Natural induction of flowering

'Smooth Cayenne' pineapple must attain some minimum plant weight before natural induction of inflorescence development can occur (Py et al., 1987). The minimum weight has not been well characterized but probably exceeds 1.0 kg in most environments and is greater in warm than in cool environments. Collins (1960) considered 'Smooth Cayenne' to be a perennial that would continue to grow as long as environmental conditions were adequate. There are, however, few regions where environmental conditions are constantly optimum for growth. Seasonal patterns in natural flower initiation are, therefore, evident in most regions. These patterns have long been recognized and were the basis for commercial pineapple production before the discovery of chemical flowering agents.

Once the minimum plant weight is reached, the factors that promote flowering are mainly those that retard vegetative growth. For 'Smooth Cayenne' grown in subtropical regions, there is a strong wave of natural induction in the autumn-winter season. Natural flower initiation can, however, occur at other times of the year (Millar-Watt, 1981; Sinclair, 1997, 1998, 1999), so those plants that do not flower naturally in the winter may flower in midsummer.

Definite peaks in natural initiation are also recognized in tropical regions (Aubert, 1977; Wee, 1978; Giacomelli et al, 1984). The main peak usually occurs when either minimum or maximum temperatures are at their lowest. This peak is also associated with a decline in the hours of solar radiation. A small peak also occurs when maximum temperatures are at their highest (Teisson, 1972). Gowing (1961) indicated that, depending on the kind of planting material used and the time of planting, natural initiation could occur in any month of the year in Hawaii (latitude 21°N).

Inhibition of natural induction is associated with those factors that promote vegetative growth. For example, vigorous growth, stimulated by excessive nitrogen and warm night temperatures, inhibits flowering (Nightingale, 1942; Bartholomew and Kadzimin, 1977; Conway, 1977).

Photoperiod

The earliest work on photoperiod by Van Overbeek (1946) showed that pineapple responded like a short-day plant. The work of Gowing (1961) later established that 'Smooth Cayenne' was a quantitative short-day plant, i.e. flowering can occur at any day length but is accelerated by short days (Taiz and Zeiger, 1991). Gowing (1961) observed that flowering of 'Smooth Cayenne' was not inhibited by day lengths in excess of 13 h. He also found that interruption of the dark period suppressed flower initiation, so it was concluded that long nights favoured induction. A subsequent study by Friend and Lydon (1979) showed that, for 'Smooth Cayenne', 8 h days were more inductive than days of 10, 12 or 16 h.

Solar radiation

There is little evidence to indicate that solar radiation per se has any direct role in natural flower induction. It seems most likely that natural induction in regions away from the equator is explained by a combination of short photoperiods and cool temperatures. However, natural induction also occurs in equatorial regions, where photoperiod is almost constant and temperatures are generally higher. In equatorial regions such as Côte d'Ivoire, West Africa (latitude 5-10°N), and tropical western Malaysia (latitude 1°17'N), the periods of greatest natural flowering coincided with decreases in maximum (Teisson, 1972) or minimum temperature (Wee, 1978). In Malaysia there was also a correlation with a decrease in hours of sunshine (Wee, 1978). Bourke (1976) found no correlation between sunshine hours and natural flower induction in 'Queen' pineapples in Papua New Guinea (latitude 4°21'S) once the variance associated with low temperature was removed.

Temperature

While 'Smooth Cayenne' pineapple is considered a short-day plant, cool temperatures in particular enhance the flowering response. It is worth remembering that the optimum temperatures for growth of pineapple are considered to be close to 30°C day and 20°C night with an optimum mean of about 24°C (Neild and Boshell, 1976; Bartholomew and Malézieux, 1994).

Van Overbeek and Cruzado (1948b) established that a night temperature of approximately 16°C over 30 days induced flowering in 88% of 'Red Spanish' plants during late summer in Puerto Rico; 28% of plants held at a night minimum temperature of 22°C also flowered. A night minimum temperature of 16°C was therefore more inducive than 22°C for 'Red Spanish'; the optimum temperature for flowering is not known.

