The growing concerns over heavy metal pollution of soil and water in the current era of industrialization have received increasing attention of researchers worldwide as its consequences can potentially endanger plant and human health. In particular, Cd is one of the most dangerous metals in agricultural soils (Dong et al., 2007). Cd concentration above 3 µg·g−1 in agriculture soil is considered to be unsafe for crop production (Lux et al., 2011). It is suggested that Cd can cause damage even at very low concentrations, and healthy plants may contain Cd levels that are toxic for humans and animals (Chen et al., 2007). Cd is a nonessential element for plant growth but can easily be taken up by plants and its accumulation in plants may result serious human health concern via food chain (Zhou and Qiu, 2005).
In plants, Cd has been reported to severely affect morphological and physiological processes such as leaf chlorosis and roots browning (Hasan et al., 2011; López-Millán et al., 2009), decline of plant growth (López-Millán et al., 2009; Moral et al., 1994), nutritional deficiency, decrease in chlorophyll synthesis, photosynthetic efficiency, nitrogen assimilation, membrane integrity, and enzymes activity (Hédiji et al., 2010; Xu et al., 2012). However, the intensity of Cd stress varies among plant species/cultivars, plant tissues, and its concentrations and duration of exposure (Alexander et al., 2006; Groppa and Benavides, 2008).
In recent years, concerted efforts focused on reducing heavy metals uptake and accumulation in the aerial plant organs. Development of cultivars able to limit uptake and translocation of heavy metals represents a constituent part of environmentally friendly technologies, which allow harvesting clean agricultural products on polluted soil (Alybayeva et al., 2014). However, it is too long process and takes years to breed a variety of desired traits. Moreover, it is still not clear whether plant’s Cd tolerance and hyper-accumulation abilities are genetically independent or related traits (Tsyganov et al., 2007).
One of the possible sustainable strategies for improving plant tolerance to Cd stress, particularly in fruit vegetables, would be by grafting them onto resistant/tolerant rootstocks. Grafting has been primarily used in fruiting vegetables to improve plant tolerance against some soilborne pathogens (Crinò et al., 2007; Lee, 1994) but its application dramatically increased over the years in mitigating the negative effects of other biotic and abiotic stresses with expanding the reasons of grafting. In recent past, grafting has emerged as useful tool to increase plant vigor and yield (Colla et al., 2008), induce higher tolerance to abiotic stress conditions such as salinity, alkalinity (Colla et al., 2010a, 2010b, 2012, 2013), heavy metal, (Kumar et al., 2015; Rouphael et al., 2008b; Savvas et al., 2013), thermal stress (Schwarz et al., 2010), water stress (Rouphael et al., 2008a), and improve fruit quality (Rouphael et al., 2010, 2012).
To our knowledge, few studies are reporting the effectiveness of grafting in reducing fruit and/or shoot Cd contents in vegetables (Arao et al., 2008; Mori et al., 2009; Savvas et al., 2013). A substantial reduction of fruit and shoot Cd content in eggplant (Solanum melongena L.) was achieved by grafting onto Solanum torvum rootstock (Arao et al., 2008). The reduced fruit and shoot Cd content was mainly due restricted root-to-shoot Cd transfer by S. torvum rootstock, as also revealed by the study of Mori et al. (2009). Similarly, Savvas et al. (2013) were able to effectively reduce fruit Cd content in cucumber (Cucumis sativus L.) by grafting onto a commercial interspecific hybrid Cucurbita rootstock, ‘Power’ (Cucurbita maxima Duch. × Cucurbita moschata Duch.), where the reduced fruit Cd content was mainly due to limited Cd uptake by the roots apparatus. However, no information is available up-to-date regarding the impact of grafting and rootstock genotype on the uptake and accumulation of Cd in tomato, which is one of the world’s most important vegetables, grown in both the open field and greenhouse conditions.
Starting from the above consideration, the present study aimed at evaluating the performance of tomato (Solanum lycopersicum L. cv. Ikram), either nongrafted or grafted onto its own roots (self-grafted) or onto selected rootstocks of tomato and eggplant under long-term Cd exposure (0, 25, and 50 µm of Cd). The relative performance of different grafting combinations under Cd stress was determined in terms of plant growth, fruit yield and quality, SPAD index and chlorophyll fluorescence, chlorophyll and carotenoid content, and mineral element composition and partitioning in different plant organs.
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