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     

Carbon Exchange Rate of Intact Individual Soya Bean Pods. 2. Ontogeny of Temperature Sensitivity

Diurnal temperature fluctuations induced change in soya bean-pod[Glycine max (L.) Merr.] carbon exchange rate (CER, where positiveCER represents CO₂ evolution). CER appeared to depend linearlyon temperature. Linear regressions of CER on temperature interceptedthe temperature axis at 5°C (i.e. zero CER at 5°C).Slopes of these regressions (i.e. temperature sensitivity) changedover the season. The CER-temperature sensitivity coefficient,K, (calculated from observed values of CER. pod temperatureand temperature intercept) rose from less than 0·02 mgCO₂ h^–1 pod^–1 °C^–1 during early pod-flll,peaked at over 0·04 mg CO₂ h^–1 pod^–1 °C^–1at mid pod-fill, and then declined during late pod-fill andmaturation. Glycine max (L.) Merr., Soya bean, carbon exchange rate, temperature     

Carbon Exchange Rate of Intact Individual Soya Bean Pods. 1. Response to Step Changes in Light and Temperature

Carbon exchange rates (CER) of individual intact field-grownsoya bean [Glycine max (L.) Merr.] pods were measured continuouslywith a mobile gas analysis laboratory. Conditions in pod chamberssimulated those experienced normally by pods except for experimentalmodification of incident radiation or pod temperature. Undernormal conditions, CER (where positive CER represents CO₂ evolution)fluctuated diurnally with a morning rise followed by a slowafternoon and evening decline which was similar among pods whichwere measured simultaneously. The frequency of measurementspermitted detection of rapid CER responses to step changes inlight and pod temperature. CER rapidly decreased and increasedwhen the chamber was alternately exposed to full sunlight andcomplete darkness, respectively. CER responded similarly tosteps up [from ambient to elevated (+ 10°C) temperature]and steps down (from elevated to ambient temperature), respectively.Thus, a temperature sensitive process which regulated pod CERwas located within the pod. CER ranged from less than 0·1to more than 1·2 mg CO₂ h^–1 pod^–1 over theperiod of rapid dry-matter accumulation. Glycine max (L.) Merr., Soya bean, carbon exchange rate, light, temperature