INTRODUCTION
Papaya (Carica papaya L.) has never been found in the wild, but is believed to have originated in tropical America, probably in southern Mexico and neighbouring Central America. It was taken by the Spanish to the Philippines in the 16th century, from where it was soon introduced to India. It has since spread to most tropical and subtropical countries between latitudes 32°N and 32°S, and up to an altitude of 1300 m near the equator (Muthukrishnan and Irulappan, Reference Muthukrishnan, Irulappan, Bose and Mitra1990; Purseglove, Reference Purseglove1968). The ripe fruit is eaten fresh and used in the preparation of soft drinks, jams and other processed products. The immature fruit contains a milky latex, which is a source of papain, a proteolytic enzyme. Papain has many practical applications, for example it is used as a meat tenderizer, by the pharmaceutical, textile and tanning industries, and in medicine.
In 2010, there were an estimated 438,000 ha of papaya in the world with an annual production of 11 million tonnes of fresh fruit. The principal producers were India (4.7 million t from 112,000 ha), Brazil (1.9 million t from 34,000 ha), Nigeria (0.7 million t from 94,000 ha), Indonesia (0.7 million t from 8,100 ha), followed by Mexico with 0.6 million t from 14,000 ha (FAOSTAT, 2012). These data suggest average yields of between only 0.7 t ha−1 (Nigeria) and 86 t ha−1 (Indonesia). The reliability of some of these estimates is questionable.
In recent years, the physiology and water relations of papaya have been well researched, particularly in Brazil and on the island of Guam (United States of America). This paper attempts to synthesise what is known, and to do this in practically useful ways. It follows the format used in previous reviews in this series, including other fruit crops, notably, banana (Carr, Reference Carr2009), coconut (Carr, Reference Carr2011a), citrus (Carr, Reference Carr2012a) and pineapple (Carr, Reference Carr2012b). The paper begins with a description of the stages of crop development (including roots), followed by reviews of plant water relations, water requirements and water productivity, and irrigation systems.
The environmental physiology of papaya, concentrating in particular on photosynthesis, has been reviewed by Campostrini and Glenn (Reference Campostrini and Glenn2007). A general description of papaya culture can be found in Morton (Reference Morton1987) and, with a focus on South Africa, in DAFF (2009).
CROP DEVELOPMENT
Vegetative
Papaya is a short-lived (it is productive for up to three or four years) large herb (not strictly a tree), although its stature is not that of a typical herbaceous plant. The hollow stem grows rapidly up to 10 m in height. It is usually unbranched. The xylem vessels are concentrated near to the periphery of the stem. Leaves emerge from the upper part of the stem in a spiral on nearly horizontal petioles. The stem and the leaves contain latex as well as immature fruits (Morton, Reference Morton1987; Purseglove, Reference Purseglove1968; Reis et al., Reference Reis, Campostrini, de Sousa and Silva2006). The trees are dioecious, although hermaphrodite forms occur. Papaya is usually propagated from seed, so there are few true varieties. The Hawaiian Solo, Blue Solo and more recent types are fairly uniform and are closest to varietal status. Consequently, most papayas are types rather than varieties, so fruiting characteristics are better considered in groups. This variability within a population makes irrigation experiments particularly difficult (see below). The planting density is within the range 1000–3000 trees ha−1
Papaya trees are vulnerable to wind damage, especially when the soil is wet and the trees are bearing a heavy load of fruit. The influence of wind on the growth processes is of particular interest. For example, the sensitivity of papaya seedlings to wind has been demonstrated in a series of experiments on the island of Guam in the western Pacific Ocean (13°26′N 144°47′E). Guam is exposed to trade winds throughout the year. Over a three-week period of exposure to a constant, one-direction wind at speeds ranging between zero and 2.5 m s−1, the rate of dry mass gain by the young plants (three to six weeks old) declined continuously with increase in wind speed (this relationship was best described by a negative quadratic function, R2 = 0.83 and 0.93 for the two cultivars tested) from an average of 1900 mg plant−1 to 900 mg plant−1. The effect of water stress was additive to that of wind, i.e. there was no interaction between the two variables tested (Marler and Clemente, Reference Marler and Clemente2006).
