Multifunctional polycultures have been theorized to provide a significant array of functions and services including enhanced landscape productivity, ecological services, and economic profitability and stability (Lovell et al., 2017; Wolz et al., 2018). The focus of these systems is on shifting our agricultural paradigm from monoculture, single-purpose, annual cropping systems toward polyculture, multifunctional, perennial cropping systems. A major issue in polyculture and mixed cropping systems is plant-to-plant competition. Light availability is a major component of plant competition resulting in reduced plant productivity and yield for shaded plants. Two distinct ecological niches exist within these larger, multilayered polyculture systems: the overstory and the understory. In the understory, the greatest competition comes from changes in light quantity and light quality (Malézieux et al., 2009). In natural ecosystems, the understory is inhabited by plants that are shade tolerant and capable of efficient utilization of the reduced light amounts and quality (Boardman, 1977; Valladares et al., 2016). By filling individual ecological niches within the built ecosystem, plant competition can be reduced while facilitating increased productivity.
Shade tolerance and intolerance exist along a spectrum, with plants ranging between high light tolerant and high light intolerant, i.e., sun plants or shade plants. The damage to plants under excessive light irradiance includes plant desiccation and ultraviolet radiation stress (Valladares and Niinemets, 2008; Valladares et al., 2016). Shade-tolerant plants tend to be more plastic in morphological adaptations to optimize light capture and increase photosynthetic efficiencies at low light levels (Bayala et al., 2015; Valladares and Niinemets, 2008). Shade-tolerant species maintain a lower relative growth rate than shade-intolerant species under both shade and open conditions (Gommers et al., 2013).
Previous research has been conducted on the effects of shading on the growth and productivity of several berry crop species. In blackberries, Gallagher et al. (2015) found optimal vegetative performance with 30% shade but incomplete or delayed floral bud differentiation with shade between 50% and 80%. When experimenting with an apple-berry polyculture, Rivera et al. (2004) found that blackberry yield was reduced when grown in polyculture, jostaberry yield was unaffected, and the yield of one cultivar of edible honeysuckle was unaffected whereas another showed enhanced yield when cultured with apple. In grapes, shade limited meristem growth and shoot biomass (Greer et al., 2010), and altered leaf morphology (Heuvel et al., 2004). In coffee, plant performance was increased due to improved photosynthesis and morphological changes favoring an increased leaf area index (Bote and Struik, 2011). In blueberries, Kim et al. (2011) found that shading decreased the number of shoots per shrub but increased shoot length, while increasing leaf area and decreasing leaf thickness. As the level of shading increased, plant yield decreased, and the authors concluded that shade levels should be no more than 60% for blueberries to remain economically viable.
Black currants (Ribes nigrum) have an interesting history in the United States (Hummer and Barney, 2002). Ribes spp. and white pines are part of a disease pathosystem caused by the fungus Cronartium ribicola (Geils et al., 2010). White pines are infected only from basidiospores produced by infected Ribes spp. In the United States, the forest and paper products industry successfully lobbied the federal government to eradicate Ribes spp. from the United States. Currants were banned by the U.S. government in the 1920s and existing plantings were eradicated. The federal ban was lifted in 1966 but individual states continued to ban or partially ban currants. For example, Massachusetts continues to ban black currants, but allows the cultivation of other Ribes spp. on a town-by-town basis, whereas Delaware, New Jersey, and North Carolina still ban all Ribes spp. Thus, most U.S. consumers have not tasted black currants nor developed an appreciation for the taste and health properties of currants. In response to the white pine blister rust disease, Canadian breeder A.W. Hunter released three black currant varieties, Consort, Coronet, and Crusader, that carry a resistance gene to the disease (Hunter, 1955).
Black currant is a highly nutritious berry crop with high levels of antioxidants, quick growth to maturity (3 to 5 years), cold tolerance, and early fruiting (Gopalan et al., 2012; Hummer and Dale, 2010). Currants are a berry crop with excellent potential as an understory crop. In the wild, black currants and other Ribes spp. naturally occur in understory environments. They are also known to grow and yield well under shaded agronomic conditions (Djordjevic et al., 2014; Šavikin et al., 2013; Toldam-Andersen and Hansen, 1993); however, empirical research to determine the optimal shade levels for black currant production is limited. Previous studies found reduced lighting affects currant physiology, including impacts on flower initiation, number and length of nodes, and yield (Toldam-Andersen and Hansen, 1993). Shade may also decrease fruit damage due to a reduction of light intensity that causes sunburn and plant stress (Djordjevic et al., 2014). Much of the previous shade research has been conducted to determine the connection between shade and temperature with the goal of reducing plant temperature to improve fruit quality and plant vigor while maintaining adequate yield. Instead of temperature modulation, our research seeks to determine the limitations to understory crops in polyculture systems.
The use of productive understory crops can help expand potential niches in agricultural systems by filling gaps in the landscape to maximize agronomic light capture. The ecological role of currants as an understory species suggests black currants may be a suitable understory crop in a polyculture system that will result in increased productivity per unit land area. The objective of this research was to explore the effects of shade on black currants in a temperate climate by assessing the impact on yield, morphology, phenology, and disease susceptibility.
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