Days after emergence

Fig. 7.2. Change in shoot height of wild mustard (Sinapis arvensis) over time at five temperature regimes (redrawn from Huang et al., 2001).

Fig. 7.3. Effect of light intensity on: (a) shoot and root dry weight, and (b) inflorescence number of Canada thistle (adapted and redrawn from Zimdahl et al., 1991).

0 20 40 60 80 100

Percentage light reduction

Fig. 7.3. Effect of light intensity on: (a) shoot and root dry weight, and (b) inflorescence number of Canada thistle (adapted and redrawn from Zimdahl et al., 1991).

thresholds) is 24.5°C (5.5-40.0°C), whereas for leaf appearance it is 27°C (l.5-48°C) (Huang et al., 2001).

Many weeds are particularly well adapted to low temperatures and this provides a potential advantage over non-weedy plants. For example, seed germination, growth, flowering and seed set all occur at 12/2°C in wild mustard (Huang et al., 2001). Weeds are often less sensitive to short periods of cold temperature in the early spring. For example, chilling reduced the level of leaf area expansion of velvetleaf (Abutilon theophrasti) and spurred anoda (Anoda cristata) in cotton (Gossypium hirsutum); however, the weeds recovered faster and subsequently gained a competitive advantage over the delayed cotton (Patterson and Flint, 1979). While specific temperatures are required for some processes, temperature fluctuations are required for others such as seed germination (see Chapter 6).

Fig. 7.4. Effect of shading on the relative growth rate (g g 1 week 1) on four native and five non-native species of Hawaiian rainforest (redrawn from Pattison et al., 1998).

Light

Light (irradiance) has a direct effect on many plant processes, such as growth, flowering, stem elongation, seed dormancy, formation of storage organs, leaf fall and on plant morphology such as leaf, number and shape (Salisbury and Ross, 1985; Lambers et al., 1998). Plants respond to changes in light quality (spectral composition), quantity (intensity), photoperiod (periodicity of light and dark cycles) and direction (phototro-pism). It is sometimes difficult to determine what type of light effect is occurring because they interact; for example, as light quantity decreases the spectral composition changes.

Reduced light quantity will have species specific effects. In Canada thistle (Cirsium arvense), for example, shading reduces shoot and root production, and the number of inflorescences produced (Zimdahl et al., 1991) (Fig. 7.3). Conversely, itchgrass (Rottboellia exaltata) plants growing in shade grow taller than those in full sunlight; this is thought to be why itchgrass is competitive with maize (Patterson, 1985). Pattison et al. (1998) compared the relative growth rates of native and non-native species grown in full sun, partial sun (30% of full sun) and full shade (5% of full sun) of Hawaiian rainforests. As light decreased, all species had lower relative growth rates, but native species were less affected (Fig. 7.4). This, incidentally, shows why non-native species are more successful in open disturbed habitats.

As light passes through the leaf canopy the quality of light is also altered. The spectrum of light wavelengths changes because plants reflect green and far-red wavelengths while absorbing blue and red. This causes the ratio of red to far-red (R:FR) light to decrease in shaded conditions (Fig. 7.5). Many species respond to this change in the R:FR ratio. For example, higher levels of FR light triggers internode extension in white mustard (Sinapis alba), Chinese datura (Datura ferox), and lambsquarters (Chenopodium album) causing plants to grow taller into better higher light situations (Alm et al., 1991). A high-

-Daylight - Filtered light

Photosynthetically active radiation

-Daylight - Filtered light

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