PINEAPPLE – Agronomy

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The pineapple is a tropical or near-tropical plant. The optimum temperature for growth is 20ºC at night and 30ºC during the day. The rate of growth and development, plant and fruit size are directly related to temperature. During the cool season growth is delayed and night temperatures below 10ºC can result in leaf necrosis. It will usually tolerate brief exposures to -2.2 °C and cool nights for short periods. Prolonged cold above freezing retards growth, delays maturity and causes the fruit to be more acidic.

Altitude has an important effect on the flavor of the fruit. In Hawaii, the ‘Smooth Cayenne’ is cultivated from sea level up to 600 m. At higher elevations the fruit is too acid. In Kenya, pineapples grown at 1371 m are too sweet for canning; between 1371m and 1738 m the flavor is most suitable for canning; above 1738 m the flavor is undesirably acid. Pineapples are grown from sea level to 2300 m in Ecuador but those in the highlands are not as sweet as those of Guayaquil.

Pineapples are very efficient water users and can collect and store water enabling them to withstand periods of drought. Pineapples are drought-tolerant and will produce fruit under yearly precipitation rates ranging from 650 – 3800 mm (ideally about 1,143 mm) depending on cultivar and location and degree of atmospheric humidity. The later should range between 70 and 80 degrees. Optimum rainfall is from 1000-1500 mm/year but poor distribution can result in prolonged dry periods, which can adversely affect growth.

Small pineapple plants also adapt well to container and greenhouse culture and make interesting potted plants.

Pineapples will grow in a wide range of soil types, provided there is good drainage and a pH of 4.5–6.0. The best soil for the pineapple is a friable, well-drained sandy loam with a high organic content. Soils that are not sufficiently acid can be treated with sulfur to achieve the desired level. If excess manganese prevents response to sulfur or iron, as in Hawaii, the plants require regular spraying with very weak sulfate or iron. Pure sand, red loam, clay loam and gravelly soils usually need organic enrichment. Filter press cake from sugar mills, worked into clay soils in Puerto Rico, greatly enhances plant vigor, fruit yield, number of slips and suckers.

The plant cannot stand water-logging and if there is impervious subsoil, drainage needs to be improved.

Commercial propagation of pineapple is not through seeds but by vegetative propagation. Except for A. macrodontes, Ananas sp. does not produce underground stolons. Pineapple seeds lack dormancy, have a very tough seed coat and a hard, flinty endosperm. Without treatment germination is slow and very irregular. Commercially, seeds are desired only in breeding programs and are usually the result of hand pollination. Although pineapple can be grown from seed, fertility in commercially grown cultivars of pineapple is very low and consequently seed production is very rare. Seeds retain germination capacity for about six months at tropical ambient temperature. The seeds are hard and slow to germinate. Treatment with sulfuric acid achieves germination in 10 days, but higher rates of germination (75-90 %) and more vigorous growth of seedlings results from planting untreated seeds under intermittent mist. The seedlings are planted when 15-18 months old and will bear fruit 16-30 months later.

Vegetative propagation is the dominant form of reproduction by the use of vegetative shoots including the crown, slips and suckers. There are four general types: slips that arise from the stalk below the fruit, suckers that originate at the axils or leaves, crowns that grow from the top of the fruits, and ratoons that come out from the under-ground portions of the stems. Crowns are the green leafy tops of the fruit. Slips are small shoots that develop on the upper part of the fruit stalk (peduncle) below the fruit and may have small fruit structures (knobs) at the base. Suckers are the large vigorous shoots that develop from axillary buds on the lower stem and called aerial suckers when they are away from the ground or called ground suckers when emerge beneath the ground. Ground or aerial suckers that emerge close to the ground are very vigorous and are often retained for the ratoon crop. Although slips and suckers are preferred, crowns are the main planting material preferred by home gardeners. Vegetatively propagated plants fruit in 15-22 months.

