Analysis and significance of gravity-induced asymmetric growth in the grass leaf-sheath pulvinus

The negative gravitropic response in the grass leaf-sheath pulvinus is a consequence of cell elongation involving all cells except those of the uppermost region of the upper flank of an horizontally oriented pulvinus. The lowermost layer of cells elongate maximally, and the regions in between elongate to intermediate extents. The resulting curvatures of a responding pulvinus can be expressed mathematically by relating the angle of curvature (theta) to the original length (L0) and the maximal length of the lower surface (L1) and the diameter of the organ (D), using the equation, theta = (L1-L0)/D, where theta is in radians. The elongation response (S) of any individual cells within the pulvinus can be expressed by the equation, S = 0.5 - r cos theta, where r is the radius of the pulvinus and theta is in degrees. Microscopic measurement of cell lengths in different regions of the pulvinus supports the mathematical predictions. Indirect support is also obtained from the use of colchicine, coumarin, dichlorobenzonitrile (DCBN) and isopropyl N-chlorophenyl carbamate which exaggerate the inherent asymmetry during gravitropic response. Coumarin and DCBN also induce thickenings in the radial walls which appear first in the statenchyma, and later, in cells located towards the outer periphery of the pulvinus. The distribution patterns of these thickenings suggest that the asymmetric growth response of the pulvinus may be due to a differential and radial, centrifugal transport of growth promotors from the central statenchyma region.