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More than 35 yr ago, observations that respiration of growing plants differed fundamentally from that of nongrowing plants led to formulation of the growth and maintenance paradigm (GMP; Thornley, 1970; de Wit et al., 1970; McCree, 1970, 1974). Early empirical studies showed that rates of respiration () were greater when plants were growing strongly, and could often be linearly related to rates of growth (RSG). The slope of the relation (the growth coefficient, g) depends on a number of environmental and developmental factors. The intercept (I) has often been interpreted as the maintenance component of respiration – that necessary to maintain cellular structures and processes in the absence of growth:
- (Eqn 1)
The GMP became a dominant paradigm for representation of respiration in plant growth models (Gifford, 2003), which seems to be a result of its putative strength of prediction. The correlation between plant growth and CO2 release enables definition of a ‘growth potential’ (1/g), that is, the increase in growth rate per unit increase of CO2 respiration:
- (Eqn 2)
where g is the growth coefficient in Eqn 1. Experimental determination of growth potential is tedious because of the need for many independent analyses of rates of growth and respiration. In part, growth potential is a genetically inherent trait, but it also depends on developmental stage, and interacts with environmental variables. Separate evaluation of each potentially influential factor (as required for modeling purposes), or identification of plant varieties with inherently high growth potential, is thus a laborious task. Consequently, growth coefficients are routinely and often unavoidably calculated with poor precision.
In general, the GMP cannot be used to determine the growth potential of an individual plant at any one point in time. As an example, measured energetic costs of constructing foliage (in units of glucose respired per gram biomass produced) are strongly dependent on developmental stage. The oxidation state of anabolic products at different times of ontogenetic development can cause large variations in growth potential (1/g), if determined according to the mass-based approach outlined in Eqns 1 and 2. Lipids and proteins, for example, are more reduced than cellulose, the latter being predominantly synthesized later in cell development (i.e. following cell extension). Simply put, more carbohydrates are required and more CO2 is respired per gram increase in biomass in early stages of cell development, resulting in comparatively large values of g (or a low, mass-based growth potential 1/g).
As pointed out by de Wit et al. (1970) and Amthor (2000), one of the most useful measures of growth within the framework of the GMP is conversion of substrate carbon (i.e. nonstructural carbohydrates) into new structure (i.e. structural carbohydrates, lignin, protein, lipid, nucleic acid, etc.), rather than changes in total dry mass. While equating growth with increases in mass is intuitively attractive and thus popular, expressing growth in terms of energy increment captures important differences among tissues and species that are not always well reflected in mass (e.g. differences between herbaceous and woody plants, between young and old tissues, or between sclerophyllous and nonsclerophyllous species; McDermitt & Loomis, 1981; Amthor, 2000).
Despite the initial popularity of the GMP, which appeared to give plant respiration a theoretical underpinning, attempts to further its development led to a range of intricate problems. Strictly, the GMP can only be applied to entire plants (Thornley & Johnson, 1990, pp. 265–284). However, most respiration measurements are conducted on leaves or stems and often only respiratory CO2 release is determined. Nonetheless, many authors have provided mechanistic interpretations of such results that frequently include division of distinct physiological process into ‘growth’ and ‘maintenance’ components. This approach has been applied despite the many physiological processes that are peculiar to functionally different plant organs that serve the entire plant. As examples, both processes of (i) sugar loading into phloem in leaves and (ii) nutrient uptake in roots require considerable amounts of respiratory energy and cannot simply be assigned to either ‘growth’ or ‘maintenance’ respiration (Cannell & Thornley, 2000; Thornley & Cannell, 2000).
Similarly, respiration rates and nitrogen concentrations are often correlated in plant tissues (Turnbull et al., 2003, Wright et al., 2004, Reich et al., 2006, Kruse & Adams, 2008a). This has been interpreted in light of respiratory energy demand for maintenance of functionally active proteins (Ryan et al., 1996). For this specific process, Amthor (2000) introduced the term ‘tool maintenance’ as a descriptor of the energy input required for the repair and maintenance of proteins associated with photosynthetic machinery (mainly). While this example might seem a precedent for ‘maintenance’ respiration, it also produces further difficult-to-resolve issues of logic.
Hansen, Criddle and co-workers (Criddle et al., 1991; Hansen et al., 1994, 1997, 1998; Criddle & Hansen, 1999) developed a calorimetry-based approach that may help to tackle some of the problems raised by the GMP. Briefly, rates of oxygen reduction are quickly and easily quantified via tissue heat rates (q) using calorimetry. Rates of oxidation of substrate (and production of CO2) are measured in the presence of an alkali trap. The techniques have been thoroughly described over the past 10 yr – including the required precautions, assumptions and caveats – and recent summaries are given in Hansen et al. (2004, 2007). Calorimetric approaches deliver measures of instantaneous rates of growth (enthalpic growth rates). They are therefore suitable for analysis of the relation between respiration and growth on the short time-scales most relevant to physiological processes. Enthalpic growth rate is determined from the energy made available by oxidation of substrate () and the loss of heat to the surroundings (q). As q is proportional to oxygen reduction in plant mitochondria (Hopkin, 1991, Ordentlich et al., 1991) and equal to (Kemp, 2000), it follows that enthalpic growth can be defined:
- (Eqn 3)
- (Eqn 4)
where q is the specific heat rate (µW mg−1) and is equal to .
