Amelioration of heavy metal and nutrient stress in fruit vegetables by grafting

The response of grafted vegetables to stress conditions owing to the nutrient status, and the presence of heavy metals in the root environment may be different than that of self-rooted plants, depending mainly on the rootstock genotype. Several studies have indicated that some rootstocks are capable of restricting the uptake and/or the transport of heavy metals (e.g. Cd, Ni, Cr) and micronutrients (e.g. Cu, B and Mn) to the shoot, thereby mitigating the stress caused by excessive external concentrations of them. However, other mechanisms driven by the root system, such as detoxification of harmful elements or hormonal signals modifying gene expression in the scion, seem to be involved in the mitigation of stress caused by excessive external nutrient or heavy metal concentrations. On the other hand, the uptake and/or utilization efficiency of macronutrients (N, P, K, Ca and Mg) by plants may be enhanced by grafting onto some rootstocks. This is ascribed mainly to the root characteristics of these rootstocks, which are more vigorous than those of highly productive cultivated varieties. However, other mechanisms implicated in the efficiency of active nutrient absorption by the roots, as well as signals arising from the scion, which are mainly governed by sink demand, may also enhance nutrient uptake and utilization. The higher efficiency of some graft combinations of fruit vegetables to take up and utilize nutrients may mitigate yield losses owing to shortages of these nutrients in the root environment of plants and restrict nutrient losses due to leaching. Nevertheless, it is important to specifically test each grafting combination and not merely each rootstock for its ability to ameliorate nutrient or heavy metal stress because in many instances the responses depend on the rootstock/scion combination. This report gives an overview on the prospects and restrictions of grafting as a means to minimize the negative effects of heavy metals, excessive nutrient availability, nutrient deficiency, and alkalinity stress on vegetable crop performance taking into consideration agronomical, physiological and biochemical aspects.