Gowing (1961), using 'Smooth Cayenne', compared night temperatures of 15, 23 and 26°C and found that a night temperature of 15°C induced flowering when in combination with a short day length for a period of

30 days. At normal day lengths (c. 12.5 h) this same low night temperature over 30 days did not induce flowering. However, exposure to a constant 18°C for 9 weeks was claimed to be more effective. The longer period of exposure was probably the main factor accounting for the improved response. A more recent study by Sanewski et al. (1998) supports this observation. Sanewski et al. (1998) found that a constant 20°C for 10 weeks induced flowering in 100% of 'Smooth Cayenne' plants but a constant 10 or 15°C for up to 12 weeks did not induce flowering. These studies were conducted during a time when day length decreased from 12.5 to 10.5 h and natural induction was most prevalent in the field. A subsequent study (G. M. Sanewski, 1999, unpublished results) confirmed that a constant 20°C was more effective than 20/15°C or 25/15°C for up to 64 days. A day/night combination of 25/20°C was as effective as a constant 20°C over 64 days. This suggests that, for 'Smooth Cayenne' at least, night temperatures below 20°C are not as effective as 20°C, and a day temperature of 25°C is as effective as 20°C when in combination with a 20°C night.

Friend and Lydon (1979) also found that, with a photoperiod of 8 h and day temperatures of 25 or 30°C, a night temperature of 20°C was more inductive for 'Smooth Cayenne' than night temperatures of 15, 25 or 30°C. Flowering was induced at night temperatures of 15 and 25°C but not at 30°C. In a review of natural flower initiation of 'Queen' pineapple in Papua New Guinea, Bourke (1976) concluded that natural induction was closely associated with minimum temperatures of 20-23°C. It appears, therefore, that exposure to temperatures of as high as 25°C for at least the nocturnal period, in combination with a short day length will induce flowering in 'Smooth Cayenne' if these plants are exposed to these conditions for 9-10 weeks. A night temperature of 20°C in combination with a day temperature of 20-25°C is believed to be close to optimal.

Nightingale (1942) suggested that sudden brief periods of very low temperature (c. 5-10°C) can induce flowering. Yow (1959) indicated that 5°C for 2-3 days was correlated with flower induction of 'Smooth

Cayenne' in Taiwan. While there are few data on the effects of a sudden drop in temperature on natural induction, it is possible that the sudden interruption in growth might induce flowering in a small percentage of plants if other conditions are also favourable.

The effect of temperature perturbations can, however, be difficult to interpret. Though conclusive data are lacking, it is also possible that, in addition to the effect of cool temperature, diurnal warming during winter may also increase the incidence of natural induction. In a comparison of six planting densities in subtropical Queensland, Scott (1992) found that the incidence of natural flowering of the ratoon crop increased consistently from 3.5 to almost 12% as plants ha-1 decreased from 80,695 to 46,112. No leaf- or plant-weight data were taken for the ratoon crop, so it is not possible to rule out differences in plant weight as a factor in this study. Further evidence of an effect of diurnal warming on flower induction is the common observation in subtropical Queensland that natural flowering is more prevalent in outside rows, where incident radiation and diurnal plant temperatures are expected to be greater. There is, however, the possibility that the increased natural initiation is a direct response to increased light interception and not the subsequent rise in plant temperature. While this is possible, data collected by Sanewski et al. (1998) suggest that warming after exposure to cool temperatures has a direct positive effect on natural initiation. In their study they collected mature suckers of 'Smooth Cayenne' from the field in late winter and placed them at either 5, 10 or 28°C for 3 days. Exposure of plants to 28°C induced flowering in 75% of plants. There is clearly a need for further study of the effects of temperature and its interaction with solar radiation on plant sensitivity to natural and forced induction.

In addition to the effects of low temperature, it is also commonly recognized that natural induction will occur after brief periods of high temperature. In Australia, these events have been associated with diurnal maxima of 40°C over 2 days (Sinclair, 1997) and is probably the result of wound-

induced ethylene production. There is good circumstantial evidence that ethylene is involved in the flower-induction process. Environmentally induced flowering is assumed to result from increased production of ethylene or heightened plant sensitivity to ethylene, or both.

Recent studies (Botella et al., 2000; J. Botella, 1999, personal communication) indicated that there is more than one gene in pineapple leaf tissues that codes for the production of ACC synthase, the enzyme that produces 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene. One gene codes for an ACC synthase that is regulated by cool and warm temperature and this enzyme is identical to the one regulated by auxin application. Apparently a different gene is involved in the production of the ACC synthase produced as a result of wounding of leaf tissues. It seems likely that temperature induces flowering by affecting different ethylene pathways. Temperature, depending on its degree and duration, could stimulate natural ethylene production and thus induce flowering by affecting both the auxin- and wound-induced pathways.

