Organic Osmolytes Regulate Substomatal Apoplast Water Potential in Pea (Pisum sativum L.) Leaves during Mild Drought.

We investigated the role of organic osmolytes in regulating substomatal apoplast water potential (ψwa) in the leaves of pea (Pisum sativum L.) plants grown under normal or doubled CO2 concentrations exposed to a 3-day drought. The sucrose content, the leading organic osmolyte, was higher in the leaves of drought-stressed plants grown under doubled CO2 concentration than in those grown under normal CO2 concentration. However, the content of proline and reducing sugars (glucose + fructose) was higher in the leaves of plants grown under normal CO2 concentration. Interestingly, the substomatal apoplast ψwa decreased 2-fold under normal CO2 concentration while remaining unchanged under doubled CO2 concentration. Similarity in the decline of ψwa and increase of proline and reducing sugars in leaves under drought indicates their specific role as apoplast osmotic agents that help diminish the turgor of guard cells by lowering ψwa, resulting in stomatal closure and diminish leaf water loss. [ABSTRACT FROM AUTHOR]