Floral Development and Breeding System of Swainsona Formosa
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HORTSCIENCE 29(2):117–119. 1994. was horizontal; G = the flag had fully reflexed and the flower was open. Flowers at each developmental stage were harvested and dis- Floral Development and Breeding sected to relate the development of internal System of Swainsona formosa parts to that of the whole flower. (Leguminosae) Results and Discussion Because of the difficulty of finding flowers Manfred Jusaitis developing in synchrony, floral stage and pe- Black Hill Flora Centre, Botanic Gardens of Adelaide, Maryvale Road, duncle length for asynchronous flowers were plotted such that onset of stage G for each Athelstone, South Australia, 5076 flower coincided. This enabled means of syn- Additional index words. Sturt’s desert pea, Clianthus formosus, stigma, pollen germination, chronized floral stages (mean of five flowers) and peduncle lengths (mean of 12 flowers) to be pollination graphed. Peduncle length followed a sigmoidal Abstract. Flowers of Swainsona formosa (G. Don) J. Thompson (syn. Clianthus formosus) growth pattern over the period of floral devel- developed through seven floral stages from buds to open flowers in 17 days. Floral stages opment, with maximal length being attained by were correlated with the sigmoidal growth pattern of the peduncle. Self-pollination was 19 to 20 days (Fig. 2). Flowers remained at prevented in the species by the presence of a stigmatic cuticle that precluded pollen stage A during the first half of this period, but germination until ruptured, exposing the receptive surface below. Cuticular rupture when peduncle growth became linear (day 9), occurred in nature during bird-pollination and was emulated manually by lightly rubbing floral development accelerated quickly through a pollen-covered finger across the stigma. The species was self-compatible, and to ensure stages B and C to reach stage D by day 13.2. cross-fertilization when breeding, emasculation before anther dehiscence was essential. Floral development slowed at stage D, while the peduncle continued to elongate. Flowers Strut's desert pea (Swainsona formosa ), a lia) were germinated by soaking in just-boiled then accelerated through stages E-G as pe- legume native to Australia, is arguably one of water until cool, and swollen seeds were planted duncle elongation rate declined. Floral stages E the world’s most striking and spectacular flow- directly in 4.7-liter black plastic pots containing and F were the most ephemeral, together occur- ering plants. Its horticultural potential as a cut a mixture of 6 gray scrub sand : 5 perlite : 2 ring within the space of a day. Floral develop- flower or as a container-grown ornamental is peatmoss (by volume). Plants were watered ment was therefore completed 17 days after the enormous (Barth, 1990). However, growth habit, twice weekly with a complete hydroponics emergence of a floral bud, while peduncle floral characteristics, and disease resistance are nutrient solution containing (mg•liter-1):212 N, growth continued slightly beyond that time. extremely variable, and ample scope exists for 31P, 218 K, 152 Ca, 41 Mg, 0.06 Zn, 0.07 Cu, Anther dehiscence commenced at floral plant improvement. This study was aimed at 0.7 Mn, 34.3 Fe, 0.018 Mo, 0.23 B, and 76 S. stage E and was completed by stage F. Thus, understanding the critical stages of floral devel- To study floral development and peduncle emasculation of flowers in breeding experi- opment in relation to the breeding system of the growth, flowers were tagged at emergence and ments should be performed during stage D, plant as the first step in a breeding program to the length of the peduncle (from point of when anthers were fully developed but had not develop new commercial cultivars. attachment to base of pedicels) was measured dehisced. Nectar production at the base of the Swainsona formosa, a member of the sub- daily. Floral stage (Fig. 1) was assessed con- filament tube commenced during stage F, and family Papilionoideae, has a large (up to 100- currently using the following scale: A = flow- by stage G, each flower contained 80-100 µl mm-long) flag-shaped flower consisting of an ers tightly clustered in bud, petals completely of nectar. Following anther dehiscence, pollen upper standard petal (or flag) that incorporates enclosed by calyx; B = tips of petals began to fell to the bottom of the keel and accumulated a shiny black boss, and a lower keel that protrude through the calyx, showing first signs immediately posterior to the stigma in the houses the sexual organs (Fig. 1). The flowers of color; C = flowers oriented horizontally region of stylar brush hairs. These anterior- are characteristically bright red and are borne during the process of downward curvature of facing hairs play an important role in pollen in umbels of up to six flowers subtended by a pedicels; D = pedicels had