Transductions for the Expression of Structural Pattern: Analysis in Sunflower.

Abstract

The transductions to initiate and propagate pattern were investigated in the sunflower head. The spiral pattern emerges as new florets form in regular centripetal sequence on the flat disc. The spiral lines of florets arise as rows of simple bumps. Each bump splits to become a small bract and a circular disc flower. This topographical progression was described by a scanning electron microscopic technique applied to living tissue. The suitability of various theories to explain the progression was examined. Because no periodic cell specialization was seen by scanning electron microscopy prior to pattern emergence, a mechanism that produces pattern in uniform tissue by spontaneous physical buckling (folding) was examined further. Key configurational changes of development were reproduced in models using the buckling assumptions. In further testing, a young head was physically constrained to cause it to grow as an oval. Pattern was modified as predicted. Unexpectedly, organ character changed as well. In localized regions, the folding was abnormal; the typical dyad floret, bract and flower, was replaced by a single large bract. This anomaly is known in mutants of sunflower; hence, the physical treatment induced a phenocopy. We concluded that (1) buckling is a strong candidate to be the process producing organ pattern, and (2) the accompanying topographical changes can be prerequisite to organ differentiation.

Phyllotaxis: Cooperation and competition between mechanical and biochemical processes

“…Experimental backing for this hypothesis includes experimental evidence in sunflowers that compressive stresses are present in the generative region where primordia form (Dumais and Steele, 2000) (although earlier experiments in Snow and Snow, 1951 indicated that the plant surface is under tension, and an extensive study of the stress state of the tunica has not been performed). Additionally, applied mechanical constraints can lead to new leaves (Green, 1999) or altered planforms (Herna´ndez and Green, 1993). The mathematical analysis of Green, Steele and Rennich was primarily a linear one, suggesting how a buckled surface might begin.…”

Phyllotaxis: Some progress, but a story far from over

“…It shows the (5,8,13) to (8,13,21) transition between R ≈ 13 and 21 but applies equally well to how the dual coordinates in the static model vary with the rise ρ. The parameter R is then inversely proportional to ρ.…”

“…Denoting the mean radius of a region byr, the vertical axis in the transformed panels is r−r, and the horizontal axis is θ. Counting the numbers of counterclockwise and clockwise spirals, the parastichy pair of the pattern are observed to be (34,55) in the annular region including region C, whereas the parastichy pair is (21,34) in the annular region that includes region A.…”

“…On the Fibonacci branch, namely the one which starts with a (5,8,13) hexagon and ends with a (8,13,21) hexagon, the path is continuous. But, the irregular branch on which the (5,8,13) goes to a (5,13,18), there is no solution over a significant gap.…”

Polygonal planforms and phyllotaxis on plants

“…Herna´ndez and Green (1993) grew a sunflower head between two fixed parallel bars. This resulted in an amplification of undulations parallel to the bars, and phyllotaxis followed this pattern.…”