Mechanism of detecting light direction in first positive phototropism in Zea mays L.

The carotenoid content of corn seedlings was reduced by 80–90% with the herbicide SAN 9789 or by using carotenoidless mutants. This caused a decrease in ‘first positive’ phototropism by about 50% without affecting geotropism. This reduction in phototropism is attributed to the decreased light gradient across the albino shoot. Decreased screening should increase the response if a focusing mechanism is used to measure the light gradient, but should decrease the response if a screening mechanism is used. Thus, these data support the hypothesis that screening establishes the light gradient used to measure light direction in ‘first positive’ phototropism.