Carica papaya

Why are the seeds of this species difficult? The seeds have uncertain storage behaviour. They tolerate drying but seem to lose viability at low temperatures. Drying and cooling the seeds may induce dormancy.

Carica papaya (Image: S. Wood, RBG Kew)

Seed Storage

It is likely that this species has Orthodox seeds. To avoid a decline in viability, remove seeds from ripe fruits soon after harvest and allow to dry slowly, under ambient conditions, before transferring to a cool dry room. Dry to 15-20% eRH and store close to -15ºC, but no lower than this. The seeds will be safe in the short-term under these conditions, but survival in long-term storage is not guaranteed.

The seed storage behaviour of this species is not fully understood. The seeds are currently classified as intermediate in storage behaviour, though drying (to 4.5-11.5% moisture content) and cooling seems to induce dormancy rather than a loss in viability, suggesting that these seeds could actually be orthodox. Furthermore, both the endosperm and embryo display an oligosaccharide to sucrose ratio within the range of values found for orthodox seed tissues.

The seeds of this species are sensitive to storage at temperatures below -15°C. Papaya seeds are oily (average oil content is 24%), so sensitivity at low temperatures could be due to lipid crystallization. Handle the seeds carefully when re-warming, to allow the lipids to slowly melt.

Germination Requirements

Germinating Carica papaya
(Image: W. Stuppy, RBG Kew)

If the seeds have been dried and cooled, allow them to imbibe on agar, germination paper or sand, at 45°C for up to 4 hours. Transfer to an incubator at 25°C for germination.

The seeds of this species may have Physiological Dormancy, induced during the processes of drying and cooling the seeds for storage. If germination is low, you can perform delicate surgery to enable the embryo to grow. Excise the tissue near the root tip.

Seed Morphology

Carica papaya section
(Image: L. Dytham, RBG Kew)

The fruit is a fleshy berry, ranging in shape from spherical to ovoid or long and cylindrical to pear-shaped. The fruit has a central ovarian cavity containing numerous black seeds. The seeds contains a small, laterally flattened embryo with ovoid cotyledons surrounded by a fleshy endosperm. The seed coat is made up of a hard, dark-brown endotesta and a translucent sarcotesta.

The thin fruit coat turns from green to yellow or orange at maturity. The flesh of the fruit is white, turning to pale orange-yellow, salmon pink or red.

See other images of this species at VIRBOGA (The Virtual Botanic Garden).

Taxonomy

Family: CARICACEAE

Accepted name: Carica papaya L.

Synonyms: various

Common name: papaya, common pawpaw

References and Links

Baskin, C.C. and Baskin J.M. (1998). Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press, USA.
Food and Agriculture Organization of the United Nations and International Plant Genetic Resources Institute (1994). Genebank Standards. FAO/IPGRI, Rome, Italy.
Janick, J. and Paull, R.E. (eds) (2008). The Encyclopedia of Fruit and Nuts. CAB International, UK.

AFPD (African Flowering Plants Database) - Conservatoire et Jardin Botaniques de la Ville de Genève, Switzerland, and South African National Biodiversity Institute, Pretoria, RSA: Carica papaya.
PROTAbase (Plant Resources of Tropical Africa) - Wageningen, Netherlands: Carica papaya.
SID (Seed Information Database) - Royal Botanic Gardens, Kew, UK: Carica papaya.
Tropicos - Missouri Botanical Garden, USA: Carica papaya.