Effects of environmental factors

Effects on photosynthesis temperature. The effects of temperature on photosynthesis of pineapple are complex and relatively few studies have been conducted. While the results of these studies are often not directly comparable, the general trends show that the intensity of CAM (phase I) is highest when the light/dark temperature differential is approximately 10°C and the night temperature is cool, about 20°C. These results are consistent with other observations that the optimum day/night temperature for growth also is about 30/20°C (Neild and Boshell, 1976; Bartholomew and Malézieux, 1994).

increasing night temperature. At a given day temperature, an increase in the night temperature decreases night net CO2 uptake (Connelly, 1972; Neales et al., 1980; Zhu et al., 1999), as well as the proportion of night fixation in a 24 h period (Connelly, 1972; Bartholomew, 1982; Bartholomew and Malézieux, 1994; Zhu et al, 1999). Neales et al. (1980) and Zhu et al. (1999) reported a 50%

decrease in night uptake as the night temperature was increased from 20 to 25°C at a constant day temperature of 30°C. The decrease could be due in part to an increase in respiration-derived CO2 fixation, but leaf titratable acidity also decreases (Min, 1995), indicating less total CO2 dark fixation. Some studies show little or no change in net CO2 uptake during the day with an increase in night temperature (Neales et al., 1980; Zhu et al., 1999), but Connelly (1972) reported a dramatic increase when night temperature increased from 15 to 30°C. It is at present not possible to resolve this discrepancy.

The relationship between net CO2 fixation and dry-matter accumulation under various thermoperiods has also not been well studied. In one study, net CO2 fixation of single attached leaves of plants grown for 8 months at 30/20°C was 1.6 times higher than that for leaves of comparable plants grown at 30/25°C, while whole-plant dry-matter accumulation was only 1.1 times higher in the cooler environment (Zhu et al., 1997a). However, assimilation was measured only on 'D' leaves, while dry-matter accumulation reflects the contribution of leaves of all ages.

increasing day temperature. At a given night temperature, the fraction of total CO2 assimilated in a 24 h period that is fixed at night increases with increasing day temperature (Neales et al., 1980; Bartholomew, 1982; Bartholomew and Malezieux, 1994; Zhu et al., 1999). This increase is due primarily to a decrease in net CO2 uptake in the day and to a lesser extent to an increase in net CO2 uptake at night (Connelly, 1972; Neales et a2l., 1980; Zhu et al, 1999). For example, Zhu et al. (1999) reported that CO2 uptake in the day decreased 50% as the day temperature was increased from 30/25°C to 35/25°C, while the increase in CO2 uptake at night was much smaller. As a result, there was a 37% increase in the fraction of CO2 fixed at night. The biochemical basis for the changes in CO2 uptake with changing temperature is not well understood. It is hypothesized that the decline in net CO2 uptake during phase IV with increasing day temperature is associated with increased respiration, but further studies are needed.

irradiance and photoperiod. Because CO2 assimilation in CAM plants, including pineapple, occurs via the photosynthetic carbon reduction cycle, it is assumed that photosynthesis will increase with increasing irradiance, and the results of Shiroma (1977) and Nose et al. (1981, 1985, 1986) support this assumption. Nose et al. (1985, 1986) found that light saturation of pineapple plants occurred at a photosynthetic photon flux (PPF) of about 500 ^mol m-2 s-1. In vitro-grown plants maintained under continuous illumination also saturated at a PPF of 500 ^mol m-2 s-1 (Côte, 1988). However, it is probable that variation in irradiance can alter the pattern of CO2 fixation. For example, high irradiance decreases the duration of phase III and increases the duration of phase IV assimilation relative to what occurs at low irradiance (Nose et al., 1986). This is because the rate of malate-decarboxylation-depen-dent CO2 assimilation is more rapid during phase III2 (Sale and Neales, 1980), so the malate pool is depleted more rapidly. Increasing irradiance probably also increases the quantity of soluble sugar formed in leaves during phase III and IV. An increased supply of sugar available for the production of PEP, the substrate required for CO2 fixation into malate at night, would increase the amount of CO2 fixed into malate during phase I. Consistent with this hypothesis, an increase in irradiance during the day increased CO2 fixation the following night (Nose et al, 1981, 1985, 1986; Shiroma et al, 1977), and leaf titratable acidity also increased with increasing irradiance (Sideris et al, 1948; Connelly, 1969; Aubert, 1971; Sale and Neales, 1980). However, increases in night CO2 fixation that follow an increase in irradiance generally are smaller than the increase in CO2 assimilation in the light. Thus, the intensity of CAM tends to decrease as irradiance increases, at least until saturating levels are attained.

