Papaya plant information

The physiology of papaya/papaw plant

The papaya (Carica papaya L.) is the most economically important fruit in the Caricaceae family and the 4th most traded tropical fruit after banana, mango, and pineapple. Papaya has won consumers thanks to its great taste (melon-banana-like sweet taste), high nutritional value, and health benefits. It is a small (12-20 feet tall) deciduous tree-form and pyramidal-shaped shrub with a quick growth rhythm. It is a fast-growing with short life with a single straight or sometimes branched stem reaching 2-10 m height, which is a cylindrical spongy-fibrous, loose, hollow, gray or gray-brown color, 10-30 cm diameter and toughened by large and protruding scars caused by fallen leaves and flowers. 

The fruit is 4 to 5 inches long with smooth custard and light yellowish to orange flesh. The fruit is usually eaten fresh but may also be used for making custard pies and preserves. 

Each plant usually bears around 15-20 leaves. The leaves are alternate, spirally arranged, bundled at the apex between the stem and branches. The size of papaya leaves is around 50 to 70 cm wide and up to 90 cm long. They are smooth, moderately palm-shaped, with thick middle irradiant veins. Papaya leaves have a life cycle of 6 to 8 months. 

The plants have different flowers that influence pollination and fruit set, which may be classified into three primary sex types: 

1) Male (staminate), 

2) Hermaphroditic (bisexual), 

3) Female (pistillate). 

Some papaya plants can produce, at the same time, more than one kind of flower. Some produce flowers that are not of these basic forms but exhibit different degrees of maleness and femaleness. This tendency to change in sexual expression seems triggered by climatic factors, such as drought and variable temperatures. The tendency to produce male flowers seems to increase at high temperatures. Since male trees are unfruitful and fruit from bisexual plants is preferred in some markets, selecting seeds that will give a maximum number of fruitful trees of the desired type is essential. This cannot be done by simply saving seeds from productive open-pollinated plants, but one can predict pretty accurately the gender by knowing the source of pollen and the kind of flower the fruit came from. The commercial grower must learn how to hand pollinate to obtain the desired combination of flower types. This is done by covering an unopened flower, either bisexual or pistillate, with a paper bag until it opens and transferring the desired pollen onto the receptive pistil. Pollination studies have shown that: 

1) Pistillate flowers pollinated by staminate flowers give equal numbers of male and female plants; 

2) Pistillate flowers pollinated by pollen from bisexual flowers give an equal number of female and bisexual plants, 

3) Bisexual flowers either self or crossed-pollinated with other bisexuals, give a ratio of one female to 2 bisexual, 

4) Bisexual flowers pollinated by staminate ones produce equal numbers of female, male, and bisexual progeny. The second and third combinations will produce the maximum number of fruit-bearing plants. 

Flowers: Six types of flowers are known in the papaya plant. 

1. a) Typical female flower. It is a rather large flower of conical shape when closed; when open, its five petals spread from the base. The ovary is large and circular and smooth or slightly raised. Fruits produced by this flower are spherical or oval. 
2. b) Similar to the above, when closed, this type has five short anthers, which correspond in their orientation with the five petals that also spread from the base. The ovary has five deep longitudinal grooves that remain until maturity. Fruit develop a form from globular to egg-shaped. 
3. c) Hermaphrodite intermediate flower. The organization is undefined; petals may be fused up to two-thirds of their length or free from the base. The number of anthers ranges from two to ten. This type of flower produces an irregularly shaped fruit known as a cat face with little commercial value. These flowers appear more frequently when prevailing temperatures are 24.5 °C during the day and 15.5 °C at night. 
4. d) Hermaphrodite elongated flower. Petals of this type of flower are fused from one-fourth to three-fourths of their total length; ten anthers are observed, five long and five short. The ovary is long, and the form of the fruit varies from cylindrical to pear-shaped. From the different types of hermaphrodite flowers, this is the most commercially important papaya flower. 
5. e) Hermaphrodite sterile flower. It is a flower that resembles the one in (d) but does not develop an ovary, and hence, it is sterile due to warm temperatures or water stress. Due to the fact that it produces pollen only, it may be considered a functional male flower. 
6. f) Typical male flower. This type of flower has a long and thin corolla containing anthers in two series of five, one series longer than the other. They have a pistil, no stigma, and are non-functional. 

In nature, papaya plants have male and female flowers found on separate plants. Male flowers are physically different from female flowers. Male inflorescences are borne in many-flowered panicles, or pendent stalks up to 1 m long. The flowers are yellowish, 2-4 cm long. The petals are fused into a long tube, have 10 fertile stamens, and a rudimentary, non-functional ovary. Female inflorescences are much shorter, only 3-4 cm long, and have fewer flowers. Female flowers are larger, usually white or cream in color, with five free petals. There are no stamens but a large ovary with 5 fan-shaped stigmas. 

NOTE: Environmental factors influence sexual expression, and the sexuality of a plant may change seasonally or over the course of its lifetime. Female flowers have a calyx formed by a crown or five-pointed star that is easily differentiated. On top of the calyx, the ovary is located by five yellowish sepals (when young, they show a purple coloration and are lost. There are five round-shaped yellow stigmas. Fruits from this flower are usually large and balloon-like. 

Male flowers grow along peduncles measuring over half a meter in length, and at the end, there are bundles formed by 15-20 small flowers. These flowers are constituted by a long tube formed by fused petals, inside which there are 10 anthers arranged in two sets of five. The flower has a small rudimentary pistil and has no stigmas. No fruit is usually produced; if formed, these are elongated and of low quality. 

