Effects of environment on vegetative growth

Specific effects temperature. The plant growth rate of pineapple is strongly influenced by temperature but few studies have been made in controlled environments and the results from field studies can be difficult to interpret. Leaf and root elongation rates measured in controlled environments over a range of temperatures (Sanford, 1962) generally show good correspondence with the results of later studies. The optimum temperature for leaf growth was 29°C and that for roots was 32°C (Sanford, 1962). Growth of both organs was greatly reduced at temperatures below 20°C and ceased at about 10°C.

Data on the long-term effects of temperature on dry-matter accumulation are sparse and such data are needed if the vegetative growth responses in the diverse field environments where pineapple is grown are to be interpreted. Pineapple plants grown in controlled environments having night temperatures of 26°C or less had greater weights than those grown in environments having 30°C night temperatures, and the differences increased over time (Fig. 5.12; Bartholomew, 1982). After about 400 days, plants grown at cooler night temperatures accumulated two to three times the dry mass of those grown at 30°C night temperatures (34/30 and 30/30°C). In environments with warm days (26-34°C) and cool nights (18-22°C), plants also had higher dry-matter contents and greater plant relative growth rates and leaf growth rates than did plants grown in warm days and warm (30°C) nights (Bartholomew, 1982). Plant leaf area at the end of the study was also greatest for plants grown at 30°C day and 26 or 22°C night temperatures (Fig. 5.13). The more rapid growth of plants at the cooler night temperatures resulted mainly from higher net assimilation rates. Net assimilation rates on a unit leaf basis ranged from about 1.0 g m-2 day-1 for plants grown at 30°C dark temperatures to 2.0 g m-2 day-1 for plants grown at dark temperatures of 26 or 22°C (Bartholomew, 1982). Friend (1981) also reported that plants grown at a 30°C night temperature were smaller than those grown at cooler night temperatures.

Fig. 5.12. Effects of day and night temperature (°C) on growth in dry mass of 'Smooth Cayenne' pineapple plants grown in controlled environments. Pineapple crowns were started in a 26/22°C day/night temperature environment and transferred to the various temperatures 100 days after planting (day 0 in the figure). Each data point is the mean for two plants. (Redrawn from Bartholomew and Malezieux, 1994.)

Fig. 5.12. Effects of day and night temperature (°C) on growth in dry mass of 'Smooth Cayenne' pineapple plants grown in controlled environments. Pineapple crowns were started in a 26/22°C day/night temperature environment and transferred to the various temperatures 100 days after planting (day 0 in the figure). Each data point is the mean for two plants. (Redrawn from Bartholomew and Malezieux, 1994.)

Fig. 5.13. Effects of day and night temperatures (°C) on growth in green leaf (white and pale green leaf basal tissue excluded) area per plant of 'Smooth Cayenne' pineapple plants grown in controlled environments. Pineapple crowns were started in a 26/22°C day/night temperature environment and transferred to the various temperatures 100 days after planting (day 0 in the figure). Each data point is the mean for two plants. (Redrawn from Bartholomew and Malezieux, 1994.)

Fig. 5.13. Effects of day and night temperatures (°C) on growth in green leaf (white and pale green leaf basal tissue excluded) area per plant of 'Smooth Cayenne' pineapple plants grown in controlled environments. Pineapple crowns were started in a 26/22°C day/night temperature environment and transferred to the various temperatures 100 days after planting (day 0 in the figure). Each data point is the mean for two plants. (Redrawn from Bartholomew and Malezieux, 1994.)

Under field conditions, the mother plant relative growth rate for the period from about 2 months after planting to induction of flowering was reduced by the cool winter temperatures of south Queensland (Sinclair, 1992). When data from Hawaii and Côte d'Ivoire are compared, plants grown from crowns in Hawaii have 0.7 m2 of leaf area 8 months after planting and would be grown another 4-6 months before forcing (Bartholomew and Kadzimin, 1977). In Côte d'Ivoire, plants of the same age grown from larger suckers would have 1.8 m2 of leaf area and would be ready to be forced (Malézieux, 1991). Some of the difference in size of plants after 8 months of growth is almost certainly due to the larger propagules used in Côte d'Ivoire, but some of the difference is also probably due to the more rapid expansion of leaf area in the tropical environment of Côte d'Ivoire. In Thailand, large suckers, some weighing up to 2.0 kg, are planted and these will be forced within 5-6 months after planting.

