Water

7.3.1 Carbon dioxide

Carbon dioxide can influence water use of plants in two ways. Firstly, CO2 directly controls stomatal conductance through its control of stomatal aperture. In most crops, including soybean, stomata are partially closed by elevated [CO2]. Morison (1987) analysed 80 sets of data and found that doubled [CO2] decreased stomatal conductance of leaves by about 40% ± 5%. On the other hand, [CO2] can promote growth, and thus produce more leaf area that will increase the transpiring surface area per unit land area. This effect on water-use rates would be especially important before crops achieve complete ground cover, or a leaf area index (LAI) of about 2.0 is obtained.

Cure (1985) summarized available soybean transpiration rate data. The ratio of water use for plants grown in doubled [CO2] compared with plants grown in ambient [CO2] ranged from 0.60 in pots inside open-top chambers (Rogers et al., 1994) to 0.94 for seasonal cumulative canopy transpiration of soybean canopies in SPAR chambers (Jones et al., 1985c).

Most of the quantitative data available on the effect of [CO2] on soybean canopy transpiration has been obtained in SPAR systems. Whole-day transpiration was calculated for several days of data at two times during the course of an experiment (Jones et al., 1985b). The average daily transpiration rate was 344 and 352 mol H2O m-2 for soybean grown at 800 and 330 |mmol CO2 mol-1, respectively. These daily rates translate to a 2% decrease in transpiration caused by elevated [CO2]. Data collected later in the season showed a daily average transpiration of 331 and 298 mol H2O m-2 for [CO2] of 800 and 330 |mmolmol-1, respectively. Over this later period, transpiration was 11% higher for the elevated [CO2] treatment. The LAI in this experiment averaged 6.0 for the elevated [CO2] treatments and 3.3 for the 330 | mol CO2 mol-1 treatments. This example shows that the effect of greater leaf area can override the effect of decreased stomatal conductance resulting from elevated [CO2] treatments. In a later study where LAI was 3.46 and 3.36 for treatments of 660 and 330 |mmol CO2 mol-1, respectively, the seasonal cumulative water use decreased by 12% for doubled [CO2] (Jones et al., 1985c). This percentage decrease in transpiration is comparable to that simulated by whole-crop energy-balance models for closed canopies (Rosenberg et al., 1990; Boote et al., 1997).

Water-use efficiency (WUE), or the ratio of CO2 uptake rate to transpiration rate (or evapotranspiration rate) of soybean, is always increased by elevated [CO2] at both leaf (Valle et al., 1985b) and canopy levels (Jones et al., 1984, 1985a,b,c,d). In leaf-cuvette studies inside SPAR chambers, Valle et al. (1985b) found that individual leaf WUE was doubled with a doubling of [CO2] from 330 to 660 |mmolmol-1, but leaf transpiration rate was decreased little because the effects of decreased stomatal conductance were offset by the effects of a concomitant increase in leaf temperature. Jones et al. (1985b) also showed that LAI over the range of 3.3 to 6.0 could have a noticeable effect on

WUE. Allen et al. (1985) developed a quantitative relationship among CER, transpiration rate and WUE which showed that CO2 enrichment increased WUE mainly by an increase of CER and secondarily by a decrease of transpiration rate.

7.3.2 Solar radiation

Solar radiation received at the earth's surface is not likely to change much unless there are also extensive changes in cloudiness and rainfall. Differences in annual incident solar radiation were only about 3% among one set of general circulation model predictions for the southeastern USA (Peart et al., 1989). Furthermore, changes in solar radiation perse may be overshadowed by other environmental changes such as temperature, air humidity, and water deficits mediated through changes in rainfall. Tanner and Lemon (1962) pointed out that water use, especially in humid climates, is very closely coupled to solar radiation. Their work preceded that of Pruitt (1964) and the Priestley—Taylor formulation (Priestley and Taylor, 1972).

7.3.3 Temperature

In theory, the temperature effect on plant water use is mediated primarily through its effect on saturation vapour pressure and vapour pressure deficit of the air. One formulation of saturation vapour pressure (esat) is given by:

where T is temperature in °K and e0 is the saturation vapour pressure (0.611 kPa) at 273°K (0°C).

Jones et al. (1985b) grew soybean at 31°C daytime air temperature and 21 °C dewpoint temperature. Daytime temperatures were changed to 28°C for several days and later to 35°C for several days. Evapotranspiration was 20% greater at 31°C (390 mol H2O m-2 per day) than at 28°C (325 mol H2O m-2 per day), and it was 30% greater at 35°C (384 mol H2O m-2 per day) than at 28°C (295 mol H2O m-2 per day) during the later comparison. Thus, over this range, transpiration increased by about 4% per °C increase in temperature. This change is less than the change in saturation vapour pressure and vapour pressure deficit over the range of 28-35°C, probably due to evaporative cooling of the leaves themselves (see Chapter 14, this volume, for further discussion on this topic).

7.3.4 Water deficits

Jones et al. (1985c) conducted an experiment with an early-season and a late-season drought-stress cycle in soybean grown at 330 and 660 |mmol CO2

mol-1. Cumulative seasonal transpiration was decreased by the drought-stress cycles, but seasonal WUE was actually improved (Table 7.5). Aggregating data, the mass ratio of seed yield to transpired water was calculated to be 0.85 g g-1 for the low [CO2] treatment and 1.36 g g-1 for the high [CO2] treatment. The elevated [CO2] caused a 60% increase in WUE (1.36/0.85). This was accomplished by a 40% increase in seed yield and a 12% reduction in water use.

7.3.5 Nutrient deficits

Few studies have been conducted on the effects of nutrient deficits on water use in soybean; however, one could hypothesize that nutrient deficits sufficient to decrease photosynthesis would decrease stomatal conductance and thereby decrease transpiration and water use. Knowledge of this type of limitation has little practical value or scientific concern.

7.3.6 Pollutants such as ozone

Few studies, if any, have been conducted on ozone effects on soybean water use. Based on foliar damage caused by ozone, one would expect that water use over the season would be decreased by ozone. However, the main issues are damage by ozone to the leaves and decreased final seed yields of soybean, rather than water use.

Table 7.5. Comparison of integrated season-long transpiration, daytime carbon dioxide exchange rate (CER), water-use efficiency (WUE) and final harvest seed yield of soybean under two drought treatments and no drought. Seasonal water-use efficiency was calculated by dividing cumulative daytime CER by cumulative transpiration. (Adapted from Jones et al., 1985c.)

Table 7.5. Comparison of integrated season-long transpiration, daytime carbon dioxide exchange rate (CER), water-use efficiency (WUE) and final harvest seed yield of soybean under two drought treatments and no drought. Seasonal water-use efficiency was calculated by dividing cumulative daytime CER by cumulative transpiration. (Adapted from Jones et al., 1985c.)

Total seasonal response (land area basis)

330 mmol CO2 mol

1 treatment

660 mmol CO2 mol 1

treatment

Latea drought

No drought

Earlya drought

Latea drought

No drought

Earlya drought

Transpiration

(kmol H2O m-2) Daytime CER

(mol CO2 m-2) WUE (mol CO2

16.1 47.1 2.93

22.2 53.0 2.39

62.8 4.62

19.3 84.7 4.39

15.5 63.3 4.08

kmol-1 H2O)

Seed yield 271 316 254 335 457 388

kmol-1 H2O)

Seed yield 271 316 254 335 457 388

aLate drought: irrigation was withheld from 70 to 83 DAP. Early drought: irrigation was withheld from 51 to 65 DAP.

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