The relationship of host plant resistance and tannin content has been studied. However, the effect of light intensity on the tannin content of plants is poorly understood. The objective of this study was to determine the effect of light intensity, stage of growth, and plant part on the tannin content of six genotypes of cotton. The experimental design was a split plot of a randomized complete block. Data were analyzed by analysis of variance procedures, and means were separated by Waller-Duncan LSD.

Main plot s consisted of two light intensities: 725 UE s^-1 m^-2 (full light) and 350 uE s^-1 m^-2 (half light). The six genotypes included four Cotton Improvement Lab High Tannin lines (86III24, SGCC2, 86E8 and 86E3) plus two commercial varieties (Arkot 5l8 and TAMCOT CD3H). The top unfolded leaf and the fourth mainstem leaf from the terminal were sampled at fourth true leaf and pinhead square stages. At first bloom stage, a six-day-old bract, or the bract from the square at the first position of the third sympodium from the terminal, was sampled in addition to the two leaves previously mentioned.

The HCl-butanol method was used to digest fresh tissue, and absorbance was determined at 550 nm. Tannin content was then calculated as a percentage of fresh weight.

Significantly higher tannin content was in plants grown under full light intensity than those grown under half light intensity. Tannin content increased from fourth true leaf stage to first bloom, and was significantly higher in the top unfolded leaf than in the six-day-old bract. The fourth mainstem leaf had the lowest tannin content. A significantly higher difference in tannin content between the two light levels was observed for the six-day-old bract compared with the fourth mainstem leaf, which was significantly higher than the top unfolded leaf. The tannin content of the top unfolded leaf remained stable across both light levels at first bloom.