Adventitious Flower and Shoot Formation by Various Cucurbits In Vitro

Cucurbit Genetics Cooperative Report 21:54-56 (article 19) 1998

P.N. Hoffman and R.M. Skirvin
Department of Natural Resources and Environmental Sciences, 258 ERML, 1201 N. Gregory, Urbana, IL 16801, U.S.A.

M. Ozgur
Department of Horticulture, University of Uludag, P.O. Box 44, 10636 Hurriyet, Bursa, Turkey

Introduction. Growth and early flowering are important aspects of crop breeding. The development of efficient methods to speed flowering can reduce expenses for both growers and customers. The effects of various growth regulators on flowering have been studied. There have also been several reports concerning in vitro flowering of various crops (Scorza, 1982) including cucumber, Cucumis sativus (Rasjasekran et al., 1983; Msikita et al., 1990; Ali et al., 1991). According to Msikita et al. (1990), flowering in ‘Burpless Hybrid’ cucumber cultures varied depending on explant type and media combinations. they reported that flower initiation occurred by the seventh week of culture in vitro and the best adventitious shoot regeneration was obtained from medium supplemented with 2 mg/liter of BA (6-benzylaminopurine). Tissarat and Galletta (1993) used this in vitro system to produce cucumbers for use as food in space.

To facilitate studies of flower development in vitro, it is important that cultivars with high rates of flower development be identified. this study was initiated to screen a number of commercially available cucurbits for their ability to differentiate adventitious flowers in vitro.

Materials and Methods. Seeds of several cucurbits (Table 1) were sorted for uniformity and surface sterilized in 10% sodium hypochlorite (p.525% v/v) with 0.1% Triton X-100 added as a surfactant for 15 minutes. The seeds were rinsed three times with sterile distilled water (5 min/rinse). The seeds were left in the final rinse bottle until they were ready for transfer. Sterilized seeds were prepared for culture by making a single cut at each end of the seed. The seed coats (outer and inner testa) were then removed. Care was taken to remove the embryonic axes so explants consisted of only cotyledons.

The medium used in these studies was the modification of the Murashige and Skoog high mineral salts (1962) used previously by Msikita et al. (1990). The medium was prepared using standard procedures, pH was adjusted to 5.7, agar was added at 7.5 g l-1 , and melted. The liquid medium was dispensed into 25 X 150 mm culture tubes and autoclaved for 15 minutes at 1.06 kg/cm2.

The explant were grown in a culture room maintained under 16 h days (cool white fluorescent light) between 20 and 22 C. The photosynthetically active radiation at the level of the agar surface was 131 μ mol m-2s-1 .

Results. Flower development was observed in Citrullus lanatus (watermelon), cucumis sativus (‘Hybrid Cucumber, Burpless’), and Cucurbita melopepo (‘Zucchini, Dark Green’ and ‘Summer Squash, Crookneck’). The most flower development was observed in ‘Hybrid Cucumber, Burpless’ where 30% of the seeds developed flowers; it was the only cultivar of Cucumis sativus to develop flowers. Two of the five Cucurbita melopepo cultivars flowered: ‘Zucchini, Dark Green’ (4.8%) and ‘Summer Squash, Crookneck’ (10.5%).

Some of the seed lots were completely contaminated by bacteria. Other lots had no contamination. The consistent contamination of some lots and lack of contamination in others suggests that some seeds harbored internal contaminants.

At least some cotyledons of each cultivar became green and expanded, indicating the seeds were alive. the cultivars of Cucumis sativus had the best germination rates (87.5% to 100%). The high rate of germination is consistent with the low rate of contamination (20% or less) in the Cucumis sativus seeds. In most cultivars with low germination rates, contamination rates were high.

Root growth was observed in four of the six cucurbit species: Cucumis sativus (‘Hybrid Cucumber, Burpless’ and ‘Burpless Hybrid Cucumber’) Cucurbita melopepo ((‘Zucchini, Dark Green’ and ‘Summer Squash, Crookneck’) and Cucurbita pepo (Squash, Table queen’). The highest rates of root growth were found in the cucumber and summer squash cultivars.

Adventitious shoot growth was recorded for some cultivars. All of the cultivars that formed flowers also had leaves and shoots, presumably of adventitious origin since care was taken to remove the embryonic axis from the explants. Of the cultivars in which this feature was recorded, shoot growth was observed in all cultivars. the highest rates of shoot growth were recorded, shoot growth was observed in all cultivars. The highest rates of shoot growth were found in Cucumis sativus. ‘Burpless Hybrid Cucumber’ had the highest rate (50%) of any cultivar. However, the sample contained only six seeds. Of the samples with the full twenty seeds, ‘Cucumber, Early Spring Burpless’ had the highest rate of shoot growth (40%).

Discussion. Cucumis sativus had the highest rates of glower development, germination, root growth and adventitious shoot formation. Cucurbita melopepo also had high rates of flower development, germination, and root development. Members of these species should be good selections for studying the regeneration response of cucurbits. When initiating a project to study flowering, we suggest that the researcher screen other members of this genus to find specific cucurbits useful to their goals.

Table 1. Growth and development of cotyledons from various cucurbit species in vitro.

Genus species (common Name)	Source	Contamination (%)	# Seeds	Cotyledon expansion & greening (%)	Flower formation (%)	Root formation (%)	Adventitious shoot formation (%)
Citrullus lanatus (watermelon)
Watermelon	Michael Leonard	90%	20	10%	5%	5%	N/A
Cucumis melo (cantaloupe)
Cantaloupe, Hale’s Best	Michael Leonard	25%	20	40%	0%	0%	N/A
Cucumis sativus (cucumber)
Hybrid Cucumber, Burpless	Henry Fields	20%	20	90%	30%	25%	N/A
Cucumber, SMR-58	Northrup-King	15%	20	90%	0%	0%	35%
Cucumber, National Pickling	American Seed	25%	8	88%	0%	0%	13%
Cucumber, Early Spring Burpless	Henry Fields	0%	20	100%	0%	0%	40%
Cucumber, Straight 8	Michael Leonard	5%	20	90%	0%	0%	30%
Burpless Hybrid Cucumber	Michael Leonard	0%	6	100%	0%	33%	50%
Cucurbita spp. (pumpkin)
Pumpkin, Prize Winner Hybrid	Burpee	0%	6	100%	0%	0%	17%
Pumpkin, Jack O’ Lantern	Michael Leonard	30%	10	80%	0%	0%	10%
Cucurbita melopepo (summer squash)
Squash, Early Golden Summer Crookneck	Burpee	100%	20	15%	0%	0%	N/A
Summer Squash Zucchini	Michael Leonard	100%	21	29%	0%	0%	N/A
Zucchini, Dark Green	American Seed	71%	21	52%	5%	19%	N/A
Summer Squash, Crookneck	Micahel Leonard	26%	19	68%	11%	21%	21%
Squash, Straightneck Early Yellow	American Seed	53%	19	68%	0%	0%	5%
Cucurbita pepo (winter squash)
Squash, Table Queen	American Seed	20%	20	95%	0%	5%	N/A
Winter Squash, Table Queen or Acorn	Michael Leonard	95%	20	10%	0%	0%	N/A

N/A = not assessed

Literature Cited

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