completed their extrusion from the keel during bird pollination long (up to 200-mm) peduncle. Flowers are downward curvature and flowers oriented ver- (Arroyo, 1981), and also restrict self-pollen omithophilous in the wild (Armstrong, 1979). tically; E = the flag began to reflex back from access to the stigma. Nevertheless, self-pollen Visiting birds probing the flower through the the keel; F = the flag in half-reflexed position was occasionally found in contact with the beak-guide in the center of the boss are re- warded with copious nectar at the base of the filament sheath. Materials and Methods Swainsona formosa plants were routinely grown in a glasshouse maintained at 25/18C (±4C) under natural daylength. Seeds (col- lected from Andamooka Station, South Austra- Received for publication 4 May 1993. Accepted for publication 7 Aug. 1993. I thank C. Schmerl and K. Holliday for technical assistance and M. Sedgley for helpful discussions. I also gratefully acknowledge the Univ. of Adelaide and M. Sedgley for allowing use of the SEM, and S. McClure for its operation. This study was funded by the Rural Industries Re- search and Development Corp. The cost of publish- ing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Fig. 1. Stages of floral development of Swainsona formosa. See text for description of each stage. HORTSCIENCE, VOL. 29(2), FEBRUARY 1994 117 from stage G flowers and incubated upright with their cut bases inserted into 1.5% agar in petri dishes. Before incubation, stigmas were either rubbed five times on the surface of a petri dish containing cross-pollen (to rupture the cuticle), or pressed lightly against pollen to enable pollen to adhere with minimal distur- bance to the stigmatic cuticle, or pollen was applied to completely undisturbed stigmas us- ing the point of a needle but without touching the stigma. After 2 h of incubation at room temperature (24 ± 1C), a subsample of pollen was removed from stigmas using the point of a needle, mounted in a drop of water on a micros- cope slide, and covered with a coverslip. Pol- len tubes were counted under × 100 magnifica- tion to determine percent germination. Viabil- ity of the pollen used in this experiment was tested on a modified Brewbaker and Kwack -1 (1963) medium [(mg•liter ) 720 Ca(NO3)2• Fig. 2. Floral development patterns (upper curve) and peduncle growth (lower curve) for Swainsona formosa. 4H2O, 200 MgSO4•7H2O, 200 KNO3, 20 H3BO3] Vertical lines indicate the onset of successive floral stages (see Fig. 1). Vertical bars represent ± SE. with 10% sucrose and 1% agar (B&Kmedium). Pollen was spread evenly on the surface of this stigma, and the important question of how Table 1. The proportion of flowers to set pods medium prepared on a microscope slide and self-pollination and fertilization were pre- following either astigmatic rub in the presence germination was assessed after 2 h at room vented within a flower required investigation. or absence of pollen, or the touch of an undis- temperature, as before. These treatments were Many papilionoid legumes reportedly pos- turbed stigma to pollen. replicated three times. sess astigmatic cuticle (membrane) covering Pollen germination was rapid, with the first the stigma, requiring rupture before pollen will pollen tubes becoming evident after 30 min of germinate (Shivanna and Owens, 1989). Such incubation on B&K medium. Nearly two-thirds rupture can be achieved by tripping the flower, of the pollen had germinated at 2 h (Table 2), subjecting the cuticle to mechanical distur- which compares favorably with earlier esti- bance, or even increasing fluid pressure in the mates (Hughes et al., 1991). Nearly half of the style (Lord and Heslop-Harrison, 1984). To pollen applied to a rubbed stigma germinated, investigate self-incompatibility and the possi- while pressed stigmas supported » 19% germi- bility of a similar cuticular barrier to self-polli- ***Significant at P £ 0.001. nation. The fact that germination was negli- nation in S. formosa, cross- or self-pollen was gible on undisturbed stigmas (Table 2) suggests applied to stigmas of stage G flowers by either other papilionoid legumes (Shivanna and that cuticle disruption to expose the stigmatic gently touching the stigma to a pollen mass, Owens, 1989), the structure of the stigma was surface, or at least cuticle bruising to release ensuring minimal disturbance to the stigma, or studied using an Electroscan ES-2 environ- stigmatic exudate, was necessary for pollen by rubbing the stigmatize times with a fingertip mental scanning electron microscope. This germination to occur. The application of size- covered in pollen. The latter treatment was microscope allowed fresh tissue to be ob- able masses of pollen to stigmas in this experi- designed to simulate contact of the stigma with served directly without prior fixation or coat- ment necessarily meant that some pollen on the the abdominal surface of a pollinating bird ing.