It seems appropriate to interject here that, while much has been written about the relative advantage of suckers over smaller propagules (shorter time from planting to forcing; larger average size of fruit), only fairly recently has it been recognized that the size of the propagule is the main factor accounting for the difference. A large propagule produces a large plant more quickly than does a small propagule.

In areas where there is considerable seasonal variation in climate, time of planting can also affect time from planting to forcing. In Hawaii, crowns will be forced 12-14 months after planting (A. Hepton, personal communication). Propagules planted during periods of cool weather and low irradiance establish and grow more slowly than do those planted when weather conditions are more favourable for growth.

Theoretically, growth in leaf area depends on the rate of both leaf initiation and expansion. Leaf expansion, as indicated by rate of elongation, increased with temperature up to 29°C (Sanford, 1962). Leaf expansion was also greater where the day/night temperature differential was large than where it was small and night temperature was high

(Bartholomew, 1982). A relatively sharp temperature optimum occurs for leaf elongation rate (Sanford, 1962), but no such sharp optimum was observed for dry-weight gain, presumably because of morphological and anatomical differences induced by temperature or the day/night temperature differential, or both.

Leaf initiation in natural environments, as indicated from rates of leaf appearance, is sensitive to temperature. In Taiwan, the equation:

where T is mean monthly temperature in °C and NLEAF is number of leaves to appear in 1 month, indicates the extent to which leaf appearance rate is influenced by temperature (Shiroma, 1972). Data on leaf emergence collected from fields at elevations of 91, 305 and 792 m in Hawaii could be described by a similar relationship (D.P. Bartholomew, 1988, unpublished results). In Côte d'Ivoire, which has a high mean air temperature and narrow diurnal variation, the number of leaves appearing per 2 months was best described by a quadratic function and showed an optimum temperature at about 26-27°C (E. Malézieux, 1992, unpublished results). Similar results showing the existence of an optimum temperature near 28°C were obtained in Hawaii (J. Zhang, 1992, personal communication).

Leaf number can be predicted using heat accumulation or daily thermal time (DTT) (Zhang et al., 1997). An enhanced model was developed which also accounted for the reduction in growth that occurs at low (below 16°C) and high temperatures (greater than 35°C). Such temperatures would normally be reached only during part of the day (Zhang et al., 1997). DTT was calculated for each day by the equation:

where TM is the mean temperature and BT the base temperature. A BT of 16°C provided the best fit for the data. For a minimum temperature < BT or a maximum day temperature > 35°C, DTT was calculated using a procedure similar to that reported by Malézieux et al. (1994) for the estimation of daily fruit thermal time. The leaf-emergence model was calibrated and validated with data from Hawaii and Côte d'Ivoire. Based on Celsius temperature, the phyllochron (interval between the appearance of two successive leaves) was 50 degree-days per leaf. Interplant competition had a significant effect on leaf number, and it was assumed that this effect was due to a reduction in meristem temperature caused by the intense shading that occurs at high plant population densities. This effect was simulated in the model using interplant competition factors calculated from leaves m-2 of ground area (Zhang et al., 1997).

In addition to the effects of root temperature on dry-matter partitioning noted above, Ravoof (1973) also showed that the total fresh mass of 40-day-old plants grown from slips increased with increasing root temperature from 15 to 30°C, though the increase in growth was not proportional to the increase in root temperature. In Okinawa, Japan, a relatively cold climate for pineapple (mean temperature ^ 17°C, minimum temperature for more than 1 month ^ 6°C) and in Hawaii during the winter (mean temperature ^ 22.3°C), growth is enhanced by black polyethylene-film mulch (Ekern, 1967; Ogura et al., 1968). The 33% increase in growth in Hawaii resulted from a soil temperature increase of only 1.6°C. In the Canary Islands, where mean average air temperature is about 20°C, both black polyethylene mulch and a polyethylene cover over the plants increased yields (Galan Sauco et al., 1988b).

irradiance and plant population density. Few studies have been conducted on the effects of irradiance on vegetative growth. Connelly (1969) found that estimated plant weight of 'Smooth Cayenne' plants grown in pots was unaffected by 25% shade but was reduced by 20% under 50% shade. Less nitrogen was required for optimum growth at low than at high light.

In most crops average plant mass decreases with increasing plant population density due to interplant competition for light, but no data illustrating this effect on pineapple were found (Bartholomew and Kadzimin, 1977; Py et al, 1987; Scott, 1992).

