Heat of combustion, degree of reduction and carbon content: 3 interrelated methods of estimating the construction cost of plant tissues

— During the last decade, degree of reduction (calculated from elemental composition), heat of combustion and carbon content have been proposed as 3 ways of estimating the construction cost of plant tissues. There is a fairly good agreement among these 3 methods, as they are all based on the link between the energy content of biomass and its reduction level. This relationship is not absolute and the regression coefficient between heat of combustion and degree of reduction may vary according to the chemical composition of biomass. The relation between degree of reduction and carbon content is implicitly based on the consequences in terms of molecular weight of the replacement of 1 oxygen atom by 2 hydrogen atoms in the process of reduction of photoassimilates (carbohydrates) into the various biomass compounds. Different trends can be observed on woody and non-woody tissues, which get richer in energy by increasing their content in lignin and lipids, respectively. Finally, the biosynthetic efficiency is also affected by the che- mical composition of biomass. Therefore, while these 3 methods provide efficient and fairly simple tools for the estima- tion of the construction cost of biomass, they should not be used without considering what the dominant compounds of the plant tissues under study are and without choosing proper parameters in consequence. construction cost / growth yield / heat of combustion / elemental analysis / carbon content / energy content / Juglans regia L = walnut Rsum — Chaleur de combustion, degr de rduction et teneur en carbone : 3 mthodes voisines d'estima- tion du cot de croissance des tissus vgtaux. Au cours des dernires annes, 3 mthodes nouvelles d'estima- tion du cot de synthse de la biomasse ont t proposes, partir de son degr de rduction (calcul grce sa composition lmentaire), de sa chaleur de combustion ou bien de sa teneur en carbone. Ces 3 mthodes donnent des rsultats comparables car elles reposent toutes sur une relation gnrale entre le contenu nergtique de la bio- masse et son niveau de rduction. On n'observe pourtant pas de corrlation parfaite entre ces rsultats car le coeffi- cient de rgression entre chaleur de combustion et degr de rduction varie avec la composition chimique des tissus vgtaux analyss. La relation entre degr de rduction et teneur en carbone est implicitement base sur les cons- quences pondrales du remplacement d'un atome d'oxygne par 2 atomes d'hydrogne au cours de la rduction des assimilats en divers constituants de la biomasse. De fait, on observe des comportements diffrents pour les tissus