Dark respiration in light as well as in dark was estimated for attached leaves of an evergreen (Heteromeles arbufifolia Ait.) and a deciduous (Lepechinia fragans Creene) shrub species using an open gas-exchange system. Dark respiration in light was estimated by the Laisk method. Respiration rates in the dark were always higher than in the light, indicating that light inhibited respiration in both species. The rates of respiration in the dark were higher in the leaves of the deciduous species than in the evergreen species. However, there were no significant differences in respiration rates i n light between the species. Thus, the degree of inhibition of respiration by light was greater in the deciduous species (62%) than in the evergreen species (51 %). Respiration in both the light and darkness decreased with increasing leaf age. However, because respiration in the light decreased faster with leaf age than respiration in darkness, the degree of inhibition of respiration by light increased with leaf age (from 36% in the youngest leaves to 81 Yo in the mature leaves). This suggests that the rate of dark respiration in the light i s related to the rate of biosynthetic processes. Dark respiration i n the light decreased with increasing light intensity. Respiration both in the light and i n the dark was dependent on leaf temperature. We concluded that respiration i n light and respiration in darkness are tightly coupled, with variation i n respiration in darkness accounting for more than 60% of the variation in respiration in light. Care must be taken when the relation between respiration in light and respiration in darkness is studied, because the relation varies with species, leaf age, and light intensity.Previous studies of the relationship between carbon balance and respiration have focused on R,, assuming that it equals R,. However, investigations using crop species suggest that respiration is partly inhibited in the light (Sharp et al., 1984;Brooks and Farquhar, 1985).There is considerable debate about the leve1 of respiration in photosynthetic tissue that occurs in the light (Graham, 1980;Turpin and Weger, 1990 (1988) reported that ETC activity is the same in light and dark, but TCA activity decreases in the light. These results are in agreement with both the results of Kromer and Heldt (1991), who showed that ETC activity is necessary in the light for optimal photosynthetic rate, and those of Geme1 and Randall(1992), who found that TCA activity is restricted in the light.The contrasting estimates of R, may also partly be the result of the different environmental conditions presented in the different studies and/or the different developmental states of the tissues used (Turpin and Weger, 1990;Villar et al., 1994). Thus, comparison of R, data from previous studies is difficult and necessitates that the effect of environmental and metabolic states on R, be determined. The dependence of R, on leaf characteristics (leaf age, leaf N, and leaf carbohydrate concentration) and environmental factors (light intensity ...

Dark respiration in the light was estimated in leaves of two woody species (Heteromeles arbutifolia Ait. and Lepechinia fragans Creene) using two different approaches based on gas-exchange techniques: the Kok method and the Laisk method. In all cases, dark respiration in the light was lower (P < 0.05) than respiration in darkness, indicating that dark respiration was inhibited in the light. Rates of dark respiration in the light estimated by the Laisk method were 52% higher (P < 0.05) than those estimated by the Kok method. Differences between the methods could be explained by the low ambient CO, concentrations required by the Laisk approach. The mean value of the inhibition of respiration by light for the two species, correded for the ambient CO, concentration effect, was 55%. Despite the differences in leaf characteristics between the species, values of the CO, photocompensation point, at which the rate of photosynthetic C 0 2 uptake equaled that of photorespiratory CO, evolution, were very constant, suggesting an excellent consistency in the results obtained with the Laisk approach.In most studies of the carbon balance of plants or plant organs it is assumed that dark respiration in the light continues at the same rate as in darkness. However, there is evidence that light inhibits dark respiration in photosynthetic tissue (Kok, 1948;Sharp et al:, 1984;Kirschbaum and Farquhar, 1987). This inhibition appears to be caused by metabolites from photosynthesis (ATP, NADPH) acting on the respiratory enzymes as respiratory regulators (Graham, 1980; McCashing et al., 1988). However, the mechanism of inhibition is not clear and appears to be complex (Gardestrom and Wigge, 1988;Kromer and Heldt, 1991).The instantaneous evolution of COz in the light is a result of at least four processes, which take place at different rates: photosynthesis, photorespiration, dark respiration in the light (Rd), and refixing of co2 from respiration (Graham, 1980 response is linear at low levels of irradiation, but near the light compensation point there is a break in the linear response, increasing markedly the slope of the light curve due to a decrease in A. This has been interpreted as a result of an increase in the respiration rate due to a progressive disappearance of the light-induced inhibition of dark respiration (Kok, 1948;Sharp et al., 1984). Extrapolation of the linear section of the light curve before the change of slope to a light intensity of zero gives an estimate of Rd.One of the main pitfalls of this method is that the decrease of PPFD during the construction of the light curves induces a gradual increase of ci and in tum a relative increase in the value of A. As a result, the slope of the regression of A versus PPFD decreases. This underestimates the true value of Rd, yielding an Rda. To cope with this problem in the estimation of the true vahe of Rd, we corrected the value of %" following the approach of Kirschbaum and Farquhar (1987). With this approach, the value of Rda obtained from the light-assimilation curve is corr...