This surprising result is assumed to be due to the fact that researchers who studied the effects of plant population density on pineapple generally did not collect data on plant fresh or dry mass, did not report such data or estimated plant mass from 'D'-leaf mass. 'D'-leaf mass is a faulty estimator of plant mass, because as plants get larger, 'D'-leaf mass reaches a maximum, while leaf number and leaf and plant mass continue to increase (Py et al., 1987).

Interplant competition does occur during vegetative growth of pineapple, and growth is reduced at high plant population densities (Malézieux, 1992a; Zhang, 1992). In a study in Hawaii, there was no reduction in plant mass with increasing plant population density for the first 200 days after planting, even at a density of 123,500 plants ha-1, and light interception did not reach 95% at that density until about 350 days after planting (Zhang, 1992). Plant mass at forcing, which occurred for plantings made in June, August and October at approximately 440, 380 and 335 days, respectively, after planting, decreased significantly with increasing density at all planting dates. Similar results were obtained in Côte d'Ivoire (Malézieux, 1992a). In the latter experiment, plant mass 8 months after planting increased by 33 and 20%, in irrigated and unirrigated conditions, respectively, as plant population density decreased from 100,000 to 20,000 plants ha-1. It is clear that competition for light occurs during vegetative growth in pineapple fields even at moderate planting densities. However, it may not be detectable where growth is limited by water or other stresses. Unless the relationship between 'D'-leaf weight and plant mass is known, 'D'-leaf weights should not be used as an indicator of plant mass beyond about 6 months after planting, or earlier than that where large suckers are planted. The effects of plant population density on fruiting are discussed in detail later (see Bartholomew et al., Chapter 8, this volume).

Lest the details obscure the important message, it is clear that total biomass and often total yield increase with increasing plant population density. However, studies often show that, at densities much above

74,000 plants ha-1 and sometimes well below that, the percentage of fruit recovered, as well as the quantity and quality of products, declines.

Allometric data help to explain the effects of plant population density on vegetative growth. Leaf appearance rate decreased with increasing planting density (Norman, 1977; Malezieux, 1992; Zhang, 1992) and, in one study, leaves were longer and narrower at the higher planting densities (Wee, 1969). Specific leaf mass (SLM) (dry-mass basis) was significantly decreased when planting density was higher than 6 plants m-2. However, at the base of the leaf, fresh weight per unit area increased at high densities (Malezieux, 1992a). Zhang (1992) reported that SLA at 300 days after planting was related to planting density (PD) by the equation:

and the effect was highly significant.

The production of vegetative propagules, such as slips and suckers, requires an input of carbohydrates, at least until they develop sufficient leaf area to become autotrophic. Competition for light at higher planting densities is assumed to reduce the supply of assimilates available for growth and for storage reserves, because the number of slips and suckers per plant decreases at higher plant populations (Lacoeuilhe, 1974; Py et al., 1987). For clones that produce slips, slip numbers decrease at a faster rate with increasing plant population density than do suckers (Py et al., 1987). Suckers per unit area may increase or, as occurred in a recent study, become normalized across planting density (Scott, 1992). Scott (1992) showed that, as plants per hectare increased from 46,112 to 80,650, suckers per plant decreased from 1.84 to 1.11. Thus, despite a wide range of planting densities, suckers ha-1 only ranged from 82,162 to 89,572 (Scott, 1992).

In addition to reducing the number of suckers per plant, sucker development, which in Australia (Sinclair, 1992) and Hawaii is initiated at the time apical dominance is broken by forcing fruit development, may also be delayed at higher planting densities. Circumstantial evidence suggests that, when interplant competition is intense, there is insufficient photosynthate for the development of both a fruit and suckers. The fruit is a stronger sink than developing suckers, since forced plants will always develop a fruit but may not develop suckers until after the fruit matures. In tropical regions, such as Côte d'Ivoire, sucker growth was increased up to 20% by cutting off the leaves that extend above the developing suckers immediately after fruit harvest (Combres, 1983). Leaf removal may improve sucker exposure, thus promoting their growth.

drought. Despite the xeromorphic characteristics of pineapple, growth and yield are significantly reduced when drought is prolonged. One effect of a water deficit is to reduce the number and length of the roots (Kadzimin, 1975; E. Malézieux and G. Godo, 1991, unpublished results). White root tips visible in the soil, expressed as a percentage, have been used as an indicator of root health and water deficit (Sanford, 1962; Fig. 5.14), and root elongation ceases and root tips suberize when the soil moisture approaches -1.5 MPa (Ekern, 1967). Suberization does not protect roots indefinitely as they will die if the soil moisture stress is severe and prolonged (Krauss, 1959). In Côte d'Ivoire, rooting depth and root numbers increase with irrigation or with polyethylene mulch (E. Malézieux and F. Zampatti, 1991, unpublished results).

