The graph shows the effect of lead nitrate concentration on seed germination of common wheat (Triticum aestivum) - Scottish Highers Biology - Question 20 - 2023

To determine the lead nitrate concentration where seed germination is 50%, refer to the provided graph. Locate the 50% mark on the y-axis, which corresponds to seed germination.

Next, trace a horizontal line from the 50% mark to intersect the graph line. From the point of intersection, draw a vertical line down to the x-axis. This will indicate the corresponding lead nitrate concentration.

From the graph, it can be seen that at approximately 20 mM, the seed germination reaches 50% of the control. Therefore, it can be concluded that the correct answer is 20 mM.