Major sources of CO2 in respiring plant tissue are: (i) oxidative decarboxylation reactions within the tricarboxylic acid (TCA) cycle of the mitochondrial matrix (coupled to reduction of NAD+), and (ii) oxidative decarboxylation of carbohydrates in the oxidative pentose phosphate pathway of the cytosol (coupled to reduction of NADP+). Reducing equivalents produced in plant mitochondria are used for subsequent reduction of O2 (generating heat), whereas NADPH is needed for reductive anabolism (thermally neutral). Some of the energy released during electron transport in the inner mitochondrial membrane is captured for the synthesis of ATP. During biosynthesis, ATP drives some anabolic reactions that are otherwise not thermodynamically feasible (anabolic reactions are thus coupled to catabolic degradation of sugars to varying degrees). Alternately put, ATP acts as a catalyst for biosynthetic reactions and is rapidly turned over, while little or no energy (that was transiently stored in ATP) is retained in the anabolic products of respiration (Macfarlane et al., 2002).
Summarizing, some of the energy made available by the oxidation of substrate carbon () is retained in anabolic products (RSGΔHB), and some is lost as heat to the surroundings. Enthalpic growth rates can thus be interpreted as the preservation of electrons in anabolic products of respiration, or as the energy stored in the chemical bonds of newly synthesized anabolic products (compare with Kruse & Adams, 2008b and Kruse et al., 2008). If carbohydrates are completely oxidized to CO2 in plant mitochondria, then and RSGΔHB = 0.
The enthalpy conversion efficiency (ηH) is a derived parameter, describing the ratio of the enthalpy retained in anabolic products (the ‘enthalpic growth’, RSGΔHB) to the sum of retained enthalpy plus the enthalpy dissipated as heat:
- (Eqn 5)
The enthalpy conversion efficiency (ηH) is proportional to the respiratory quotient CO2:O2, assuming carbohydrates as substrate (see Kruse et al., 2008). It also follows from Eqn 5 that:
- (Eqn 6)
Hence, enthalpic growth rates (i.e. the difference between rates of decarboxylation reactions and those of oxygen reduction; compare Eqn 3) depend on both rates of CO2 respiration and on the ratio of CO2 release to O2 reduction. We must note that Eqn 3 represents a static view or a snapshot of a distinct metabolic state of respiring tissue. Physiological mechanisms that relate cell growth (increases of enthalpy) to CO2 respiration might be elucidated by a more dynamic approach, such as an approach using the expected change in enthalpic growth per unit increase (or decrease) in CO2 respired; that is, and by analogy with Eqn 2:
- (Eqn 7)
where 1/gH is ‘enthalpic growth potential’ (compare with Eqn 2).
Combining features (or at least some of the broad concepts) of the GMP and the ‘enthalpy balance approach’ might enhance our knowledge of the physiology of respiration. For example, it is not clear if correlation methods originating in the GMP can be applied to lower levels of biological organization (i.e. to plant cells or organs; Matheson et al., 2004). Kruse & Adams (2008a) demonstrated that fast rates of respiration by foliage from the upper canopy of pine trees were not related to instantaneous rates of enthalpic growth in early and late summer. Instead, upper canopy foliage had switched from carbohydrate ‘sink’ to ‘source’ and rates of respiration reflected the large energy demand for phloem loading of sugars and maintenance of existing proteins, rather than synthesis of new cell structure (Kruse & Adams, 2008a).
Our first aim was to clarify whether rates of respiration of pine needles () could be related to rates of enthalpic growth, when cell metabolism is dominated by growth processes during the ‘sink stage’ of foliage development in early spring. We also investigated whether respiration and growth were related to pools of soluble sugars and amino acids during this remobilization period. Secondly, we explored variation in the relations among respiration parameters RSGΔHB, , ηH and dηH (equivalent to 1/gH) attributable to environmental and developmental factors. For this purpose we used as our model apical buds (plus young, growing needles), differing in branching order and taken from different canopy positions of Pinus radiata plantations that were either thinned or left untreated as a control. Thirdly, we sought to develop a method that could simplify determination of ‘enthalpic growth potential’ (1/gH or dηH), without the necessity of traditional correlation methods. We hoped to enhance our knowledge of the physiological mechanisms that link foliage respiration to cellular growth. This might in turn be useful in modeling plant growth and identifying plant varieties with large growth potential.