Most of the quantitative data on the effects of water stress on growth were obtained on plants grown in pots; few data were found on the effect of water deficit on vegetative growth rate. Sideris and Krauss (1928) found that plants did not grow in soils containing 5% moisture and, relative to well-watered soil, growth decreased significantly in soils containing 10 or 15% moisture. Growth was not different in soils maintained at 20, 25 and 30% moisture. In another pot study, growth in a loamy soil continued at a soil moisture content of 10% but almost stopped when soil moisture fell to 5% or less (Shiroma, 1971). In a pot study where the irrigation interval was varied from twice weekly to bimonthly, 'D'-leaf weight, leaf area, dry weight of all plant parts and fresh

Fig. 5.14. Variation in the percentage of white root tips used to diagnose root health and water availability (modified from Nightingale, 1949).

fruit weight all decreased significantly as the irrigation interval was extended (Chapman et al., 1983). After an early establishment period, Kadzimin (1975) grew pineapple in black polyethylene bags for 7 months at average soil water potentials (^soil) of -0.1, -0.5, -1.0 and -1.5 MPa and in an unirri-gated treatment. In that study, plant total dry mass decreased 27.6, 32.4 and 46.9%, respectively, in the -1.0 and -1.5 MPa and unirri-gated treatments. Only leaf and stem growth were reduced at -1.0 MPa; in the -1.5 MPa and unirrigated treatments, leaf, stem and root growth were reduced. The root system as a proportion of the whole plant decreased significantly in the -1.5 MPa and unirrigated treatments. It was suggested that the decrease was due to a reduction in meristem-atic activity associated with the internal water deficit (Kadzimin, 1975). In the clay soils of Hawaii, where water-holding capacity is high, irrigation begun when ^soil reached -0.4 MPa was equal to or better than irrigating at a ^soil of -0.03 or -0.07 MPa (Thorne, 1953). However, where soils are sandy and soil water-holding capacity is low, growth was reduced as soon as the ^soil fell below -0.015 MPa at a 15 cm depth (Combres, 1979). Combres (1979) concluded that a ^ ., of -0.015 could be used as a soil threshold value to initiate irrigation (Combres, 1983).

Plant moisture content decreases with increasing soil moisture stress and the thick ness of the leaf water-storage tissue (Fig. 5.15) decreases as the plant water deficit increases (Fig. 5.16). However, leaf water content changes slowly and pineapple tolerates long periods of drought with minimal plant loss. Nose et al. (1981) observed only a 0.1% decrease in leaf water content as soil pF - a measure of soil water status - decreased from 1.1 to 4.5. Chapman et al. (1983) reported that the water content of 'D' leaves was not significantly reduced by any irrigation treatment until fruit harvest and, even then, differences in the 'D'-leaf moisture contents for the high and low water treatments were small. Even uprooted plants lose water slowly. Fruiting suckers removed from their mother plants at flowering and kept without water for 3 months on a glasshouse bench in sunlight lost 44.4% of their fresh weight by transpiration, while comparable shaded ones lost 41.6% (Sideris and Krauss, 1955). Despite the high degree of dehydration, significant fruit growth occurred. Water losses from the leaves due to translocation to the fruit were estimated at 18 and 26.8% of leaf weight for the sun and shade plants, respectively. Pineapple plants show a remarkable ability to transfer water and other nutrients from the leaves to the fruits when water stress is extreme.

Though normal values of pineapple leaf water potential (^L) were about -0.6 MPa at a ^soil of -0.1 MPa, reached -2.3 MPa at a ^soil of -1.8 MPa (Kadzimin, 1975). George et al. (1984) found that of pineapple leaves varied from -0.2 to about -1.8 MPa after 12 weeks without any water supply, while leaf relative water content (RWC) decreased from about 96% to 42% over the same time period. Leaf RWC and were linearly correlated with a 10% change in RWC corresponding to a 0.28 MPa change in (George et al, 1984). Similarly, Kadzimin (1975) found that, as decreased from -0.1 MPa for well-watered plants to -2.2 MPa for unirrigated plants, leaf RWC decreased from 96% to 70%.

