A New Technology Separating Allelopathy From Competition in Pot Experiments

A new technology is developed to separate allelopathy from competition in pot experiments. Square pots of any sizes may be used. Each pot is divided by a metal or pvc mesh (100-150 micrometer) barrier fi xed from the bottom to pot surface. The barrier divides the pot into two sections fi lled by the same amount of soil mixture. Seeds or seedlings of the donor and receiver species can be planted separately each in a pot section. Density of the donor plant can be varied as required. Nutrient solution can be equally and uniformly added to both pot sections. At an extended period, another similar barrier can be also inserted from the above pot surface to prevent shoot competition. In the control, both receiver and donor plants can be separately grown in divided pots and treated similarly as in other treatments. The technique prevents donor invasion into receiver section and differentiate non-allelopathic from allelopathic species. Failure of receiver species to perform or attain normal growth may be regarded as an indicator on possible allelopathy effects of the donor species. All kinds of comparisons and combinations can be tested and under both normal and stress conditions. A diagram of the new technique separating allelopathy from competition is illustrated. Research Article A New Technology Separating Allelopathy From Competition in Pot Experiments Jamal R Qasem* Department of Plant Protection, Faculty of Agriculture, University of Jordan, Amman, Jordan Dates: Received: 07 March, 2017; Accepted: 18 March, 2017; Published: 20 March, 2017 *Corresponding author: Jamal R. Qasem, Department of Plant Protection, Faculty of Agriculture, University of Jordan, Amman, Jordan, Email:


Introduction
Allelopathy is not accepted among ecologists and many have argued that its effects cannot be separated from other mechanisms of plant interference mainly competition.
Allelopathy is a direct negative chemical effect on one plant resulting from the release of allelochemical into the environment by another while competition is a struggle between both on one or more growth factors in limited supply [1]. A great effort was spent to differentiate between competition and allelopathy effects by many researchers during 1970s, while in the 1990s others appeared convinced in that the effects were often interdependent and could not readily be separated [1,2]. In spite of all these diffi culties, from fi eld studies [3]. Many articles were concluded by similar implications on allelopathy effect as were early reported. This violate researchers, worldwide, deep concern and criticism on results claimed as due to allelopathy. Most results were theoretical, descriptive and based on observed symptoms and the morphological severity of allelopathic effects with no supporting fi rm evidence from nature's allelochemicals [4] but a rather refl ection of deep realization and speculation. Some were entirely reported cases from natural or agroecosystem (most based on visual observations) on the suffering of certain plant species from another and explained as due to allelopathy. However, one of the misleading conclusions is that based on results derived from laboratory studies as in general were mainly conducted using plant extracts, root exudates or foliage leachates of dead plant tissues on artifi cial media.
Extracts are commonly used in allelopathy studies to explain the observed harmful effects of certain plant species on other/s under fi eld conditions [1], and different pioneer researchers are not convinced by the results obtained using this technique [5,6]. Although it is simple, widely used and preferable but its credibility is highly questionable since extracting all cell materials including releasable and unreleasable chemicals under normal conditions [7,8] disadvantage of plant box method [9], relay seeding technique [10] and equal compartment agar method [11] is the use of agar as the culture medium, which assumed that allelochemicals would directly act on the target plants, thus overlook any possible modifi cation of these chemicals in soil. Fujii et al. [12], developed "rhizosphere soil method" for the evaluation of allelopathic activity of plants, which is agar medium included dry leaves of donor plants or culture soil and seeds of the target plant are placed on the surface of agar medium opposing the spirit of allelopathy science.
Rigorous proof of such experimental results under glasshouse and fi eld conditions is necessary to avoid any misleading results and false conclusion on the allelopathic activity of certain plant species. Willis [13], listed conditions required to provide evidence on alleloapthy operation and even though all do not prove that allelopathy is operative but only offers the most reasonable explanation of the observed pattern.
Although workers claimed "based on techniques used" or assumed that results obtained are due to allelopathy under specifi c conditions but critical analysis and deep thinking in the used techniques may reveal more involvement of competition [5,14]. Since separation or complete isolation of competition from allelopathic effect is diffi cult in the fi eld but a well and carefully designed technique remained possible to develop considering positive and negative aspects involved in plants interactions in nature. However, as mentioned earlier the available techniques for allelopathy studies are many and varied [5,15,16] and some have been already reviewed, evaluated and previously criticized [5,15]. While most, if not all, followed in laboratory studies are artifi cial and do not represent actual natural situation, others failed to eliminate allelopathy interference/overlapping with competition or lack natural conditions or requirements to operate. In addition, different factors may be also involved in the effect obtained using such techniques. For example, with use of glasshouse potted soils or hydroponics designs more problems emerged which may negate allelopathy existence and thus failure to eliminate competition interferences. For example, pot screening method [17] without the use of pre-germinated seeds may result in varied densities of both donor and receiver species, whereas allelopathic activity is density dependant [18,19]. Considering all these diffi culties, a drip irrigation technique was developed and introduced to separate competition from allelopathy between weeds and tomato plants grown in pots placed in channels and irrigated by nutrient solution [20]. These workers demonstrated that one weed species has an allelopathic effect through root exudates released into the circulated nutrient solution while similar effect of the other weed species was not possible although both were reported as of allelopathic infl uence.
Field studies also failed to separate allelopathy from competition using smother or highly competitive crops in forms of living or dead mulches or their cropping systems in intercropping design and more complications were added and challenge separation of allelopathy from competitions. A recent thorough review of applied allelopathy for weed management is available (14], allelopathy techniques were also reviewed and evaluated [15] and new proposed methodologies and techniques were suggested. Moreover, differences in results obtained between laboratory and glasshouse experiments and between these and some observed natural phenomenon on certain weed species were reported [15].
In the present work, a new technology that allows better separation of allelopathy from competition between different plants species in potted-soil under glasshouse condition is introduced, sketched illustrated and discussed.

