Effects of Plant Density and Cow Manure Levels on Growth Criteria of Bean (Phaseolus vulgaris L.) Cultivars under Mashhad Climatic Conditions

Legumes after cereals are the second source of human food and in Iran they are the second most important food after the wheat. Legumes protein is four times as much as that of grains and 10 to 20 times as much as that of glandular plants. In addition, beans are planted in Iran in a wide area and knowing optimal farming factors can be an important step in increasing them.
One of the most important factors determining the yield of cowpea (Vign asinensis L.) is appropriate plant density. Plant density defines the number of plants per square meter, which in turn determines the area available to each individual plant. For most crops, plant density has a major influence on biomass, crop yield and economic profitability.
In common bean (Phaseolus vulgaris L.), plant density can affect canopy architecture, light conversion efficiency, duration of vegetative growth, dry matter production, seed yield and ultimately, the economic productivity of a crop. Therefore, optimizing plant density, which may be defined by both the number of plants per unit area and the arrangement of plants on the ground, is a pre-requisite for obtaining higher productivity of common bean. However, the other yield components such as number and weight of pods and seeds per plant and 100- seed weight which are established at a later stage in the course of the crop cycle are significantly affected by environmental conditions. Furthermore, the contribution efficiency of these components in the final seed yield is also associated with the number of plants per unit area. Therefore, varying plant density may be a viable alternative to manipulate the productivity of bean under different environmental conditions through their changes in physiological processes. Madani et al. (2008) showed that plant density had significant effect on LAI and shoot dry weight of bean. Moeinit et al. (2009) reported the increase in common bean seed yield with the increase in plant density. Another important factor determining growth indices is manure. Integrated supply of nutrient to plants through planned combinations of organic and inorganic sources is becoming an increasingly important aspect of environmentally sound agriculture. There are reports which show that the application of manure on bean has improved yield and growth indices.
In order to study the effects of plant density and cow manure levels on four common bean cultivars an experiment was conducted as a factorial arrangement based on complete randomized block design with three replications at the Agricultural Research Station, Ferdowsi University of Mashhad, during two growing seasons of 2011-2012 and 2013-2014. The experimental treatments were plant density in three levels (13.13, 20 and 40 plants.m-2), three cow manure levels (0, 15 and 30 kg.ha-1) and four common bean cultivars (Gholi, Akhtar, Naz and D-81083). The plot size was 5×2 meter and the spacing between rows was 50 cm and the seeds were planted in four rows. In order to measure the growth indices, the destructive samplings were carried out every 10 days from 50 cm of row in each plot. All common bean plants were harvested by cutting at the soil surface. Dry matter (DM), leaf area index (LAI), crop growth rate (CGR), relative growth rate (RGR), net assimilation rate (NAR), of bean (Phaseolus vulgaris L.) were measured and calculated accordingly. Plants were then divided into leaf and stem. The areas of green leaves were measured using a Delta-T leaf area meter (Delta-T Devices, Cambridge, England). Then the samples, including stems and leaves were dried in a forced-air oven at 80 ˚C for 48 h and after witch total dry matter (TDM) was measured. The leaf area data was divided to ground area and the leaf area index (LAI) was obtained. The LAI data was fitted to the Gaussian equation (Equation 1) to predict the LAI of common bean in growth season: x (t)= a . Exp {-0.5((x-x0)/b) ^2} (Equation 1)
Where is the time (day), x (t): predicted LAI; a, the maximum LAI during growth season; b is the time that after that the LAI increase exponentially; x0, the time (day) that common bean had the highest LAI.
The sigmoid equation (Equation 2) was fitted to the TDM data and by derivation from this equation, the crop growth rate (CGR) (Equation 3) and relative growth rate (Equation 4) were obtained (Steinmaus and Norris, 2002): W (t) = a / 1 exp {-b (t-m)} (Equation 2)
CGR = b. w (t) {1-(w (t)/a)} (Equation 3)
RGR = b {1-(w (t)/a)} (Equation 4)
Where t is the time (day), W(t), common bean dry matter at time t; a, the maximum total dry matter of common bean; b, the slope of increasing the dry matter; m, the time that common bean had the maximum growth rate and CGR is crop growth rate. The regression analysis was performed by SAS 9.1 and the graphs were prepared by Excel.
The results showed that during the days after planting in all treatments, leaf area index first increased until 56 days after planting and then it had a descending trend. The highest slope of leaf area increase is related to plant density (40 plants.m-2). Although, decreasing plant density from 40 plants.m-2 to 13.13 decreased LAI 29% plants.m-2. The results revealed that the maximum of LAI was obtained in cow manure (30 t.ha-1) (2.57). LAI for four common bean cultivars were different. The cultivar Goli with 2.61 had the greatest LAI. Crop growth rate (CGR) in all treatments first increased slightly and then increased more quickly until 56 days after planting. Then CGR decreased with a sharp slope. Gradual increase of CGR at first was due to insufficient vegetative meristems; however, as the plant canopy was completed and due to more efficient application of light and increase of leaf area the rate of CGR increased quickly so that it was maximized and then it decreased due to increase of interplant competition, decrease of light penetration into canopy photosynthetic organs’ getting late and also assimilates mobilization into grains. In this study the lowest rate of plant growth recorded during growth stage belonged to 13.13 plant.m-2 density, 0 t.ha-1cow manure and Akhtar cultivar) (8.32 g.m-2.day, 7.90 g.m-2.day and 4.26 g.m-2.day, respectively). Relative growth rate decreased as the plant age increased so that at the end of growth season RGR was close to zero. At the beginning of growth stage, due to more penetration of light into the canopy and less shadow of the leaves on each other and the less respiration, RGR is more and its reduction slope is less. As time passes and vegetative and reproductive organs grow more, the shadow of leaves on each other increases and RGR decreases. Plant density with 40 plants.m-2 shows the highest primary RGR and the plant density with 3.13 plants.m-2 shows the lowest one. The highest RGR in the 56th day has been related to 30 t.ha-1cow manure.
We can conclude that indeterminate common bean cultivars such as Goli and Naz showed the greatest growth rate and these findings indicate that the common bean (Phaseolus vulgaris L.) crop has the ability to alter plant size and canopy structure in response to changes in plant density. These strategies can be used as cultural methods to reduce the competitive ability of weeds and maintain common bean growth at acceptable levels. However, there is a need to evaluate numerous common bean cultivars in different locations and years to find cultivars with high competitive ability and stability in yield.
Acknowledgments
The authors acknowledge the financial support of the project by Vice President for Research and Technology, Ferdowsi University of Mashhad, Iran.