BIOACCUMULATION OF TRACE METALS AND THE COMPROMISED NUTRITIONAL INTEGRITY OF TOMATO SOLANUM LYCOPERSICUM

Abstract

Heavy metal contamination of soils and irrigation water is a major problem for food security and crop quality worldwide. Tomato (Solanum lycopersicum) is a high value vegetable fruit that is very popular in both fresh and processed forms and is readily contaminated by the uptake of toxic metals (cadmium, lead, arsenic and nickel) from contaminated growing media. This review aims to critically discuss the pathways of accumulating, translocating and accumulating heavy metals in tomato and to review the latest scientific data to assess the impact of metal stress on fruit quality attributes. More than 150 peer reviewed studies have shown that there is a specific pattern for heavy metal accumulation in the roots, leaves and fruits. Cadmium and lead, however, are mobile in the phloem and can be directly transported to developing fruits. Physiologically, metal-induced oxidative stress hinders the photosynthetic efficiency and affects phytohormone signaling. With respect to fruit quality, there have been a number of studies indicating that higher Cd and Pb levels significantly reduced the lycopene content by downregulating the expression of phytoene synthase gene. The soluble sugars and titratable acidity are also decreased which reduces the flavor. Cell wall pectin metabolism changes and causes fruit firmness to decrease. While this may be a stress response to some phenolic compounds, this does not counterbalance the loss of some important nutraceuticals. Fruit pulp critical threshold values are often less than maximum permissible limits for fruit quality degradation. Finally, high level of accumulation of heavy metals in tomato fruits represents a threat to human health because of their consumption as food and significantly reduces the organoleptic and nutritional quality. To ensure consumer health and marketable quality of tomatoes, integrated mitigation measures such as biochar amendments, resistant cultivars and microbial bioremediation are urgently required.