CHAPTER 58. POLYGONACEAE

The Polygonaceae comprise roughly 800 species of herbaceous plants, shrubs and trees within 30 to 40 genera which provide grain (Fagopyrum esculentum Moench, buckwheat), edible stems (in fact the petioles of radicle leaves) (Rheum raponticum L., rhubarb), leaf vegetables (e.g. Rumex acetosa L., garden sorrel), and several medicinal products. The fruits are usually achenes and are sometimes enclosed in perianth, forming a berry-like structure. The seeds show orthodox storage behaviour. For example, Erigonum and Polygonum spp. are maintained in the long-term seed store at the Wakehurst Place Gene Bank.

SEED DORMANCY AND GERMINATION

B.R. Atwater classifies seed morphology as endospermic seeds with peripheral linear embryos (see Table 17.1, Chapter 17). The seeds can exhibit a high degree of dormancy. Light, treatments to the seed coat (particularly removal), pre-chilling and alternating temperatures promote the germination of dormant seeds.

Detailed information on seed dormancy and germination is provided for the genus Fagopyrum in this chapter. Recommendations for germination test procedures and dormancy-breaking treatments for other species are summarised in Table 58.1. In addition the algorithm below may be helpful in developing suitable germination test procedures.

RBG Kew Wakehurst Place algorithm

The first step in the algorithm is to test the seeds at constant temperatures of 16°C and 26°C, with light applied for 12h/d. If full germination is not achieved but a trend is apparent in the response of germination to constant temperatures then test at more extreme constant temperatures. For example, if a greater proportion of seeds germinate at 16°C than at 26°C then test further samples of seeds at constant temperatures of 6°C and 11°C with light applied for 12h/d. If, however, the proportions of seeds germinating at 16°C and 26°C are similar then test a further sample of seeds at the intermediate constant temperature of 21°C with light applied for 12h/d.

If the above constant temperature regimes do not promote full germination then the second step in the algorithm is to test seeds in an alternating temperature regime of 23°/19°C (12h/12h) with light applied for 12h/d during the period spent at the upper temperature.

TABLE 58.1 Summary of germination test recommendations for species within the Polygonaceae

Species and Authority	Substrate	Temperature	Duration	Additional directions	Source
Antigonon leptopus Hook. & Arn.		20°C	15d		Atwater
Coccoloba uvifera L.			21d	pre-soak, 24h, then warm stratification	Riley
Eriogonum fasciculatum Benth.		20°C	14d	remove outer coats	Atwater
Polygonum convolvulus	TP	23°C	6d	light, remove fruit coat	R&S
Polygonum lapathifolium	TP	20°/30°C	21d	test at 2°-10°/35°C, 10d, then 20°/30°C	Everson
	TP	15°/25°C		light, pre-chill, 2w	M&O
Polygonum pennsylvanicum	TP	10°/35°C	28d	test at 2°/35°C, 21d, then 10°/35°C	Everson
	TP	23°/30°C	21d	light	R&S
Polygonum persicaria	S	20°/30°C	21d	test at 2°-10°/35°C, 14d, then 20°/30°C	Everson
	TP	23°C	6d	light, remove fruit coat	R&S
Polygonum scandens L.		3°-6°C	21d	remove outer coat	Atwater
Rheum palmatum L.	TP; BP	20°/30°C; 20°C	21d		ISTA
Rheum rhabarbarum L.	TP	20°/30°C	21d	light	AOSA
Rheum rhaphonticum L.	TP	20°/30°C	21d		ISTA
	TP	20°/30°C	14d	light	Heit
Rumex acetosa L.	TP	20°/30°C	14d	pre-chill	ISTA
	TP; S	20°/30°C	14d	light, test at 15°C	AOSA
	TP	15°C	10d	light, potassium nitrate	Heit
Rumex crispus L.	TP; S	15°/30°C	10d	light	Everson
	TP	20°/30°C		light	M&O
Rumex maritimus	TP	20°/30°C		light	M&O
Rumex obtusifolius L.	TP	20°/30°C			M&O

FAGOPYRUM

F. esculentum Moench [F. sagittatum Gilib.]	common buckwheat
F. tataricum Gaertn.	India-wheat, tartary buckwheat

I. Evidence of dormancy

Although viviparous germination is sometimes observed in both ripe and unripe seeds of F. esculentum (5), a high degree of dormancy is normally exhibited by seeds of F. esculentum (6) and F. tataricum (1,3) at harvest. At 25°C, 40 (1) or 60-70 days (3) after-ripening are required to remove dormancy from seeds of F. tataricum, but at 2°-3°C as many as 6 months may be necessary (1).

