Mesophyll-Driven Blade Expansion in <i>Pisum sativum</i> var. <i>argenteum</i> Leaves

Abstract

The growth rate of any multilayered plant organ is limited by the cell layer with the least extensibility. The dicot leaf blade has two epidermal layers covering the mesophyll layers, in which the vascular network is embedded. There has been a lingering uncertainty about which layer limits the rate of blade expansion in dicot leaves. The current study made use of leaf strips cut from the <i> argenteum </i> variety of <i> Pisum sativum </i> L., in which the epidermal layers can easily be removed with minimal damage. After this procedure, the mesophyll showed accelerated growth in short-term and long-term experiments and light and darkness. Extension of both layers is strongly promoted by acidic solutions. Isolated mesophyll layers expand in response to light. This effect depends on turgor pressure, photosynthesis, and the plasma membrane H+-ATPase. The data allow concluding that expanding leaf blades share with axial stem organs a similar arrangement of tissue tension: an expanding core tissue pushing against a restrictive epidermal envelope. In complete leaves, partial removal of the epidermis from only one side of the blade causes a strong epinastic or hyponastic response. Removal of matching epidermis strips from both sides of complete blades causes the exposed mesophyll strip to elongate in excess of the neighboring tissue: it buckles.