Previously, Clemente and Marler (Reference Clemente and Marler2001) had shown how wind exposure reduced several measures of seedling growth, including plant height, and leaf and stem dry weight, but had no effect on root dry weight. Not all the genotypes tested responded to the same degree, indicating genetic variability within the species for tolerance to wind. Subsequently, Marler (Reference Marler2011) described how wind reduced stem and leaf expansion rates, but not root extension. Adaptive responses to directional wind exposure included asymmetric increases in stem diameter on the side exposed to the wind.
Close spacing of papaya trees results in tall plants with thin stems, making them more susceptible to wind damage than plants grown at a wider spacing. In the US Virgin Islands (18°N 65°W), Zimmerman (Reference Zimmerman2008) compared the responses of three papaya cultivars to three plant densities, corresponding to 6000, 3000 and 2000 plants ha−1. In order to obtain a marketable crop during the holiday season, the trees were established in the field in double rows in February during the dry season, using drip irrigation. After six months, plants grown at the highest density had fewer fruits (c. 25 tree−1) than those planted at the other two densities, both of which had a similar number (c. 41 tree−1). The fruits were also set closer to the soil surface. The three cultivars, Maradol (compact), Tainung 5 and Yuen Nong 1 (both standard size trees), were chosen because they set fruit early, and responded to plant density in similar ways.
Flowering
After a juvenile period, flowers begin to develop in leaf axils. Some trees bear only female flowers, or bisexual flowers, while other trees only have male flowers. Some plants may have both male and female flowers. Male flowers are clustered on long panicles. Seedlings usually begin flowering about two months after planting out in the field, although the length of the juvenile period depends in part on the cultivar (Sansom, Reference Samson2003). Flowering continues throughout the year as new leaves emerge.
Fruiting
Papaya trees begin to bear fruit within a year after planting, sustaining high yields for two years (up to 100 fruits tree−1 year−1) before yields decline. The fruit is a large, bulbous, hollow berry. It takes from four to six months after flowering for fruits to mature. Fruits formed from female flowers are oblong to nearly spherical. Those formed from bisexual flowers are pear shaped, cylindrical or grooved. Marketable fruits weigh from 0.3 to 3 kg, although small fruit are preferred for export. After three to four years, papaya plantations are usually uprooted and replanted, as yields decrease and the trees become too tall for economical harvesting. Fruit is harvested in most weeks, except during cool weather (DAFF, 2009; PIP, 2011; Samson, Reference Samson2003). When papaya trees are experiencing severe water stress, the leaf area declines, the fruits shrink in size and the new internodes for flowers are compressed.
In South Africa, the responses to temperature of clonal plants (cv. Honey Gold) were compared in controlled temperature greenhouses. A very hot regime (day/night temperatures 36/28 °C respectively) led to rapid rates of development (nine to ten leaves emerging each month), tall spindly plants and small, early maturing fruits (170 days from flowering to ripening). In the coolest regime (20/12 °C) leaves emerged at half the rate (four to five leaves month−1), while fruits took 312 days to mature (Allan et al., Reference Allan, Chlery and Biggs1987). The authors concluded that the optimum air temperatures for papaya, growth and fruit production are in the range 25–30 °C (day) and 11–16 °C (night).
Roots
The root systems of young papaya plants were described by Marler and Discekici (Reference Marler and Discekici1996a) at intervals of three and six months after being transplanted to the field in Guam. At the three-month stage the spread of roots covered an area of 3.5 m2. After six months, this had increased three-fold to 10.2 m2. The corresponding total root lengths were 9.6 m and 975 m respectively, a 10-fold increase.