Pineapple Leaves Microscope
Fig. 5.15. Light and scanning electron microscope cross-sections of a mature pineapple leaf showing the prominent water-storage tissue on the adaxial side of the leaf and the chlorenchyma on the abaxial half of the leaf (photos of D. Bartholomew).
Fig. 5.16. Leaf water-storage tissue deficiency (%) at inflorescence emergence (red bud) with decreasing availability of water (modified from Nightingale, 1949).

However, leaf RWC was concluded to be a rather insensitive indicator of water stress in pineapple, especially under minimal stress situations (Kadzimin, 1975; George et al., 1984).

Pineapple plants subjected to water stress have significantly lower ET values compared with unstressed plants. In central Côte d'Ivoire, ET of complete pineapple leaf canopies averaged 3.8 mm day-1 when irrigated and 2.9 mm day-1 when unirrigated (Combres and Perrier, 1977). The ET/ETg ratio, where ETg is the potential ET of a grass cover, is about (g.45 for an irrigated pineapple crop but the ratio decreases during drought (Combres, 1983).

ET of a pineapple crop depends both on the stage of development and climatic conditions. When well watered, the ET of complete canopies ranged from 0.6 to 0.7 of the potential evaporation of a grass cover (Combres and Perrier, 1977). The crop parameter Kc, in the equation:

where ETR is actual ET and ETo is standard evaporation (Priestley and Taylor, 1972), generally ranges between 0.8 and 1.3 for most crops. An average value of 0.74 was observed for pineapple (Combres, 1983), but

Kc can vary significantly over the plant cycle. Kc decreased significantly from 0.93 5 months after planting (50-60% of soil cover) to 0.41 by 11 months after planting, at which time there was 80-90% of soil cover by the canopy (N'Guessan, 1985). Where pineapple was planted through a polyethylene mulch in Hawaii, a 50% decrease in the daily rate of consumptive use of water occurred by the time of 60% canopy closure (Ekern, 1964). It is a striking feature of pineapple that the ET rate of a pineapple crop decreases as the plant develops and the canopy cover increases, whether in mulched or unmulched conditions (Ekern, 1964). With mulch, 15% of the incident energy was used for ET after the canopy closed. The very low water-use rate of pineapple, which is due to the inverted pattern of stomatal conductance and xero-morphic anatomical features, is further reduced by water stress.

flooding. Excess water can reduce the growth and yield of pineapple, mainly when waterlogging occurs during root initiation and at fruit filling. As with most crop plants, root growth and efficiency are restricted by inadequate aeration associated with excess soil moisture. In solution cultures, root growth was increased by aerating the solution (Iwaoka et al, 1935) or by removing roots from the solution for 2 h day-1 (Tisseau, 1971). The formation and persistence of root hairs seem to depend upon an oxygen supply to the roots (Py et al., 1987). Root aerenchyma is also increased by a diminished oxygen supply and a comparable effect is seen with an increase in soil compaction (Rafaillac et al., 1978). Excessive water produces a leaf colour change similar to that seen with water stress, in that leaves first become pale yellow and then red, and leaves are reduced in length and are more erect (Py et al, 1987).

General effects temperature. Although there are few specific data on the effects of temperature on the growth and development of pineapple, temperature appears to be one of the most important environmental factors determining pineapple distribution and productivity in the world. Pineapple survives in hot, dry environments where other crops would be non-productive, but is also cultivated in the cool subtropics, where freezing temperatures may occur (Table 5.1). The lowest annual average temperature where pineapple is grown on a commercial scale appears to be 17.2°C in Port Elizabeth, South Africa (Bartholomew and Kadzimin, 1977). In southern Queensland the mean monthly maximum/minimum temperature ranges from 29/19°C in summer to 20.5/6°C in winter (Wassman, 1990). The plant does not tolerate frost, but temperatures have been reported to drop below 0°C for short periods of time in the pineapple-growing areas of south Queensland, Australia (Swete Kelly and Bartholomew, 1993), South Africa, Sao Paulo, Brazil (Giacomelli and Py, 1982; Py et al., 1987), and southern Florida. As was noted before, prolonged exposures to temperatures less than 0°C can destroy the canopy and lead to the loss of the crop, as has happened in some areas where freezing temperatures can occur (Giacomelli and Py, 1982; Swete Kelly and Bartholomew, 1993). Overhead irrigation has been used to protect against freezes in Florida (J. Tenbruggencate, 1986, personal communication). In the absence of frost, the plant is quite productive in both cool subtropical and tropical environments, though the crop cycle is prolonged in such environments.

