Plant species and pH dependent responses to copper toxicity

High copper (Cu) levels in soil caused by anthropic activity, as in vineyards, can cause phytotoxicity and reduce plant productivity. However, the distribution of Cu chemical-species in the soil and then, the level of its availability for crops can be considerably altered by pH and root exudates in the rhizosphere. The study aimed at investigating the influence of both pH and excessive Cu concentrations on physiological and morphological responses of two different plant species, cucumber (Cucumis sativus L.) and oat (Avena sativa L.). The experiment was conducted with two plant species, cucumber (Cucumis sativus L.) and oat (Avena sativa L.), three pH levels (4.5, 6.0 and 7.5) and three Cu concentrations (0.2, 5 and 50 µM). Both plant species showed the capacity to change the pH of the solution, but oat plants showed higher capacity to alkalize the nutrient solution than cucumber plants to reduce Cu availability. Excluding a few exceptions, exudation increased with increasing Cu supply in both plant species studied. Exudation resulted also pH dependent, particularly visible for flavonoids in cucumber plants. Overall, flavonoid release resulted higher in cucumber than in oat while the release of phenolics was higher in oat compared to cucumber. Cucumber and oat plants supplied with 50 µM Cu and pH 4.5 exhibited the lowest growth and development because of higher Cu availability. On the other hand, nutrient solution at pH 7.5 and high Cu concentration stimulated root development especially in cucumber plants. Copper interfered with the accumulation of both macro and micronutrients, yet the synergisms and/or antagonisms were species and pH dependent. Both plants presented the highest Cu concentration in shoots and roots at 50 µM Cu in all the pH values solution, especially in the roots. Oat plants showed more resistance to high Cu concentrations in solution over a wide range of pH exhibiting a better plant development and more Cu-induced synergistic responses to nutrients than cucumber. Oat plants can be used in Cu-contaminated soils as phytostabilizer plants.