In Bahia State, Brazil (12°48′S 39°06′W; alt. 225 m), Coelho et al. (Reference Coelho, Saantos and Filho2005) compared the root distribution of papaya (cv. Sunrise Solo) irrigated by three different methods, namely micro-sprinklers and drip irrigation, either buried (at a depth of 0.25 m) or on the soil surface. The soil was a Latosol (sandy clay) and the average annual rainfall was 1143 mm. Root samples were collected from a grid of points on the wall of 0.75 m deep trenches, 19 months after planting in the field. By using a scanner, root length densities were calculated and lines plotted showing the distribution of lateral roots of similar densities. For surface drip irrigation, using a root length density of 0.04 cm−3 as the threshold value, 80% of roots were found within a depth of 0.25 m below the dripper, and within a width of 0.35 m. Although some roots were present at depths of 0.75 m, 80% (this is an arbitrary value chosen to represent the limits for an ‘effective’ root system) of the total root length was within 0.45 m of the soil surface. For the buried drip system, the greatest root density extended from the soil surface to a depth of 0.35 m and laterally over a width of about 0.55 m. Over 90% of the total root length was within a soil depth of 0.25 m. For the micro-sprinkler, the ‘effective’ root system occupied a larger volume of soil than it did for either drip system: the maximum root length density was between 0.25 and 0.45 m depth with 80% of the total root length within 0.55 m depth. These data give a useful indication of where to site soil water sensors for irrigation scheduling purposes.
The results of a similar study, this time on Guam, were summarized by Marler and Discekici (Reference Marler and Discekici1997a). During the dry season, roots proliferated under the drip irrigation lines in inverse proportion to the number of drip lines (1, 2 and 3). Where there was a single line, the density of roots, as recorded on the trench profile walls, was 3.7 times greater than in an unirrigated zone, 2.3 times greater where there were two lines, and 1.9 times greater where there were three lines. The authors concluded that papaya roots are highly morphoplastic, proliferating in wetted zones when only part of the root zone is irrigated. One dripper line per row was considered to be adequate.
In an unusual experiment on Guam, Marler and Discekici (Reference Marler and Discekici1997b) investigated root distribution of papaya (cv. Red Lady) growing on a steep slope (60–70%). Roots were exposed and mapped on the uphill and downhill faces of trenches excavated seven and 17 weeks after planting. Afterwards they were separated from the soil and their length and dry mass determined. There was a greater concentration of lateral roots on the downhill side on both sampling dates (64% and 69% by mass, 56% and 71% by length, after seven and 17 weeks respectively). In addition, after 17 weeks, 34% of the roots on the uphill side were located above a horizontal plane positioned at the stem base. The authors considered that, when monitoring crop water use, these differences in root distribution justified installing soil water sensors both sides of a papaya plant growing on a steep slope.
Campostrini and Glenn (Reference Campostrini and Glenn2007) stressed the importance of a papaya tree having a mycorrhizal network for improved water and nutrient uptake.
Summary: crop development
1. Papaya is a short-lived large herb, although its stature is not that of a typical herbaceous plant.
2. The plants are dioecious.
3. Papaya seedlings are sensitive to wind, and mature trees are vulnerable to wind damage.
4. Flower buds develop in the leaf axils and flowering begins about two months after planting.
5. Papaya trees begin to bear fruit within a year after planting, sustaining high yields for two years before yields decline.
6. Leaf production, flowering and fruiting are continuous processes.
7. The optimum air temperatures for growth and fruit production are in the range 25–30 °C (day) and 11–16 °C (night).
8. Papaya roots can reach depths of at least 0.75 m, but the ‘effective’ root depth (80–90% of the total root length) varies with the irrigation method, e.g. 0.45 m (surface drip), 0.25 m (subsurface drip) or 0.55 m (micro-sprinkler).
PLANT WATER RELATIONS
In this section the role of the stomata in controlling gas exchange (particularly photosynthesis and transpiration) and the influence of dryness of the air, light intensity and soil water availability on these processes are considered.
Stomata
The stomata are found only on the lower leaf surface of papaya at densities of about 400 stomata mm−2 (El-Sharkawy et al., Reference El-Sharkawy, Cock and Hernandez1985). Stomatal conductance is sensitive to the saturation deficit of the air, declining by 47% when the saturation deficit increases from 1–1.5 kPa to 3.5–4.5 kPa. There is a similar reduction in the rate of photosynthesis. This value compares with 85% reduction in conductance for cassava, a very sensitive plant, and 30% for maize, which is less sensitive (El-Sharkawy et al., Reference El-Sharkawy, Cock and Hernandez1985).
