**4. Papaya production**

Papaya can be grown from seeds and vegetative tissues such as cuttings, grafting, and in vitro culture. The somatic embryos and somatic tissues can also be

micro-propagated [49]. In many countries including India, Bangladesh, and Malawi papaya seeds are collected by growers from open-pollinated varieties of both female and hermaphrodite types and cultivated with little or no fertilization, irrigation, insect, or pathogen control. As a result, the yield and quality generally are variable as too the phenotype. Fruits are harvested and consumed within the household and are an important source of dietary fiber [50–52].

#### **4.1 Open field**

Commercial production of papaya is traditionally done under large open-field conditions. Selected cultivars, both inbred and hybrid varieties with the desired market characteristics of fruit color, weight, size, shape, and texture, are cultivated from seedlings. The plants are well maintained being fertigated, rouged to remove both off types as well as competing for vegetation, and sprayed with pesticides, fungicides, and other protective applications against insects and pathogens which negatively impact yield. Commercial papaya plantations are either rain-fed or irrigated by furrow, drip, sprinkler, or other mechanical means [53]. Integrated farm management has been practiced in some countries and has been shown to enhance papaya fruit yields and net return for growers even when compared to traditional management techniques [54]. Papaya fruit is harvested by hand using experienced pickers or, in some more developed countries, using mechanical harvesters. Fruit can be treated before packaging for long-distance transport. The use of cold room storage provides for the extended availability of fruit in the market and allows for farmers and wholesalers to also penetrate nontraditional markets and seasons offering significantly higher returns.

Large-scale open field conditions are the preferred cultivation for most papaya commercialization as it provides large-scale production with low to medium investment and operating costs. The drawbacks are less consistent fruit quality and yields as a result of seasonal variations and unexpected weather conditions such as flooding, in addition to physical damage to fruit as a result of environmental conditions, insects, and disease. Favorable cultivation conditions for papaya plants include cultivation temperatures of between 20°C and 30°C with a relative humidity of 66%, well-drained soil with a pH of between 6.0 and 6.5, low wind, adequate irrigation, and a balanced fertigation regime preferably via a drip irrigation system. Papaya plants are sensitive to frost with yields being negatively affected both through reduced temperatures and fruit quality. Generally, plants are productive after approximately nine months of transplanting and will yield for between two and four years, depending on the variety as well as weather conditions and inputs. Papaya is susceptible to a range of diseases and pests depending on the region. Papaya ringspot disease, caused by PRSV, is one of the most severe diseases and results in significant losses. Genetically modified PRSV resistant papaya varieties have been developed and have been found to be effective in controlling the disease [55]. Resistant varieties have been developed in a number of countries both on a research and commercial basis including the United States, Australia, Taiwan, China, India, Thailand, and The Philippines. In the United States two cultivars, Rainbow and SunUp, have been released on a commercial basis [56]. More recently, the use of gene mutation technologies, including CRISPR-Cas9, has allowed the mutation of cell receptors in papaya that facilitate cell infection by PRSV (as well in a range of potyvirus susceptible plants including species of *Capsicum*), thus rendering the mutated plant resistant.

### **4.2 Protected cultivation**

Papaya production under protected conditions has been widely adopted using a variety of modifications applicable to local climatic conditions and cultivars used. The use of greenhouses with full climate control environments provides for yearround, high-quality fruit with maximum yields, albeit at very high capital input in addition to higher operating costs compared to open or protected field cultivation. In India, "Red Lady" papaya growing under greenhouse conditions performed well with reduced insect infestation and disease, and improved fruit quality [57, 58]. A combination of short stature cultivars and greenhouse conditions in Argentina and similar temperate regions allows for year-round papaya cultivation [59]. The use of closed plastic tunnels in the subtropical areas of Europe, including the Mediterranean and Canary Islands, has been successful in producing high yields. Similar methods have also been employed in Turkey where there is a widespread use of protected cultivation and greenhouses for a large range of crops [60]. A comparison of papaya cultivation and harvesting periods throughout the year indicated that greenhouse conditions result in the production of more uniform fruit quality in part due to a uniform season [61]. In southeastern Spain, five locally grown commercial papaya varieties were cultivated in multi-tunnel greenhouses covered with low-density polyethylene over fixed periods of 456 days and were considered a commercial success [62]. Under similar conditions and in the same region, five commercial cultivars of various geographic origins, have different plant and fruit types. Two varieties, "Siluet" and "Sensation", have been specially selected under greenhouse conditions and are both high yielding with the fruit of optimal quality for the European market including size, shape, weight, and importantly, total soluble solids (TSS), a major factor in determining sweetness. The greenhouses, with active climate control (ACC) incorporating both cooling and heating systems, enhanced "Siluet" papaya plant growth, flowering, fruit set, and yield resulting in doubling yields with both more and heavier fruit. Additionally, fruit quality factors including skin color, acidity, and TSS were not affected. Protected cultivation and the use of greenhouses offer an affordable and cost-effective strategy for papaya cultivation, especially in regions where open field cultivation, whether due to climate, soil, or other factors, is not feasible [63]. The compactness of the protected system producing large volumes of high quality, uniform fruit, aligned with readily available packaging and transport facilities lends itself to supplying both long and short distance high-value markets.
