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

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 developmentof plants. With the stable plant growth in environmental chambersand glasshouses, the assumptions used to derive these termshave been validated. However, more recently these expressionsare being used to characterize growth under the unstable conditionsresulting from the imposition of stress. This study examinesthe validity of the assumptions used to derive PI and PR forfield-grown soya beans [Glycine max (L.) Merrill] subjectedto drought stress. Under stress conditions, the assumptionswere not satisfied. In fact, observing change in PR appearedto be a good method for detecting drought stress in these plants.An alternate method for calculating PI based on a single, youngleaf was developed. This alternate method appeared to be a moresensitive indicator of changes in leaf emergence rate underunstable conditions. Plastochron index, plastochron ratio, Glycine max (L.), soya bean, drought, leaf growth