The sensitivity of stomatal conductance in papaya (Formosa group) to the saturation deficit of the air was also demonstrated by Reis and Campostrini (Reference Reis and Campostrini2008) in field measurements taken in Brazil (21°27′S 41°15′W). On cloudless days in October, when the saturation deficit averaged 2.2 ± 0.7 kPa, the stomata were virtually closed throughout the day (despite irrigation with a centre pivot). By contrast, on cloudy days in January, during the rainy season, the stomata were wide open early in the day (0800 h), before partially closing during the morning. The stomatal conductance then remained relatively constant from midday through the afternoon. The average saturation deficit during the day was only 1.4 ± 0.7 kPa. On both occasions the air temperature averaged 29–30 °C. Rates of photosynthesis followed the same trends as stomatal conductance.
Photosynthesis
Papaya has a C3 photosynthetic pathway. According to Campostrini and Glenn (Reference Campostrini and Glenn2007), citing others, photosynthetic rates in papaya approach zero at saturation deficits of 6–7 kPa, even when the tree is growing in a wet soil. The stomata of papaya also respond rapidly to changing light conditions.
On the island of Guam, stomatal conductance was observed to decline within 60 s of reduction in irradiance (from 2000 μmol m−2 s−1 to 325 μmol m−2 s−1). Recovery began within one minute of return to full sunlight, reaching its original value after three minutes. Photosynthesis responded even more quickly, declining within 20 s of the reduction in light intensity, and recovering rapidly. This tracking response by the stomata is important in the lowland tropics where there is intermittent cumulus cloud cover leading to rapid changes in photosynthetic photon flux density. During the period of low irradiance, instantaneous water-use efficiency increased. Mild water stress enhanced the rapidity of light tracking response (Clemente and Marler, Reference Clemente and Marler1996).
On Guam, Marler and Mickelbart (Reference Marler and Mickelbart1998) observed partial stomatal closure during the middle of the day in well-watered field-grown plants (cv. Red Lady), but only when there was a clear sky. This was associated with air temperatures of 34 °C and saturation deficits of the air peaking at 2.4 kPa. On overcast days, stomatal conductance remained high throughout the middle of the day. With drought-stressed plants, gas exchange remained low and relatively constant during daylight hours on both clear and overcast days. Instantaneous water-use efficiencies followed similar diurnal trends. Leaf chlorophyll fluorescent characteristics were not influenced by drought stress.
Again on Guam, Marler and Discekici (Reference Marler and Discekici1996b) conducted an experiment in which the effect of partial irrigation of the root system of papaya, grown in containers, on stomatal conductance was evaluated. Initially the roots were trained to grow into four compartments and kept well watered until the plants were established. Water was then withheld from one, two or three of the quadrants. Restricting access to water in this way had no effect on midday stomatal conductance.
Compacted soil has been shown to reduce gas exchange in papaya (Caampostrini and Yamanishi, Reference Campostrini and Yamanishi2001). All four genotypes tested in Brazil (two from the Solo group and two from the Formosa group) responded in similar ways to root restriction with reduced photosynthetic rates and stomatal conductance, even when the soil was close to field capacity. Since conductance was reduced more than net photosynthesis, instantaneous water-use efficiencies were increased in the compacted areas, especially for one cultivar (Sunrise Solo TJ).
Transpiration
After successful calibration, the Granier heat dissipation method was used to measure sap flow in papaya trees growing in a commercial orchard in Brazil (21°27′S 41°15′W; alt. 12 m) (Reis et al., Reference Reis, Campostrini, de Sousa and Silva2006). Measurements were made over a period of four days in October and again in January. There was a positive, but nonlinear relationship between instantaneous transpiration rates measured by porometry and xylem sap flow. This relationship was explained, in part, by the time lag early in the morning between water loss by transpiration and the resultant sap flow. This was because water was taken initially from that stored in leaves, petioles and small branches. The maximum sap flow values recorded corresponded to 15.6 L d−1 m−2 (leaf area).
