Plantain-Tree Rubber Intercropping Systems Improved Productivity in the Tropical Humid Zone of Ghana, West Africa

A three-year field trial was conducted between 2014 and 2017 in the Ellembelle and Jomoro districts of the Western region of Ghana where rubber production is common to determine the optimum population density of plantain when grown in combination with immature rubber tree crops. &e trials were arranged in a Randomized Complete Block Design with 3 replications. &e treatments were sole rubber, sole plantain, and three intercrops of one row of plantain in between two rows of rubber, two rows of plantain in between two rows of rubber, and three rows of plantain in between two rows of rubber. &e rubber clone used was GT1 while the variety of plantain used was false horn. &e results showed that population density of plantain had significant effect on the growth of the associated rubber. Growing plantain at closer spacing of 1.5m under the high-density plantain treatment significantly increased plantain yield compared to the other cropping systems. &ere was a significant positive relationship between population density of plantain and the rubber tree growth and development.&e optimum population density of plantain when intercropped with rubber was 1,666/ha. &e study showed intercropping was advantageous over sole cropping for both crops.


Introduction
Rubber tree (Hevea brasiliensis [Willd. ex A. Juss.] Müll. Arg.) produces latex used in the manufacture of rubber products. e tree grows best under humid and semi-humid tropical conditions. Areas suitable for natural rubber production in Ghana include the forest zones of the Western, Central, Eastern, and Ashanti regions. e tree requires a minimum rainfall of 1,200 mm per annum and is evenly distributed on lower slopes, uplands, and flatlands.
Rubber plantation development is one of the lucrative farming ventures in the Western and parts of the Central regions of Ghana. Besides cocoa, oil palm, and coconut, rubber cultivation now stands out as one of the most profitable farming activities, despite the long gestation period of six to seven years. Coconut farmers whose farms were affected by the Lethal Yellowing Disease (LYD) as well as those who were not affected are cutting down their coconut trees to make way for the development of rubber plantations in the region [1]. e cultivation of rubber is associated with a long immature period of six years, under good management conditions and possibly longer under low input conditions during which no latex is harvested. e provision of an alternative source of income is particularly important to the smallholder low-income farmers. Similarly, land-poor/landless farmers also have the opportunity to provide labor in rubber plantations by way of intercropping arable crops [2]. Smallholder farmers in Ghana and some of the rubber growing countries like Nigeria, Côte d'Ivoire, and Liberia, amongst others, intercrop rubber with crops with a short gestation period to offer them the most practical means of addressing the gap in income suffered after planting of the rubber. ere are varying opinions by smallholder farmers about the benefits of intercropping with some farmers suggesting the associated crop exerts a negative effect whilst others reported a positive effect on the growth of rubber trees [3,4].
Studies that focused on the evaluation of productivity showed improvement in the growth of rubber trees in the intercrop relative to the sole crop [5][6][7][8][9]. e choice of intercrop by smallholder farmers depends on the local needs, the amount of capital required, market access, and agroclimatic conditions. Rubber cultivation in Ghana is predominantly under smallholder systems. It thus creates employment for a large number of farmers, and it is a sustainable source of income for them when latex production begins after six years of cultivation. However, the primary constraints faced by the farmers who are mostly resourced poor are the nonavailability of dependable income sources prior to latex production. To ameliorate this situation, farmers are advised to intercrop rubber plantations with desirable annual or short-duration cash/food crops to generate income to fill the income gap and also enhance land-use efficiency.
Intercropping in rubber cultivation offers a means of increasing income and land-use efficiency during the unproductive immature phase of rubber [9]. In Ghana, plantain appears to be the most popular intercrop with rubber cultivation. Plantain is a major food crop grown in the Western region of Ghana. It is a very important food crop in Ghana because plantain is eaten in every household in Ghana. Due to the high demand for plantain, cultivating plantain is seen as a sure way to have a regular income for farmers [10,11].
Also, it is anticipated that the adoption of an appropriate rubber-plantain intercropping system would encourage the unemployed, especially the youth in the plantation areas to acquire spaces in the rubber plantation to intercrop with plantain to earn some revenue [2]. e plantation owner can benefit through savings on weed control and enhanced growth of the rubber trees. Food crops such as plantain, pepper, eggplant, cassava, cocoyam, and maize are important food and cash crops that feature prominently in the farming systems of Ghanaian farmers as sole crops or intercrops in mixed cropping systems. Farmers would be willing to intercrop rubber with crops that are proven to be compatible and profitable, which plantain is considered to be one of the best in Ghana. Good agronomic management for intercropping systems is the possible way by which farmers can be assisted to make maximum use of the lag period to the maturity of the rubber to maximize their profits [12]. Several studies have shown that rubber agroforestry systems improve the soil [13], improve the rate of growth of the rubber [14], as well as reduce the cost of the management of the plantation by ensuring the early generation of revenue to the farmer in the immaturity period of the rubber [15][16][17]. us, rubber agroforestry systems could be a suitable approach in Ghana and could add to the Ghana Poverty Reduction Strategy effort aimed at increasing the production of natural rubber, ensuring food security, reducing rural poverty, and creating employment opportunities for the rural dwellers [18].
However, all the studies pointing to the positive effect of rubber agroforestry were conducted outside Ghana and therefore the need to conduct site-specific studies to validate these claims. e study hypothesizes that intercropping young rubber with plantain at the optimum population density would improve productivity and returns to smallholder farmers during the lag phase to the maturity of the rubber. e specific aim was to determine the optimum population density and productivity of plantain used as intercrop in young rubber plantation.  1).

