Tomato (Solanum lycopersicum L.) and bell pepper (Capsicum annuum L.) are important vegetable crops that belong to the Solanaceae family. Florida is the leading state in fresh market tomato production and the second-leading state in bell pepper production, with an average production of 0.4 and 0.2 million metric tons, respectively (USDA-NASS, 2016). In the United States, the total tomato production has decreased from 0.7 million metric tons in the year 2000 to 0.4 million metric tons in 2015, and bell pepper production has also declined significantly in recent years (Biswas et al., 2017). Potential reasons for this decline are the loss of methyl bromide as a pest management tool, increased pest management costs, and increased competition from foreign markets (Guan et al., 2017). As a result, Florida growers are faced with low prices for fresh market produce, high production costs, and decreased profits. Intercropping may be a potential management option to address some of these challenges.
Intercropping is the planting of more than one crop on the same bed at the same time, a method that lowers the input use and costs per crop. Intercropping increases resource use efficiency, crop yield, and income stability (Raseduzzaman and Jensen, 2017). It lowers the chances of complete crop failure, as it is highly unlikely that two or more crops have the same susceptibility to pest and disease incidence, and the method thus ensures crop stability (Boudreau, 2013). Monocrops are more likely to be subjected to crop failure due to biotic or abiotic stresses than intercrops (Rao and Willey, 1980; Raseduzzaman and Jensen, 2017). Intercropping has also proven to increase the economic returns for growers compared with monocropping (Njoroge et al., 1993; Sandhu et al., 2020b, 2020a). However, there are some challenges associated with intercropping, such as the management of competition for nutrients, water, and light, which varies with many factors, such as plant population density, resource use efficiency, as well as temporal and spatial plant interactions (Gebru, 2015).
Plant population density is an important factor that can determine the success or failure of intercropping. It is defined as the number of plants per unit area and can affect the vegetative growth and yield of crops grown as a monocrop or an intercrop. The selection of the optimum plant density or spacing ensures optimal crop yield can be achieved (Fortin et al., 1994; Gebru, 2015). Competition can be among plants of the same species (intraspecific) or plants of two or more species (interspecific). A meta-analysis of multiple cropping studies found that intraspecific competition tends to be much stronger than interspecific competition. In some cases, interspecific competition can result in positive effects, whereas intraspecific competition tends to have negative consequences (Adler et al., 2018; Sandhu et al., 2020b). This could be because of “resource complementarity,” which occurs when two intercropped plants are not competing for the same resource due to niche differentiation (Snaydon and Satorre, 1989). Intraspecific competition increases with population density because plants that share the same niche compete for resources such as water, nutrients, and light (Goldberg, 1990). Plant density needs to be optimized to establish effective intercropping systems.
Standard spacing for tomato and bell pepper production, when planted as sole crops, is well established (Freeman et al., 2019; Ozores-Hampton et al., 2015), but little information exists on the optimal plant spacing when intercropping. Given that intercrops have better resource use efficiency than sole crops, there is the potential that the overall plant density could be higher with an intercrop than with a monocrop (Hauggaard-Nielsen et al., 2006).
A study on tomato and black nightshade competition reported that nightshade competed for light, nutrients, and reduced fruit yields by ≈36%, but only when nightshade height surpassed the tomato height (Gonzalez Ponce et al., 1996). The same authors reported that nightshade competed with bell peppers at a much greater extent than tomatoes and reduced pepper yields by 93%. These findings suggest that pepper is more susceptible to competition than tomato and that this difference might be associated with the height differential. There are no known published research papers that evaluate planting density for intercropped tomato and bell pepper. However, bell peppers have been successfully intercropped with different vegetables, including vegetables from the Bromeliad, Rose, Citrus, Legume, and Nightshade families, with improved insect management and higher yields (Kahn, 2010).
We hypothesized that tomatoes or bell peppers planted as a sole crop would have higher yields compared with intercropped bell peppers and tomatoes, and that yields would decrease with increased planting density. The objectives of the study were a) to compare the growth and production of tomatoes and bell peppers at different plant densities both as an intercrop and a monocrop, and b) to determine the competitive effects of intercropping on the yields and morphology of tomatoes and bell peppers.
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