In a follow-up study by Ferraz et al. (Reference Ferraz, Campostrini, Torres-Netto, Reis and de Sousa2011) in Brazil (19°12′S 40°06′W), sap flow measurements again showed a time lag in the morning due in part to the large water reserve in the trunk. It was only post-midday that there was a close relationship between sap flow and whole-plant transpiration rates, as measured in the field with gas-exchange chambers on five-month-old plants (cv. Golden). There was though a good linear correlation between hourly transpiration rates and calculated reference crop evapotranspiration (ETo, r2 = 0.89 n = 18). The actual water use over four days of measurement equated to 2.46 L d−1 m−2 (leaf area) or 8.6 L d−1 plant−1. The average instantaneous water-use efficiency was 154 g water lost for each gram of CO2 assimilated.
The observation by Clemente and Marler (Reference Clemente and Marler2001) on Guam that evapotranspiration during daylight hours was less from plants exposed to wind than from those that were sheltered is of particular interest because it records a situation that theory predicts, but which is contrary to common perceptions. In contrast, at night wind exposure increased water loss (probably due to direct evaporation from the soil surface). Assimilation was reduced when plants were exposed to wind, but only on sunny days. Respiration rates increased regardless of sky conditions.
There is some evidence that cultivars differ in their physiological responses to the environment. For example, in a comparison of five different genotypes in Brazil, one cultivar (Golden) was identified, which, during the middle of the day, had stomatal conductance (and transpiration rates) higher than other four genotypes. However, over the 12-week period of measurements, photosynthetic rates were similar for all five genotypes (Torres-Netto et al., Reference Torres-Netto, Campostrini, Azevedo, de Souza, Ramalho and Chaves2009). In a similar comparison of five cultivars in South India, one (Co7) stood out from the others in many of the attributes measured in the field during the summer months, including gaseous exchange processes, cell membrane integrity and various chlorophyll indices. Instantaneous water-use efficiency was also higher in this cultivar than with the others (Jeyakumar et al., Reference Jeyakumar, Kavimo, Kumar and Soorianathasundaram2007). Net photosynthesis increased linearly with air temperature over the range from 20 °C up to 35 °C for all five cultivars, before declining rapidly. Similarly, net photosynthesis peaked at a photosynthetic photon flux density of about 1600 μmol m−2 s−1.
By contrast, in a comparison between three transgenic lines (genetically modified to confer resistance to ringspot virus) at the University of the Philippines, transpiration rates and stomatal conductance were similar among transgenic lines, and between transgenic lines and the non-transgenic control treatment (Cruz et al., Reference Cruz, Mylalulex, Magnaye, Magdalita, Hautea and Hautea2009).
Papaya is reported to be sensitive to waterlogging (low oxygen availability) (Campostrini and Glenn, Reference Campostrini and Glenn2007 citing others). Papaya is classified as moderately sensitive to salinity, with a threshold level of 3 dS m−1 for the saturated extract (Maas, Reference Maas, Baligar, Duncan and Yohe1993).
Summary: plant water relations
1. Stomata are found only on the lower leaf surface of papaya.
2. Stomatal conductance is sensitive to the saturation deficit of air.
3. The stomata of papaya respond rapidly to changing light conditions.
4. Midday suppression of photosynthesis has been observed in papaya as a result of partial closure of the stomata (but only on clear days).
5. Net photosynthesis peaks at an air temperature of 35 °C.
6. Intermittent spraying of leaf canopy with water can prevent the stomata from closing.
7. Early in the morning, there is a time lag early between water loss by transpiration and the resultant sap flow. This is because water is taken initially from storage in the stem.
8. There was a good linear correlation between hourly transpiration rates and calculated reference ETo.
9. Evapotranspiration during daylight hours was less from papaya plants exposed to wind than from those that were sheltered.