II. Germination regimes for non-dormant seeds

F. esculentum

BP; TP: 20°/30°C (16h/8h); 20°C: 7d (ISTA)

BP: 20°/30°C (16h/8h): 6d (AOSA)

Constant temperatures: 25°C (2)

III. Unsuccessful dormancy-breaking treatments

F. esculentum

Light: blue (6); dark, at 28°C (6)

F. tataricum

Alternating temperatures: 20°/30°C, -20°C/room temperature (1)

Removal of seed covering structures: pericarp only (1,3); prick (1); cut, at base of seed (1)

Scarification: concentrated sulphuric acid, 2 min-24h (1); ethyl alcohol, 95%, 2 min-24h (1)

GA₃: pre-applied, 2 min-24h, 10-1,000 ppm (1); pre-applied, 24h, 1000 ppm, after seedcoat removal (1)

Pre-dry: pre-wet/pre-dry, cycle (1)

IV. Partly-successful dormancy-breaking treatments

F. esculentum

Constant temperatures: 10°-25°C in light, continuous, 200 lux (6); 45°-50°C in light, continuous, 200 lux (6)

Light: sunlight, at 28°C (6); diffuse daylight, at 28°C (6); diffuse daylight plus incandescent, continuous, 400-1200 lux (6); diffuse daylight plus incandescent, 2-12h/d, 200 lux (6); violet (6); green (6); yellow (6); red (6); far red (6)

Kinetin: 10 ppm (4); plus N-chloroacetate-M-iodoaniline (4) N-Chloroacetate-M-iodoaniline: (4)

F. tataricum

Removal of seed covering structures: pericarp and seedcoat (1)

V. Successful dormancy-breaking treatments

F. esculentum

Constant temperatures: 30°C, 35°C, 40°C, light, diffuse daylight plus incandescent, continuous, 200 lux (6); 28°C, light, diffuse daylight plus incandescent, 16-24h/d (6)

Light: diffuse daylight plus incandescent, continuous (6); orange, continuous (6)

F. tataricum

Removal of seed covering structures: pericarp and seedcoat (3)

Pre-dry: 80°C, 2,3d (1); 70°C, 3d (1); 40°C, 2-6w (1)

VI. Comment

Whilst treatment with light promotes the germination of dormant seeds of F. esculentum (6), high doses can inhibit germination (6). It is suggested that seeds of F. esculentum be tested for germination in the alternating temperature regime prescribed by AOSA/ISTA, 20°/30°C (16h/8h), with light applied at 200 lux or as described in Chapter 6. If it is not possible to provide an alternating temperature regime, it is suggested that 30°C would be a suitable constant temperature germination test regime.

It is probable that the germination of seeds of F. tataricum will prove more difficult to achieve. Removal of both the pericarp and seed coat will promote full germination if the seeds have been partly after-ripened (1), but the promotion is meagre for non-after-ripened seeds (1). Similarly treatment with gibberellins may have some promotory effect for partly after-ripened seeds but fails to promote the germination of non-after-ripened seeds (1). For the present it is suggested that the seeds be tested for germination in the manner described for F. esculentum after the removal of pericarps and seed coats. Although pre-dry treatments have been reported to be successful (1), the treatments themselves are severe and if applied should be used with great caution.

VII. References

1. Born, W.H.V. and Corns, W.G. (1958). Studies on seed dormancy, plant development, and chemical control of tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) I. Seed dormancy. Canadian Journal of Plant Science, 38, 357-365.

2. Born, W.H.V. and Corns, W.G. (1958). Studies on seed dormancy, plant development, and chemical control of tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) II. Germination, growth, flowering and seed production. Canadian Journal of Plant Science, 38, 367-373.

3. Cormack, R.G.H. (1952). A note on the dormancy of tartary buckwheat seeds. Scientific Agriculture, 32, 170-172.

4. Ismagilov, F.S. (1981). [On stimulation of buckwheat seed germination by kinetin and N-chloroacetic-m-iodoaniline.] In Rost i Produktivnost' rastenii. Ufa. USSR, pp. 16-22. (From Field Crop Abstracts, 1982, 35, 9219.)

5. Katoch, P.C., Baksh, S., Bhardwaj, S.D. and Kaushal, A.N. (1979). A report of vivipary in buckwheat (Fagopyrum spp.). Current Science, 48, 446-447.

6. Singh, V.P. and Mall, S.L. (1977). Seed germination studies in Fagopyrum esculentum Moench. I. Role of light and temperature. Proceedings of the Indian National Science Academy, B, 43, 37-43.