Effects of Added Carbon Dioxide on Iodine Staining of Soybean Leaves

A Pilot Study Toward Early Detection of Magnesium Deficiency

Abstract

Magnesium (Mg) deficiency lowers crop yield and the nutritional value of food crops, but early-stage deficiency produces few visible symptoms and is difficult to diagnose. One of the earliest changes in Mg-deficient leaves is the accumulation of carbohydrates, including starch, which can be detected with iodine staining. Rising atmospheric carbon dioxide (CO2) has also been associated with lower mineral concentrations in plants. In this pilot study, we tested whether iodine staining and simple image analysis could detect changes in soybean leaves exposed to added CO2. Three groups of 20 soybean plants were grown in enclosed containers: a control group and two experimental groups that received 7 g or 14 g of gaseous CO2 each week for four weeks. Each week, leaves were collected from the lower nodes, cross sections were stained with Lugol’s iodine solution, and microscope images were analyzed in MATLAB by separating each image into its red, green, and blue channels and recording the peak count of each channel’s intensity histogram. Stained sections from both experimental groups appeared darker than those of the control group, and their peak blue-channel counts rose to values five to six times higher than any control value, generally reaching a maximum in week 3. Because the most common blue value in these images was near zero, this pattern reflects darker staining rather than more blue color. These results are consistent with greater starch accumulation in CO2-treated leaves, but Mg was not measured, and elevated CO2 can increase leaf starch directly through enhanced photosynthesis. Future studies should measure leaf Mg directly and include Mg-deficient controls to test whether iodine staining can serve as a low-cost early indicator of Mg deficiency.