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 argenteum variety of Pisum sativum 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.
Highlights
In a single plant cell, the rate of expansion depends on two factors: the extensibility of the cell walls, and the expansive push of turgor pressure [1]
When multicellular plant organs expand, cell expansion depends on the cell layer with the lowest extensibility since the cells are interconnected by common cell walls
The question remains as to which of the cell layers restricts the growth rate in the dark and in the light. To approach this question we investigated the contribution of epidermis, mesophyll, and midvein in blade expansion of the argenteum variety of Pisum sativum
Summary
In a single plant cell, the rate of expansion depends on two factors: the extensibility of the cell walls, and the expansive push of turgor pressure [1]. When multicellular plant organs expand, cell expansion depends on the cell layer with the lowest extensibility since the cells are interconnected by common cell walls. The situation changes and the wall extensibility of the outer layer increases greatly [3]. These phenomena are known under the name of “tissue tension” and a description of its role in axial organs has been reviewed [4] [5]. The generation and role of tissue tension in flat leaves, have remained uncertain [6]
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