Sites of Action of Elevated CO(2) on Leaf Development in Rice: Discrimination between the Effects of Elevated CO(2) and Nitrogen Deficiency

Elevated CO(2) concentrations (eCO(2)) trigger various plant responses. Despite intensive studies of these responses, the underlying mechanisms remain obscure. In this work, we investigated when and how leaf physiology and anatomy are affected by eCO(2) in rice plants. We analyzed the most recently fully expanded leaves that developed successively after transfer of the plant to eCO(2). To discriminate between the effects of eCO(2) and those of nitrogen deficiency, we used three different levels of N application. We found that a decline in the leaf soluble protein content (on a leaf area basis) at eCO(2) was only observed under N deficiency. The length and width of the leaf blade were reduced by both eCO(2) and N deficiency, whereas the blade thickness was increased by eCO(2) but was not affected by N deficiency. The change in length by eCO(2) became detectable in the secondly fully expanded leaf, and those in width and thickness in the thirdly fully expanded leaf, which were at the leaf developmental stages P(4) and P(3), respectively, at the onset of the eCO(2) treatment. The decreased blade length at eCO(2) was associated with a decrease in the epidermal cell number on the adaxial side and a reduction in cell length on the abaxial side. The decreased width resulted from decreased numbers of small vascular bundles and epidermal cell files. The increased thickness was ascribed mainly to enhanced development of bundle sheath extensions at the ridges of vascular bundles. These observations enable us to identify the sites of action of eCO(2) on rice leaf development.