Healthy, disease-free planting materials from bona fide cultivars are important for the production of high yield, uniform and good quality pineapples. Where the source of planting material is limited as in the case of newly released cultivars, planting materials are usually propagated using the rapid quartering technique, stem sectioning, leaf bud cutting or tissue culture. Sometimes growth regulators called morphactin containing chloroflurenol are used to induce proliferation of small slips. It induces axillary bud growth after apical dominance has been broken with ethephon application. Planting materials can be increased ten-fold with morphactin induction. Very rapid multiplication of pineapple plants has been reported to be possible through tissue culture. About 5,000 plantlets from a single crown and 100,000 plantlets from a single shoot can be obtained in 12 months. Dormant lateral buds excised from crown leaves when cultured on Murashige and Skoog basal media containing NAA, IBA and kinetin, produce multiple shoots or plantlets. Plantlets can be rooted in MS salts and vitamins, 0.3 % Phytagel and 1 mg/l napthalene acetic acid (NAA). After four weeks of root initiation, rooted plantlets can be transferred to peat pots and hardened for 8 weeks before planting into the nursery. It will take another 3 months before nursery plantlets can be field-planted. Even though the tissue culture technique is very efficient for rapid multiplication, it has some shortfalls. Mass propagation by this technique produces quite high variability (soma-clonal variation) among the progenies especially in traits like leaf spine. First-generation tissue culture plants showed less vigor in field performance and bear significantly smaller fruits.

For established cultivars, standard planting materials are usually used without resorting to rapid propagation techniques. The conventional planting materials are taken from the vigorous aerial and ground suckers, large slips and occasionally crowns. The suckers are usually harvested monthly within a period of 2-5 months after the fruit is picked. Suckers must be uniform in size, with a height of at least 45 cm. The suckers after harvest are left to dry for a few days. The leaves are then trimmed and the suckers dipped in pesticides before planting in the field. The source of planting materials affects the time from planting to fruit maturation. For suckers which are usually the largest propagules, it takes about 14-17 months to maturation while for slips it is 15-20 months. The crowns which are usually the smallest propagules will take 18-24 months from planting to harvesting.

In Queensland, closely stored upside down tops and slips from the summer crop of ‘Smooth Cayenne’ are used for planting in the fall. Some growers salvage the crowns from the largest grades of fruits going through the processing factory to be assured of high quality planting material.

Workers in Johore, Malaysia, report, without specifying cultivar, that large crowns give highest yield and more slips, followed by small crowns, big slips, small slips, large and small suckers in descending order.

With the ‘Red Spanish’ in Puerto Rico, the utilization of large slips for planting in the first quarter of the year, medium slips during the next six months, and small slips in the final quarter, provides fruits of the maximum size over an extended period of harvest. South African experiments with ‘Smooth Cayenne’ have shown medium-size slips to be the best planting material. Next in order of yield were large crowns, medium-size suckers, medium-size crowns and large suckers. However, medium and large suckers fruited earlier. Trimming of basal leaves increased yields. Storage of slips until optimum planting time prevents premature bloom and diminished fruit size.

The ‘Red Spanish’ reaches shipping-green stage (one week before coloring begins) inPuerto Rico150 days after natural blooming.

In South Africa the ‘Queen’ is grown mainly from stumps, secondly from suckers. The stumps which have fruited are detached from the mother plant as soon as possible to avoid their developing suckers of their own. In comparison with suckers, the stumps are consistently heavier in yield after the 4th crop. When suckers are used, those of medium size, approximately 18 in (45 cm) long, planted shallow and upright, yield best.

In the past, growers preferred plants that supplied abundant basal slips for planting, not recognizing the fact that such plants gave smaller fruits than those without slips or suckers. Also, breeders aim toward elimination of slips to facilitate harvesting. Because of the increased demand for planting material, a new method of mass propagation received wide attention in 1960. During the harvest, plants that have borne single-crowned, superior fruits without basal slips are selected and marked. Following harvest, these plants are cut close to the ground, the leaves are stripped off and the stems-usually (30-60 cm) long and 3 to 4 in (7.5-10 cm) thick-are sliced lengthwise into 4 triangular strips. The strips are disinfected and placed 4 in (10 cm) apart, with exterior side upward, in beds of sterilized soil, semi-shaded and sprinkler-irrigated. Shoots emerge in 3 to 5 weeks and are large enough to transplant to the nursery in 6 to 8 weeks. ‘Smooth Cayenne’ yields an average of 3 shoots/slice whilst ‘Red Spanish’ and ‘Natal Queen’, 4 shoots/slice.

This use of the stem is a major improvement over the former practice of allowing it to develop suckers high up after the fruit is harvested. If such suckers bear fruit in situ they are not strong enough to support it and collapse. They are better removed for planting, but repeated removal of suckers weakens the mother plant.