Large areas are planted to pineapple in hot, wet intertropical regions, in low-altitude areas and, more specifically, along coastal plains, where the climate is moister and hotter than in continental areas. In most of the tropics, and especially in hot and wet regions close to the equator, the annual range of monthly temperature is very small, sometimes not more than ±1°C. Thus, the diurnal temperature cycle is often more important than the seasonal cycle (Monteith, 1977).

Optimum day and night temperatures for vegetative growth of pineapple are near 30 and 20°C, respectively, with an optimum mean temperature of 23-24°C (Neild and Boshell, 1976). Plant growth decreases rapidly at mean temperatures below 15°C or above 32°C (Neild and Boshell, 1976). In Hawaii, the slower vegetative growth rates observed from December to April are mainly due to a decrease in the average temperature, especially soil temperature (Ekern, 1967). In Australia, low temperatures from May to October reduce or even stop plant growth in the midwinter period (Glennie, 1981; Wassman, 1986).

The large environmental variation among areas where the crop is grown (Table 5.1) accounts for much of the large variation in the time from planting to maturation of the mother plant crop. Within a given environment, fruit weight at harvest is determined in large part by plant weight at forcing (Py and Lossois, 1962; Gaillard, 1969; Tan and Wee, 1973; Malézieux, 1988; Malézieux and Sébillotte, 1990a). Consequently, an important objective of growers is to obtain a given plant weight at the time plants are to be forced. In the absence of other stresses, plant growth is determined by temperature. For that reason, the interval from planting to forcing varies considerably over the wide range of latitudes and altitudes where pineapple is grown (Table 5.1). In the East London area (33°S, South Africa), where average temperature is 18.8°C (mean minimum temperature of 14.7°C, mean maximum temperature of 22.8°C), the average period from planting to forcing is 24 months (Bouffin, 1991), whereas in areas near the equator, such as in West Africa, the vegetative growth period is only 6-8 months.

The planting-to-forcing interval also varies with date of planting, especially in areas where the seasonal temperature variation is large. In the Canary Islands, the vegetative cycle ranges from 9 months when planting is done in the spring and plants develop in summer, to 14 months when the crop is planted in winter (Galan Sauco et al., 1988b); similar large variations are encountered in Australia and South Africa. A plant fresh mass of 2.5 kg - a common plant weight for forcing - can be reached within 8 months after planting in Côte d'Ivoire (5°N, mean temperature of 26.6°C), within 10-11 months in Hawaii (21°N, mean temperature of 23°C) and within 13-14 months in Queensland, Australia (26°S, mean temperature of 19°C). As a result, the time from planting to harvest ranges from 12 months near the equator (Gaillard, 1969), where the average annual temperature is 26-27°C, to 32 months in Swaziland (26° 30'S), where mean annual temperature is 16-17°C (Dodson, 1968). While there is wide variability in the length of the vegetative growth phase for 'Smooth Cayenne' in the pineapple-growing areas of the world, there is also considerable variation in the length of the reproductive phase (Table 5.1; see Bartholomew et al., Chapter 8, this volume).

In some pineapple-growing regions, it is common to produce one or two ratoon crops from suckers borne on the mother plant. In equatorial climates, sucker development is commonly delayed until after the fruit matures. The first ratoon crop takes about 1 year in both Hawaii and Thailand, even though growth is more rapid in the warm, humid climate of Thailand. The long period of time required for ratoon-crop development in Thailand results from the fact that suckers do not begin to develop until after the plant-crop fruit is harvested, whereas, in Hawaii, development begins at the time of forcing. In general, the plant and one ratoon crop are harvested after about 2 years in Thailand, 3 years in Hawaii, 3-4 years in Australia (Vuillaume, 1986) and as many as 5 years in Swaziland (Dodson, 1968).

temperature injury. High temperatures are generally of minor concern during vegetative growth of pineapple. Although leaf and plant temperature may reach very high values due to the low transpiration rate and poor air circulation around the leaves within the plant canopy, leaves tolerate high temperatures well. Leaf sunburn is sometimes seen, but it is not a serious problem in any region where pineapple is grown.

