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Total chlorophyll and nutrients content in bean plants and weeds in competition

Plants compete for several resources on soil, including water, light and nutrients which can affect crop yield and also weeds growth. Under the hypothesis that in different densities and weed communities, crops and weeds undergo changes in their nutritional contents of macro and micronutrients, the aim of this study was to quantify the nutritional contents of bean plants and weeds cultivated in increasing densities (0, 74, 147, 221 and 295 plants m-2) and the effect of the association of plants (isolated bean, bean + Urochloa plantaginea, bean + Bidens pilosa and bean + Urochloa plantaginea + Bidens pilosa). The contents of macro and micronutrients were evaluated. The increase in plant density promoted a decrease in the total chlorophyll content of bean plants and weeds, as well as a decrease in macronutrients (K and P) and micronutrients (Zn, Fe, Mn and Cu). In the arrangement where the greatest number of species were cultivated simultaneously and in increasing densities, a greater reduction in total chlorophyll content and in macro and micronutrient contents of the three species (crops and weeds) were observed.


Introduction
Despite being the world's largest producer of beans (Phaseolus vulgaris), Brazil presents low average yield (CONAB, 2016). The inadequate management of weeds is among the several reasons for the low yield in the country. Several studies, such as the ones performed by Salgado et al. (2007) and Borchartt et al. (2011), confirm that weeds can promote losses of up to 93% of bean production.
Bean plants are cultivated in several types of production systems and periods of the year. They suffer interference from a wide variety of weeds (Pereira et al., 2015, Teixeira et al., 2009). This interference represents the sum of negative interactions among plants, including competition and allelopathy (Agostinetto et al., 2008;Sausen et al., 2009). It is necessary to understand the effects of each individual component of the interference to evaluate the best management strategy. Plants compete for a wide variety of soil resources, including water and at least 20 essential nutrients (White, 1995. Therefore, the competition for nutrients can reduce the scarce crop resources (Carvalho et al., 2007).
According to Procopio et al. (2004), although there is great knowledge and technological advances regarding mineral nutrition of cultivated species, the lack of studies in mineral nutrition of weeds undermines the understanding of factors that interfere in the competition for nutrients between crops and weeds. Desmodium tortuosum (Procópio et al., 2005) and Nicandra physaloides populations (Ronchi et al., 2003) were able to accumulate, respectively, up to 10 and 15 times more the relative quantity of phosphorus than weeds. Pereira et al. (2012) observed that cassava plants showed better responses to the increase of phosphorus doses than Bidens pilosa and Urochloa decumbens.
Several authors report morphophysiological characteristics that may be directly related to the superior competitive ability of crops, such as: shoot dry matter and structure (Barroso et al., 2010), spatial distribution (Dias et al., 2010), germination and emergence (Carvalho & Christoffoleti, 2008), growth habit (Teixeira et al., 2009), soil cover (Braz et al., 2006) and interception of solar radiation (Carvalho & Christoffoleti, 2008). However, the capacity of supply and allocation of nutrients vary with the species involved.
Under the hypothesis that in different densities and weed communities, the crop of interest and weeds undergo changes in their nutritional contents of macro and micronutrients, the aim of this study was to quantify nutritional contents of bean plants and weeds cultivated in increasing densities and the effect of plant association (community).

Materials and Methods
The experiment was performed from The soil texture was classified as medium.
The chemical analysis showed the following results: pH (water) of 5.0; organic matter content of 4.7 daq kg -1 ; P, K and Ca of 1.3; 47 and 2.4 mg dm -3 , respectively; Mg, Al, H + Al and effective CEC of 1.2; 0.1; 7.1 and 3.8 cmolc dm -3 , respectively. The soil sample was corrected and fertilized according to the analysis, incorporating limestone and fertilizing one month before the experiment implantation. 3.0 g dm -3 of dolomitic limestone and 3.0 g dm -3 of 4-14-8 (N-P2 O5-K2 O) were applied. Data were submitted to analysis of variance, and means, when significant, were compared by Tukey´s test at 5% probability.

Results and Discussion
In bean + brachiaria and bean + The determination of relative chlorophyll content through chlorophyll meter is used to predict the need for nitrogen fertilization in several crops, including coffee (Godoy et al., 2008;Reis et al., 2006); potato (Coelho et al., 2010) and tomato (Ferreira et al., 2006). The chlorophyll content in the leaf is used to predict the nutritional level of nitrogen (N) in plants, due to the fact that the amount of this pigment correlates positively with N content in the plant (Godoy et al., 2008).
In relation to potassium content ( (Table 3). The nutrient competition is affected by water content in the soil, by specific aspects of the competitors and also by differences in growth habit and nutrient requirement of the species (Ferreira et al., 2013).
The ability to remove nutrients from soil and the required amounts vary not only with the cultivar, but also with the degree of competition (Cury et al., 2013). However, when bean plants were cultivated with beggartick, no difference was observed among the densities (Table 5).   (Table   6). It is known that micronutrients are limiting to bean production. This limitation may be due to its unavailability, deficient supply or presence of weeds (Ronchi et al., 2003). Thus, the last two processes may have occurred possibly due to the omission of this nutrient in the substrate suitability for nutrition, and / or the competition of this resource with beggartick and brachiaria. It can be verified that the crop negatively affected the accumulation of nutrients of weeds.
Thus, the superiority of weeds in the competition for these resources may be due to the occurrence of high density of these weeds in the area (Santos et al., 2003). The interaction of a single weed species at different densities interferes according to the weed density Carvalho & Christoffoleti, 2008). The competition also depends on the crop and weed population and it is also important to emphasize that intraspecific competition may represent greater losses to the crop (Dias et al., 2010).

Conclusion
The increase in plant density promoted a decrease in total chlorophyll content of beans and weeds, as well as reduction in the values of macronutrients (K and P) and micronutrients (Zn, Fe, Mn and Cu).
The arrangement in which the highest number of species was cultivated simultaneously and in increasing densities promoted a greater reduction of total chlorophyll, macro and micronutrient contents of the three species.