In Sri Lanka, the shortage of planting material inspired experiments at first utilizing stem cross-sections 2.5 cm thick-15 to 24 from each stem. These sprouted in 4 weeks but plant growth was slow and fruiting was delayed for 30 months. Most of the cuttings developed a single sprout, some as many as 5, others, none at all. Accordingly, this technique was abandoned in favor of a system developed for purposes of reproducing a selected strain in Hawaii. Stems are cut into segments bearing 3 to 5 whorls of leaves. The leaves are trimmed to 10-12.5 cm and the disinfected cuttings set upright in beds until each gives rise to one strong plantlet which is then transferred to the nursery.

The butts, or bases, of mother plants, with leaves intact, are laid end to end in furrows in nurseries and covered with 5-7.5 cm of soil. Sprouting occurs in 6 to 8 weeks. The butts give an average of 6 suckers each, though some have put forth up to 25. A 0.4 ha nursery of 25,000 butts, therefore, yields between 100,000 and 200,000 suckers.

The Pineapple Research Institute in Hawaii has also employed axillary buds at the base of crowns. Each crown segment may develop 20 plantlets. This method has been adopted in Sri Lanka for perpetuating superior strains but not for commercial cultivation because the resulting plants require 24 months or more to fruit.

In India, because of low production of slips and suckers in ‘Smooth Cayenne’, crown cuttings (15-16 per crown) have been adopted for propagation with 95% success, and this method is considered more economical than the utilization of butts.

Vegetative propagation does not assure facsimile reproduction of pineapple cultivars, as many mutations and distinct clones have occurred in spite of it.

Before planting, the land should be well prepared at the outset because the pineapple is shallow-rooted and easily damaged by post-planting cultivation. Fumigation of the soil contributes to high quality and high yields. Planting is normally staggered all the year round to guarantee a regular supply of fruits to the market. On mineral soils and large estates, planting is semi-mechanized with a worker sitting at the back of a tractor and feeding seedlings into slots of a slow rotating disc that makes the hole, pushing the seedlings in and tamping the earth around the seedlings at the same time.

On peat, it is done manually, usually conducted by a team of two workers. Planting rows are first marked out, and holes are made with a pointed stick as one worker moves along the line. The other worker pushes the sucker into the hole, usually at a depth of 10-15 cm. The soil around the plant is then tamped by the planter’s feet. The recommended and most popular planting pattern is the double-row bed system of ‘1-2-3′ i.e. 30 cm x 60 cm x 90 cm, giving a plant density of 44,400 plants per ha. Each bed consists of two rows of plants spaced at 30 cm between plants within rows and 60 cm between rows. The beds are spaced 90 cm apart. Traditionally, plants are spaced 30 cm apart. Set crowns about 5 cm deep; suckers and slips 7.5 to 10 cm deep. Close spacing gives highest total crop weight- e.g. 43,200 plants/ha = 69.12 tons. However, various trials have shown that overcrowding has a negative effect, reducing fruit size and elongating the form undesirably, and it reduces the number of slips and suckers per plant.

Density trials with ‘P.R. 1-67’ in Puerto Rico demonstrated that 51,265/ha yielded 86 tons/ha in the main crop and 45.43 tons/ha in the ratoon crop, but only one slip per plant for replanting. Excessively wide spacing tends to induce multiple crowns in ‘Smooth Cayenne’ in Hawaii and in ‘Red Spanish’ in Puerto Rico.

Some plantings are mulched with bagasse. In large operations, asphalt-treated paper, or black plastic mulch is regarded as essential. A ground cover of black plastic works very well for pineapples, both as protection from weeds and for the extra heat it seems to absorb. It also helps to conserve moisture. It retains warmth in cool seasons, reduces loss of soil moisture, and can be laid by machines during the sterilization and pre-fertilization procedures. Mulch necessitates removal of basal leaves of crowns, slips and suckers and the use of a tool to punch a hole at the pre-marked planting site for the insertion of each plantlet. The mulch is usually rolled onto rounded beds 1 m wide.

The plant is surprisingly drought tolerant, but adequate soil moisture is necessary for good fruit production and watering should not exceed 2.5 cm semi-monthly.

Fertilizer is the most important input accounting for about 30% of the total operating costs. The fertilizers are applied in the form of foliar sprays and as well as ground broadcast of straight or compound fertilizers.

