The cultivation of crops has
imposed a new selective pressure on plants, in particular on weed species. While
crops have been genetically modified and selectively bred, many species of
weeds have been able to keep up, adapting to the changing morphology and
phenology of crop plants to the point where they are almost indistinguishable. This
mistaken identity has continued the survival of crop mimics (Vavilovian
mimicry); they reap the benefits of fertilisers, pest management and
irrigation, all whilst being undetected by the farmer. These mimetic forms of
weeds are most likely subject to the pressures of hand weeding practices in
Asian crops, and in some cases have been found to be more similar in attributes
to the crop than to its closest species relative. This high degree of specialization
developed in weed mimicry largely restricts species distribution, as the weeds
can only occur where the crops are grown and have often developed to rely
heavily on the cultivation conditions. This is considered strange for a weed
species which are generally characterised by their ability to live in a variety
of environments, natural and disturbed. Some weed populations have even developed
the ability to mimic a specific phase of the life history of a crop when weeds
are most likely to be removed, preventing eradication (Barrett, 1983).
Mechanization and herbicides are
the most influential pressures on crop mimics in the modern era. Chemical selection
has vastly altered the composition of crop weed floras, and the increased use
and sometimes incorrect use of these chemicals has led to the alteration in the
genetic structure of weed populations. There is evidence that weed populations
have evolved phonological patterns which aid in the optimization of their
survival. For example a weed of maize, Zea
Mexicana, has developed a germination inhibitor which prevents the plant
from germinating in fallow years, protecting the plant from herbivores (Barrett, 1983).
Crop mimics have developed such
strong relationships with their model crops that in many cases they are unable
to survive without the crop. As agriculture has become more mechanized and
chemically controlled there has been a decline in crop mimics, and it is
hypothesised that with the decline of these mimics other agroecotypes of weeds
will take their place (Barrett, 1983).
Very interesting. If some of these weeds are so difficult to detect, are they also producing substances similar to the crop plants in terms of what we would consider “food” (e.g. structures similar to corn, or structures similar to wheat)? Do these crop mimics cause any damage to the crops themselves (e.g. through competition of resources)? Intriguing.
ReplyDeleteSome varieties have been observed to provide food products and other don't. Occasionally the food product is good (for example the modern variety of rice now used was once considered a weed), but more often it is of inferior quality and can lower the value of the crop harvests. They do cause competition to other plants, but it not of high significance because if plants are seen to be suffering the farmer can always just apply more resources.
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