Assumptions of Plastochron Index: Evaluation With Soya Bean Under Field Drought Conditions

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Abstract
Erickson and Michelini (1957) derived the plastochron index (PI) and a term sometimes referred to as the plastochron ratio (PR), as quantitative expressions of the vegetative development of plants. With the stable plant growth in environmental chambers and glasshouses, the assumptions used to derive these terms have been validated. However, more recently these expressions are being used to characterize growth under the unstable conditions resulting from the imposition of stress. This study examines the validity of the assumptions used to derive PI and PR for field-grown soya beans [Glycine max (L.) Merrill] subjected to drought stress. Under stress conditions, the assumptions were not satisfied. In fact, observing change in PR appeared to be a good method for detecting drought stress in these plants. An alternate method for calculating PI based on a single, young leaf was developed. This alternate method appeared to be a more sensitive indicator of changes in leaf emergence rate under unstable conditions.