Among the three major nutrients, nitrogen is required mostly during the first five months of growth. Nitrogen is essential to increase fruit size and total yield, which could be applied in the form of urea [CO(NH₂)₂] solution at every four months.  A limited amount of phosphorous is normally applied to provide balanced nutrition for the crop. Potassium, on the other hand, is required particularly during fruit development. Fruit weight can be increased by the addition of magnesium. Iron is the most important, particularly in high pH soils, which may be supplied by foliar sprays of ferrous sulfate (Fe₂SO₄).

Fertilizer trials in Kenya show that a total of 471.7 kg N/ha in 4 equal applications during the first year is beneficial, whereas no advantage is apparent from added potassium and, phosphorus. Puerto Rican studies have indicated that maximum yields are achieved by urea sprays supplying 151 kg N/ha.

In Queensland, total yield of mother plants and ratoons was increased 8% by urea spraying. Normal rate of application is 13.3 liters per 1,000 plants. On acid Bayamon sandy clay in Puerto Rico, addition of magnesium to the fertilizer mix or applying it as a spray 327 kg magnesium sulfate/ha increased yield by 7 tons/ ha. On sloping, stony clay loam high in potassium,Queensland growers obtained high yields of ‘Smooth Cayenne’ from side dressings of NPK mixture 5 times a year.

On poor soils, nitrogen and potassium levels of the plants may become low toward the end of the crop season. This must be anticipated early and suitable adjustments made in the application of nutrients. Potassium uptake is minimal after soil temperatures drop below 20°C. On fine sandy loam in Puerto Rico, the cultivar ‘P.R. 1-67’ performed best with 13-3-12 fertilizer applied at the rate of 3.74 tons/ha. In this experiment, 32,167 plants/ha produced 23,717 fruits/ha, weighing 75 tons/ha. In Venezuela, 6,250 medium-size 15,000 fruits/ha is considered a very good crop.

Fruit weight has been considerably increased by the addition of magnesium. In Puerto Rican trials, magnesium treatment resulted in 54% more total weight providing an average of 64.8 tons/ha than in control plots. Fruit size and total yield have been enhanced by applying chelated iron with nitrogen; also, where chlorosis is conspicuous, by accompanying nitrogen with foliar sprays of 0.10% iron and manganese.

The recommended schedule for fertilizer application is as shown in Table on Fertilizer Application below:

On peat soil 

For the first six months of a new planting, attention should be given to weeding. Once the pineapple plants are bigger the weeds would be shaded out and less weeding is required. Weeding should be done at least once in two months during the first six months of the plant crop, and later every three to four months. However, this depends on the performance of the crop growth and this schedule should be used as a guide only. Weeding should be done as and when it is necessary as weed spread faster during wetter months. Weed control is essential as noxious weeds can reduce pineapple yield by 50%. Not only do weeds compete for water, light and nutrients, they also harbor nematodes and other pests.

Atrazine, bromacil, diuron and fluometuron are very active pre-emergent herbicides used for land preparation. It is normal practice to use a combination of manual and chemical weeding. Ametryne, a pre-emergence herbicide, is applied at 1.7 kg a.i./ ha at planting. This is repeated 3 months later. The treatment keeps the area relatively weed-free for about 6 months. The effectiveness of the chemical is influenced by weather conditions. During dry periods, Ametryne can give up to 20 weeks control. Other pre-emergence herbicides used are Diuron, Atrazine, and Prometryne. Manual weeding at monthly intervals follows chemical control until close to harvesting.

Mulching is an effective way of weed control especially on mineral soils. It also provides other benefits such as prevention of soil erosion, reduces leaching of nutrients and reduces soil moisture loss. In some countries where manual costs for weed management is prohibitive, mulching is widely practiced using black polythene sheets. This significantly reduces manual weeding costs and increases yield by as much as 25%. Mulching with polythene sheets is especially useful on sandy soils where soil erosion and nutrient leaching are serious during wet seasons. In large operations, these sheets are laid on the ground by tractor mounted equipment. However, the covered ground makes broadcast of fertilisers inconvenient. The sheets may also be blown away if they are not securely pegged to the ground. Green manure (chopped pineapple residues) and organic matter, such as sugarcane bagasse, may also be used as mulch.