As noted above, low-temperature injury is of greater concern where freezing temperatures are encountered. With mild radiation frost, the upper surface of horizontal leaves develops a red/white-flecked, scorched appearance. If the injury is severe, the whole leaf dies and the leaves become papery. Depending upon the age of the plant when frost injury occurs, vegetative plants may recover and produce an acceptable crop (Swete Kelly and Bartholomew, 1993). Without some sort of frost protection, prolonged exposure to temperatures less than 0°C can destroy the aerial parts of the plant, leading to loss of the crop. As noted above, sprinkler irrigation has been used to protect pineapple plants from freezing in Florida (J. Tenbruggencate, 1986, personal communication).

irradiance. The significance of irradiance as a factor affecting pineapple yield was recognized at least by the mid-1930s in Hawaii (Sideris et al., 1936). However, there is little evidence that irradiance limits pineapple production in most areas where the crop is grown. Sanford (1962) attributed to Sideris an observation that yield decreases about 10% for each 20% decrease in solar radiation. Shiroma (1977) concluded that irradiance could be limiting for 'Smooth Cayenne' pineapple in Okinawa, but his conclusion was based on lower average global radiation values for Okinawa than for Taichung, Taiwan and Hawaii, rather than data obtained in situ. A linear relationship between plant weight 8 months after planting and radiation intercepted by the canopy was established in an experiment in Côte d'Ivoire, where plants were planted every month for 6 years (E. Malézieux, 1992, unpublished results). In this experiment, an empirical relationship that associated average irradiance during the entire crop cycle and an indicator of drought from planting to harvest explained 64% of the variation in plant weight at harvest (72 monthly plantings with similar cultural practices). However, interpretation of the results of such studies is very difficult because of the long-term nature of the crop and the fact that changes in irradiance are confounded with simultaneous changes in air temperature. In areas where the variation in temperature is small compared with variation in irradiance, such as near the equator, irradiance may be of importance in determining plant growth.

Where both irradiance and temperature change significantly with season, it is very difficult to evaluate the effects of irradiance on pineapple growth and productivity. The quality and average size of the available planting material can vary with time of year (Louis and Nightingale, 1937; Zhang, 1992) and these factors are important because they determine the initial plant size. At a given planting density in the same environment, plant mass at a given time after planting is determined by the mass of the propagule (Py et al., 1987). Even modest differences in propagule mass between plantings in different seasons can obscure the effects of seasonal changes. After a 2-3 month establishment period, which can be further prolonged by cool temperatures or lack of rain, vegetative growth is the product of accumulated irradiance over a period of 6-15 months. Fruit development requires an additional 5-8 months. Since plant mass at forcing is an important determinate of fruit mass at harvest (Py and Lossois, 1962; Gaillard, 1969; Tan and Wee, 1973; Malézieux, 1988; Malézieux and Sébillotte, 1990b), anything that retards vegetative growth in one season relative to another will either delay harvest or reduce yield.

Seasonal variations in productivity can also be due to the seasonal distribution of rainfall and seasonal and day-to-day variations in forcing success (Wee and Ng, 1968; Wassman, 1991) and in pest and disease pressure. While it is difficult to demonstrate an effect of irradiance on productivity, in Côte d'Ivoire, where temperature is rela tively stable and irradiance fluctuates significantly throughout the year, potential plant weight at forcing was correlated with irradi-ance intercepted during vegetative growth (Malézieux, 1988). In these unirrigated conditions, yield was depressed when drought was prolonged.

drought and water excess. Because pineapple has a great capacity to survive drought (Sideris and Krauss, 1928), it has been grown successfully on Molokai, Hawaii, in an environment with less than 600 mm year-1 of rain (Noffsinger, 1961). Where the soil is well drained, pineapple has also been cultivated in relatively wet areas, such as Guadeloupe, where rain exceeds 3500 mm year-1 (Py et al., 1968). Hence, depending on the location and season, the climatic water balance over the crop cycle or some part of it can be largely positive or negative. In Wahiawa, Hawaii, an almost ideal location for pineapple culture without irrigation, pan evaporation averages 1850 mm year-1 while rainfall averages 1000 mm year-1 (Noffsinger, 1961; Ekern, 1965).

