The Relationship Between the Plastochron Index and the Structure of the Apical Meristem of Dianthus chinensis L.

Abstract

The purpose of this study was to characterize the structural changes of the shoot apex of Dianthus chinensis during the production of a pair of leaf primordia and to determine if the plastochron index (P.I.) previously developed for this plant could be used to predict the morphological stages of the apex during a plastochron. Height and volume determinations were made on greenhouse-grown D. chinensis shoot tips prepared for light microscopic examination. These measurements were plotted against the plastochronic fraction of each shoot to evaluate the relationship between apical stage and the P.I. In general, lower fractional plastochron values correspond to smaller apical height and volume values while higher fractional plastochron values correspond to larger apical height and volume values. For the greenhouse plants, there is a 3-fold increase in height and a 5-fold increase in volume during a plastochron. Height and volume determinations were made on growth-chamber-grown D. chinensis shoot tips prepared for scanning electron microscopic examination. Largely plants in Plastochron 6 were sampled; plants in Plastochrons 3, 4, and 9 were also sampled for comparative purposes. The curves of apical height and volume versus Plastochron 6 show a sigmoidal pattern of growth. The corresponding curves of the Plastochron-4 and -9 plants show a similar pattern of increase. The Plastochron-6 curves also show that the integer value of the P.I. corresponds to the minimal apical height and volume when a pair of leaf primordia are just discernible, while the integer plus high fractional value of the P.I. corresponds to the maximal apical height and volume just prior to the emergence of the next pair of primordia. During Plastochron 6 there is a 2-fold increase in height and a 3-fold increase in volume. Apical height and volume increase was examined by regression analysis to determine whether growth is better described as linear or exponential. The results indicate that the sigmoidal pattern of increase might be better described by some more complex function. It is concluded that during certain stages of a shoot's vegetative growth the P.I. will accurately predict apical meristem structure in D. chinensis two-thirds of the time.