The pineapple fruit develops from the inflorescence formed at the terminus of the plant. The early sign of inflorescence emergence is the base of young leaves turning red (‘red-heart’ stage). The inflorescence is carried on a short peduncle and is made up of about 100-200 small flowers. At anthesis, one to several flowers open each day, beginning at the base of the inflorescence, over a period of 3-4 weeks. Each flower develops into a fruitlet or ‘eye’, therefore, the pineapple is considered a multiple fruit. It takes about 4 months from the end of last open flower to fruit maturity and a total time from floral initiation to harvest takes about 5-6 months. The elongated, large fruit is usually topped by a crown made up of a short fleshy stem densely packed with small leaves.

Natural flowering in pineapple is rather variable and unpredictable. Commercially produced pineapples are induced to flower in synchrony so that harvesting can be done in a ‘once-over’ operation. Therefore, fruiting can be forced when the plant is mature by using compressed acetylene gas acetylene gas or a spray of CaC₂ solution (7.92 gm/liter water), which produces acetylene. Or CaC₂ (10 -12 grains) can be deposited in the crown of the plant to be dissolved by rain.

A more advanced method is the use of the hormone, a-naphthaleneacetic acid (ANA) or β- naphylacetic acid (BNA) which induces formation of ethylene. In recent years, β-hydroxyethyl hydrazine (BOH) came into use. Treatment is given when the plants are 6 months old, 3 months before natural flowering time. The plants should have reached the 30 leaf stage at this age.

Spraying of a water solution of ANA on the developing fruit has increased fruit size in ‘Smooth Cayenne’ in Hawaii and Queensland. In West Malaysia, spraying ‘Singapore Spanish’ 6 weeks after flowering with Planofix, an ANA-based trade product, delayed fruit maturity, increased fruit size, weight and acidity. Similar results have been seen after hormone treatment of ‘Cayenne Lisse’ on theIvory Coast.

Trials with ‘Sugarloaf’ in Ghana showed CaC₂ and BOH equally effective on 42-to 46-week-old plants, and Ethrel performed best on 35-to 38-week-old plants. ‘Sugarloaf’ seems to respond 10 days earlier than ‘Red Spanish’.

In Puerto Rico, treatment in ‘Cabezona’ can be done to induce flowering at any time of the year.

However, ethephon is commonly used in pineapple for flower induction and crop synchrony. Several flower-inducing hormones are available in the market such as ethephon (2-choloroethylphosphonic acid), calcium carbide, and ANA tablets (α-naphthylacetic acid). For greater efficacy and ease of application, ethephon solution is generally preferred. Ethrel, which has the active ingredient ethephon, is the most popular product for pineapple flower induction in the market. It works by releasing ethylene gas, which is absorbed by the plant and induces flowering. Normally, 50 mL of the Ethrel solution (with active ingredient ethephon at 100-500 ppm) is applied into the heart of the plant. In order to increase its effectiveness, urea at 2.5-5.0% is usually added into the solution. Fruitone 3-CPA (2-3 chloro-phenoxypropionic acid) is regularly used for increasing fruit size and yield of canning pineapple but its use for fresh pineapple is limited because it damages the crown, which is important for the fresh market. Fruitone is sprayed onto the crown just after anthesis when the petals are dried. It reduces the size of the crown or removes it altogether, makes the flesh of the fruit more compact and increases fruit weight. However, there may be a reduction of sweetness and an increase in fruit acidity as well as increasing susceptibility to black rot.

A safer and more practical method for home growers is a foliar spray of α-naphthaleneacetic acid (26 mg in 100 liters water) or β-hydroxyethyl hydrazine. The latter is more effective. The plants usually produce for about four years, but they may last longer in cooler weather since the life cycle is slowed down.

Split peduncle is prevalent in the Queen type of pineapple but not in Smooth Cayenne or Spanish. In the early part of the fruit development, a horizontal split occurs at a node of the peduncle, about 1-2 cm from the base of the fruit. Upon maturity the fruit is smaller than average and tend to bend towards the side of the split. Plants that flower naturally tend to have much lower incidence compared with those induced by hormones. This defect is also related to late application of fertilizer and copper deficiency.

Multiple crown is seen in a number of cultivars. This is a condition where the fruit produces two or more crowns bunched together. It does not seem hereditary because plants propagated from multiple crown may not express this character and plants established from normal crowns can also develop multiple crowns. Multiple crown appears to be seasonal in occurrence and high nitrogen fertilizer increases its incidence.