Materials and Methods
Ellembelle falls within the wet semi-equatorial climatic zone of the West African subregion and Axim belt where there is rainfall throughout the year. e maximum mean monthly rainfall of 36 mm (ranges between 26.8 mm and 46.6 mm annually) [19]. e average temperature is about 29.40°C with monthly temperature variation between 4°C and 5°C.
e relative humidity is about 90% during the night and about 75% during the afternoon, especially in June and July [20] (Figure 2). e soil type is mainly of the Ferric Acrisols and Dysric Fluvisols type [21]. e vegetation is made up of the moist semi-deciduous rain forest.

Field Preparation and Experimental
Design. An old and abandoned rubber plantation with regenerated tree species was cleared and used at Ellembelle. At the Jomoro site, the land was an abandoned regenerated oil-palm plantation. e field size was 102 m × 102 m for each experimental setup.
e treatments were laid out in a Randomized Complete Block Design (RCBD) with three replications. Five treatments, consisting of sole rubber (R), sole plantain (P), and three intercrops consisting of an additive series of one row of plantain between two rows of rubber (i.e., low-density plantain intercropping or PR), two rows of plantain between two rows of rubber (i.e., mediumdensity plantain intercropping or PPR), and three rows of plantain between two rows of rubber (i.e., high-density plantain intercropping or PPPR). e rubber clone used was GT1 obtained from Ghana Rubber Estates Limited (GREL) while the variety of plantain was false horn. e population density of plantain was 555, 1,111, and 1,666 plants/ha in the low-density, medium-density, and highdensity treatments, respectively (Figures 3 and 4). In all intercrop treatments, rubber was planted at a spacing of 3 m and 6 m intra-and inter-rows, respectively. Population densities of 1,666 plants/ha and 555 plants/ha were achieved by planting at inter-and intra-row spacing of 2 m × 3 m and 6 m × 3 m for sole plantain and sole rubber, respectively. Intra-row spacing for both rubber and plantain was kept constant at 3 m whilst varying the inter-row spacing according to the number of plantain rows, ranging from 3 m in the low density, 2 m in the medium density, to 1.5 m in the high-density treatments (Figures 3 and 4). N : P 2 O 5 : K 2 O 15 : 15 : 15 fertilizer was applied at a rate of 100 and 200 g/plant for plantain and rubber, respectively, after one month and 6 months after planting.

Data Collection and Analysis
2.3.1. Plantain. Growth data were collected on plant height, the number of leaves, stem girth, months to harvest after flowering for the plantain in each treatment within the central rows. Data were collected on the yield and yield components of plantain at harvest. ese included number of functional leaves, number of hands per bunch, number of fingers per hand, number of fingers of second hand per bunch, number of suckers per plant, weight of second hand of bunch, height of tallest sucker, and bunch weight of plantain.
e plant height was measured from the soil level to the latest matured leaf with tape measure. Stem girth was measured with electronic calipers at 0.1 m from the soil level.
e leaf area (a) was determined following the approach described by Obeifuma and Ndubizu, [22] and Potdar and Pawar, [23] as shown below:    (1) e value 0.8 is a correction factor. Leaf area index (LAI) is defined as the one-sided green leaf area per unit ground surface area in broadleaf canopies.
is was determined by the following equation: LAI � mean leaf area of plant area occupied per plant .
Since the latex yield of the rubber trees were not ready during the experimental period, only the Partial Land Equivalent Ratio (PLER) of the plantain was used to determine the agronomic productivity of the plantain in the various rubber-plantain intercropped systems following the approach of Willey as shown in the following equation: [24]

