Plants like tobacco can produce more proline when exposed to too much copper, which can harm them. This happens because the excess copper affects how proline is made and broken down in the plant. However, the increase in proline is not directly caused by the copper itself, but by changes in water levels and a hormone called ABA. This was shown by studying tobacco plants in pots and floating leaf discs treated with copper. The excess copper made the plants produce more proline, but in the leaf discs, it actually decreased proline levels. So, the way plants respond to copper stress can depend on how they are treated and the conditions they are in.