Pineapple plant nitrogen and potassium requirements are low until about 4 months after planting (Lacoeuilhe, 1978; Ingamells, 1981), after which requirements increase with growth until flower induction. In an experiment where 8 g of N and 20 g of K2O were provided for each plant prior to forcing, at the time of forcing at 10 months after planting plants of 375 g dry mass contained 5 g N and 11 g K (Lacoeuilhe, 1978). Plant nitrogen content remained constant during the period from forcing until harvest. However, K absorption continued to increase from forcing to harvest, and 13 g were accumulated in a plant dry mass of 800 g (Lacoeuilhe, 1978). Factors that reduce potential growth, such as drought, low temperature and root anoxia, reduce the plant's nutrient requirement (Py et al., 1987).
Total uptake of a particular nutrient does not necessarily indicate a plant's requirements for that nutrient. Pineapple plants may take up more, the same as or less than the amounts required for optimum growth, depending on the factors indicated above and the specific nutrient in question. In a field high in available N and K, total uptake over a 30-month period was highest for K, followed by N and phosphorus. The efficiency of pineapple in extracting K from the soil is high and, if readily available, the plant accumulates K in greater amounts than are required for optimum growth, often referred to as luxury consumption.
The uptake of nitrogen also shows luxury consumption, generally being proportional to the amount of nitrogen fertilizer applied during vegetative growth (Scott, 1993). Juice nitrate is an important quality factor in canned pineapple because high levels detin cans; 8 p.p.m. is considered the critical level in Australia (Scott, 1994). In Thailand, fruits that exceed 25 p.p.m. nitrate in the pineapple juice are rejected at the cannery (P. Chairidchai, 2000, personal communication). Juice nitrate levels can be highly correlated with nitrogen applied with fertilizers (Scott, 1993). In one study, juice nitrate averaged 1.0, 6.0 and 23 p.p.m. when N applied was 200, 600 and 1200 kg ha-1 (Scott, 1993). Heavy N fertilization and application of N fertilizers after flowering is more likely to result in elevated fruit nitrate levels (Chongpraditnun et al., 1996). However, nitrate levels in leaves sampled at different stages of plant growth were not well correla ted with juice nitrate levels (Scott, 1994). Fruit from plants with a low level of molybdenum in the 'D' leaf had a high nitrate content in juice in Thailand (Chairidchai, 2000; Chongpraditnun et al., 2000). In these experiments, high fruit nitrate content and low 'D'-leaf molybdenum content were thought to be due to increased absorption of molybdenum by soil particles at the low soil pH.
Luxury consumption of calcium (Godefroy et al., 1971), magnesium, sulphur, boron, chlorine, copper and manganese also occurs in pineapple when these elements are readily available, whether they are taken up from the soil or applied to the leaves in nutrient solutions (W.G. Sanford, personal communication). Conversely, low amounts of phosphorus are extracted by the plant and uptake is not proportional to the supply available, but reflects the plant's requirement for P. The levels of iron, zinc and molybdenum also do not generally increase with the available supply, but reflect the plant's requirement for these nutrients. However, as was noted above, leaf levels of iron and manganese can be above 500 mg kg-1 dry mass in soils that have a low pH (3.5-4.5) and high levels of soluble manganese.
Other elements that are accumulated by pineapple plants when levels are high in the soil are silicon, sodium and aluminium (Table 7.6). In Hawaii, relatively high concentrations of Na and Cl are found in soils in close proximity to the ocean, where sea-water sprays are prevalent (Sideris and Young, 1954; Sideris, 1955). Sea-water sprays can cause leaf-tip dieback, characterized by alternating dark brown and light brown bands running perpendicularly to the length of the leaves (Sideris, 1955) and it was concluded that chloride in the spray rather than in the soil is the cause of leaf dieback. High levels of Cl in the soil can also result from the application of potassium chloride fertilizer to the soil and from soil fumigants, such as dichloropropene, which contain large amounts of chlorine. Leaf injury was not observed in soils high in Cl, perhaps due to the fact that the Cl taken up by the roots remains diluted in the plant, while sea spray concentrates on the leaves as the spray dries, causing salt injury. Salt injury on pineapple leaves was also not observed in response to increasing sodium chloride in the soil in pot studies (Wambiji and El-Swaify, 1974, 1976), even though leaf concentrations of both Na and Cl increased and growth decreased slightly. However, in a recent study (Marinho et al., 1998), where plant establishment and early growth at 25°C were studied at salinity levels in irrigation water of 0-7 dS m-1, establishment and growth were greatly reduced at salinity levels above 3 dS m-1.
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