Little is known about the effect of pho-toperiod on pineapple photosynthesis. Nose et al. (1986) reported that as day length increased from 10 to 16 h, total uptake in the light increased while uptake at night decreased. However, expressed on an hourly basis, the increase in CO2 uptake in the light as day length increased was small, while the decrease at night was relatively large. A 40-50% reduction in CO2 uptake at night can be deduced from the data of Nose et al. (1986) as photoperiod decreased from 16 to 8 h. In natural environments, e.g. Hawaii, the decrease in photoperiod from summer to winter is only 21%, while the decrease in irradiance is about 50%.

co2 concentration. The effect of elevated CO2 concentration on pineapple photosynthesis has not been well studied, but generally increases in response to elevated CO2 have been observed. At a concentration of 1700 ^mol mol-1 CO2, net CO2 fixation increased two- to threefold during phase IV (Côte, 1988) and a CO2 concentration greater than 1000 ^mol mol-1 was required to saturate photosynthesis in that phase. Saturation at such a high CO2 level indicates that pineapple has a high resistance to CO2 diffusion during phase IV.

Zhu et al. (1997b) reported that the dry mass of pineapple plants grown for 4 months at 730 ^mol mol-1 CO2 was 1.2 times higher than that of plants grown at ambient CO2 (330 ^mol mol-1). In that study, 'D'-leaf titrat-able acidity at the end of phase I was more than 1.3 times greater for plants grown at elevated than at ambient CO2. Net CO2 uptake in phase I was also enhanced, but only where average temperature was above 25°C. Uptake of CO2 during phase IV was also enhanced and the effect of enrichment was greater as the average temperature increased from 25 to 30°C (Zhu et al., 1997b). Nocturnal CO2 fixation has been reported to be insensitive to high CO2 levels in other CAM plants, so the origin of these different responses is unknown. Further studies are required to understand the combined effect of thermo-period regime and elevated CO2 partial pressure on pineapple photosynthesis.

water stress. When CAM plants are subjected to drought, CO2 fixation during phase IV soon ceases. If the period of water stress is extended, net CO2 fixation ceases but CAM plants continue to refix respired CO2, a phenomenon referred to as CAM idling (Kluge and Ting, 1978). Pineapple plants subjected to water deficit exhibit a similar response (Fig. 5.4).

When well watered, CO2 fixation during the day and night was normal, with 45% of the CO2 being fixed at night. Depending on plant size and perhaps moisture supply, drought reduced CO2 uptake in the light rapidly (Fig. 5.4) or relatively slowly (Zhu, 1996). With small plants, uptake in the day was nil after 4 days without water, while, for large plants, net uptake in the day by attached 'D' leaves ceased after about 15 days. For small plants (Fig. 5.4), transitory net CO2 evolution occurred as the period of water stress lengthened. Nocturnal CO2 fixation rate was not affected at the beginning of water stress, but decreased progressively as the duration of water stress lengthened (Fig. 5.4; Zhu, 1996). After rewater-ing, CO2 uptake in both phase I and phase IV resumed rapidly (Fig. 5.4). The reduced sensitivity of night CO2 uptake to water deficit, as compared with day CO2 fixation, and the reversibility of the effects of water deficit in pineapple are consistent with observations for other CAM plants (Kluge and Ting, 1978).

Water deficit also alters the pattern of CO2 assimilation. At the onset of water deficit, there was a transient stimulation of net CO2 uptake at the beginning of the night period. As drought was prolonged, the maximum rate of CO2 assimilation shifted progressively towards the end of the night period (Côte et al., 1993; Zhu, 1996), and the shift was more marked at 25°C night temperature than at 20°C. Zhu (1996) also showed that the rate of decline of net CO2 assimilation during drought was related to the day/night temperature regime. Fluctuations in leaf titratable acidity indicated that significant assimilation - about 35% of the maximum -still occurred after 70 days of drought at 30/20°C, while at warmer temperatures (35/25 and 30/25°C), titratable acidity was 20% or less of the maximum after only 40 days of drought. While pineapple is highly tolerant of drought, assimilation declines fairly rapidly with drought, and warm temperatures hasten the rate of decline.

Effects on plant water relations

Transpiration rate is closely related to net CO2 uptake and to the differences in water-vapour pressure and CO2 partial pressure c ^ ra >-S

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