Final answer:
The statement is true with respect to incomplete dominance, where a single allele produces less enzyme resulting in a lighter petal color than in homozygous dominant plants.
Step-by-step explanation:
The statement regarding petal color being lighter due to a single allele for red petals causing the production of about 50% of the normal enzyme is a reference to incomplete dominance. In incomplete dominance, a heterozygous organism with one dominant and one recessive allele will exhibit a phenotype that is a blend of both parental traits. An example of this is when a red flower is crossed with a white flower, resulting in offspring with pink flowers. This occurs because the single allele for red petals does not produce a full complement of the enzyme necessary for creating the red pigment, resulting in a lighter color than the red petals of a homozygous dominant plant.