Description of the technique
PVC square pots of 20 by 20 cm may be used in this technique with holes made for drainage in the bottom. A compartment (barrier) of the same pot height, made from metal or net mesh of 50-150 micro-millimeter pores and hard enough to withstand soil pressure can be inserted from pot surface down to the bottom. This compartment divides the pot into two identical sections. The net/perforated compartment may be also made from different materials (leather, jute, PVC or plastic}. If made from PVC, then it can be built in with the pot or slides inside from the bottom to increase or decrease space allocated to donor or receiver plants. The pore size can be varied according to roots diameter or thickness of studied species with the purpose of prevention root physical intermingle or penetration into the section of the partner species. This would keep roots of receiver plants separated from those of the donor and the only contact between both is mainly through root exudates possibly containing allelochemicals from donor plants, partially leached along the opposite pot section and received by the receiver plants through the mesh compartment. The pot may be vertically placed or made more or less sloppy toward the receiver section to allow normal fl ow of exudates into the pot section grown by the receiver species. Flaw of exudates may occur along the whole perforated barrier. This technique also allows planting donor and receiver plants at different levels (heights) in the pot and comparison of receiver plants performance when donor plants planted at different heights or in different section sizes.
In order to avoid any limitations in mineral nutrient supply, a full strength nutrient solution can be added weakly to each pot section and plant responses may be kept under observation throughout the whole study period. Moisture level in the soil can be maintained by adding water for each section, separately. The same amount of water is also applied to all pots and sections or may be made differ between receiver and donor plants based on their requirements. The technique allows studying different densities of donor species and examining their infl uence on germination and growth of the receiver plants. It may be also used to study the effects of root exudates of donor plants introduced into the system as transplants and examine their effects against receiver species grown from seeds or seedlings.
Since stress conditions are important in production of alleloachemicals [21], nutrient solution may be added at different strengths or suspended only on donor plants to DOI

Results
The diagram of the technique is drawn and illustrated in Pot screening [17] without use of pre-germinated seeds may result in varied densities of both donor and receiver species, whereas allelopathic activity is density dependant [18,19].
Grinding plant tissues and their addition into potted soil may change soil structure, texture and compaction and thus soil water holding capacity, emergence and growth of the small seedlings or may exert high pressure on imbibed seeds ready to germinate. Regulation of allelopathy by soil factors such as soil texture, soil microbes, and soil chemical components were well documented [29][30][31][32][33][34][35].
Addition of plant materials as a percentage or ratio of the total soil volume/weight also create undetectable problems at which certain soil volume with its contents of nutrients is replaced by similar volume/weight of dried/fresh residues of the allelopathic species of lower or higher content or concentration of nutrients than that of the replaced soil portion.
This creates a reduction gradient in the amount of nutrients available, increased with amount/volume of soil removed and is negatively correlated with the amount of plant tissues added. In the fi eld and in different cases the applied allelopathic plant materials were harmful to both weed and crop plants or even more to crop plants [37,38] and many of the reported work failed to separate allelopathy from competition. Low density of Cucurbita pepo intercropped with Zea mays reduced weed biomass while high density was detrimental to both weeds and Zea mays [39]. Some or all benefi ts of soil mulch were eroded by phytotoxic leachates from residues of Secale cereale and Trifolium subterranean as cover crops [40]. Water soluble toxic substances of wheat straw mulching leached into the soil under natural conditions inhibited Zea mays growth and the effect was more pronounced under wet conditions [41] while wheat residues stimulated germination and growth of summer weeds [42].
Intercropping system ignored the complementary value of intercropped species and their differential responses and requirements for growth factors, while root exudates, or the symbiotic relationship, are other factors to be considered.
Cover crops may act through allelochemicals, competition or other mechanism including stimulation of microbes' allelochemicals, physical barriers, shading effects and changes in soil physical properties [43]. These are characterized by strong abilities to cover the soil surface and to effectively smother weeds [44]. Legumes covers elevate soil nitrogen level, availability and uptake. These crops are less competitive than others like cereals and crucifers. Soil mulch with living Vicia villosa has been reported to improve various soil physical properties including increase moisture, stabilization of soil temperature, increase water permeability and drainage and decrease soil hardness [45]. Changes in physical environment were responsible for the reduction in Circium vulgare seedlings emergence in the presence of leaf litter [46]. Good summer weed suppressions was achieved using Mucuna prurience,

Cortalaria juncea, Cortalaria spectabilis, Panicum maximum and
Glyine max as green manure while the effect was explained as due to the quick growth and good ground cover of these species [47].
Soil cover act by denying light, and prevention of photosynthesis. The ability of emerged seedlings to establish highly depends on thickness of the straw mulch layer.
All of the above mentioned factors should be taken into consideration dealing with role of allelopathy using cover crops for weed management.

Conclusion
The new developed technique has taken into considerations all diffi culties encountered using presently and widely