dynamic method assumes that maintenance respiration is represented by the dark CO 2 efflux when the net
daytime uptake is zero. With net CO 2 uptake being zero, there are few readily available assimilates for
growth during the following night. With the zero growth rate (or alternatively gross uptake), it is assumed
that maintenance occurs when the growth rate or the daily carbon accumulation rate is zero; thus, the to-
tal daily assimilate production is utilized in maintenance. In the latter two methods, the maintenance co-
efficient is calculated by extrapolation because practically zero uptake in the light or zero growth rates,
respectively, would not occur.
Schwarz and Gale [77] pointed out that the consumption of assimilates for maintenance processes
and possibly their diversion from growth requirements may increase under the demand of environmental
stress leading to higher respiration rates. Under conditions of environmental stress, plants may shift en-
ergy expenditure from growth to maintenance respiration and repair in order to accommodate the ener-
getic costs of stress [78,79]. This increase in maintenance respiration may be a characteristic feature of
salt tolerance insofar as it suggests an ability to divert assimilates and respiratory energy to maintain the
biomass [77,80]. For instance, a native salt-tolerant species of Lycopersicon pennelliiexhibited an in-
crease in maintenance expenditure and a domesticated salt-tolerant species (L. esculentum) showed a de-
crease in root maintenance respiration under exposure to saline conditions [81]. However, Stavarek and
Rains [82] described reduced values of maintenance respiration in Medicago sativaunder conditions of
salinity. Wild barley is more tolerant to sulfate salinity than is cultivated barley [83]. Thus, the increase
in maintenance respiration may serve as a criterion for evaluation of the ability of a plant to cope with
stress. Moreover, it may help to evaluate the cost that the plant must pay for adaptation in terms of allo-
cation of resources [77].
Stress factors other than salinity were also found to affect maintenance and growth respiration. In this
respect, Amthor and Cumming [84] found that leaves of Phaseolus vulgarisexposed to ozone exhibited
a 15% increase in maintenance respiration. Similar patterns have been reported with Cucumis sativusun-
der conditions of chilling stress [85]. However, low dark respiration rates and low specific leaf area of the
tropicalPandanusspecies have been regarded as important characteristics for growth and survival in en-
vironments where resource levels are low and the likelihood of tissue damage is high [86]. Ahmed [87],
working with Chlorella fusca, reported that maintenance respiration increased when the algae had been
exposed to salinity. Huang and Redmann [88] reported similar results based on experiments with wild and
cultivated barley plants, depending on the Ca^2 availability. On the basis of maintenance respiration co-
efficient values, the sensitivity of three plants can be arranged as follows: broad bean sunflower
maize. Broad bean was the most sensitive one and exhibited the highest value of maintenance respiration
[76]. Maize, however, exhibited no response of maintenance respiration to water stress but reacted to
salinity, whatever the level used [77]. Ca^2 lowered the values of maintenance respiration in sunflower,
broad bean, and maize plants. Accordingly, Ahmed [87] found that salinized Chlorella fuscadecreased
its maintenance respiration when exposed to Ca^2 . In barley, maintenance respiration was significantly
reduced by low Ca^2 treatment but was slightly increased by high Ca^2 treatment [88], which might in-
dicate that maintenance respiration can be minimized by appropriate concentrations of Ca^2 . The expla-
nation might be that Ca^2 is a structural component of cell walls and membranes and an indispensable
cofactor of photosystem II in addition to its physiological role in the regulation of enzyme activities. It
was found that Ca^2 reduces respiration in general and maintenance respiration in particular, not least be-
cause of its importance for maintaining membrane integrity.
In addition to environmental factors, a variety of internal plant factors affect the magnitude of the
portion of respiratory energy that is used for maintenance. For instance, in field populations of Phrag-
mites australis, respiratory activity was inversely related to the age of the rhizomes. In the case of 3-year-
old rhizomes, values went down to about 2.66 0.40, 2.28 0.40, and 2.72 0.40mol CO 2 (g dry
wt)^1 hr^1. The specific location played only a minor role in this context [89]. Maintenance respiration
rates varied with the tissue size of stems and branches of 9-year-old loblolly pine (Pinus taedaL.) but
were constant with respect to the nitrogen content of the tissue [90]. In this context, root respiration may
account for as much as 60% of total soil respiration [91]. Small lateral roots at the distal end of the root
system have much greater tissue nitrogen concentrations than larger roots, and this led to the hypothesis
that the smallest roots have significantly higher rates of respiration than larger roots. Nitrogen content in
the roots might explain 70% of the observed variation in respiration in sugar maple (Acer saccharum
Marsh.). The nitrogen values in any case appeared to be a better indicator of root function than, e.g., mor-
phological parameters such as the root diameter. The carbon budget of the lowest Scots pine (Pinus
314 BADER AND ABDEL-BASSET