2.3.2.
Rubber. Data were collected on height, stem girth, and leaf area of rubber. e stem girth of the rubber plant was measured with a digital caliper 10 mm above the bud-grafted union of the plant. e height was taken from the budgrafted union to the tip of the plant using a tape measure. e leaf area of rubber was taken using the area meter AM300 developed by ADC Bio Scientific Limited, SG12 9TA U.K. e leaf area index (LAI) of rubber was derived using (equation 1) above. e data collected were subjected to analysis of variance (ANOVA) using the GenStat statistical package. Separation of means was done using Standard Error of the Difference between the means (SED) at 5% significant level (P ≤ 0.05).

Effect of Plantain Population Density on the Stem Girth and Height
Growth of Rubber. Generally, stem girth and height of rubber increased significantly with increasing population density of plantain in the order of high-density treatment (PPPR) >medium-density treatment (PPR) >lowdensity treatment (PR) >sole rubber (R) at various stages of the plantain growth (Tables 1-3). At the same age and same intercropping pattern, stem girth of rubber trees at the Jomoro site was consistently higher than that of Ellembelle site. e values of height of the rubber trees at both the Ellembelle and Jomoro sites were significantly higher under the intercrop than the sole crop. At the Jomoro site, the increasing plantain population significantly increased the height of rubber while at the Ellembelle site, the increase was significant only at 4 and 11 months after planting (Table 1). Abdul Razak and Barizan [14] found that rubber agroforestry enhances the growth rate of associated rubber trees. Several other studies have also reported improved growth of rubber when cultivated as an intercrop compared to those that are planted as sole crops [4,25,26]. ese are consistent with the enhanced girth and height of rubber trees under the intercrop relative to the sole rubber. e growth parameters of the rubber tree especially the stem girth is very important because the latex tapping begins only when the rubber tree has attained a stem girth of 50 cm and above. e beginning of tapping is evaluated by the percentage of the rubber trees which have attained a stem girth of 50 cm at a height of 90 cm from the bud-grafted union upwards [26,27]. e use of above and below ground resources in intercropping systems could facilitate (positive effect) or result in competition (negative effect) between the crops involved [28]. e negative effect of competition in rubber intercropping systems reported by others [3,20,29,30] was contrary to the results of this study. In Sri Lanka, 50% of smallholders practice intercropping during the immature phase of the rubber and farmers were generally positive about intercropping [4]. Improved performance of the associated rubber in rubber-banana [4,9,25] and rubbersugarcane [31] intercropping system are consistent with this study.
e rubber crop could have benefit from the microclimatic condition created by the associated plantain which could have reduced weed competition and soil moisture loss in the cropping system to facilitate resource use [4,9,[31][32][33].
e stem girth and height of rubber at harvest of plantain was better in the intercrops relative to the sole rubber ( Figures 5 and 6, Tables 2 and 3) and this trend could be maintained till the attainment of the tappable stem girth of 50 cm. is stem girth increment could be due to a positive carry-over effect which has been proved by many scientific outputs. Rodrigo et al. [4] observed that an increase in stem girth of intercropped rubber was maintained throughout the immature phase resulting in an earlier onset of tapping in the intercrop than in the sole rubber.
e improved rubber growth with the increasing plantain densities in the intercrops shown in the correlation analysis suggests the stem girth of rubber trees in the highdensity plantain intercropping systems could attain earlier tappable stem girth of 50 cm faster than when rubber is planted as sole rubber (Figures 5 and 6). ere is intercropping advantage over sole rubber planting [9,26]. Rodrigo et al. [4] found out that after 5½ years of growth, the high-density banana intercrops, double banana rows between two rubber rows, and triple banana rows between two rubber rows showed an increase in tappable number of rubber trees of 69% and 72%, respectively, compared to sole rubber.
Other studies have reported the positive effect of intercropping on rubber latex production even at mature phase [19,26]. is is very important to the plantation sector such as the Ghana Rubber Estates Limited (GREL) as well as the numerous smallholder rubber farmers in other parts of the world where rubber is cultivated.

