Changing the genotypic properties of the cultivated crop can result in tremendous increase in the nutritional value of certain nutrients that are beneficial for humans. However, there have been examples of disproportional increase reaching up to 10–35 fold in some target phytochemicals through genetic selection, which is concerning because such high concentrations are toxic for humans (Stopper et al. 2005; Poiroux-Gonord et al. 2010). An alternative approach that avoids these undesirable ones can be facilitated by grafting. That is independent selection and usage of rootstock and scion traits. Kyriacou et al. (2017) provide a good review of the effects of grafting on fruit quality in vegetable crops, and the biological mechanisms implicated. Initially grafting was mainly used as crop protection against soilborne diseases and is widely used particularly for Cucurbitaceae and Solanaceae crops. Grafting can also be used to alleviate abiotic stresses on vegetable crops, i.e. salinity, nutrient stress, water stress, organic pollutants and alkalinity (Savvas et al. 2010; Schwarz et al. 2010; Borgognone et al. 2013). Grafting of tomato (Solanum lycopersicum L.) is a nice example of elevated nutrient content by using rootstocks. As these rootstocks can reduce the incidence of blossom-end-rot by increase in calcium uptake and calcium fruit content (Khah et al. 2006). In pepper (Capsicum annuum L.) there are examples of increase in antioxidant capacity, iron, β-carotene content and phenolic content (Chávez-Mendoza et al. 2013; Sánchez-torres et al. 2016). Note that the opposite can also be true, that is, grafting can reduce the nutritional value of crops. Especially when grafting enhances growth this can lead to a dilution effect as described in (§5.1.7).
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