10. There is some evidence that cultivars differ in their physiological responses to the environment.
CROP WATER REQUIREMENTS
The water requirements of papaya do not appear to have been studied in any detail. There is one report from Cuba (22°46′N 82°37′W; alt. 6 m), where papaya is one of the main irrigated fruit crops, by Chaterlan et al. (Reference Chaterlan, Hernández, López, Paredes, Pereirab, Martínez and Puig2010), which describes an attempt to derive crop coefficients (Kc) based on a statistical, modelling approach. The Kc values obtained were all in the range 0.90 to 1.10, almost regardless of the stage of crop growth, and the estimate for the allowable depletion of the total available soil water was a consistent 0.40. Although the authors claimed that the indicators of goodness of fit between the observed and estimated data were good, there must remain some doubt about the validity of these Kc values for papaya elsewhere in the world.
Despite the paucity of experimental data, irrigation recommendations are still made to growers. For example, in subtropical South Africa, Government advice (DAFF, 2009) on the amount of water to apply to papaya in each season (in L d−1 tree−1, including rainfall) was as follows:
Establishment 6–13; first autumn 6–13; first winter 4–9; first spring 9–17 first summer 13–26; second autumn 9–17; second winter 6–13; second spring 13–26; second summer 17–34.
It is not known on what evidence this advice is based.
Summary: crop water requirements
1. Ways of estimating the water requirements of papaya on an orchard scale have yet to be evaluated.
2. There are no reliable published values for Kc.
3. There have been two attempts to measure transpiration by individual trees (based on sap flow).
WATER PRODUCTIVITY
Several papaya irrigation experiments have been reported, all of which were designed to identify the minimum depth of water needed (deficit irrigation) to obtain the highest water productivity. The research was undertaken in India, Puerto Rico, Brazil (two reports) and the Canary Islands.
Srinivas (Reference Srinivas1996) reported the results of one such experiment conducted in Bengaluru, India (13°58′N; alt. 868 m) over three years (1990–1992) on a sandy loam soil. There were six watering treatments: 20, 40, 60, 80, 100 and 120% replenishment of evaporation from a USWB Class A evaporation pan. Irrigation was applied on a daily basis based on a simple water balance calculation (evaporation minus ‘effective’ rainfall), except at weekends. The rainy season extended from May to November, and over the three years the rainfall totalled 265, 1292 and 803 mm a−1. Water was applied by drip irrigation, either on the surface or buried (at a depth of 250 mm). After the seedlings (cv. Coorg Honey Dew: spacing 2 × 2 m) were transplanted in September 1992 the crop was irrigated uniformly for 60 days before the treatments were imposed. The total yields of fresh fruit recorded over 28 months (36 months from planting) increased linearly with depth of water applied from 97 t ha−1 at 20% replacement to 121 t ha−1 at 60% replacement. At 80, 100 and 120% replacements, yields were similar, averaging 130 t ha−1. The yield differences were the result of both an increase in the number of fruits (from 15 to 28 tree−1) and individual fruit size (from 0.8 to 1.5 kg). It is difficult to interpret the water-use efficiencies cited in the paper (‘water use’ is not clearly defined) but, when recalculated, the incremental values, in terms of yield response to irrigation water applied, are 17.9 kg ha−1 mm−1 (from 20 to 60% replacement) and 7.8 kg ha−1 mm−1 (from 60 to 80% replacement).