Effect of Population Density on the Stem Girth and Height Growth of Plantain.
e growth in stem girth and height of plantain was monitored on a monthly basis three months after planting till harvest of the plantain.
is was to ascertain the influence of the intercropping systems on the growth of the plantain. At Ellembelle, the plantain population significantly influenced plantain stem girth from 4 MAP to 7 MAP, while the differences observed from 8 to 11 MAP were not significant. At Jomoro, the increasing plantain population significantly increased plantain stem girth throughout the 15 months (Table 4). e stem girth of plantain ranged from 48.34 cm (PR) to 55.27 cm (P) at Ellembelle. e ranking order for girth was PPPR (64.  e umbrella formed by the leaves of the high-density plantain intercropping system (PPPR) could have reduced the extent of water loss from the cropping system. is could have led to high moisture retention under the high-density plantain intercropping systems, thereby enhancing stem and height growth of the plantain. High-density banana intercrops conserved more soil moisture relative to the low-density banana intercrop [4].
At the flowering stage of plantain, the stem girths recorded for plantain were 50.4 cm for sole plantain, 51.7 cm for low-density treatment, 58.1 cm for medium-density treatment, and 61.3 cm for high-density treatment, while the recorded heights were 226.5 cm for sole plantain, 230.9 cm for low-density treatment, 241.3 cm for medium-density treatment, and 260.8 cm for high-density treatment. At plantain harvest, plantains had attained stem girth of 55.3 cm for sole plantain, 55.6 cm for low-density treatment, 62.7 cm mediumdensity treatment, and 65.4 cm for high-density plantain. is finding is consistent with the report that intercropping, even at high densities, resulted in improved growth of both the component crops of banana and rubber [9].   International Journal of Agronomy 7

Population Density Effect on the Number of Leaves and
Functional Leaves of Plantain. e number of leaves and functional leaves of plantain were monitored at both Ellembelle and Jomoro trials till flowering and harvest of plantain (Tables 5 and 6). is was to determine how the intercropping systems influence leaves and canopy formation of the plantain. At flowering of plantain, the number of functional leaves obtained at Ellembelle was highest under the PPPR system. At flowering and harvest at Jomoro, there was no significant difference in the number of functional leaves among the treatments. Generally, the number of leaves fluctuated monthly with the PPPR (high-density treatment) recording significantly higher values at 4, 5, and 11 MAP at Ellembelle and throughout the growth period at Jomoro (Table 6). e number of functional leaves of plantain obtained at flowering in both trials was consistent with the required functional leaves for a good bunch yield. According to Banful [11], at good vegetative growth, 8 functional leaves are required at flowering to produce a good bunch yield. e number of functional leaves obtained at flowering at Ellembelle (9-12) and Jomoro (8)(9) for the various treatments (Table 5) were within the number of functional leaves required for a good bunch yield.

