Herbicides and Plant Physiology. Andrew H. Cobb

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Название Herbicides and Plant Physiology
Автор произведения Andrew H. Cobb
Жанр Биология
Серия
Издательство Биология
Год выпуска 0
isbn 9781119157700



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1.8 in risk assessment relating to alien species, prior to their becoming major weed problems.

       1.5.2 Germination time

      Further major problems are evident in sorghum, radish and sugar beet crops. Hybridisation of cultivated Sorghum bicolor (L.) Moench with the weed S. halepense (L.) Pers. results in an aggressive perennial weed that produces few seeds, but demonstrates vigorous vegetative growth. Similarly, hybridisation between the radish (Raphanus sativus L.) and the weed R. raphanistrum (L.) has produced a weedy form of R. sativus with dormant seeds and a root system that is more branched and penetrating than the crop. Lastly, hybridisation of sugar beet (Beta vulgaris (L.) subsp. maritima) has created an annual weed‐beet that sets seed, but fails to produce the typically large storage root. In each of these examples of crop mimicry by weeds, chemical weed control is extremely difficult owing to the morphological and physiological similarities between the weed and the crop.

       1.5.3 Germination depth

      Most arable weeds germinate in the top 5 cm of soil and this is the region that soil‐acting (residual) herbicides aim to protect. Where minimum cultivation or direct drilling is carried out, the aim is to avoid disruption of this top region of soil in an attempt to minimise weed‐seed germination. A small number weed species can germinate from greater depths and this may be due to these species possessing larger seed. A good example of this is wild oat (Avena spp.), which can successfully germinate and establish from depths as low as 25 cm.

      Source: Hance, R.J. and Holly, K. (1990) Weed Control Handbook: Principles, 8th edn. Oxford: Blackwell. Reproduced with permission of John Wiley & Sons.

       1.5.4 Method of pollination

      The survival and growth of weed populations are dependent upon successful pollination. Weed species tend to rely on non‐specific insect pollinators (e.g. dandelion) or are wind pollinated (e.g. grasses), so their survival is not dependent on the size of population of specific insect pollinators. Annual weeds are predominantly self‐pollinated and, when outcrossing does take place, pollination is achieved by wind or insects. This means that a single immigrant plant may lead to a large population of individuals, each as well adapted as the founder and successful in a given site. Occasional outcrossing will alter the genotype, which may aid the occupation of a new or changing niche. Furthermore, many weeds, unlike crops, begin producing seed while the plants are small and young, and continue to do so throughout the growth season. In this way the weed density and spectrum in an arable soil may change quickly.

       1.5.5 Seed numbers

       1.5.6 Seed dispersal

      Source: Adapted from Radosevich, S.R. and Holt, J.S. (1984) Weed Ecology: Implications for Vegetation Management. New York: Wiley; containing information from Hanf (1983).

Weed Common name Seed production per plant
Veronica persica Common field speedwell 50–100
Avena fatua Wild oat 100–450
Galium aparine Cleavers 300–400
Senecio vulgaris Groundsel 1100–1200
Capsella bursa‐pastoris Shepherd’s purse 3500–4000
Cirsium arvense Creeping thistle 4000–5000
Taraxacum officinale Dandelion 5000 (200 per head)
Portulaca oleracea Purslane 10,000
Stellaria media Chickweed 15,000
Papaver rhoeas Poppy 14,000–19,500
Tripleurospermum maritimum