The results of an irrigation experiment conducted in the semi-arid zone of the Caribbean island of Puerto Rico (17°44′N 66°38′W), where the 28-year mean annual rainfall total was 917 mm, have been reported by Goenaga et al. (Reference Goenaga, Rivera and Almodóvar2004). The five irrigation treatments were again based on applying a fraction of the evaporation from a USWB class A pan (Epan), this time over a two-year period (1996–1998). The way in which the water requirements were calculated is complicated and open to question. For example, to determine potential evapotranspiration (ETc) values were chosen for the pan coefficient (Kp = 0.70), and for a ‘modified average crop coefficient’ (Kc = 0.42). Thus,
\begin{equation*}
{\rm ETc} = 0.42\; \times \;0.70\; \times \;{\rm Epan}{\rm .}\end{equation*}
The depth of water applied to each of the five treatments was then calculated, at weekly intervals, as a fixed proportion of ETc, namely 0.25, 0.5, 0.75, 1.0 and 1.25 ETc. Treatment 1.0 ETc was considered to be the theoretical optimum. Allowance was made for rainfall. The cultivar was Red Lady (spaced 1.8 × 1.8 m; 3100 ha−1), the soil was a Mollisol (fine loam), which was covered with a silver polythene mulch. Water (and fertilizer) was applied by trickle irrigation on alternate days. There was a linear response between both the number of harvested fruits as well as the total fresh weight of fruit and the value of the pan factor (a surrogate for the amount of water applied). This implies that even a pan factor of 1.25 did not equate to the optimum quantity of water. Marketable fruit (yield range from 50 to 78 t ha−1) made up 78% of the total fruit number in both extreme treatments. There was no effect of treatment on the sweetness (Brix index) of the fruit. The water productivity, calculated from the data presented in the paper, equated to 28 kg (fresh marketable fruit) ha−1 mm−1 (irrigation water applied), or 2.8 kg m−3.
A similar type of experiment was undertaken in the Northern Fluminense Region of Brazil (21°45′S 41°19′W). This time there were seven water replacement treatments, ranging from rainfall only (the control) to water applications (with micro-sprinklers) equivalent to 0.40, 0.80, 1.20, 1.60, 2.00 and 2.40 times the reference ETo, where ETo is the evapotranspiration from a USWB Class A pan. Total yields of fresh fruit (cv. Improved Sunrise Soil 72/12) obtained over a 16-month period increased with the depth of irrigation water applied from 7.4 t ha−1 (80 mm irrigation; total rainfall 1268 mm) to 53.0 t ha−1 at 1.60 ETo (1749 mm irrigation). Yields declined if more water than this was applied. When averaged over the first five treatments (rain only to 1.60 ETo), the water productivity (for irrigation) was again 28 kg ha−1 mm−1 (Almeida et al., Reference Almeida, de., Bernado, de Sousa, Marin and Grippa2003).
In the state of Espirito Santo, Brazil, a similar selection of watering treatments was imposed over a 14-month period (1996/1998). There was a linear yield response over the range of water applications from 0.40 to 1.20 times Epan (USWB Class A × Kp). Irrigation amounts were supplementary to rainfall, which averaged 1249 mm a−1. It is difficult to judge the validity of the data as no points are shown on the graphs, only best fit lines for each of the three irrigation intervals that were compared. The commercial fruit yield response to the water applied 9.6 (at two-day intervals), 16.2 (three-day intervals) and 29.4 (five-day intervals) kg ha−1 mm−1. The largest yield obtained was 31 t ha−1, with a total water application (rain plus irrigation) of 2730 mm. Yield increases were due to the combined effects of larger fruit and more fruit per tree (Da Silva et al., Reference Da Silva, Ferreira, Costa, Melendes and Cecon2001).
Another similar irrigation experiment of a similar format was undertaken on the island of Teneriffe, Canary Islands (28°14′N 16°50′W) (Santana et al., Reference Santana, Sousa, Rodriguez and Suarez2008). The four treatments were all based on applying a proportion (0.2, 0.5, 0.8 and 1.1) of Epan (USWB Class A). Allowance was also made for the area of the ground shaded by a tree at midday. It is not stated how the timing of an irrigation was determined. Treatments began when the first flower had formed. The experiment was repeated thrice between 2000 and 2006. Allowance was made for ‘effective rainfall’. Actual rainfall totals during the duration of each experiment ranged between 182 mm (13 months duration) and 285 mm (12 months duration). The corresponding Epan totals were 1824 mm and 1583 mm. The trees (a dwarf cultivar known as Baixinho of Santa Amalia) were spaced 2.5 × 2.5 m (1600 ha−1); the soil was clay loam. The depths of irrigation water applied (by trickle) in one experiment ranged from 173 to 948 mm. In all three experiments the treatment with the most water applied (1.1 Epan) yielded the most. The yield (fresh fruit) response per unit of irrigation water applied plus effective rainfall was similar between treatments and between years, averaging about 1.1 kg m−3. The highest yield achieved was 11.5 kg plant−1. There was only one harvest per plant. The yield differences were due to greater numbers of fruits, not to their size.
