The Caricaceae comprise roughly 30 species of small trees and shrubs within four genera: Carica spp. provide edible fruits. The fruits are generally large berries. The seeds possess fleshy arils and show orthodox seed storage behaviour.
SEED DORMANCY AND GERMINATION
Germination is epigeal and dormancy can be a severe problem. Information is provided in this chapter for the genus Carica only.
CARICA
C. papaya L.
papaya, papaw, pawpaw
I. Evidence of dormancy
The germination of seeds of C. papaya is frequently reported to be slow, erratic, and incomplete (2,5,7). For example, in one investigation freshly harvested seeds gave only 6% germination (4). The seed is enclosed within a gelatinous sarcotesta (aril, or outer seed coat) which is formed from the outer integment. Whilst this sarcotesta can prevent germination (5,10,11), dormancy is also observed in seeds from which the sarcotesta has been removed (5,10,11).
II. Germination regimes for non-dormant seeds
Constant temperatures: 28°-29°C (2); 28°-30°C (3); 30°C, 30d (10)
Alternating temperatures: 20°/30°C (15h/9h), 90d (1)
III. Unsuccessful dormancy-breaking treatments
Constant temperatures: 30°C, in dark, after arils removed, 30d (11)
Pre-chill: 11°C, 3d (8); 15°C, 10-40d (10)
Warm stratification: 40°C, 50°C, 1-4d (10)
Pre-soak: 24h (8)
QA3: dust (7); pre-applied, 50-100 ppm (2)
Napthaleneacetic acid: pre-applied, 10-10000 ppm (5); pre-applied, 24h, 100, 200 ppm (8)
Indoleacetic acid: pre-applied, 1-10000 ppm (5)
Maleic hydrazide: pre-applied, 10, 10000 ppm (5)
Dimethyl aminosuccinamic acid: pre-applied, 24h, 10000 ppm (8) 2-Chloroethyl trimethyl ammonium chloride: pre-applied, 24h, 10000 ppm (8)
Ethylenediaminetetraacetic acid: pre-applied, 24h, 100, 200 ppm (8)
Benzyladenine: pre-applied, 20h, 1-1000 ppm, after arils removed, germinate at 30°C in dark, 30d (11)
IV. Partly-successful dormancy-breaking treatments
Pre-chill: 5°C, 30,40d, arils removed, germinate at 30°C (10); 11°C, 3d, then pre-soak, 24h (8); 15°C, 50d (10); 15°C, 10-40d, arils removed, germinate at 30°C (10)
Warm stratification: 25°C, 10-40d, arils removed, germinate at 30°C (10)
Pre-soak: 12h (9); 24h (6); 70°C, 15s (6)
GA3: (4); pre-applied, 100 ppm (5); pre-applied, 24h, 100, 200 ppm (8); pre-applied, 20h, 10-1000 ppm, after arils removed, germinate at 30°C in dark, 30d (11)
Removal of seed covering structures: arils (5,6,10); arils, then pre-soak (5); arils, then pre-wash (5); arils, then GA3 pre-applied, 10-1000 ppm (5)
Potassium nitrate: pre-applied, 24h, 1.5%, plus potassium phosphate, 1.5% (6)
Sodium phosphate: pre-applied, 12h, 200 ppm (9)
Ethylenediaminetetraacetic acid: pre-applied, 12h, 200 ppm (9)
V. Successful dormancy-breaking treatments
Pre-chill: 15°C, 50d, arils removed, germinate at 30°C (10)
VI. Comment
The flesh of papaya fruit contains an inhibitor which can prevent germination (5), the sarcotesta can prevent germination (5,6,10), but drying freshly extracted seeds results in increased germination in subsequent tests (10,11). Consequently it is suggested that, as a general practice at harvest, the freshly extracted seeds are rubbed to remove the gelatinous sarcotestae and thoroughly washed in running water before being dried for storage. Pre-soaking the seeds in water for 24 hours is reported to promote germination in seven Carica spp. (12).
Treatment with gibberellins may (4,5,8,11) or may not (2,7) promote germination, but even where it does not promote the germination of a greater proportion of seeds the time taken to germinate is reduced (2). Indeed the major effects of both gibberellin treatments and removal of the sarcotestae is a reduction in the time taken to germinate (5,11). Pre-application for 24 hours with 500 ppm GA3 is suggested.
Temperature is reported to affect greatly the percentage of seeds which will germinate (5), but unfortunately no details have been provided. On the basis of the evidence available at present, it is suggested that the seeds be tested for germination at 30°C or 20°/30°C (16h/8h), but the temperature requirements for germination require further work. If a combination of aril removal, washing and drying prior to storage, followed by pre-treatment with GA3 at 500 ppm and testing at either 30°C or 20°/30°C for 30-42 days is insufficient to promote germination, then a pre-chill treatment should also be considered: 15°C for 40 or 50 days was extremely promotory in one case (10), and is suggested as a possible standard procedure.
VII. References
1. Bass, L.N. (1975). Seed storage of Carica papaya L. HortScience, 10, 232.
2. Chacko, E.K. and Singh, R.N. (1966). The effect of gibberellic acid on papaya seeds and subsequent seedling growth. Tropical Agriculture, Trinidad, 43, 341-346.
3. Chacko, E.K. and Singh, R.N. (1971). Studies on the longevity of papaya, phalsa, guava and mango seeds. Proceedings of the International Seed Testing Association, 36, 147-158.
4. Koyamu, K. (1951). A preliminary note on the germination of papaya seed. Madras Agriculture Journal, 38, 348-349.
5. Lange, A.H. (1961). Effect of the sarcotesta on germination of Carica papaya. Botanical Gazette, 122, 305-311.
6. Pérez, A., Reyes, M.N. and Cuevas, J. (1980). Germination of two papaya varieties: effect of seed aeration, K-treatment, removing of the sarcotesta, high temperature, soaking in distilled water, and age of seeds. Journal of Agriculture of the University of Puerto Rico, 64, 173-180.
7. Ramirez, O.D. (1961). Effects of gibberellic acid on germination of papaya (Carica papaya L.) seed. Journal of Agriculture of the University of Puerto Rico, 45, 188-190.
8. Sen, S.K. and Ghunti, P. (1976). Effect of pre-sowing seed treatments on the germination and seedling growth in papaya. Orissa Journal of Horticulture, 4, 38-43.
9. Veeraragavathatham, D., Vadivelu, K.K. and Ranganathan, T.B. (1980). Seed invigoration in Co2 papaya. South Indian Horticulture, 28 69-71. (From Seed Abstracts, 1981, 4, 2822.)
10. Yahiro, M. (1979). Effects of seed-pretreatments on the promotion of germination in papaya, Carica papaya L. Memoris of the Faculty o Agriculture, Kagoshima University, 15, 49-54.
11. Yahiro, M. and Oryoji, Y. (1980). Effects of gibberellin and cytokinin treatments on the promotion of germination in papaya, Carica papaya L., seeds. Memoris of the Faculty of Agriculture, Kagoshima University, 16, 45-51.
12. Riley, J.M. (1981). Growing rare fruit from seed. California Rare Fruit Growers Yearbook, 13, 1-47.