Papaya Fruit characteristics

The fruit exhibits a melon or pear-like appearance, with an oval to nearly round or elongated, club-shaped form, measuring 15-50 cm long and 10-20 cm thick. It can weigh up to 9 kg. The fruit’s skin is characterized by its waxy and thin texture, which is relatively tough. The fruit contains a substantial amount of white latex in its unripe, green state. The outer surface transforms into a light or deep-yellow hue as it matures. At the same time, the thick layer of succulent flesh develops an aromatic profile, showcasing shades of yellow, orange, or even varying tones of salmon or red. At this stage, the fruit becomes juicy slightly sweet, with a flavor reminiscent of a cantaloupe, and in some varieties, it can have a distinct musky quality. Each fruit encloses, on average, 100-500 black, round, corrugated, black, peppery seeds lightly attached to the fruit’s wall through soft, white, fibrous tissue.

Cultivars There are many strains and varieties of papaya fruit, and the variation in size, form, and color is great. Because of its complex genetic makeup, there are few, if any, true cultivars of papaya that are as uniform in horticultural characters as the cultivars of other herbaceous crops. When seed results from open pollination, in most cases, it is impossible to obtain selections that are reasonably uniform in flower type and fruit characteristics. Though there may be a lack of recognized cultivars, growers can maintain satisfactory strains by controlled pollination of selected plants. Parent plants should be carefully selected for early and heavy fruit production and should have fruit of desirable shape and size.

For Further reading

Papaya Interesting facts, Nutritional value, and Health benefits

Papaya plant information

Papaya Soil Preparation, Planting, and Plant density

Papaya propagation and Pollination

Papaya Plant Care – Irrigation and Fertilization of Papaya plants

How to cultivate papayas for profit – Complete papaya production guide

Papaya Plant Major Pests, Diseases and Weed Management

Papaya Harvest,  Yield and Storage

Papaya Handling, Grading, and Packing

References

https://gsconlinepress.com/journals/gscbps/sites/default/files/GSCBPS-2021-0073.pdf

https://edis.ifas.ufl.edu/publication/MG054

https://biosafety.icar.gov.in/wp-content/uploads/2016/10/3_Biology_of_Carica_papaya_Papaya.pdf

https://journals.ashs.org/hortsci/view/journals/hortsci/55/11/article-p1861.xml

Carvalho FP. Agriculture, pesticides, food security and food safety. Environ Sci Policy. 2006; 9(7–8):685– 92. 

FAO. Food and Agriculture Organization of the United Nation. Sustainable Food Systems. Concept and Framework. 2018. 

Kuhfuss L, Préget R, Thoyer S, Hanley N (2016) Nudging farmers to enrol land into agri-environmental schemes: the role of a collective bonus. Eur Rev Agric Econ 43:609–636. 

Lamichhane JR, Dachbrodt-Saaydeh S, Kudsk P, Messéan A (2015) Toward a reduced reliance on conventional pesticides in European agriculture. Plant Dis 100:10–24. 

Le Gal P-Y, Dugué P, Faure G, Novak S (2011) How does research address the design of innovative agricultural production systems at the farm level? A review. Agric Syst 104:714–728. 

Lechenet M, Bretagnolle V, Bockstaller C et al (2014) Reconciling pesticide reduction with economic and environmental sustainability in arable farming. PLoS ONE 9:e97922. 

Lefebvre M, Langrell SRH, Gomez-y-Paloma S (2015) Incentives and policies for integrated pest management in Europe: a review. Agron Sustain Dev 1:27–45 

Lesur-Dumoulin C, Malézieux E, Ben-Ari T et al (2017) Lower average yields but similar yield variability in organic versus conventional horticulture. A meta-analysis. Agron Sustain Dev 37:45. 

Liu B, Li R, Li H et al (2019) Crop/weed discrimination using a field imaging spectrometer system. Sensors 19:5154. 

MacMillan T, Benton TG (2014) Agriculture: engage farmers in research. Nat News 509:25. 

Mahlein A-K (2015) Plant disease detection by imaging sensors – parallels and specific demands for precision agriculture and plant phenotyping. Plant Dis 100:241–251. 

Maria K, Maria B, Andrea K (2021) Exploring actors, their constellations, and roles in digital agricultural innovations. Agric Syst 186:102952. 

Mariotte P, Mehrabi Z, Bezemer TM et al (2018) Plant–soil feedback: bridging natural and agricultural sciences. Trends Ecol Evol 33:129–142. 

Martinelli F, Scalenghe R, Davino S et al (2015) Advanced methods of plant disease detection. A review. Agron Sustain Dev 35:1–25. 

Sapkota, T.B.; Mazzoncini, M.; Bàrberi, P.; Antichi, D.; Silvestri, N. Fifteen years of no till increase soil organic matter, microbial biomass and arthropod diversity in cover crop-based arable cropping systems. Agron. Sustain. Dev. 2012, 32, 853–863. 

Muller, A.; Schader, C.; Scialabba, N.E.H.; Brüggemann, J.; Isensee, A.; Erb, K.; Smith, P.; Klocke, P.; Leiber, F.; Stolze, M.; et al. Strategies for feeding the world more sustainably with organic agriculture. Nat. Commun. 2017, 8, 1290. 

Seufert, V.; Ramankutty, N.; Foley, J.A. Comparing the yields of organic and conventional agriculture. Nature 2012, 485, 229–232. 

Tal, A. Making conventional agriculture environmentally friendly: Moving beyond the glorification of organic agriculture and the demonization of conventional agriculture. Sustainability 2018, 10, 1078.