Trade-off Between Salt Secretion and Gas Exchange by Stomata in the Leaves of Glycyrrhiza uralensis

Our previous study found that stomata in the leaves of Glycyrrhiza uralensis (licorice) could secrete salt crystals. In theory, secretion of salt should affect the normal functioning of stomata, thereby affecting the growth and development of G. uralensis; however, its population grows well. We suspect that there may be a trade-off between stomatal salt secretion and gas exchange from the leaves at different positions. Therefore, we compared stomatal salt secretion capacity, chlorophyll content, anatomical structure, net photosynthetic rate and stomatal conductance from the leaves at different positions of licorice. The stomata of lower leaves exhibited strongest salt secretion capacity, whereas the stomata of upper leaves did not secrete any salt. Additionally, the upper and middle leaves had significantly higher chlorophyll content than the lower leaves. The arrangement of mesophyll cells in the upper leaves was densest, and that in the lower leaves was least dense. The net photosynthetic rate and stomatal conductance in the upper leaves were highest, and those in the lower leaves were lowest. We conclude that the stomata of upper leaves are mainly used for gas exchange. In contrast, stomata of lower leaves, showing weak photosynthesis, are responsible for secreting excessive salt to maintain the inner ion balance and ensure normal metabolism in G. uralensis.