Night fixation2

0 5 10 15 20 25 Time after cessation of watering (days)

Fig. 5.4. Effect of water deficit on the daily photosynthetic CO2 exchange by a 'Smooth Cayenne' pineapple plant (Côte et al., 1993). The plant was placed on an inert culture support aluminium silicate). Watering with the nutrient solution was stopped on day 0 and was resumed on day 26. Environmental conditions include: photosynthetic photon flux density, 650 ^mol m-2 s-1; photoperiod, 12 h night/12 h day; night/day temperature, 22°C/28°C. The plant (fresh weight of aerial parts = 100 g) was obtained from in vitro culture.

0 5 10 15 20 25 Time after cessation of watering (days)

Fig. 5.4. Effect of water deficit on the daily photosynthetic CO2 exchange by a 'Smooth Cayenne' pineapple plant (Côte et al., 1993). The plant was placed on an inert culture support aluminium silicate). Watering with the nutrient solution was stopped on day 0 and was resumed on day 26. Environmental conditions include: photosynthetic photon flux density, 650 ^mol m-2 s-1; photoperiod, 12 h night/12 h day; night/day temperature, 22°C/28°C. The plant (fresh weight of aerial parts = 100 g) was obtained from in vitro culture.

between the stomatal cavity and the atmosphere (equation 3). Thus the effects of environmental factors on plant water relations are closely linked to those of environmental factors on photosynthesis (Côte, 1988; Zhu, 1996; Zhu et al, 1997a).

temperature. For pineapple held at a constant day temperature, daytime transpiration rates decreased as the night temperature increased, while transpiration rates at night remained constant (Yoder and Bartholomew, 1969a,b). These results are consistent with the results of CO2 assimilation studies, which show that larger day/night temperature differentials decrease assimilation in the light. Mean values for WUE were higher when night temperatures were lowest (Zhu, 1996). For example, WUE at night was more than 1.3 times higher for pineapple plants maintained in a day/night temperature of 30/20°C compared with those maintained at 30/25°C. In the light, WUE was more than 1.5 times greater at 30/25 than at 35/25°C (Zhu, 1996).

Stomatal conductance (gs) of pineapple is typically lowest during the day and highest at night, referred to as an inverted pattern of stomatal opening. However, in a 10/25°C day/night temperature regime, gs was much higher during the day (51 mmol m-2 s-1) than at night (8.1 mmol m-2 s-1) (Neales et al., 1980), reversing the typical inverted pattern of stomatal opening.

Few data on gs are available under field conditions. In Côte d'Ivoire, gs data indicated that the stomata opened earlier in the afternoon when the day/night temperature differential was 9°C than when it was 14°C (J.C. Combres, 1981, personal communication). Conversely, a dry wind, such as the Harmattan in West Africa, induced prolonged closing of the stomata, with opening occurring late in the night (Py et al., 1987). Based on these data, we predict that consumptive use of water by pineapple would be higher in tropical environments, with high night temperatures and a small day/night temperature differential, than it would be in more subtropical environments, where larger day/night temperature differentials prevail.

irradiance. Because pineapple transpires little or not at all during the day, it is expected that irradiance would have a limited effect on transpiration. However, high irradiance tends to increase the rate and duration of CO2 fixation during phase IV because the stomata open earlier in the afternoon. This would be expected to increase transpiration during the day, and Nose et al. (1981) reported an increase in transpiration with increasing irradiance.

co2 concentration. Zhu et al. (1999) reported a 1.1-1.2-fold increase in WUE during the night and a 2.3-2.7 fold increase in the day for pineapple plants grown at 700 ^mol mol-1 CO2 compared with those grown at 350 ^mol mol-1. Depending on the day/night temperature regime, the 24 h WUE was 1.3-1.6 times higher at 700 ^mol mol-1 than at 350 ^mol mol-1. Zhu et al. (1999) clearly demonstrated that higher WUE was associated with a two- to threefold decrease in stomatal conductance during phases IV and I.

water stress. The patterns of gs during the onset of water stress are similar to those of net CO2 uptake. Zhu (1996) observed a rapid decrease in gs for pineapple plants subjected to drought during phase IV and a somewhat slower decrease during phase I. After 15 days of drought, gs was near zero during phase IV. In phase I, the time when gs reached a maximum shifted from early in the light period to towards its end. In ambient CO2, WUE decreased progressively as water stress progressed. However, plants maintained at 700 ^mol mol-1 CO2 exhibited a steady increase in WUE (Zhu, 1996). In a field study, J.C. Combres (1981, personal communication) observed that, after rainfall following a period of drought, the stomata open progressively earlier in the morning and in the afternoon in the following days.

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