Besides its physical function of supporting the leaf blade and transporting water and nutrients, the cotton petiole has been found to have a nutritional role:reducing the nitrate nitrogen (NO3-N) which flows through it. Using an in vivo assay technique adapted for the cotton petioles, the authors found that there was a great amount of nitrate reduction (NR) in the cotton petioles. The NR in the cotton petioles was temperature dependent, varied by leaf rank on the main stem, and varied from one section to another in the same petiole. The adapted in vivo assay also detected 98% of total NO3-N in the cotton-petioles.