PLANT CALORIMETRY .2. MODELING THE DIFFERENCES BETWEEN APPLES AND ORANGES

In a previous review we discussed calorimetric methods for the study o f plant metabolism. Since that review, a number of papers describing c alorimetric measurements examining plant growth, stress responses and effects of temperature have appeared. This recent work is reviewed her e. In addition to the experimental work, a mechanistic model linking r espiration rates to growth has been published. This model is derived f rom both mass and enthalpy balance equations. It describes specific gr owth rate and substrate carbon conversion efficiency as functions of t he metabolic heat rate, the rate of CO2 production, the mean oxidation state of the substrate carbon produced by photosynthesis, and enthalp y changes for conversion of photosynthate to biomass and CO2. Applicat ion of this model to understanding the basis for variation in growth r ates among individual genotypes in plants is reviewed. The effects of environment on the plant respiration-growth relation has been an impor tant focus for plant calorimetry studies. Climatic temperature is one of the most important variables determining growth. Extremes of temper ature determine limits of growth, and diurnal variation and mean tempe rature have a major influence on growth rate. Calorimetric measurement s of respiratory rates as a function of temperature can be used to rel ate the temperature influence on respiratory metabolism to the tempera ture influence on growth rate. These studies have also discovered the existence of an isokinetic point within the range of normal growth tem peratures. Studies of temperature dependence are reviewed and the resu lts analyzed in terms of the recently published mechanistic model.