Transpiration and Water

Water is the key variable that affects cotton production, since the crop is grown mostly in arid and semi-arid regions of the world under rain-fed conditions. Transpiration from individual leaves growing in high [CO2] is usually lower than that from leaves growing in ambient [CO2] (e.g. Kimball and Idso, 1983; K.R. Reddy et al., 1996b) because the elevated [CO2] causes partial stomatal closure. Changes in stomatal density in herbarium samples collected over several centuries indicate that increasing [CO2] may also be affecting stomatal density. However, there is insufficient information to have confidence that changes in stomatal density are occurring. Growing cotton plants for an entire season in elevated [CO2] produced leaves throughout the season that had similar stomatal densities to leaves produced in ambient [CO2] (K.R. Reddy et al., 1998a). Therefore, if stomatal densities have changed because of elevated [CO2], alterations must have occurred over many generations rather than within a single generation.

Several factors affect the degree to which a reduction in leaf transpiration will change water use per unit of land area. As the elevated [CO2] causes partial stomatal closure, the resultant decreases in transpirational cooling increases the foliage temperature of cotton (Idso et al., 1987; Kimball et al., 1992a). The increased foliage temperature increases the partial pressure of water vapour inside the leaves and increases leaf transpiration, thereby partially counteracting the CO2-induced stomatal closure. At the same time, the CO2 stimulation of growth results in larger plants (e.g. Kimball, 1983a,b) with larger leaf areas, which would also tend to increase whole-plant transpiration. Samarakoon and Gifford (1995) recently conducted an interspecific comparison among cotton, wheat and maize, using glasshouses, that nicely illustrated the dependence of water use on the relative effects of CO2 on changes in leaf area and stomatal conductance. Cotton had a large increase in leaf area and a small change in conductance so that water use per pot actually increased. Maize had very little photosynthetic or leaf area response to elevated [CO2] and so the reduction in conductance resulted in significant water conservation. Wheat was intermediate between the other two species.

Cotton plants grown in ambient and twice-ambient [CO2] in the SPAR units were gradually allowed to develop increasingly more severe water deficits while other conditions remained uniform (K.R. Reddy et al., 1997c). Measurements taken at midday showed that leaf water potentials of plants grown in ambient and twice-ambient [CO2] were not different. Also, when placed in environments of varying evaporative-demand, with adequate soil moisture, transpiration rates (per unit of land area) of canopies grown under the two CO2 environments were similar (Fig. 8.7; V.R. Reddy et al., 1995b). Kimball et al. (1993) found a slight decrease (4%) in seasonal water use of cotton as measured with lysimeters in OTCs at 650 |J.mol CO2 mol-1 in 1983 (Table 8.2). Determinations of water use as a residual in the soil water balance indicated that water use changed from -5 to +28% in 1983 and 1984. Being devoid of walls, the FACE approach has the least disturbance of wind flow and other micrometeorological factors. For this reason, the most definitive data on the effects of elevated [CO2] on cotton water use are probably those from the FACE experiment in 1991 (Table 8.2). In that experiment, cotton water use was determined from sap flow measurements (Dugas et al., 1994), as a residual in the soil water balance (Hunsaker et al., 1994) and as a residual in the energy balance (Kimball et al., 1994). The conclusion from all three independent measurements was that, under ample water supply, elevated [CO2] at 550 mmol mol-1 did not significantly affect water use of cotton per unit of land area under open-field conditions. These results are similar to those found for plants grown in the SPAR units.

Air temperature also affects transpiration rates. In the SPAR experiments, transpiration rates of individual leaves strongly interacted with [CO2] and temperature. Transpiration rates increased linearly on a leaf area basis as

E x1

Canopy transpiration

-

° o

-

apÇfS,

O CD60

Ocfi oQ

rgr

pf? o

o

o J*

y= 2.7799 + 0.9234* x, r20.83

Transpiration at ambient [C02] (kg H20 rrr2 day-1)

Transpiration at ambient [C02] (kg H20 rrr2 day-1)

Fig. 8.7. Relation between daily total transpiration of cotton canopies grown at ambient (350 or 360 |mmol mol-1) and elevated [CO2]. The data are from five temperature treatments of Upland cotton and three water-deficit treatments of Pima cotton collected over several days during fruiting.

temperature increased from 26 to 36°C (A.R. Reddy et al., 1998). At 26°C, the transpiration rate of leaves in 700 |mmol CO2 mol-1 was only 70% as great as the transpiration rate of leaves in 350 |mmol mol-1. Leaves at 36°C and 700 |mmol CO2 mol-1 transpired only 62% as much as leaves at 350 |mmol mol-1. The average transpiration rates of individual leaves at 36°C were over twice those at 26°C. Therefore, one should avoid extrapolating [CO2] effects on canopies from data on individual leaves.

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