Physiological effects of short-term copper stress on rape (Brassica napus L.) seedlings and the alleviation of copper stress by attapulgite clay in growth medium

Rape (Brassica napus L.) seedlings grown in vermiculite-based medium were subjected to short-term copper stress. With the increase of exogenous CuCl2 concentration, photosystem II (PSII) photochemistry and the chlorophyll (Chl) content of rape seedling leaves decreased, while Cu (copper) content of the seedlings, the levels of the soluble protein content, reactive oxygen species (ROS) production, and activities of antioxidant enzymes of the seedling leaves increased. Under the condition without copper stress, there was no significant differences in the measured physiological indexes between the seedlings grown in vermiculite and those grown in the attapulgite clay (AC)/vermiculite mixture with the volume ratio at 1:80, 1:50, or 1:30. After subjected to short-term copper stress, the rape seedlings grown in the mixture of AC/vermiculite had higher levels of Chl content and PSII photochemistry but lower levels of the Cu content, soluble protein content, ROS production, and activities of antioxidant enzymes, compared with the leaves of the seedlings grown in vermiculite. The volume ratio of AC/vermiculite at 1:50 or 1:30 seemed to be the most optimized content in alleviating the physiological effects by the short-term copper stress. These results indicate that AC at moderate content in the growth medium can alleviate the physiological stress of the rape seedlings when excess Cu emerges in the growth medium.