The plastochron index: still useful after nearly six decades

The plastochron index (PI) introduced by Erickson and Michelini in 1957 provides a solution to a long-standing problem, of how to measure time in growing plant populations, such that the occurrence of critical developmental events can be more readily detected, compared, and analyzed, than if chronologic time is used. The PI reduces the rather large variation associated with chronologic time in measuring such events by taking advantage of the growth characteristics of stem organs that repeat at regular intervals (the plastochron) and has found widespread application in botanical research. The original formulation and derivation of the PI and associated leaf plastochron index (LPI) is reviewed. Additional formulations that have been developed to overcome some of the limitations of the original PI formulation are examined. Major advancements that have been achieved in understanding the physiology, growth, and development of agriculturally important and current model plant species are reviewed to illustrate how various researchers have used the PI in such studies. Potential uses to which the PI and LPI might be applied in emerging frontiers of plant science are suggested. A searchable bibliography of most all the primary research studies that cite the original PI article is provided.