The diagram shows four experimental setups (A\text{A}A to D\text{D}D) used to investigate the distribution of the plant hormone auxin (indole-3-acetic acid, or IAA) in oat coleoptile tips. In each setup, the coleoptile tips were incubated on agar blocks, and the relative concentration of IAA that diffused into the blocks was measured (indicated by the numerical values).
Which of the following statements provides the most direct evidence that unilateral light causes lateral transport of IAA from the illuminated side to the shaded side, rather than the chemical destruction of IAA on the illuminated side?
Comparing setup A\text{A}A and setup B\text{B}B shows that the total amount of IAA produced remains constant, but does not provide evidence of lateral transport.
Comparing setup C\text{C}C and setup D\text{D}D shows that blocking lateral transport at the apex prevents unequal distribution, while allowing it results in an asymmetric distribution (35:6535 : 6535:65) without reducing the total IAA yield (100100100).
Setup C\text{C}C shows that unilateral light directly destroys 50%50\%50% of the IAA on the illuminated side, which is prevented by the glass barrier.
Setup D\text{D}D shows that unilateral light destroys IAA on the illuminated side, reducing its relative concentration to 353535 while leaving the shaded side unaffected.
