffin, sectioned and examined, there was no significant difference in the height of the median sections of buds from untreated controls, P E G ~- ethanol controls, and kinetin treatments. Table 2. Mean increase in length of buds of Microgramma treated with 5.8 X 10-9 mmol kinetin for 7 days Values are mean ~=standard error in mm Bud No.

No treatment

PEG controls

Kinetin

1 9

0.033 • 0.630 •

0.077 =]=0.050 0.670 •

0.086 :j=0.040 0.500 ~=0.100

Application of kinetin directly to the ventral rhizome surface also had no effect on the rate of bud outgrowth or root elongation. I n previous studies, correlative inhibition in ferns has been explained either by an auxin-mediated apical dominance or by competition between the lateral buds and the apex (or the first leaf and root) for nutrients. Wardlaw (1946) found that decapitating the rhizome of Matteuccia struthiopteris and Onoclea sensibilis cansed the lateral buds to expand while replacement of the decapitated shoot apex resulted in their continued inhibition. On the other hand, Allsopp (1956) found that in Marsilea drummondii, lateral buds expanded when the decapitated shoot apex was replaced with 1 and 10 mg/1 IAA and concluded that their correlative inhibition was due to a lack of nutrients which had been diverted to the shoot apex. Laetsch and Briggs (1963) likewise proposed that the dominance of the primary leaf and root over the formation of secondary organs of sporelings of Marsilea vestita resulted from the diversion of available nutrients. I n M. vaccinii]olia, excision of the rhizome apex has no effect on the elongation of lateral buds. Thus, the classical concepts of apical dominance do not appear to apply to M. vaccinii/olia. Although the apical bud of the shoot is growing 10-15 times faster than the lateral buds, this rate of growth is associated

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neither with the production of an inhibitor of lateral bud growth nor with the sequestration of available nutrients. Rather, lateral buds expand continuously but slowly until their apex reaches a certain size. This occurs either when the bud is displaced 7-12 cm behind the rhizome apex, or when roots located on the ventral surface of the rhizome make contact with and branch into a moist substrate. At this time, the rate of development of the lateral bud accelerates and approaches that of the terminal bud whether or not the latter is present. Thus, it appears that once the lateral bud has achieved an apical size and thus a nutritional status nearly equivalent to that of the terminal bud, it will produce leaf, root, and bud primordia at the same rate as the terminal bud, and will form a secondary shoot system which is independent of the growth of the main shoot. I t appears that branching in this fern does not involve a release of lateral buds from correlative inhibition. The branching pattern is the result of differing rates of growth of the main and lateral buds. Two extremes in the expression of branching pattern are evident in plants as well as the two types of control of branching. Many herbaceous dicotyledons represent one extreme; once the apical bud (a source of IAA) is removed, each lateral bud quickly expands and assumes an orthotropie habit until one of the more vigorous laterals becomes the new leading shoot. Many gymnosperms, on the other hand, represent the opposite extreme in pattern of lateral branching. For example, in Araucaria spp., the laterals retain their plagiotropie habit even after decapitation of the main apex, and continue to grow horizontally for many years with no lateral assuming the role of the main leading shoot (Goebel, 1900). M. vacciniifolia can be described as intermediate between these extremes of branching pattern since the lateral buds remain as laterals whether or not the apex is removed until a certain developmental stage is reached; at this time the branch begins to grow as a leading shoot. With regard to control of branching, this fern appears to more closely represent the nutritional mode via the increase in apical size and nutritional status of the lateral buds. Thanks are due to Drs. Russell Jones, D. R. Kaplan and S. J. Kirchanski for their critical reading of the manuscript and helpful suggestions. My thanks to J. Croxdale also for her helpful comments and assistance.

References Albaum, H. G. : Inhibitions due to growth hormones in fern prothaUia and sporophytes. Amer. J. Bot. 25, 124-133 (1938) Allsopp, A. : Apical dominance in Marsilea, with particular reference to the effects of 3indolylacetic acid, 3-indolyl-acetonitrile, and coumarin on lateral bud development. J. exp. Bot. 7, 14-24 (1956) Eliasson, L.: Growth regulators in Populus trem~tla. I. Distribution of auxin and growth inhibitors. Physiol. Plantarum 2~, 1288-1301 (1969) Goebel, K.: Organography of plants. I. General organography. (Engl. transln.) Oxford: Clarendon Press 1900 Hirsch, A. M., Kaplan, D. R. : Organography, branching and the problem of leaf versus bud differentiation in the vining epiphytic fern genus Microgramma. Amer. J. Bot. 61, 217-229 (1974) Kende, It.: Preservation of chlorophyll in leaf sections by substances obtained from root exudate. Science 145, 1066-1067 (1964)

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Laetsch, W. M., Briggs, W . R . : Correlative inhibition and the primary organs of Marsilea vestita. Bot. Gaz. 124, 317-324 (1963) Lang, W. H. : Presidential Address, Section K, Botany. In: Rep. Brit. Assoc. Adv. Sci., Manchester, p. 701-718. London: John Murray 1915 Sachs, T., Thimann, K. V.: The role of auxins and cytokinins in the release of buds from dominance. Amer. J. Bot. 54, 136--144 (1967) Steeves, T. A., Morel, G., Wetmore, R. H. : A technique for preventing inactivation at the cut surface in auxin diffusion studies. Amer. J. Bot. 40, 534-538 (1953) Troll, W.: Vergleichende l~orphologie der hSheren Pflanzcn. Vol. 1. Vegetationsorgane, Pt. 1. Berlin: Borntraeger 1937. (Reprinted Koenigstein-Taunus: O. Koeltz 1967) Wardlaw, C.W.: Experimental and analytical studies of pteridophytes. VIII. Further observations on bud development in Matteuccia struthiopteris, Onoclea senaibilis, and species of Dryopteris. Ann. Bot. 9, 117-132 (1946) Williams, S.: Correlation phenomena and hormones in Selaginella. Nature (Lond.) 139, 966 (1937)

7 Planta (Bed.), Vol, i24

Control of branching pattern in Microgramma vacciniifolia, an epiphytic fern.

The rate of expansion of lateral buds of Microgramma vacciniifolia (Langsd. et Fisch) Copeland (Polypodiaceae) does not differ in intact plants, in pl...
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