Growth factors, competition

Weeds directly compete with the crop for water, nutrients, light and other growth factors. Competition for water begins when root systems overlap as they absorb water and nutrients (3). Competition for water depends on the rate and completeness with which a plant utilizes the soil water supply (4). Competition for water usually occurs with other forms of competition. For example, competition between weeds and peas (Pisum spp.) centered on light and water depending on weed height (5). 

The combination of water usage, continual water loss, and potential for a soil depletion zone extending beyond the roots suggests that competition for water might occur earlier in the crop life cycle than for any other soil-supplied growth factor. Competition for water may account for a major part of the crop yield reduction from weeds. Realistically, it seems unlikely that competition for water explains the earliest observed reduction in crop yield. 

Stantchev TS, Broder CC. Human immunodeficiency virus type-1 and chemo-kines beyond competition for common cellular receptors. Cytokine Growth Factor Rev 2001 12(2-3) 219-243. 

According to the law of the minimum. P should only influence the growth and competitive strength of plants if it is the limiting factor. The niche dimension hypothesis predicts that a larger number of limiting resources in a habitat leads to... 

Figure 8.6 Competitive inhibition. Sulphonamide drugs compete with p-amino-benzoic acid, which is an essential growth factor for many bacteria.

A competitive assay could also be used for quantitation. In a competitive assay, unlabeled antigen competes for labeled antigen. Examples include ELISAs for vaccine product antigens, such as recombinant proteins from viruses, or nonvaccine antigens such as growth factors or cytokines. 

Why weeds reduce crop yields cannot be adequately answered. Considerable data have accumulated which relate duration of weed presence and weed density to crop yield. However, such data provide little explanation for why crop yields are reduced. The objectives of this paper are to 1) provide an overview of the time relationship of competition for growth factors and of allelopathy as factors in crop yield reduction and 2) suggest a direct feedback effect on reproduction in response to light as a possible third direct factor in explaining effects of weeds on crop yield. 

Critical periods may vary depending on the area of the brain and the activity involved. Despite the evidence that patterns of neural activity influence the organization of neuronal circuitry, the mechanisms involved remain elusive. Neuronal activity drives the selective survival and sprouting of branches, accompanied by the local addition of synapses, within appropriate areas furthermore, the lack of activity promotes the pruning of synaptic connections from inactive areas (Katz and Shatz, 1996). These competitive processes increase the refinement and precision of maps and require the activity of excitatory receptors (Constantine-Paton et ah, 1990 Antonini and Stryker, 1993) and locally released growth factors (Thoenen, 1995 Inoue and Sanes, 1997). 

The dependence of growth forms on overpotentials originates from the potential dependence of nucleation and growth processes. Competition between nucleation and growth processes is strongly influenced by the potential of the cathode. Thus, major factors determining growth forms are as follows ... 

---