riety of conditions [6]. Plants may play a role in modifying their environment and climate [7]. In addition,
plants have complex relationships with other organisms in their communities including herbivores,
pathogens, parasites, symbiotic or free-living nitrogen-fixing bacteria, and mycorrhizae. All of these fac-
tors can affect the rate of plant growth.
Photosynthesis supports all life on earth and in eukaryotes occurs exclusively in chloroplasts. All
green tissues contain chloroplasts, but most photosynthesis, by far, occurs in leaves. C 4 plants have a pho-
tosynthetic rate that is two to three times faster than that of C 3 plants and 100-fold faster than that of Cras-
sulacean acid metabolism (CAM) plants [8], but within each of these groups there is much variability in
photosynthetic rate. Despite much effort to correlate this variation with growth rates, no consistent results
have been obtained [9]. Thus, although photosynthesis is absolutely necessary for plant growth, the rate
of photosynthesis does not predict the rate of plant growth [9]. Insufficient carbon assimilation does not
explain why alpine plants are so small and why biomass accumulation per unit land area is so low [10].
Several investigators have suggested that respiration is a better predictor for plant growth [11].
III. RESPIRATION
McCree [12] reported that specific respiration rate and specific plant growth rate are linearly correlated
with a positive slope and positive intercept. Thornley [13] then borrowed a model from microbiology [14]
that equates the slope of such a plot to a growth coefficient and the intercept to a maintenance rate. This
is represented in Eq. (1).
RRMRG (1)whereRis total respiration, RMis maintenance respiration or that necessary to maintain life processes,
andRGis the respiration responsible for growth. This model has been widely used for 30 years but pro-
vides only an empirical fit to the data [15–17]. This model cannot predict plant growth rates from
metabolic rates.
This chapter discusses another model linking plant respiratory metabolism with growth [18] that is
testable, based on first principles, allows predictions of growth rates from metabolic rate measurements,
and defines responses to subtle changes in environmental stress. The theory will be presented followed
by several examples of applications.
IV. GROWTH AND RESPIRATION
Consider the overall growth reaction (2).
Csubstratex(compounds and ions of N, P, K, etc.) yO 2 →
Cstr.biomass(1)CO 2 yH 2 Oheat (2)Reaction (2) is the sum of two reactions, that is, the catabolic reaction (3)
CsubstratezO 2 →CO 2 heat (3)and the anabolic reaction (4)
heatCsubstratex(compounds and ions of N, P, K, etc.) →Cstr.biomass (4)that occur in the condition-dependent ratio (1 )/, where is the substrate carbon conversion effi-
ciency. Reactions (3) and (4) are energy coupled through cyclic production and hydrolysis of ATP and
redox cycling of NADH. Because the ratio of the rates of reactions (3) and (4) varies with conditions, re-
action (3) must always produce an excess of ATP and NADH, as clearly explained in the book Introduc-
tion to the Thermodynamics of Biological Processes[19]. This necessitates both an ability to change the
efficiency of production of ATP through such pathways as the alternative oxidase and a third reaction, the
futile hydrolysis of ATP and oxidation of NADH as in reaction (5).
aATPbNADH→aADPaPibNAD (5)Note that aandbmust always be greater than zero and that the rate of reaction (5) varies with conditions
because catabolism and anabolism are not stoichiometrically coupled [19].
2 SMITH ET AL.