Distribution of relative growth rates and variation of cytohistological zonation in apical meristems of seedlings of two contrasting open-pollinated jack pine (Pinusbanksiana) families

Abstract

A general method is developed to obtain relative growth rates in different parts of an apical meristem, bare apical dome, internode, and whole apical dome. Expressions are given for transverse, longitudinal, and volume growth. It is shown that these growth rates depend on only four parameters, radius (Ro) and height (Ho) of the bare apical dome and transverse (r) and longitudinal (h) plastochron ratios. A method is described to obtain relative volumes of different zones in apices showing cytohistological zonation using relative area measurements on median longitudinal sections. Both methods are applied to a practical example, on ontogenetical variation of seedlings originating from two open-pollinated families of jack pine (Pinusbanksiana Lamb.) that exhibited a contrasting pattern of height growth in previous studies. Both families differ in their plastochron duration (PD) and this difference can be related to different distributions of growth rates in the apical meristem. PD values range, between 16–6 h in the first family, and between 12–4 h in the second one. Most family differences appear on bare apical dome growth rates and on allometric coefficients between bare apical dome growth and intermode growth. As an example, bare apical dome radial relative growth rates per PD varies during ontogeny from 0.004 to 0.008 and then decreases to −0.011 during the first 70 days after germination for the first family. Corresponding values for the second family are 0.001, 0.007, and −0.004. There are also ontogenetical variations in cytohistological zonation. Both families show an increase in the apical initial plus central mother cell zone relative volume following the decrease of plastochron duration. However, no significant differences between families are obtained for relative volumes of cytohistological zones. Relationships between distributions of growth rates and changes of cytohistological zonation are discussed.