Cadmium toxicity in tomato (Lycopersicon esculentum Mill.) : deepenings of a morphological, physiological and biochemical characterisation

Cadmium (Cd) toxicity was investigated in a non-tolerant plant : Lycopersicon esculentum Mill. We tried to gain a deeper understanding of the relationship between growth and development on the one hand and cadmium accumulation on the other hand with emphasis on some specific morphological, physiological and biochemical parameters. Under a sudden increase of Cd concentration in the nutritive solution (250 µM CdCl2), an increase of Cd concentration occurred in the shoot part (until 2.5 mgCd gDryWeight-1) and growth was completely inhibited mainly through its Leaf Area Ratio component. Pigments concentrations, stomatal conductance, water potential and water content were decreased. However, impact on net CO2 assimilation, transpiration rate and fluorescence parameters remained limited. Osmotic adjustment was detected and could allow tomato to avoid a secondary Cd-induced water stress. Tomato also decreased net transport of Cd from roots to shoots thus avoiding heavy metal toxicity in the shoot part. This showed that despite growth inhibition, tomato developped several strategies linked to both avoidance and tolerance mechanisms. Secondly, we characterised the resistance mechanisms involved in roots, young and mature leaves of tomato in response to toxic Cd concentration (100µM CdCl2). The most striking result concerned the increase in root growth under Cd treatment. We hypothesised a link between root growth and auxin metabolism, and/or water transport. Lipid peroxidation increased in both mature leaves and roots, while protein carbonylation increased in mature leaves and decreased in roots. Protein carbonylation was correlated with growth inhibition but was not correlated with malondialdehyde concentrations. Roots seemed to privilege the primary defensive response such as phytochelatins sequestration, thus limiting the oxidative stress in this part of the plant. On the contrary, the shoot part (young and mature leaves) seemed to use the antioxidative mechanisms to cope effectively with Cd stress. Finally, we tested the possible counteracting role of gibberellic acid (GA) on growth and its interaction with the antioxidative system under Cd stress (100 µM CdCl2). We demonstrated that although GA had no impact on the antioxidative mechanisms, it restored the growth of plants treated with Cd, without any decrease in Cd accumulation. We postulated a possible action of GA on the cell wall extensibility and/or the regulation of the cell cycle.