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Leaves of cottonwood (Populusdeltoides, Bartr.) plants grown under constant conditions are initiated at relatively uniform rates. Because of this uniformity, the plastochron index (PI), developed by Erickson and Michelini (1957), can be applied to quantitatively identify the developmental stage of the vegetative shoot. In a preliminary test, the PI was shown to be linearly related to time, with plastochron intervals ranging from 2.00 to 2.76 days. In a second test, leaf plastochron indexes (LPI) were calculated for a series of trees representing five size classes. Statistical models were developed for each size class, relating each of the dependent variables (leaf length, leaf area, leaf dry weight, number of vessels per internode, and number of vessels per petiole) to LPI. These models were used to demonstrate that the PI and LPI can serve two useful purposes when applied to developmental studies of woody plants: (1) to adjust plants of different developmental stages to a standardized morphological time scale, and (2) to predict developmental processes and events from simple, nondestructive measurements.
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http://www.nrcresearchpress.com/doi/abs/10.1139/m91-149#/toc/cjfr/46/11
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