starting with one molecule of isocitrate and ending with fumarate, what is the maximum number of atp molecules that could be made through substrate-level phosphorylation?

Ascience research group is testing a new type of plant food. one trial is conducted in a controlled experiment. the data from the trial show that the plant that received the new food grew faster than the control plant. the group announces that the new plant food makes plants grow faster. what is the main weakness in this scientific claim?