Despite the capacity to survive drought, growth and yield are reduced in all areas where water stress occurs (Foote, 1955; Black, 1962; Ekern, 1964; Py, 1965; Huang and Lee, 1969; Malézieux, 1988). Hence, irrigation or polyethylene mulch or both are regularly used (Hawaii) or are used by some growers (Queensland, Côte d'Ivoire). Though polyethylene mulch has several benefits, its impact on water economy is one of the main reasons it is used in Côte d'Ivoire (Combres, 1983). Several studies report no effect of irrigation on pineapple growth, even where drought occurs (Rao et al., 1974; Tay, 1974; Senanayake, 1978; Htun, 1986). Despite such results, the benefits of irrigation during prolonged drought (Medcalf, 1950; Py, 1965; Huang and Lee, 1969; Combres, 1979, 1983; Kuruvilla et al, 1988; Malézieux, 1992) are sufficient to cause it to be a widely adopted practice. In Guinea, growth of unir-rigated plants was delayed by up to 3.5 months by the end of a 5-month dry season (Py, 1965). Despite good evidence of the benefit of irrigation, no quantitative studies were found that would provide the data needed to predict growth or yield reduction in response to water stress.

In regions of high rainfall, water excess can also limit growth, as well as increasing susceptibility to disease (see Rohrbach and Johnson, Chapter 9, this volume). Waterlogging impedes gas exchange, resulting in elevated CO2 and depleted O2 levels in the soil, which lead to decreased growth by inducing root anoxia. In the Mekong delta, for instance, beds may become submerged (Le Van Thuong, 1991), and pineapple plants died after 15 days of submersion of the bed beneath 10-15 cm of water. Where rainfall is high, as in the humid tropics, soils used for pineapple cultivation are generally lower in clay and higher in sand to ensure good drainage. Such soils have low water-holding capacity and are susceptible to erosion (Ducreux et al., 1980; Ciesiolka et al., 1993; El-Swaify et al, 1993; Ciesiolka, 1994). Ridging practices, widely used in South and West Africa and some areas of Australia, also decrease the risk of waterlogging during rainy periods.

Plant growth and fruit weight are decreased by waterlogging that occurs in small field depressions in Côte d'Ivoire. While few quantitative data on the effects of water excess on growth under field conditions were found, until relatively recently, when effective fungicides became available to control Phytophthora spp., loss of plants to root and plant rots was a greater concern than reduced growth due to waterlogging (Rohrbach and Apt, 1986; see Rohrbach and Johnson, Chapter 9, this volume).

miscellaneous factors. Wind is a minor concern for pineapple growers, though prolonged winds have been reported to reduce plant size by 25% (Nightingale, 1942). It is not known if the reduction was due to the physical effect of wind, to an altered water balance or to reduced temperature within the canopy. Though wind has little effect on ET when the plant cover is complete, hot, dry winds, such as the Norte in Mexico or the Harmattan in Côte d'Ivoire, can lead to leaf-tip drying (Py et al., 1987). The influence of wind on thermal exchanges in the canopy is significant (Py et al., 1987). Strong winds cause leaves to rub against each other and the physical damage provides points of entry for fungi (see Rohrbach and Johnson, Chapter 9, this volume), but neither the physical nor the fungal damage is considered economically significant in most instances (Py et al., 1987). Although wind damage is generally minor, wind-breaks of napier grass or sugar cane have been used in some regions of South Africa (Anon., 1956). In coastal areas, wind-borne salt spray can cause burns, leading to blackish spots near the tips of the leaves (Sideris, 1955). Exceptionally strong winds caused by hurricanes can severely damage all parts of the plant or uproot it, leading to significant reductions in yield (Py et al., 1987).

Hail is rare is most areas where pineapple is grown but occurs in southern Queensland (Swete Kelly and Bartholomew, 1993) and southern Africa (Anon., 1956). Hail can seriously damage the rigid and brittle pineapple leaf, significantly reducing leaf area. After a severe hailstorm in Swaziland, most of the leaves wilted and died off, leaving only a small growing point and leading to a loss of 1 year's growth (Anon., 1956).

Few data on the effects of salt on pineapple were found (see Malezieux and Bartholomew, Chapter 7, this volume), but pineapple is grown along windward sea coasts in Hawaii with minimal adverse effects, even though the north-easterly trade winds can be both persistent and strong.

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