Gay and Hauck (1994) studied L. temulentum L. (a C3 plant), also called ryegrass or darnel (Bailey 1974). Darnel is a poisonous weed resembling rye, often found in grain fields. This is not to be confused with L. perenne L. (perennial or English ryegrass) or L. multiflorum Lamarck (Italian ryegrass). [The word "darnel" probably is derived from the Old French darnu, which means "stupefied," so called from its supposed stupefying or intoxicating qualities (Friend and Guralnik 1959). In L. temulentum, the outer glume produces a narcotic poison (Bailey 1974).] Plants grew in controlled environments at two CO2 concentrations (ambient CO2 or 345 |mol/mol and elevated CO2 or 575 |imol/mol) and at two light intensities (low photosynthetic photon flux density or 135 |imol/ m2/s and high photosynthetic photon flux density or 500 |imol/m2/s). Plants were harvested every 2 or 3 days until about 40 days after sowing to determine leaf lengths and leaf growth rates. Leaf growth curves were fitted using a logistic equation. Leaf length (Figure 15.23a) and leaf growth rate (Figure 15.23b) were increased under elevated CO2, but only under the low-light conditions. (For leaf length, where data points are shown in Figure 15.23a, compare the solid triangles for high CO2 with the solid circles for low CO2.) In high light, the effects of increasing CO2 were small. They concluded that L. temulentum is limited in its ability to respond to increased CO2 in high light, but it can respond in low light.

The genus Rumex [Polygonaceae (buckwheat or knotweed family); common name, dock or sorrel] consists of weedy herbs. Pearson and Brooks (1995) studied Rumex obtusifolius L. (broad-leaved dock; C3), a perennial herb that grows to be 40-150 cm tall. In many countries, it is the most troublesome weed in intensively managed permanent grasslands. The possession of a large root system explains much of the persistence and aggressive nature of R. obtusifolius. They grew the plants in pots containing compost and grit placed in day-lit environmental chambers with air containing 350 or 600 |imol/mol CO2. During the experiment, there were five flushes of new leaves, which they labeled A, B, C, D, and E. They determined dry weights of roots and shoots, and weighed senescent leaves. Data from gas exchange measurements were used to calculate the instantaneous water use efficiency (photosynthetic rate divided by transpiration rate). Plants grown under 600 ||mol/mol CO2 had higher aboveground (Figure 15.24a) and root (Figure 15.24b) dry weights than plants grown in 350 ||mol/mol CO2. For plants grown in 350 ||mol/mol CO2, water use efficiency was relatively constant over the first five measurement days and then declined (Figure 15.25). By contrast, plants

FIGURE 15.23 (a) Time course of leaf length of four leaves of L. temulentum L., a type of ryegrass, grown in ambient CO2 (345 ||L/L) and low light (135 ||mol/m2/s) (solid circle and solid line); increased CO2 (575 ||L/L) and low light (solid triangle and short-dashed line); ambient CO2 and high light (500 ||mol/m2/s), (solid square and long-dashed line); and high CO2 and high light (upside down solid triangle and dotted line). Lines are fitted logistic curves and points are means. Standard errors are shown by vertical lines where larger than the symbol. (b) Leaf growth rates derived from the fitted lines in (a). (Gay, A.P. and Hauck, B., J. Exp. Bot, Fig. 4, 1994 by permission of Oxford University Press.)

FIGURE 15.23 (a) Time course of leaf length of four leaves of L. temulentum L., a type of ryegrass, grown in ambient CO2 (345 ||L/L) and low light (135 ||mol/m2/s) (solid circle and solid line); increased CO2 (575 ||L/L) and low light (solid triangle and short-dashed line); ambient CO2 and high light (500 ||mol/m2/s), (solid square and long-dashed line); and high CO2 and high light (upside down solid triangle and dotted line). Lines are fitted logistic curves and points are means. Standard errors are shown by vertical lines where larger than the symbol. (b) Leaf growth rates derived from the fitted lines in (a). (Gay, A.P. and Hauck, B., J. Exp. Bot, Fig. 4, 1994 by permission of Oxford University Press.)

FIGURE 15.24 The average dry weight of component parts of R. obtusifolius grown in 350 (open circle, open square, open triangle) and 600 (closed circle, closed square, closed triangle) |J,mol/mol CO2. (a) Green tissue, open circle and closed circle; senescent tissue, open triangle and closed triangle; total aboveground tissue (green + senescent), closed square and open square. (b) Roots, open and closed squares. The solid lines connect data points for the 600 |J,mol/mol CO2 treatment and the dotted lines connect data points for the 350 |lmol/ mol CO2 treatment. Symbols are shown ± one standard error of the mean (n = 12). (Reproduced from Pearson, M. and Brooks, G.L., J. Exp. Bot, Fig. 7, 1995 by permission of Oxford University Press.)

FIGURE 15.24 The average dry weight of component parts of R. obtusifolius grown in 350 (open circle, open square, open triangle) and 600 (closed circle, closed square, closed triangle) |J,mol/mol CO2. (a) Green tissue, open circle and closed circle; senescent tissue, open triangle and closed triangle; total aboveground tissue (green + senescent), closed square and open square. (b) Roots, open and closed squares. The solid lines connect data points for the 600 |J,mol/mol CO2 treatment and the dotted lines connect data points for the 350 |lmol/ mol CO2 treatment. Symbols are shown ± one standard error of the mean (n = 12). (Reproduced from Pearson, M. and Brooks, G.L., J. Exp. Bot, Fig. 7, 1995 by permission of Oxford University Press.)

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