Effect of Plantain Population Density on the Yield and Yield Components of Plantain.
e population density of plantain generally had a significant effect on the plantain yield in the rubber-plantain intercropping system (Table 7). e yield of plantain recorded at the Ellembelle site was 9,972 kg/ha for P, 3,983 kg/ha for PR, 7,036 kg/ha for medium-density (PPR), and 11,453 kg/ha for high-density (PPPR). e yield recorded at Jomoro was 9,765 kg/ha for P, 3,287 kg/ha for PR, 7,851 kg/ha for medium-density (PPR), and 11794 kg/ha for high-density (PPPR) ( Table 7). e highest yield of 11,453 kg/ha and 11,794 kg/ha were recorded in the high-density treatment at Ellembelle and Jomoro, respectively. Plantain yield from both the sole plantain (P) and high-density rubber-plantain intercropping (PPPR) were not significantly different. e number of hands per bunch was seven each for medium and highdensity treatments and there was no significant difference between the two treatments. e sole plantain (P) and the PR treatments each recorded six hands which were similar to the  [33]. e mean plantain yields recorded at both sites from the high-density rubber-plantain intercrop at both sites were higher than that recorded by FAOSTAT [33], whiles that of the sole plantain crop (P), low-density and medium-density were within or belowaverage yield. us, intercropping plantain holds a key in improving plantain yield in Ghana.
At both Ellembelle and Jomoro, increasing plantain population in the inter-row did not affect soil moisture content at 0-15 cm soil depth (Table 8). However, increasing plantain population significantly increased soil moisture content at 15-30 cm soil depth at both Ellembelle and Jomoro study sites. e high soil moisture content under the high-density plantain intercropping system (PPPR) could be a major contributing factor to the improved growth of rubber under the high-density plantain intercropping system relative to the sole rubber (Tables 2 and 3).
Rubber is a deep-rooted plant, with rooting depth (taproot) of 3-4 m, and higher soil moisture at 15-30 cm depth could benefit the tree crop [34]. e improved rubber growth at a higher plantain population (mediumdensity and high-density plantain) compared with sole rubber (R) could be attributed to the availability of soil moisture at lower soil depth. In intercropping systems, radiation-use efficiency (RUE) becomes less important in terms of intercrop advantage under condition of limited soil moisture supplies [35]. Soil moisture, therefore, plays a significant role in the improved growth of the rubber in the high-density plantain intercrop relative to the sole rubber crop.
Even though other workers like Rodrigo et al. [4] attributed the improved growth of intercrop relative to the sole  rubber to radiation-use efficiency, Willey and Reddy [35] found out that radiation-use efficiency (RUE) alone becomes less important in terms of intercrop advantages under conditions of limited soil moisture supplies. e soils under the sole rubber and low-density plantain intercrop treatments tend to be exposed to high temperatures due to the bigger gaps between the canopies of the plants and this can eventually lead to higher water loss through evaporation. However, due to the closeness of the canopy of leaves from the high-density plantain intercropping system (PPPR) coupled with their large leaf area, moisture retention was high under the high-density treatment and this could have benefitted the associated rubber, resulting in its enhanced growth relative to the sole rubber. e difference in the rooting depth of both crops might also have contributed to a reduction in competition for soil nutrients and moisture, with the rubber absorbing from a deeper depth compared to the plantain. Soil moisture content could therefore be a major factor for enhanced growth of associated rubber and yield of plantain in highdensity rubber-plantain intercropping system.
Since plantain does not have a taproot system like rubber coupled with the fact that plantain is a water-loving plant, it might have taken most of its water around the 0-15 cm layer. Soil moisture at the 15-30 cm depth could therefore be available to the associated rubber. e improvement in growth (rubber and plantain) and yield of plantain could be attributed to reduced competition for soil resources as a result of rubber picking from deeper layers of the soil compared with the plantain in the shallow layers [35]. e improved growth of plantain and rubber under the intercrop (low, medium, and high density) relative to the sole rubber crop (R) could also be attributed to the symbiotic relationship of Arbuscular Mycorrhizal Fungi (AMF) with the plantain.
ere is a symbiotic relationship between plantain plants and indigenous mycorrhizal fungi in Ghanaian soils [36]. In their study, False Horn plantain was found to have a higher frequency and intensity of mycorrhizal colonization than French plantain in all root samples. In a related study, it was also found that AMF could increase nutrient content and growth parameters of banana plants [37] and improve soil structure by releasing glomalin when hyphae die and decay, hence improving soil stability and increasing water retention [38]. ese advantages of the

Effect of Population Density on Plantain Sucker
Development. Increasing plantain population in the plantain-rubber intercrop did not significantly influence the number of suckers in the first seven months after planting and at harvest of the plantain (Table 9). However, on the 8th, 9 th , and 10th months after planting at Jomoro, the number of suckers obtained from the medium-and high-density treatments were significantly higher than that from the sole plantain (P) and the PR treatments (Table 9). Plantain suckers obtained by a farmer is important for securing planting materials for next season's planting and income generation from surplus. At both study sites, there was no significant difference in the number of plantain suckers/plant after harvesting of plantain. After the harvest of the plantain, the height of suckers differed significantly among the different plantain densities. e height of the plantain sucker under the medium-and high-density plantain intercrops was significantly higher than the sole plantain and low-density plantain intercrop at both sites. e medium-and high-density plantain intercrops were likely to obtain an early yield from their ratoon crops relative to the sole plantain and low-density plantain intercrop due to the early height attainment which can give them a competitive advantage.

