Stem unit growth analysis of Linum usitatissimum (Linaceae) internode development

How overlapping nodes in shoot apical meristems become vertically separated on mature stems was investigated via analysis of spatial and temporal characteristics of growth rate fields of Linum usitatissimum stem units. Linum grown under constant environmental conditions exhibited constant spiral phyllotaxis for nodes 35-80, during which time the stem unit was delimited vertically and tangentially by the boundaries of four successive leaf primordia along the 3-, 5-, and 8-contact parastichies and radially by these boundaries extended to the centroid of the stem. Stem unit age was assessed by a stem unit plastochron index (SUPI) based on a 5 mm reference length. Growth characteristics of -32-0 SUPI stem units were investigated using cylindrical coordinates. Vertical growth velocity was uniform within stem units, increasing basipetally as the units expanded. Vertical strain rate of nodes was nonlinear and consistently lower than the linear rate of subjacent stem units. Radial and tangential stem unit velocity fields were steady and uniform up to -25 SUPI; thereafter, these fields were characterized by uniform spatial and temporal patterns. Stem unit analysis suggests that the stem is a steady-state growth structure up until -25 SUPI, but thereafter it should be viewed as a repetitive growth structure.