Water availability

Water stress and water excess are often implicated in a flowering response in tropical and subtropical species. An increase in ethylene production is often associated with water stress in plants (Yang and Hoffman, 1984), but there are conflicting reports on this point. For example, Morgan et al. (1990) found only slight, transient ethylene production on rewatering cotton subjected to water deficit but no response in beans or roses. Nevertheless, it is generally believed that pineapple flowering may be induced in response to a seasonal water deficit and induction also occurs in response to water excess (Py, 1964), due to growth inhibition or enhanced ethylene production, or possibly both. Tay (1974) reported that both water excess and water deficit delayed natural induction of pineapple grown in peat soil, while growth and natural induction were enhanced by increasing watering frequency. Consistent with the results of Tay (1974), Min

(1995) found that a severe water deficit decreased the plant's susceptibility to ethep-hon and did not induce natural flower initiation. Mild water deficit was not studied. Water excess increased ethylene production of the basal white tissue of 'D' leaves by approximately 100% but did not induce flowering. The fact that excess water increased ethylene production so substantially suggested it should increase the plant's susceptibility to flowering and may therefore be implicated in some situations. Waterlogging, a more severe degree of water excess, had no effect on ethylene production or flowering (Min, 1995).

Much of the inconsistency in the effect of plant water status on flower initiation is probably due to differences in the severity of deficit or excess imposed. Severe and sudden deficit or excess are likely to result in a cessation of all enzymatic processes, including those involved in flower initiation.

Genetic variation

Most pineapple genotypes appear to be more susceptible to natural flower initiation than 'Smooth Cayenne'. The natural flowering cycle in wild types, such as Ananas ananas-soides and Ananas parguazensis, is usually annual (Leal and Coppens d'Eeckenbrugge, 1996). In an Ananas spp. germplasm collection held in subtropical Australia (latitude 27°S), plants of 'Pernambuco' and 'Mordilona Queen' and the species A. ananassoides all flower naturally more frequently than does 'Smooth Cayenne'. Williams (1987) reported that some hybrids flower when they reach a plant weight of about 1.5 kg, regardless of the time of year.

Van Overbeek and Cruzado (1948b) reported that 'Red Spanish' was less responsive to day length than 'Cabezona', but more responsive to low temperature. 'Cabezona' is a triploid cultivar of the 'Spanish' type (Samuels, 1969). In Malaysia, 'Smooth Cayenne' is considered to be a vigorous plant that it is more difficult to force than is 'Singapore Spanish' (Dunsmore, 1957), the main canning variety there.

It is assumed that the relatively low incidence of natural flowering in 'Smooth

Cayenne' and the associated ease of crop control in comparison with other types of pineapple account to a substantial degree for its popularity and may be the most important factor allowing large-scale production of this variety.

Geotropic stimulation

Van Overbeek and Cruzado (1948a) reported that plants of 'Cabezona' pineapple could be geotropically stimulated to flower, as indicated by a strong flowering response when plants were held horizontally for 3 days or more. It was theorized that flowering was induced by the accumulation of endogenous auxin in the lower longitudinal half of the stem apex, though Salisbury and Ross (1992) note that many gravitropic responses can be attributed to increased sensitivity to auxin rather than an increase in concentration.

In summary, both environmental and cultural factors can contribute to natural induction in 'Smooth Cayenne' pineapple. 'Smooth Cayenne' shows a clear response to day length and temperature and probably a weaker response to solar radiation. Surprisingly, very low temperatures are not commonly implicated in the incidence of natural induction. Flowering is more likely to occur as a result of moderate (around 20°C) temperatures, not much lower than the optimum for growth, sustained over many weeks in combination with a short day length. Ethylene production resulting from mild to moderate wounding of plant tissues due to sudden exposure to short periods of very high or very low temperatures appears a minor cause. Plant water status is probably also implicated in a minor way, though supporting data are lacking.

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  • Sofia Ruotsalainen
    How to naturally induce flowering in pineapples?
    5 months ago

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