To determine if a particular plant is homozygous or heterozygous, you would have to test cross with a to make the check.
homozygous recessive
heterozygous dominant
homozygous dominant
purebred

You would have to test cross with a HOMOZYGOUS RECESSIVE to make the check

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature