Final answer:
A Neurospora valine auxotroph requires valine for growth because it has a mutation that prevents it from synthesizing valine on its own. So, the correct answer is option 1.
Step-by-step explanation:
The correct statement about a Neurospora valine auxotroph is: 1) It requires valine for growth. An auxotroph is an organism that cannot synthesize a particular organic compound required for its growth. In the case of a valine auxotroph, this means that the organism cannot produce valine on its own due to a mutation and thus must obtain it from the environment to grow.
This condition was observed in experiments like those conducted by Beadle and Tatum in their seminal work on Neurospora crassa, which helped formulate the one gene-one enzyme hypothesis.
They investigated various metabolic mutants, or auxotrophs, by using a minimal medium that lacked certain amino acids. When spores were not able to grow on minimal medium but showed growth upon the addition of a specific amino acid, like valine, it was clear that these spores lacked the ability to synthesize valine due to a genetic mutation in the biosynthetic pathway of that amino acid.