Eliminating interference by anthocyanin in chlorophyll estimation of sweet potato (Ipomoea batatas L.) leaves

Spectral reflectance was evaluated for its usefulness as a nondestructive estimation of chlorophyll (Chl) content from three cultivars of sweet potato (Ipomoea batatas L.) with green, yellow, and purple leaves grown in a greenhouse for 22 days. While the green and yellow leaves contain variant amount of photosynthetic pigments without or with little level of anthocyanins, the purple leaves, except large amount of photosynthetic pigments, have high quantity of anthocyanins.

For green and yellow leaves, the reciprocal reflectance (R^-1) and derived indices incorporating near infrared (NIR) reflectance, [(R_λ)^-1 - (R_NIR)^-1] and [(R_NIR/R_λ) - 1], in the green and red edge spectral ranges were shown to be strongly correlated (r² = 0.8 ~ 0.9) with the chlorophyll content. The root mean square error (RMSE) of the chlorophyll content estimation using these indices was < 50 mg m^-2. However, when purple leaves containing high levels of anthocyanins were included in the sample, R^-1 in the green spectral range and the above-mentioned indices displayed much weaker correlations with the chlorophyll content. The RMSE of chlorophyll estimation using these indices in the green spectral range sharply increased to > 110 mg m^-2 when the sample included purple leaves. The new index, [1 - (R_λ/R_NIR)], was therefore inferred and developed to eliminate the distorting effect of anthocyanins on chlorophyll content estimation using reflectance in the green spectral range. For leaves with high levels of anthocyanins, the correlation between [1 - (R_λ/R_NIR)] and the chlorophyll content remained strong (r² = 0.8 ~ 0.9) in the green spectral range, and the RMSE was minimal.

The reflectance index, [1 - (R_λ/R_NIR)], therefore represents a new and useful parameter for estimating leaf chlorophyll content in leaves with any level of anthocyanins such as purple rice leaf.