Limitations in plant calorimetry measurements and interpretations

This di.scussion. so far has discussed limitations to plant calorimetry based only on calorimeter sensitivity, sample size, and Oo depletion. Additional sample-specific problems are encountered, however, and incautious consideration of data can lead to errors in interpretation. 

In some cases, plant tissues are sensitive to gas phase inhibitors or activators. The most evident of these is CO produced via metabolism so that concentration is increased in the sample ampule. Respiration in some plants appears very sensitive to CO . For example, i.sothermal values of soybean leaf tissue metabolic rates are not constant, but rapidly decrea.se with time [33 j. Opening the ampule, flushing with air, and resealing re.stores the initial heat rate and initiates the process of decline once more. When a CO trap is included in the ampule to maintain CO nt near zero pressure, stable heat rates are obtained. Thus, the buildup of CO appears to be responsible for the declining rate. Cauliflower tissue also shows decreased metabolic rate in sealed ampules [54], but for different reasons. Emission of a metabolic modifier from the cauliflower flowerets appears responsible for the decline in this case. 

Problems have also occurred from over-interpretation of the consequences of metabolic heat production in plants. In a series of papers, Raskin and coworkers [55-57] postulated that a temperature increase from metabolic heating protects 

Studies by Skubatz and Meeuse [59]and Skubatz et al. [60] illustrate another problem with interpretation of calorimeter data. These workers pre.sented data that were interpreted to indicate temperature cycling in the reproductive tissues of cycads. They failed to balance masses in sample and reference ampules. This resulted in a cyclic overshoot during equilibration with a periodicity equal to the instrument time constant. Interpretation of the cycles in terms of a periodicity in tissue metabolism is obviously incorrect. Also, in some of their work, samples were not left in the ampules long enough to reach steady-.states of heat evolution. 

These errors in interpretation illustrate a general concern with any calorimetry study. Heat production occurs from nearly all chemical and physical events. Thus, careful interpretation, proper controls, and theoretical evaluation of results is essential for all studies. 

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