Population Density Effect on Days to Flowering and Leaf Area Index (LAI).
e number of days to flowering of plantain was not influenced by plantain population density in both sites. At the Ellembelle site, it took 10 months for plantain to flower in all the plantain densities and 3 months between flowerings to harvesting. At Jomoro, it took an average of 15 months for flowering to occur after planting but the number of months from flowering to harvest remained 3 months (Table 6). e longer duration of flowering recorded at Jomoro might be due to the phosphorus (P) deficiency in the soil. Soils that are inherently low in phosphorus can delay the gestation period of some plants [39].
Most plants need about 0.2-0.5% P (on a dry matter basis) for normal growth [40]. Roper et al. [41] reported that plants do not need to take up new P for every cell function because phosphorus existing in plant cells is recycled over and over again. However, early plant growth is dependent on P because of the need for rapid cell division and expansion. e primordial for future roots, stems, leaves, flowers, and seeds are produced very early during plant growth and therefore P deficiency during the early growth of plants and germinating seedlings can greatly affect the yield potentials of crops and pastures [39].
e observed values for leaf area (LA) and leaf area index (LAI) differed significantly among treatments. High-density treatment obtained a significantly higher value at both sites (Tables 10 & 11). e results show that the leaf area of rubber increased with increasing population density of plantain under the different intercropping systems. Generally, the leaf area of rubber in the intercrop was significantly higher than that of the sole rubber (R). With plantain, a significant difference in the leaf area was observed under the treatments. e leaf area values increased with increasing population density with the PPPR treatments recording higher values in both sites (Tables 10 & 11). e high-density plantain in the rubber-plantain intercropping system (PPPR) recorded a significantly higher leaf area relative to the other treatments. e greater leaf area index in the high plantain density rubber-plantain intercropping could lead to an increase in fractional light interception and ground cover [4]. e growth of weeds could also be reduced under the high-density treatment due to the higher LAI [42].
ere was a positive correlation between the plantain population density and the leaf area and leaf area index of the plantain in the rubber-plantain intercropping systems (Figures 7 and 8). e leaf area (LA) and leaf area index (LAI) increased with the increasing population density of the plantain in the rubber-plantain intercropping systems at both Ellembelle and Jomoro sites (Figures 7 and 8).
e increased ground cover could also lead to reduced soil erosion [43], resulting in higher soil moisture being recorded under the high-density plantain intercrop relative to the sole rubber or plantain (Table 8). is could result in protecting the long-term sustainability of the farmland [43]. Due to the expected immediate yield from plantain, the farmers' attention and care would be better under the intercrops than the sole rubber crops due to the absence of immediate yield from the sole rubber crop [44].

Partial Land Equivalent Ratio (PLER).
e total land area required under sole cropping to give the same yield obtained in the intercropping is called Land Equivalent Ratio (LER). In this study, only the yield of the plantain described as Partial Land Equivalent Ratio (PLER) was used. is is because the rubber yield was not ready during the study period. e PLER values for plantain at Ellembelle showed no intercrop advantage in the low and the medium density treatments, but the high-density treatment was advantageous (Table 12). In the PR system, intercrop yield was only 40% of the sole plantain crop and 60% of the land was needed to get the same yield as in the sole plantain. In the medium-density treatment, the intercrop yield was 71% of the sole plantain and 29% of the land was needed to produce the same yield as the sole plantain. e highdensity treatment system was however advantageous, recording a PLER of 1.15. e yield from the high-density treatment was 15% more than that that of the sole plantain. It can be concluded that, with the high-density system, 15% more land would be needed for the sole plantain (P) to get the same yield.
Generally, the PLER at Jomoro Ellembelle followed a similar trend as observed at Ellembelle Jomoro. e PLER values for low-, medium-density, and high-density treatments were 0.34, 0.80, and 1.21, respectively. Intercrop yield was 34% of the sole crop yield in the low-   density treatment, while the yield in the medium-density system was 80% of the sole plantain yield. Sixty-six percent of the land was needed under the low-density treatment to produce the same yield as in the sole plantain. In the same scenario, 20% of the land was needed under the medium-density intercrop treatment to produce the same yield (9,765 kg/ha) as the sole plantain. Both the low-and the medium-density systems  PR � low-density plantain; PPR � medium-density plantain; PPPR � high-density plantain.