Copper toxicity and sulfur metabolism in Chinese cabbage are affected by UV radiation

Biomass production, dry matter content, specific leaf area and pigment content of Chinese cabbage were all quite similar, when plants were grown in the absence or presence of UV-A + B (2.2 mW cm−2). Elevated Cu2+ concentrations (2–10 μM) in the root environment and UV radiation had negative synergistic effects for Chinese cabbage and resulted in a more rapid and stronger decrease in plant biomass production and pigment content. The quantum yield of photosystem II photochemistry (Fv/Fm) was only decreased at ≥5 μM Cu2+ in the presence of UV radiation, when leaf tissue started to become necrotic. The enhanced Cu toxicity in the presence of UV was largely due to a UV-induced enhanced accumulation of Cu in both roots and shoots. An enhanced Cu content strongly affected the uptake and assimilation of sulfur in plants. The total sulfur content of the root increased at ≥2 μM Cu2+ in presence of UV and at 10 μM Cu2+ in absence of UV and that of the shoot increased at ≥2 μM Cu2+ in presence of UV and at ≥5 μM Cu2+ in absence of UV. In the shoot it could be attributed mainly to an increase in sulfate content. Moreover, there was a strong increase in the water-soluble non-protein thiol content upon Cu2+ exposure in the root and, to a lesser extent in the shoot, both in the presence and absence of UV. The regulation of the uptake of sulfate responded to the occurrence of Cu toxicity directly, since it was more rapidly affected in the presence than in the absence of UV radiation. For instance, the expression and activity of the high affinity sulfate transporter, Sultr1;2, were enhanced at ≥2 μM in the presence of UV, and at ≥5 μM Cu2+ in the absence of UV. In the shoot, the expression of the vacuolar sulfate transporter, Sultr4;1, was upregulated at ≥5 μM Cu2+ in the presence and absence of UV whilst the expression of a second vacuolar sulfate transporter, Sultr4;2, was upregulated at 10 μM Cu2+ in the presence of UV. It is suggested that high Cu tissue levels may interfere/react with the signal compounds involved in the regulation of expression and activity of sulfate transporters. The expression of adenosine 5′-phosphosulfate reductase in the root was hardly affected and was slightly down-regulated at 2 μM in the presence of UV and at 10 μM in the absence of UV. The expression and activity of sulfate transporters were enhanced upon exposure at elevated Cu2+ concentrations; this may not be simply due to a greater sulfur demand at higher Cu levels, but more likely is the consequence of Cu toxicity, since it occurred more rapidly in the presence compared to the absence of UV.