Summary: water productivity
1. Only five irrigation experiments with papaya have been reported.
2. All five compared yield responses to a selection of irrigation treatments based on applying a fraction of ETc, which was calculated from USWB Class A evaporation pan data.
3. In all cases, different Kp values were chosen for calculating ETc, making it difficult to compare results.
4. The experiments were of variable and limited duration – one, two or three seasons.
5. With one exception, there were linear responses between the yield of fresh fruit and the water applied expressed as a fraction of ETc.
6. Where the responses were linear, water productivity values were in the range of 1.8 to 2.8 kg fresh fruit m−3 irrigation water applied.
7. Yield benefits were the result of increase in fruit size and sometimes also fruit number.
IRRIGATION SYSTEMS
Little has been published on irrigation systems or scheduling methods that are specific to papaya. In the paper summarised above by Srinivas (Reference Srinivas1996), surface and sub-surface trickle systems were compared at six different watering levels. Across all treatments, there was a significant +9% yield advantage in favour of sub-surface irrigation (from 111 to 121 t ha−1, totalled over the 36 months from planting) as a result of larger fruit (1.5 kg compared with 1.3 kg). Srinivas also commented on the need in India to replace surface (basin) irrigation of papaya with drip systems.
In Queensland, both under-tree micro-sprinklers and drip irrigation methods are recommended for papaya, but not overhead irrigation. The maximum water requirements for the two methods are given as 160 and 50 L tree−1 week−1 respectively, while the conductivity of the irrigation water should not be above 1.2 dS m−1 (DAFF, 2012).
USAID (2008) published an interesting pamphlet on drip irrigation of papaya. Designed for the Ghana export horticultural industry, this well-illustrated training guide covers the installation, operation and maintenance of a one hectare drip irrigation system for papaya. The design peak water requirement for trees more than seven months old is 5 mm d−1 or, for a plant population of 2000 ha−1, 25 L tree−1 d−1. This equates to five drip-emitters per tree, each delivering one litre per hour for five hours per day.
Irrigation scheduling
An irrigation experiment with papaya in south Florida was reported in great detail by Migliaccio et al. (Reference Migliaccio, Schaffer, Crane and Davies2010), but it is difficult to see what, if anything, it contributed to our understanding of how best to irrigate this crop. A selection of different methods for scheduling irrigation was compared in two trials: three were based on soil water suction, one was based on historic ET and one was a fixed time schedule. The conditions were unusual in that the trees were grown in raised beds but with a maximum soil depth above bedrock of only 200–250 mm. Automated-switching tensiometers were used to control irrigation in the soil-based treatments. These often malfunctioned (power cuts, major leaks, equipment failure), while natural events (tropical storms and hurricanes) also contributed to the problems faced by the researchers in managing the experiments. Nevertheless, the authors still felt it justified to list water productivities, as tonnes (fresh fruit) m−3 (water applied), to an ‘accuracy’ of three decimal places. It is difficult to understand how this paper came to be published in a refereed journal.
Summary: irrigation systems
1. Drip and micro-sprinklers are recommended for irrigating papaya.
CONCLUSIONS
Although papaya is generally considered to be drought sensitive and responsive to irrigation, there is limited experimental evidence to support this view. Papaya is unusual among tree fruit crops in that it produces leaves, flowers and fruits on a continuous basis. Its productive life is also relatively short-lived. It should therefore be quite easy to establish cause and effect when developing relationships between water stress and yield. A uniformity of approach to irrigation experimentation and terminology would be a useful starting point. The sensitivity of the stomata to dry air is reasonably well established, but the actual water requirements have yet to be quantified with any precision. Unusually, a lot of effort has gone into explaining the effects of wind on the growth and development of papaya. Root systems are also well described. There is still a need to establish practical irrigation schedules for this remarkable crop.
Acknowledgements
I thank Dr Wayne Hancock for helpful comments on this paper.
