Wateruse efficiency related to CAM in pineapple

Transpiration rhythm and transpiration rate in pineapple

In pineapple, the diel rhythms of stomatal conductance and transpiration are closely linked to the net CO2-uptake rhythm (Neales et al, 1968; Nose et al, 1981; Bartholomew, 1982; Côte et al, 1993; Fig. 5.3). The lowest rates of transpiration occurred during phases I, II and III and the highest during phase IV.

WUE is two times greater during phase I than during phase IV and reaches a minimum during phase III (Fig. 5.3). The transpiration rates of pineapple ranged from 0.05 to 0.23 mmol m-2 s-1 (Ekern, 1965; Joshi et al., 1965; Neales et al, 1968; Nose et al, 1981), while values for sunflower and tobacco were at least tenfold higher at about 3.1 mmol m-2 s-1 (Neales et al., 1968). Transpiration by pineapple in the light in controlled conditions was only 4% of that of sunflower leaves grown in a similar environment (Neales et al., 1968) and only 6% of that for wheat, and WUE was 3.3-fold greater (Côte et al, 1993).

Leaf and canopy transpiration rates for pineapple are lower than those for most cultivated crops. At midday, there was no measurable water-vapour loss from a full pineapple canopy with an LAI of 7 (Ekern, 1965) and transpiration from pineapple plants in pots was 1.0 mm day-1 (monthly average) (Shiroma, 1971). The transpiration ratio (TR) (units water lost per unit dry-matter gain; kg kg-1), a measure of the WUE, for pineapple was about 50 (Sideris and Krauss, 1955; Joshi et al., 1965; Côte, 1988) to 116 (Neales et al., 1968), while the range for C3 crops was 450-950 and that for C4 crops 250-350 (Kluge and Ting, 1978). A consequence of the low rate of water use is that the crop can sustain an LAI of greater than 7 (Malézieux, 1991; Zhang, 1992) over long periods under low-rainfall conditions (Ekern, 1965; Aubert, 1973).

Origins of the high water use efficiency related to CAM

Diffusion of water vapour from and CO2 into plant leaves can be described by the equations:

where AW is the difference in water-vapour pressure between the stomatal cavity and the atmosphere, r is the resistance to water-vapour diffusion, T is the transpiration rate, AC02 is the difference in CO2 partial pressure between the stomatal cavity and the atmosphere, 1.6 is the ratio of the diffusion

Oo Cm m

CO2 fixation Transpiration

CO2 fixation Transpiration

Night

0.20

0.15

0.10

0.05

Fig. 5.3. Transpiration and net CO2 exchange during a night/day cycle by a 'Smooth Cayenne' pineapple plant (Côte et al., 1993). 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 = 120 g) was obtained from in vitro culture. Data are the averages for 3 consecutive days.

0.25

0.20

0.15

0.10

0.05

nh m

Night

Fig. 5.3. Transpiration and net CO2 exchange during a night/day cycle by a 'Smooth Cayenne' pineapple plant (Côte et al., 1993). 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 = 120 g) was obtained from in vitro culture. Data are the averages for 3 consecutive days.

coefficients for water vapour and CO2 in air and P is the rate of net CO2 fixation. Combining equations (1) and (2) gives:

The low transpiration rate in CAM plants is related to morphological and physiological adaptations that influence AW, P and AC02 of equation (3) to reduce T:

• Net CO2 fixation (P) occurs mainly during the night when AW is low.

• During the day, when AW is high, malate decarboxylation increases the internal CO2 concentration; consequently P is low or even zero.

• During phase IV AC02 is high (Côte et al., 1993), reducing P and T.

The highest leaf conductances recorded for pineapple were about 3% of those observed for cotton (Neales et al., 1968). This low conductance was attributed to a thick mesophyll, low stomatal frequency and sunken stomata located beneath multicelled trichomes (Bartholomew and Kadzimin, 1977).

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